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The carbon dioxide free power plant
Global climate change resulting from emissions of carbon dioxide and other
green house gases is one of the greatest environmental problems of our time.
Capture and storage of carbon dioxide has the potential to contribute to a
significant and relatively quick reduction in CO2 emissions from power
generation, allowing fossil fuels to be used as a bridge to a non-fossil future
while taking advantage of the existing power-plant infrastructure. One
capture technology is the oxy-fuel combustion process, which combines a
conventional combustion process with a cryogenic air separation process.
In the first part of this work, the oxy-fuel process is applied to commercial
data from an 865 MWe lignite-fired reference power plant in Lippendorf,
Germany. The second part focuses on the experimental and modeling work on
the process carried out at the Chalmers 100 kW oxy-fuel test facility.
The carbon dioxide free power plant
large scale capture and storage of carbon dioxide
process evaluation and test-facility measurements
This report is a result from the project pathways to Sustainable european energy Systems – a five year project within The aGS energy pathways flagship program.
The project has the overall aim to evaluate and propose robust pathways towards a sustainable energy system with respect to environmental, technical, economic and social issues. here the focus is on the stationary energy system (power and heat) in the european setting.
The aGS is a collaboration of four universities that brings together world-class ex-pertise from the member institutions to develop research and education in collaboration with government and industry on the challenges of sustainable development.
paThwaYS To SUSTainable eUropean enerGY SYSTeMS
The
aG
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waY r
ep
or
TS 2006:e
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foUr UniVerSiTieS
The Alliance for Global Sustainability is an international part-
nership of four leading science and technology universities:
chalMerS Chalmers University of Technology, was founded
in 1829 following a donation, and became an independent
foundation in 1994.Around 13,100 people work and study at
the university. Chalmers offers Ph.D and Licentiate course pro-
grammes as well as MScEng, MArch, BScEng, BSc and nautical
programmes.
Contact: Alexandra Priatna
Phone: +46 31 772 4959 Fax: +46 31 772 4958
E-mail: [email protected]
eTh Swiss Federal Institute of Technology Zurich, is a science
and technology university founded in 1855. Here 18,000 people
from Switzerland and abroad are currently studying, working or
conducting research at one of the university’s 15 departments.
Contact: Peter Edwards
Phone: +41 44 632 4330 Fax: +41 44 632 1215
E-mail: [email protected]
MiT Massachusetts Institute of Technology, a coeducational,
privately endowed research university, is dedicated to advanc-
ing knowledge and educating students in science, technology,
and other areas of scholarship. Founded in 1861, the institute
today has more than 900 faculty and 10,000 undergraduate
and graduate students in five Schools with thirty-three degree-
granting departments, programs, and divisions.
Contact: Karen Gibson
Phone: +1 617 258 6368 Fax: +1 617 258 6590
E-mail: [email protected]
UT The Univeristy of Tokyo, established in 1877, is the oldest
university in Japan. With its 10 faculties, 15 graduate schools,
and 11 research institutes (including a Research Center for
Advanced Science and Technology), UT is a world-renowned,
research oriented university.
Contact: Yuko Shimazaki
Phone: +81 3 5841 7937 Fax: +81 3 5841 2303
E-mail: [email protected]
The carbon dioxide free power plant
Large scale capture and storage of carbon dioxideProcess evaluation and test-facility measurements
AGS Pathways report 2006:EU2
This report is based on a thesis work made by Klas Andersson
at the Department of Energy and Environment, Division
of Energy Technology, at Chalmers University of Technology
PAT H WAY S TO S U STA I NA B L E E U RO P E A N E N E RG Y S Y ST E M S
AG S , T H E A L L I A N C E F O R G L O BA L S U STA I NA B I L I T Y
Göteborg 2006
C o p y r i g h t 2 0 0 6
P r o d u c e d b y K r e a t i v M e d i a A B
E d i t i n g L a r s M a g n e l l
L a y o u t M å n s A h n l u n d
P r i n t e d b y P R - O f f s e t , M ö l n d a l
I S B N 978 - 9 1 - 976 3 27 - 0 - 6
This report can be ordered from:
AGS Office at Chalmers
GMV, Chalmers
SE - 412 96 Göteborg
The carbon dioxide free power plant
Large scale capture and storage of carbon dioxide
Process evaluation and test-facility measurements
Table of contentIntroduction 1
Part 1Method 6
Results 8
Plant Efficiency and Emissions 13
Economic evaluation 15
Conclusions 18
Part 2Aim of Experimental work 20
Experimental facility and test conditions 24
Measurements 27
Results and discussion 30
On-going work on oxy-coal combustion 37
Conclusions 39
Appendix 40
Pathways to sustainable European energy systems 45
The Alliance for Global Sustainability 47
1
Introduction
Carbon dioxide, CO2, is by far themost dominant green house gas, andapproximately 80 percent of theanthropogenic emission of CO2 is dueto fossil fuel combustion.
Capture and storage of carbondioxide has the potential to contributeto a significant and relatively quickreduction in CO2 emissions frompower generation, allowing fossil fuelsto be used as a bridge to a non-fossil
future while taking advantage of theexisting power-plant infrastructure.
Oil recoveryAt present there are no power plantswith carbon dioxide capture availableon a commercial scale, but long timeaquifer storage is currently applied andevaluated in the North Sea showingpromising results. Carbon dioxidecould also be stored in connection to
enhanced oil recovery or enhancedcoal bed methane recovery to make upfor some of the economic lossesassociated with the energy penalty ofthe capture processes.
The oxy-fuel combustion processNew technologies can be used incapture plants for combustion of fossilfuel with subsequent capture andstorage of carbon dioxide. One such
Global climate change resulting from emissions of carbon dioxide and other greenhouse gases is one of the greatest environmental problems of our time.
In order to stabilize the atmospheric concentrations of greenhouse gases drastic cuts in particularly carbon dioxide emissions are required.
2
Coal-mining in GermanyThe International Energy Agency (IEA) pre-dicts that renewable energy sources,such as wind power, will continue to represent less than 15 per cent of worldconsumption for another 20 to 30 years. In the EU (EU-25) the use of fossil fuels isforeseen to increase. According to the IEA reference scenarios, 50–60 per cent ofthe electricity and heat will be based on fossil fuels in 2030, compared to 55 percent in 2002.
3
technology is the oxy-fuel combustionprocess, which combines a conventio-nal combustion process with a cryo-genic air separation process, so that thefuel is burnt in oxygen and recycledflue gas. This results in a high concen-tration of carbon dioxide in the fluegas, which reduces the cost for itscapture. Other commonly studiedprocesses are amine-based absorptionprocesses and the integrated gasifica-tion combined cycle.
The Lippendorf plantIn the first part of this work, the oxy-fuel process is applied tocommercial data from an 865 MWelignite-fired reference power plant inLippendorf, Germany, and large airseparation units (ASU).
A detailed design of the flue gastreatment pass, integrated in the overallprocess layout, is proposed. Essentialcomponents and energy streams of thetwo processes have been investigatedin order to evaluate the possibilities forprocess integration and to determinethe net efficiency of the capture plant.
The electricity generation cost and
the associated avoidance cost for thecapture plant have been determinedand compared to the reference plantwith investment costs obtained directlyfrom industry. Although an existingreference power plant forms the basisof the work, the study is directedtowards a new state-of-the-art lignite-fired oxy-fuel power plant. The boilerpower of the capture plant has beenincreased to keep the net output of thecapture and the reference plant similar.
The second part of the reportfocuses on the experimental andmodeling work on the combustionprocess carried out at Chalmers. Theexperiments are performed in theChalmers 100 kW oxy-fuel test facility.The overall aim of this work is toincrease the knowledge on thecombustion fundamentals and togenerate data and modeling toolsrequired in the design of a commercialscale oxy-fuel fired power boiler.
Combined captureBoth absorption based capture pro-cesses and oxy-fuel combustion areoften considered as alternatives for
retro-fitting of existing coal-fired unitsmaking it possible to take advantage ofalready invested capital which to acertain extent can be considered assunk costs. However, existing units areoften old units with rather modest netefficiencies and with the increasedparasitic losses of the capture, theretrofit concept results in comparative-ly high fuel costs. It should thereforebe pointed out that although anexisting reference power plant has beenthe basis of the process design of thisstudy, the present work should beconsidered as a process evaluation for anew oxy-fuel fired power plant, with allits components and costs. Furthermore,as a comparison the associated costsare determined for a capture plant bothwith and without a sulphur dioxideremoval system (wet flue gas des-ulphurization) in order to show thepossible economic benefit fromcombined capture of sulphur dioxideand carbon dioxide – in the case thiswill be environmentally approved andapplicable to the type of storage.considered.
4
The oxy-fuel combustion process is applicable to different types of fuels and boilers. It involves
burning the fuel in an atmosphere of carbon dioxide and recycled flue gas instead of in air, as
outlined in the figure above.
The mixed flow of oxygen and recycled flue gas is fed to the boiler together with the fuel,
which is burnt as in a conventional plant. Typically 70-80 percent of the flue gas is recycled from
down stream the economizer and mixed with new oxygen. The remaining part of the flue gas is
cleaned, compressed and later transported to storage or to another application.
cooler andcondenser
cooler andcondenser
fly ash
particleremoval
sulphurremoval
recycle (mainly CO2,water vapour)
sulphurwater
water
O2/CO2 recycle (oxyfuel) combustion capture
boiler
bottom ash
steamcondenser
steam turbine
cooling water
oxygen
carbondioxide
mechanicalenergy
lowtemperature
heat
carbon dioxidecompressor
fuel
Electricity
lowtemperature
heat
air
nitrogen
energy
airseparation
Part 1
5
6
MethodProcess data and schemes wereobtained directly from the plant owner.Based on these data, a processevaluation was carried out in order toidentify new components needed aswell as components that can beexcluded from the reference boilerscheme.
The process layout is based on anexisting air separation unit with anoxygen production of 50,000 cubicmeters per hour.
Different coal qualitiesTwo different chemical processsimulation programs, ChemCad and
Hysys were used to simulate thechemical reactions in the flue gascondenser for cross comparison ofresults. ChemCad, which in this case isthe most accurate one, uses electrolytereactions together with thermo-dynamic models (Peng-Robinson).Hysys does not consider the electrolytereactions and hence, for dissolvedcompounds the accuracy is lower thanin the Chemcad program.
The actual fuel flow and coalcomposition at the Lippendorf plant isadapted to three different coal qualities:a guarantee coal and the so-called maxwater and max ash coals. The guaran-
tee coal is used for all calculationsregarding efficiency and flue gas flows.The plant must nevertheless be able tohandle other coal as well. Thus “maxash” and “max water” coals are usedfor dimensioning the flue gas cleaningequipment.
Flue gas desulphurationAll investment costs have beendetermined in co-operation withindustry. The specific investment costand variable operation and main-tenance cost of the power plant withcapture has been assumed equal to thatof the reference power plant. In the
The 2 x 865 MW lignite-fired Lippendorf powerplant in northern Germany is used as reference inthis study. This is a modern state of the art powerplant commissioned in the year 1999.
Hysys and Refprop softwaresDifferent compressor configurations for the air separation unit,
as well as for the flue gas compression were analyzed.
Calculations of the adiabatic compressor work for different
compressor cycle designs were performed using the software
RefCalc from which pressure enthalpy diagrams and their
corresponding numeric values are obtained.
The properties of the various gas mixtures are obtained
from the NITS standard reference database with the software
Refprop (Reference Fluid, Thermodynamic and Transport
Properties).
Hysys was also used to determine the total heat rejection
and the temperature curve of the condensation unit. In the
proposed scheme, nitric oxide is separated in a liquid/gas
separator since it is assumed not to be soluble in the liquid
carbon oxy-fuel mixture.
Part 1
case where the flue gas desulphuriza-tion is included, the investment costhas been reduced to 60 percent of thecorresponding flue gas desulphuriza-tion for the reference plant, since theflue gas flow will drastically decrease inthe oxy-fuel scheme. According to theliterature wet flue gas desulphurizationtechnology can be applied althoughthe sulphur dioxide removal is to beperformed in a carbon dioxideatmosphere with three times as highvolumetric concentrations of sulphurdioxide compared to normal con-ditions in air firing.
95 percent oxygen purityThe investment and running costs ofthe air separation unit has been up-dated for the increased plant size.These costs were obtained from threedifferent unit sizes ranging from276,900 to 528,700 mn3/h of oxygen.The specific power consumption ofthese air separation units ranges from
0.34 to 0.36 kW/mn3/h for a unitproducing oxygen of 99,6 percentpurity. The power consumption andthe investment costs are reduced whena lower purity is required. In this studyall the calculations are made for anoxygen purity of 95 percent. Thereduction in power demand is about
1.6 percent whereas the reduction forthe investment costs is 4 percent.
A cost analysis for each process part(power plant, air separation unit andFGCC – Flue Gas Cleaning andCompression) was set up in order toobtain the electricity generation costand the avoidance cost.
7
Table 1. Process data for the reference power plant Lippendorf (identical for both blocks).
Gross electricity output 933 MW
Net electricity output 865 MWeBoiler power 2030 MW
District heat extraction 115 MW
Electricity net efficiency 42.6 %
Fuel Raw lignite
Steam flow 672 kg/s
High pressure steam 554/258.5°C/bar
Intermediate steam pressure 583/49.7°C/ bar
Steam pressure at condenser discharge 0.038 bar
Part 1
The electricity generation cost andthe associated avoidance cost forthe capture plant have beendetermined and compared to thereference plant, the 2 x 865 MWlignite-fired Lippendorf plant, withinvestment costs obtained directlyfrom industry. Although an existingreference power plant forms thebasis of the work, the study isdirected towards a new state-of-the-art lignite-fired oxy-fuel powerplant.
8
ResultsFigure 2 gives a principal processscheme of the lignite fired oxy-fuelplant as obtained from the processevaluation, using the Lippendorf asreference plant.
The air separation unit is based oncryogenic air separation, which is theonly separation technique, which canprovide the oxygen flows required inlarge-scale coal combustion plants.
An oxygen production rate of528,000 mn3/h is required and can besplit into either 2 or 4 air separationunit production lines with a corre-
sponding maximum productioncapacity per line of 277,000 or 138,000mn3/h. An oxygen purity of 95 percentis selected as the most favorable, sinceit gives an exchange rate (oxygen tooxygen) of 1.0 without nitrogen in theproduct gas, but with near 5 percentargon content. Thus, for oxygenpurities lower than 95 percent, theoxygen contains nitrogen in addition toargon. The final power consumption ofthe air separation unit with the abovefeatures becomes 181 MWe. Thecompressor(s) in the air separation
unit, with intercooling in four steps,operates between ambient temperatureand about 60°C. Without intercoolingan air temperature of about 210°C isreached, with a heat rejection of about1 MWt per MWe consumed, whichcould be used for feed water pre-heating or district heating. However,this would result in a significantdecrease in compressor efficiency ofapproximately 20 percent, whichmakes this alternative unattractive.
The main result from the process study is thedetailed overall process layout shown in the figurebelow. Based on this layout, which includespossibilities for process integration, the net efficien-cy of the oxy-fuel power plant was determined.
Figure 2The three main parts of the plant are the air separation
unit (A), the boiler island (B) and the flue gas treatment
pass (C) with the essential features and components
described below following the numbering of Figure 2.
Part 1
1. Air compressor2. Compressor cooling3. Direct contact air cooler4. Ev aporative cooler5. Molecularsieves6. Heat exchanger7. Expansionturbine8. Destillationcolumn9. Boiler
10. Super heater11. Economizer
12. TEG heat exchanger13. Flue gas condensation unit14. Flue gas cooler or FGD unit15. Compressor unit 1, 30 bar16. TEG17. Compressor unit 2, 58 bar18. CO2 condenser19. Heatexchanger (CO2/CO2)20. Gas/Liquid separator21. Subcooler22. High pressure pump
23. HP Steam turbine24. IP Steam turbine25. LP Steam turbine26. Condenser27. Cooling tower28. District heating29. Feed water preheater (FPH)30. FPH31. Optional heater,
district heat/FPH32. Nitrogen heaterr
CoolingThe cooling of both the carbon dioxidecompressors and the air compressor iscarried out with cooling water fromthe plant cooling tower. In addition,the carbon dioxide condensation alsorequires cooling water from the coolingtower.
In total, there must be an almost 50percent increase in mass flow ofcooling water produced in the coolingtowers compared to the reference plantin order to attain the low temperaturesfor the compressor inter cooling andthe carbon dioxide condensation.However, if the temperature levels areincreased only a few degrees thecoolant mass flow will decreasesignificantly due to the narrow tempe-rature intervals set in this study for thecooling of the components. To mini-mize losses in power transmission tothe air and carbon dioxide compres-sors, these can be directly driven bysteam turbines on a joint shaft.
However, the main steam turbine shaftcannot be used for this purpose, sinceit would cause problems both in thecompressor units, such as surge at thestart up, and in the shaft itself becauseof too large thermal motions.
The extra investment cost of theseparate powering of the compressors
is to be compared with the powersaving corresponding to about 0.7percent of the net output.
The heat required in the molecularsieves are provided by the nitrogenheater, which exchanges heat from theflue gas with a minimum temperatureof 200°C. A cooling capacity of about
9
Table 2. Design composition of the flue gas during oxy-fuel combustion
Components [kg/s] [wt%] [mn3/s] [vol%]
H2O 176.4 38.4 222.3 60.4
CO2 253.9 55.3 0.0 35.4
SO2 6.7 1.5 2.3 0.6
O2 6.4 1.4 142.1 1.2
N2 0.6 0.2 22.1 0.2
Ar 14.6 3.2 10.1 2.2
Total 458.7 100 416.9 100
Part 1
Enormous amounts of carbon are naturally stored in the forest in trees andother plants, as well as in the forest soil. As part ofphotosynthesis trees absorbcarbon dioxide from theatmosphere and store it ascarbon while oxygen isreleased back into theatmosphere.
Photo: Måns Ahnlund
10
25 MW at a temperature of 8°C, canbe generated in the evaporative coolerthat can be used in the flue gas treat-ment for sub cooling of the carbondioxide
Table 2 (above) shows the flue gasmass and volume flows entering theflue gas treatment pass and the flue gasrecycle loop in the oxy-fuel powerplant. As seen wet recycling conditionsare applied in the present process lay-out. This recycle approach will resultin an increased level of sulphur dioxideand other impurities in the boiler.However, the advantage is that the fluegas flow entering the cleaning equip-ment is drastically reduced, whichleads to savings in investment costs.Given that the flue gas desulphuriz-
ation is included in the process, nodrastic changes on the overall design ofthe flue gas treatment would berequired if, for fuel specific reasonsmainly, dry and clean flue gas recyclingconditions need to be applied. Onesuch reason may be high concen-trations of sulphur trioxide (SO3)causing the sulphuric acid dew point todrop. In principle the only difference isthat the recycle line need to bechanged to include flue gas condenserand a flue gas desulphurization unitand that the size of these two unitsmust be increased to enable treatmentof the total flue gas flow.
Preheating the feed waterVarious options for the utilization of
the flue gas heat, available downstream of the economizer in thecapture plant, have been evaluated.Heat available at a minimum temper-ature of 90°C could be used in anabsorption cooler to produce a coolantstream at 5°C, for example to be usedfor sub-cooling purposes of the carbondioxide or cooling of the compressors.Especially the large amount of heatrejected from the flue gas condenser,approximately 365 MW between 88°Cand 60°C where the most part of theheat of evaporation is released, willrequire a heat sink. Part of this heatcould be used in the available districtheating system, but the one alternativerepresented in the results of this workis the integration with the steam cycle
Figure 3 shows the mass flow of the
flue gas components throughout the
treatment steps with respect to
excluding and including a flue gas
desulphurization unit. Both options are
provided since sulphur dioxide may
cause problems depending on the type
of storage considered. For example,
storing of the sulphur dioxide below
ground level can cause problems due to
its sulphation behaviour in contact with
calcium present in the storage environ-
ment. If sulphates are produced, in for
example a saline aquiphere, the porosity
will decrease, which directly will affect
the storage capacity.
Part 1
11
to preheat the feed water instead ofusing low-pressure steam in order toincrease the electricity output of theplant.
Combined storageThe flue gas treatment basicallyinvolves the removal of water and non-condensable gases. A more or lesscomplete dehydration of the flue gas isrequired to avoid problems in the finalflue gas treatment pass and in thetransportation of the captured carbondioxide. Provided that the gas is driedto ppm level there is a technicalpossibility for combined storage ofcarbon dioxide and sulphur dioxide.This, since the physical behaviour ofthe two compounds is similar duringsupercritical conditions, for example
conditions required for the transportand the injection at the storage site.Such a capture plant could thenexclude a sulphur dioxide removalsystem, which would results in someeconomic benefit.
Complete dehydrationBesides technical feasibility, a com-bined underground storage of carbondioxide and sulphur dioxide alsodepends on political decisions withrespect to dumping conventions. Acomplete dehydration of the flue gas isimportant since it will reduce the massflow, inhibit corrosion and hydrateprecipitation. In the case of a com-bined storage of carbon dioxide andsulphur dioxide, provided that the fluegas is dehydrated to a dew point five
degrees below the temperaturerequired for transport conditions, thesulphur dioxide will in principle behaveas the carbon dioxide and the twogases will not cause any corrosionproblems. For both storage options, thegas must nevertheless be dehydratedbefore reaching the high-pressure stepsin the compression to make thecompression of the gas mixturepossible.
Sweet corrosionCarbon dioxide alone can be corrosivein the presence of water and cause socalled sweet corrosion, that is whenwater vapour in the gas form solid ice-like crystals called gas hydrates. Thehydrates are formed when waterencages gas molecules smaller than 1.0
Part 1
Tight rock – Cap rock
Porous rock –Sandstone
Rock (ca 1000 m)
Soil
Coal power plant
Coal
Electricity
CO2
CO2 Storage
The idea is to capture CO2from a coal-fired power plant,transform it into a liquid, andpermanently store it deepunderground in suitablegeological formations. Thestorages are of the same kindas where oil and gas havebeen stored for millions ofyears, a porous rock with animpermeable layer on top. Atpresent several such storagetests are performed all overthe world, for example be-neath the sea or in the NorthSea since 1996 (the SACSproject) and also outsideBerlin, where an abandonedgas storage exists (theCO2SINK project).
12
nm (which is the case for both carbondioxide and sulphur dioxide) at lowtemperatures and elevated pressures(below 25ºC and above 15 bar).Various mechanisms for the carbondioxide corrosion process have beenproposed, which all involve eithercarbonic acid or bicarbonate ionformed when carbon dioxide isdissolved in water. Also in this case,dehydration to dew point five degreesbelow the transport temperature issufficient to avoid the problem. This,since dry carbon dioxide is notcorrosive at temperatures below 400°C.The maximum water content in thegas prior to compression should there-fore not exceed 60 to 100 mg/mn3,whatever the content of other possibleacidic compounds. For pipelinetransportation with presence of water,serious corrosion can be expected if
the partial pressure of carbon dioxideexceeds 2 bar.
Tri ethylene glycol unitIn the process chosen the gas isdehydrated in two steps. The first is ina traditional flue gas condenser wheremost of the water is removed, togetherwith remaining particles (sulphurtrioxide etc). The second dehydrationstep is the tri ethylene glycol unit,which will remove the remaining waterdown to a value of 60 mg/m3n, corres-ponding to a dew point of –5°C at 100bar under transport conditions. Sincethe tri ethylene glycol unit requires apressure of 30 bar to be efficient, acompressor step with intercooling isinstalled before the unit. Some water isseparated in the cooling steps in thecompressor.
To reduce the power consumption
of the flue gas compressors, compres-sion is carried out, up to the transportpressure, by a high-pressure pump.This, since the pressure should only beincreased to a sufficient level, allowingtransfer of the flue gas (mostly carbondioxide) into a liquid state at a reason-able cost. The first compressor stepraises the pressure from 1 bar to 30bar, which is the inlet pressure of thetri ethylene glycol unit. The flue gas isthen compressed in the second stepfrom 30 bar to 58 bar. At a pressure of58 bar, the carbon dioxide and sulphurdioxide will be liquefied, if cooled to20°C by the main cooling system.When the carbon dioxide is liquefied ahigh-pressure pump is used for the lastpressure increase up to 100 bar beforetransportation to the injection site.
Carbon dioxide
Besides technical feasibility, a combined underground
storage of carbon dioxide and sulphur dioxide also depends
on political decisions with respect to dumping conventions.
Part 1
Sulphur dioxide
13
Plant Efficiencyand Emissions
The net electrical efficiency of theplant becomes 33.5 percent, which isto be compared with 42.6 percent forthe reference plant. The energy penaltyof the capture plant consequentlybecomes 21.5 percent. The plant in theSankey diagram represents the option
including sulphur dioxide removal. Inthe case of combined capture ofsulphur dioxide and carbon dioxide theinternal electricity demand is reduced,with a boiler power of 2524 MW and anet efficiency is slightly increased to34.3 percent (see the power plant
specifications in Table 5 on page 17).The Sankey diagram of the captureplant includes the reduced powerdemand from a reduced flue gas flowas well as the integration possibilities asdiscussed previously. The electricityproduction is increased by about 10
The Sankey diagram below (figure 4) shows the reference power plant (a) and theoxy-fuel power plant (b). The figure illustrates the energy losses in the oxy-fuelplant with the same net electricity output as in the reference plant.
Part 1
Oceans are natural carbon dioxide sinks.
Table 3. Comparison of emissions to atmosphere between the reference plant and the oxy-fuel power plant.
Reference plant oxy-fuel-plant Emissions to air [mg/mn3] [kg/h] [kg/MWhe] [mg/mn3] [kg/h] [kg/MWhe]
SOx <350 <1120 1,28 <6 <13 0,015
NOx <145 <460 0,53 <141 <190 0,220
CO2 <235 <740 000 855,2 <4 <5000 5,8
Dust <2 <6 0,007 <1 <1 0,001
Estimation on nitric oxide with a reduction of the emission of about 60 percent. The reduction can be attributed to the
absence of thermal nitric oxide as well as a drastic reduction in the conversion ratio of fuel-N to exhausted nitric oxide. The
emitted nitric oxide is ventilated to air in a concentrated stream in the gas/liquid separator (as shown in figure 2, in section
C) since it is assumed non-soluble in the carbon dioxide/sulphur dioxide mixture. In order to reach further reduction in nitric
oxide emissions the high nitric oxide concentration stream should be well suited for a nitric oxide catalyst.
14
Part 1
Figure 4. The Sankey diagrams of
a) the reference power plant and
b) The capture plant with the same power output as in the reference power plant.
MW due to feed water preheating withheat from the flue gas condensation.
Table 3 summarizes emissions to theatmosphere from the oxy-fuel -firedplant obtained in the present study and
compares these with those of thereference plant. The combustionconditions are considered to bestoichiometric with an oxygen excessof 1.5 percent on a dry basis.
The sulphur oxide, nitric oxide andcarbon dioxide emissions are leakageflows and ventilated flows from theflue gas treatment pass.
a b
15
Economic evaluation
Table 4 lists the overall cost parametersused together with the capture andavoidance costs.
An interest rate of 10 percent hasbeen assumed as a mid range valuecompared to previously performedstudies on the economics of carbondioxide capture where interest ratesbetween 7 and 15 percent have beenassumed. This is also in line with thestandard power plant economic andassessment criteria, introduced by IEA,
which suggests an interest rate set at10 percent and an assumed load factorof 85 percent. However, according toOECD the long-term interest rate (10year basis) is forecasted to around 4percent in the Euro area (March,2005), why Table 4, below, gives thecarbon dioxide avoidance cost fordifferent interest rates and fuel pricesand obviously these parameters have asignificant impact on the results.
It should hence be noted that this
figure is likely to be lower in Europeanprojects and in a previous work by theauthors an interest rate of 6 percentwas applied. Lignite price, economiclifetime of the plant and distribution ofcosts during construction are based oninformation from industry. The energyavailability of the plant is 7 500 hourper year at full capacity, whichcorresponds to a load factor of 85percent.
The electricity generation cost with
The following pages give a brief outline of the economic parameters that determinethe carbon dioxide capture and avoidance costs. An interest rate of 10 percent hasbeen assumed as a midrange.
Part 1
Table 4. Input cost parameters
Cost parameters
O&M cost assumptions Power plant - variable [ /MWh] 1.0
Power plant - fixed [%] 1.5
air separation UNIT - fixed [%] 4.0
Flue gas treatment fixed [%] 4.0
Distribution of costs over 1st yr 0.15
construction period: 4 yrs 2nd yr 0.30
3rd yr 0.35
4th yr 0.20
Interest rate [%] 10.0
Lignite price [ /MWh] 4.00
Exchange rate [$/ ] 1.30
Plant economic life time [yrs] 20
16
Part 1
flue gas desulpuration increases from42.1 to 64.3 USD per MWh, whichcorresponds to a carbon dioxideavoidance cost of 26 USD per toncarbon dioxide (or 20 euro per toncarbon dioxide).
Without flue gas desulphurizationthe avoidance cost decreases about 4 USD per ton (3 euro per ton carbondioxide).
Marginal effectAs previously discussed, storage ofcarbon dioxide contaminated withsulphur dioxide may be difficult fromboth a legal and public acceptancepoint of view. However, the resultsshow that the combined storage has amarginal effect on the overall costsituation. It should be pointed out thatthe resulting cost is strongly dependent
on the economical parameters for theannuity cost calculations (although thisshould be rather obvious). Theavoidance cost differ substantially bothdepending on the interest rate chosenfor the invested capital and on the fuelcost. Thus, the latter should be kept inmind when comparing the results fromthe cost studies presented in theliterature.
Photo: Måns Ahnlund
17
Table 5. Plant performance and costs of the reference plant and the oxy-fuelpower plant with and without flue gas desulphurization (the cost of the flue gasdesulphurizationis included in the power plant cost).
Plant performance oxy-fuel w. flue gas oxy-fuel w/o flue gasand costs Reference desulphurization desulphurizationBoiler power [MW] 2026 2585 2524Gross electrical output [MW] 933 1203 1176Net electrical output [MW] 865 865 865Operating time [h] 7500 7500 7500Fuel demand [kg/s] 192.9 240.4 246.2O2 demand [kg/s] - 221.6 226.9Total investment costs [106x ]
Power plant 1100 1403.50 1310.74Air separation UNIT - 304.59 297.06Flue gas treament - 35.46 34.70
Annualized capital cost [106x yr] 126.55 200.59 188.97O&M costs: fixed + variable [106x yr]
Power plant 22.99 29.33 27.39Air separation UNIT - 12.18 11.88Flue gas treament - 1.42 1.39
Annualized O&M cost [106x yr/yr] 22.99 42.93 40.66Fuel costs [106x yr/yr] 60.77 77.55 75.72Total annualized costs [106x yr/yr] 210.31 321.07 305.35Electricity generation cost [ /MWh] 32.4 49.5 47.1Electricity generation cost [$/MWh] 42.1 64.3 61.2Emitted carbon dioxide [ton/MWh] 0.855 0.0058 0.0058Avoidance cost [ /ton carbon dioxide] - 20.0 17.1Avoidance cost [$/toncarbon dioxide] - 26.0 22.3
Part 1
CO2 storageThe capacity to store carbon dioxide inEurope seems to exeed the needs basedon available amounts of fossil fuels. Theresearch on CO2 storage is focused onthree case scenarios:1. On-shore geolocical storage near the
powert plant.2. On-shore geological storage far away
from the plant.3. Off-shore geological storage beneath
the sea floor in the North Sea.
18
ConclusionsThis study proposes an overall processscheme of an oxy-fuel plant based oncommercial data for key componentsrequired in the process. With all inte-gration possibilities considered the netefficiency becomes approximately 33.5percent which should be compared to42.6 percent in the reference plant. Analmost complete dehydration of theflue gas is of great importance to avoidproblems in the final flue gas treatmentand in the transportation of the carbondioxide. The fixed and running costsassociated with an 865 MWe lignite-fired oxy-fuel power plant have been
evaluated in order to obtain the carbondioxide avoidance cost for a new state-of-the-art capture plant. The captureplant has been scaled up to yield thesame net capacity as the Lippendorfreference plant and the carbon dioxideemissions to the atmosphere arereduced with 99.5 percent. With alignite price of 5.2 USD per MWh (4.0euro per MWh) and an interest rate of10 percent, the electricity generationcost increases from 42.1 to 64.3 USDper MWh which corresponds to a car-bon dioxide avoidance cost of 26 USDper ton carbon dioxide (or 20 euro/ton
carbon dioxide). Furthermore, thestudy shows that if combined captureand storage of carbon dioxide andsulphur dioxide is environmentallyapproved and applicable to the type ofstorage considered, the economicbenefit for the plant studied is stillsmall with a reduction in the electricitygeneration cost of 3.1 USD per MWh.In summary, by using commercial datafrom existing plants and componentsthis study shows that oxy-fuel com-bustion is a realistic and a near futureoption for carbon dioxide reductions inthe power sector.
Carbon dioxide Capture and Storage (CCS) technology in brief The carbon dioxide (CO2) is captured from thepower plant combustion process and compressedto liquid phase to make transportation easier. TheCO2 is then transported to a storage site andinjected into an underground porous rock formationfor permanent storage at a depth of 800 metres ormore. Here the carbon dioxide is maintained in aliquid state by the ambient pressure at that level.
Part 1
Part 2
19
20
Aim of Experimentalwork
The overall objective of this work is toincrease the knowledge on thecombustion fundamentals and togenerate data and modelling toolsrequired in the design of a commercialscale oxy-fuel fired power boiler.
With this aim the Chalmers groupworks in close collaboration togetherwith the University of Stuttgart,Vattenfall Utveckling AB and FluentEurope in projects funded by the EU,the European power industry and theSwedish Energy Agency. One im-portant near future activity, in whichChalmers participates, is the designand testing of the Vattenfall 30 MWoxy-fuel pilot plant, which is plannedto be commissioned in the beginningof 2008.
Radiative heat transferThe work presented here summarizesthe results from a recent study onflame characteristics of gas fired oxy-
fuel combustion where the focus is puton the radiative heat transfer of theflame and the combustion gases in theoxy-fuel environment. A description ofthe experimental equipment used forthe measurements and the theory (seeappendix) used for evaluating andanalysing the radiative properties of theflame is given, followed by a des-cription of the results from the study.Furthermore, a brief summary of theon-going work on oxy-coal com-bustion is given in the later part of thissection. The experimental test unitfacilitates oxy-fuel combustion withreal flue gas recycling conditions. Thegas-fired tests comprise a reference testin air and two oxy-fuel test cases withdifferent recycled feed gas mixtureconcentrations of oxygen (21 and 27volumepercent) and carbon dioxid (79and 73 volumepercent). In-furnace gasconcentration, temperature and totalradiation (uni-directional) profiles are
presented and discussed. The changein total emissivity and gas emissivity ofthe oxy-fuel environment is discussedby means of available models and theexperimental data.
Dry or wet mixturesIn existing coal-fired power plants it iscommon that a small portion of theflue gases are externally recycled as ameans to control the combustiontemperature level in order to reducethe nitrogen oxide formation. For theoxy-fuel concept a high amount ofrecycled flue gas is required in order tomaintain important combustionparameters, such as temperature, heatexchange, residence time and mixingconditions etc, on similar levels tothose of air-fired combustion. Thecombustion will therefore take place indry or wet mixtures of oxygen andcarbon dioxide instead of in oxygen/nitrogen. The overall combustion
The second part of this report focuses on the experimental and modelling work onthe combustion process carried out at Chalmers. The experiments are performedin the Chalmers 100 kW oxy-fuel test facility.
Part 2
21
Front side of the Chalmers 100 kW oxy-fuel combustion test unit showingthe top fired 3.0 meters high combustor. In the background to the left thefabric filter can be seen with the surrounding piping system.
Part 2
Part 2
22
behavior will, to a varying extent, bedifferent from conventional com-bustion with possible changes in forexample fuel burnout, ignitionbehavior, heat transfer and pollutantemissions. Apparently the differencesare caused by the different physicaland chemical properties of carbondioxide compared to nitrogen. Theexperimental work performed else-where on oxy-fuel combustion mainlyconcerns the combustibility andemission formation of different type ofcoals for different mixtures of oxygenin either dry or wet recycled flue gastogether with in-furnace measurementsof species concentrations, temperaturesand heat flux profiles. The work onsuch combustion characteristics hasbeen carried out both in laboratory andsemi-industrial scale test facilities.
Nitrogen oxidePrevious work has concluded that coal-fired oxy-fuel combustion could bemaintained under satisfactoryconditions, that is conditions thatwould be applicable to existing utilityboilers. More recent experimental workhas focused on the gas phase reactionsand overall emission characteristics fordifferent coal types, and, especially sowhen it comes to formation and re-duction of nitrogen oxide during eitherreal or simulated flue gas recyclingconditions. These studies indicate thatthe nitrogen oxide conversion ratiofrom fuel-N under oxy-fuel conditionscan be reduced significantly compared
to air-fired conditions. One obviousreason to these results is that the nothermal nitrogen oxide is formed dueto the absence of nitrogen in the feedgas. More detailed studies on thenitrogen chemistry for different typesof coals and combustion atmospheresare however required to get a deeperunderstanding on the nitrogen oxideformation during oxy-fuel combustion.
Little data is available on theradiative properties of different mixtu-res of oxygen and carbon dioxid andfuels.
Difference in radiative propertiesSuch data is required in order to assessthe difference in radiative properties interms of emissivity between air andoxy-fuel combustion, for different typesof fuels with the overall aim ofevaluating the accuracy of radiationmodels presently used in commercialCFD codes. Tests on both natural gas-fired and coal-fired conditions in airand oxy-fuel have been performed,which include measurements of theaxial distribution of the hemisphericheat flux using an ellipsoidalradiometer. Since, for refractory linedfurnaces, such as the one used in thework presented here as well as in theexperimental studies carried outpreviously, the dominating con-tribution to the local incident radiativeflux originates from the surroundingwalls, which makes the analysis ofhemispheric heat flux data lessinteresting. This due to that the inner
wall surface temperature betweendifferent test conditions is bound tochange from air to oxy-fuel conditions,and, that the wall emissivity, for thesetypes of materials and temperatures,will approach unity. The walldominance on hemispheric radiationmeasurements is further accentuatedfor moderate and non-sooty flameswith low emissivity of the flameenvelope. In one of the previousstudies this set back of the ellipsoidalradiometer measurements wasrecognized and it was concluded that,in order to determine the flameradiation characteristics of oxy-fuelcombustion in a more accuratemanner, wall effects should be mini-mized to facilitate a direct comparisonbetween oxy-fuel and air-firedconditions.
Detailed profilesHence, in this work the aim is to re-duce the influence of the hot furnacewalls on the radiation data. The studycomprises uni-directional measure-ments of the radiation intensity inorder to facilitate the possibility tointerpret the difference between thethree test environments. In addition,detailed profiles on temperatures andgas concentrations have been measuredto characterize the properties of thedifferent flames and to facilitate a moredetailed discussion of the flameemissivity in relation to the radiationprofiles.
The upper part of the Chalmers oxy-fuel combustor. To the left the pipes ofthe primary and secondary streams are seen, which feed the top mountedburner with either air or a mixture consisting of oxygen and carbon dioxide.
Part 2
23
24
Experimental facilityand test conditionsFigure 2 shows the 100 kW oxy-fuelcombustion test unit located inconnection to the Chalmers 12 MWresearch boiler, thus taking advantageof the existing measurement infra-structure. The test unit has specificallybeen designed for the study of oxy-fuelcombustion under real flue gas re-cycling conditions. In this work, gas-fired tests were carried out with a fuelinput of about 80 kW. The testconditions and fuel properties arespecified in Table 2 and Table 3,respectively. The cylindrical refractorylined reactor is down-fired with aninner diameter of 0.8 meters and totalheight of 3.0 meters. The tests can berun under oxy-fuel conditions (withdifferent mixing ratios between oxygenand carbon dioxide/water), with air orwith oxygen enriched combustion,with the latter condition by means ofdirect oxygen injection in the burner.The flue gas recycle loop includes oneseparate temperature controlled coolerand one condenser enabling both wetand dry flue gas recycling (Figure 2). In this work, dry flue gas recyclingconditions was applied. The flue gastemperature after the condenser isreduced to about 25 to 30ºC corres-ponding to 2 to 2.5 volumepercent
moisture content in the recycled fluegas, and thus similar to non-preheated air-fired conditions. The ratio of thevolumetric flow between the recyclestream and the stack gas is set by twofrequency controlled ID-fans.
For the experiments given in thispaper, the oxygen is mixed into therecycled flue gas up-streams the ID-fanin the recycle loop. After passingthrough the recycle fan the feed gas isdivided into a primary and a secondarystream which then enters the tworespective registers of the burner.Approximately 40 percent of the totalflow enters the primary register. Theratio between the primary andsecondary stream is adjusted byindividual valve settings and thevolumetric flows are continuouslylogged by turbine flow meters. Bothoxygen and propane is supplied frombundles of gas tubes. The oxygen andpropane flows are controlled byelectronic mass flow meters.
In order to reduce the time for thecombustor to reach thermal steadystate conditions, cooling tubes withvariable length and an outer diameterof 50 mm are inserted from the top ofthe combustion chamber at fourpositions close to the wall, equally
distanced from the flame. Thetemperature and flow rate of theincoming and outgoing cooling wateris continuously measured for heatbalance and modeling purposes. Walltemperature conditions are monitoredby means of thermocouples measuringthe temperature 20 mm from the innerwall surface at 2x7 positions at eachside of the probe ports R1 to R7 (Table1). The ports along the side of thefurnace are used for in-furnace datacollection, by means of water-cooledprobes. A number of ports are availabledown stream of the reactor exit and atvarious positions in the flue gas recyclepass, which allow for flue gas as well asrecycle feed gas composition measure-ments.
Table 1. Measurement positions onthe Chalmers 100 kW oxy-fuel unit
Port Distance number from burner [mm]R1 46R2 215R3 384R4 553R5 800R6 998R7 1400
Part 2
25
TC TC
TI
SC
FI
TI
FI
FI FI
FI
FI
PIC
TI
C3 H
8
O2
SC
air/O2/CO2 fanmixing pointO2/flue gas
Wet flue gasrecycle
Dry flue gasrecycle
Flue gascooler
Flue gascondenser
Air inlet
Stack gas
Cooling water
Primary/secondaryregister
Measurementports R1, R2...R7
Direct O2 injection
pre-heater
Pilot burner
Cylindricalfurnace
800 mm
2400
mm
Cooling tube 1/4
Figure 2. Schematic of the CHALMERS 100 kW oxy-fuel combustion test unit, which has been adopted for both gas and coal firing.
Primaryoxidant
Secondaryoxidant
Propane
Oxygen
Replacebleregisters
34 mm
52 mm
92 mm
Figure 3. Schematic side viewof the gas-fired burner.
Part 2
The gas-fired burner (Figure 3) consistsof two separate lances for fuel anddirect oxygen injection (as mentioned,the oxygen register is not used in thesetests). The primary and secondary swirlregisters are replaceable in order tofacilitate a change of the swirl numbersettings and the corresponding mixingconditions of the flame. Table 2 liststhe settings used in the present worktogether with the overall testconditions of the measurements.
The tests comprise a reference testin air and two oxy-fuel test cases withdifferent recycled feed gas mixtureconcentrations of oxygen (21 and 27volume-percent) and carbon dioxide
(79 and 73 volume-percent). The 21volume-percent oxygen case (OF 21case) was chosen to keep the volume-tric flow conditions similar to the aircase whereas the 27 volume-percentoxygen case (OF 27 case) should give atemperature level similar to air-firedconditions. In-furnace gas concentra-tion, temperature and total radiation(uni-directional) profiles are presentedand discussed.
All tests were performed at astoichiometric ratio of 1.15 resulting ina target oxygen excess of 3.0 volume-percent for the air and the OF 21cases, and 3.8 volumepercent for theOF 27 case. The higher oxygen excess
in the latter case is due to that thevolumetric flow of recycled flue gas ismore than 20 percent lower than inthe OF 21 case. For the OF 27 case theresidence time is increased by thereduction in volumetric flue gas flowand to some extent also by the changein temperature distribution from air tooxy-fuel fired conditions. From theexperimental results it is seen that thein-flame temperature distributions ofthe OF27 and the reference case arerather similar, that is the difference intemperature will only moderatelyinfluence the relative change in resi-dence time from the air to the OF 27case, whereas this change will be morepronounced under the OF 21conditions. All in-furnace measure-ments have been repeated twice inorder to ensure satisfactoryrepeatability for each test condition.
26
Part 2
Table 4. Fuel composition and oxygen quality
Fuel analysis [average mole content]Methane 0 mol %Ethane < 0.1 mol %Propane 98.3 mol %Butanes 1.1 mol %Pentanes & higher < 0.1 mol %Total Unsaturated HC < 0.1 mol %Sulphur <1 mg/kgHi 46.4 MJ/kg
OxygenQuality (industrial std) 2.5 -Oxygen content 99.5 vol%
Table 2. Test conditions for the gas-fired tests
Table 3. Test conditions for the three test cases
Test Combustion Temp Feed gas composition Theoretic CO2case media feed (vol %) conc. @ stack
gas [ºC] O2 N2 CO2 (vol %)
Air air 25-30 21 79 - 12
OF21 O2/CO2dry recycle 25-30 21 - 79 97
OF27 O2/CO2dry recycle 25-30 27 - 73 96
Heat input 80 kWth
Fuel C3H8Stoich ratio 1.15
Primary register Fin angle 45Swirl No 0.79
Secondary register Fin angle 15Swirl No 0,21
27
MeasurementsGas concentration measurementsMeasurements of in-furnace gasconcentrations, temperatures and uni-directional radiation intensity werecarried out to characterize the threetest cases specified in Table 3. Gascomposition data consisting of oxygen,carbon monoxide, carbon dioxide andtotal hydrocarbon concentrations arewithdrawn from the furnace by meansof a gas suction probe inserted in theprobe ports and at the furnace exit.The hydrocarbon is measured asmethane equivalents by means of anFID (flame ionization detector) basedinstrument. The analysis of oxygen,
carbon monoxide, and carbon dioxidewere each performed for two differentinstruments with different measure-ment ranges, adapted for reference andoxy-fuel conditions respectively. Themeasurement principles of both oxygeninstruments are of paramagnetic type,carbon monoxide is analyzed based onNDIR (non-dispersive infraredabsorption) and carbon dioxide ismeasured based on NDIR and TC(thermal conductivity).
The gas suction probe is shown inFigure 4 and consists of a heated innertube with controlled temperature toprevent condensation inside the probe
and an outer protecting water-cooledjacket with a diameter of 45 mm. Thegas is dried and filtered before it is fedinto the on-line gas analyzer train. Thegas analysis instruments used arespecified in Table 5. In order to avoidair-infiltration during the measure-ments, the furnace is maintained at anexcess pressure of 20 to 50 Padepending on the test condition. Theoxygen excess in the stack gas ismonitored throughout the suctionmeasurements in order to control andmaintain stable test conditions.
Part 2
Table 5. Gas analysis instruments
Component Manufacturer/ Measure- Measure- Detection Span gasmodel ment ment limit (of concen-
principle range full range) tration
O2 Fisher Rosemount Para- 0–25 % ≤1% 9.00 %(NGA 2000) magnetic
Binos Para- 0–50 % ≤1% 21.0 %(100 2M) magnetic
CO2 Hartmann & NDIR 0–20 % ≤1% 18.0 %Braun (Uras 10p)
Binos (100 2M) TC 0–100 % ≤2% 86.0 %
CO Hartmann & NDIR 0–1.0 % ≤1% 8998 ppmBraun (Uras 10P)
Hartmann & Braun NDIR 0–10 % ≤1 % 8.98 %(Uras 3K)
HC Rosemount FID 0–10 % ≤2% 4.94%Analytical (400 A)
28
Temperature measurementsThe in-furnace temperature measure-ments were performed with a water-cooled suction pyrometer, shown inFigure 5. The probe is specificallydesigned for measurements of flametemperatures. The tip of the probeconsists of a 200 mm long ceramictube with an inner diameter of 10 mmin which a Pt-PtRh 30 percent-thermo-
couple (ANSI Type B) is placed, whichmeasures temperatures up to 1700ºC.The gas is sucked through a 6 mmhole on the side of the tube. To mini-mize the error due to radiation lossesfrom the thermocouple the gas passesthe thermocouple at high velocity. At700ºC, i.e. the lower furnacetemperatures, the suction velocity isapproximately 130 m/s. The velocity
increases up to about 230 m/s at1500ºC, that is the highest furnacetemperatures represented in the flamezone of the present tests. Given thatthe probe is properly designed and thatsuction velocity is kept above 100 m/s,it is considered that suction pyrometryproduces a maximum measurementerror of about 50 K.
Figure 4. Schematic illustration of the gas sampling probe.
A
A
1900 mm
200mm
Cooling water outlet
Cooling water inlet
Suction inletflue gas: D=6mm
Thermocouple
Type B: D=3mm
Section A-A
Ø45,0/41,0mm
Ø30,0/26,0mm
Ø17,2/13,0mm
10,0x1,0 mmØ13,0mm
Cooling water inlet
A
A
1620 mm
Cooling water outlet
Heated tube:
to preventcondensationGas suction tube
4 mm
Section A-A
Ø45,0/41,0mmØ33,0/29,0mmØ18,0/14,0mmØ8,0/6,0 mm
Figure 5. Schematic illustration of the suction pyrometer used for temperature measurements.
Part 2
29
Radiation intensity measurementsThe radiation intensity measurementswere carried out using a narrow angleradiometer of IFRF-type (Figure 6).The probe consists of a straight tube,termed collimating tube, of 10 mminner diameter, approximately 2 mlong covered by a protecting water-cooled jacket. A temperature con-trolled receiver unit is located at therear end of the collimating tube. Theradiation beam passes through thecollimating tube in which a number ofparallel baffles are mounted to limit thebundle of rays and to restrict the vie-
wing angle of the radiometer. Inaddition, the inner surface is blackenedto minimize internal reflections. Thebeam reaches a high broadbandreflectance mirror at the receiver endof the probe, which focuses the beamonto a thermopile. A shutter is placedbetween the receiver unit and thecollimating tube to avoid an increase inthe detector temperature and to enablezero check during traversing measure-ments of the flame. A known purge gasflow is applied during both calibrationand measurements to prohibit radiatingspecies and other contaminants to
enter the probe. The sensitivity of thesignal to different purge gas flow rateswas evaluated both during calibrationand measurements in order to find theminimum applicable flow to restrictthe influence from the purging on thein-situ measurement conditions. In thepresent probe setup Argon was used aspurge gas at a flow rate of 10 ln
3/min.The calibration of the narrow angleprobe was performed in a black-bodyfurnace before and after each measure-ment series to check the zero drift ofthe thermal detector.
ThermopilePT-100
Coolingwater
Collimating tube
Coolingwater
Electronicshutter
High broadbandreflectance mirror
Figure 6. Schematic illustration of the narrow angle radiometer (IFRF type) used for total radiation measurements.
Part 2
Results and discussionFigure 7 shows a typical start-upsequence from air-fired conditions tooxy-fuel combustion, in this case forthe OF 21 case with a target oxsygenexcess of 3 volume-percent in the fluegas.
Ninety percent carbon dioxide within thirty minutesThe oxygen and carbon dioxideconcentrations are measured at thefurnace exit. The step-wise changes insome locations are due to instrumentadjustments (flow or pressure). It canbe seen that a high level of carbondioxide enrichment in the flue gas (85to 90 vol %) is reached within the first30 minutes.
Stable O2 – more time consumingThe final depletion of the nitrogencontent to reach stable oxygen excessconcentration is rather time con-suming. Satisfactory experimentalconditions are typically reached aftersome four to five hours. Any dis-crepancy between the theoretic carbondioxide concentration level (97volume-percent) and the one measured(typically about 94 to 95 volume-percent) is therefore due to nitrogenstill left in the recycling media, and, tosome extent also due to the instrumentprecision of the carbon dioxideanalyzer which has a maximum errorof ±2 percent of the full measurementrange (0–100 percent).
Temperature profilesThe centerline andradial temperatureprofiles are measuredfor the three differentcases: • air • OF 21 • OF 27
It has been well established experi-mentally, that for a similar feed gasmixture in terms of volumetric con-centrations oxy-fuel conditions yieldssignificantly lower peak as well asoverall combustion temperature levelsthan oxygen/nitrogen conditions (i.e.reference conditions). This is also thegeneral impression obtained from acomparison of the OF 21 case and theair case.
The highest measured combustiontemperaturesFor example, the highest measuredcombustion temperatures for the OF21 and air-fired case in port R2 areapproximately 1370ºC and 1550ºC,respectively. The lower temperature inthe OF21 case is mainly a result fromthat carbon dioxide has higher specificheat capacity than nitrogen and to acertain extent, that there is an increasein radiation losses caused by thecarbon dioxide which increases the gasemissivity. The significance of the lattereffect obviously depends on theradiative properties of the flameconsidered, that is to what extent thecontribution from moisture, soot and
30
Figure 7. Measured O2 and CO2 concentrations at the
furnace exit during a start-up sequence from air to
OF21-fired conditions.
Part 2
0 60 120 180 240 300 360
Time [min]
0
1
2
3
4
5
6
7
8
9
10
O2
conc
e nt ra
ti on
[vol
%]
0
4
8
12
80
84
88
92
96
100
CO
2co
ncen
t rati o
n[v
ol%
]
Furnace exit concentrationsCO2
O2
Shift from air toO2/CO2 combustion
0 60 120 180 240 300 360
Time [min]
0
1
2
3
4
5
6
7
8
9
10
O2concentration[vol%]
0
4
8
12
80
84
88
92
96
100
CO2concentration[vol%]
Furnace exit concentrationsCO2O2
Shift from air toO2/CO2 combustion
31
particle content need to be included.The change in emissivity of the oxy-fuel environment is further discussedbelow, and related to the radiationintensity profiles.
Improvement of the combustion intensityA reduction of the recycled flue gas isan obvious way to increase reactortemperature levels in order to improvethe combustion intensity, in this casefrom the OF 21 to the OF 27 case. Asa result the combustion temperaturesof the OF 27 case and the air case arerather similar, but different from the
OF 21 case which yields significantlylower temperatures of the combustingflow due to the cooling from therecycled carbon dioxide. Both the airand the OF 27 cases exhibit a strongnegative gradient in temperaturebetween 50 and 100 mm from thecenterline, which represents thelocation of the flame front. In the OF21 case the temperature starts to de-crease already between the centerlineand 50 mm distance away from thecenterline (0 mm), and no parabolicshape of the temperature profile can beseen close to the centerline (the distrib-utions are assumed axi-symmetric).
A shift in position of the flame frontAlthough a weak trend of such aparabolic temperature distribution mayexist between 0 and 50 mm, thetendency in ports R2 and R3 clearlyshows a shift in position of the flamefront. In addition, already between 50and 100 mm the temperature isbrought down to a level of about700ºC, which corresponds to the gastemperature near the wall (400 mm) of both the OF 27 and the air-firedcase.
0 100 200 300 400
Distance from centreline [mm]
600
800
1000
1200
1400
1600
Gas
tem
pera
ture
[oC
]
Measurement port: R2
Figure 8. Measured mean, minimum and maximum radial
temperature profiles in measurement ports R2-R3.
Part 2
AirOF 21OF 27Air, MinAir, MaxOF 21, MinOF 21, MaxOF 27, MinOF 27, Max
0 100 200 300 400
Distance from centreline [mm]
600
800
1000
1200
1400
1600
Gas
tem
pera
ture
[oC
]
Measurement port: R2
0 100 200 300 400
Distance from centreline [mm]
600
800
1000
1200
1400
1600
Gastemperature[oC]
Measurement port: R3
AirOF 21OF 27Air, MinAir, MaxOF 21, MinOF 21, MaxOF 27, MinOF 27, Max
32
Burn-out rate This shows that for the OF 21 case,the physical volume of the high tem-perature zone is smaller for the OF 21case compared to the two other casesand, as will be discussed later on, thisinfluences the burn-out rate in the OF21 environment. It should be notedthat the measurement data represen-ting the first position, R1, close to theburner, is accompanied with a highdegree of uncertainty since the suctionflow from the temperature probe isbound to disturb the flame, which isconfirmed by a heavily fluctuatingoxygen signal in the flue gas leavingthe furnace when measurements arecarried out in this position. Thus, thevalues from this position should beconsidered only as an indication ofignition not yet being initialized. Also,
the exact point of ignition cannot bedetected, since for all cases the point issomewhere between the first and thesecond measurement port.
The furnace wall temperaturesThe furnace wall temperatures for thethree cases, measured with thermocou-ples at a distance 20 mm from theinner furnace wall surface togetherwith the minimum and maximum gastemperature, measured close to theinner furnace wall surface with thesuction pyrometer. The furnace walltemperatures are measured at the rightand left hand side of the probe portson the same axial distances from theburner as shown in Table 1. Thedistribution of gas and wall temper-atures correlate well for all three caseswith the highest gas and wall temper-
atures obtained in the OF 27 case at adistance of 800 mm from the burner,where the temperature drops fasterthan for both air and OF 21.
Gas concentration profilesRadial gas concentration profilesconsisting of oxygen, carbon monoxideand hydrocarbon. The difference inmass feed rate of air compared to themixtures of oxygen and carbon dioxideresults in different mass flux profiles.Still, the gas profiles and the burnoutrates show clear trends and areconsistent with the temperatureprofiles presented.
Part 2
Figure 9. Mean values of the measured radial radiation intensity profiles
with hot refractory furnace wall as background. The grey dotted profiles
show the corresponding gas temperatures (also shown in Figure 8).
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Rad
iatio
nin
tens
ityq
[kW
/m2 s
r]
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
Gas
tem
pera
ture
T g[o
C]
Measurement port: R2
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Rad
iatio
nin
tens
ityq
[kW
/m2 s
r]
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
Gas
tem
pera
ture
T g[o
C]
Measurement port: R3
Air: q1
OF 21: q1
OF 27: q1
Air: Tg
OF 21: Tg
OF 27: Tg
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Radiationintensityq
[kW/m
2 sr]
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
GastemperatureTg[oC]
Measurement port: R2
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Radiationintensityq
[kW/m
2 sr]
500
600
700
800
900
1000
1100
1200
1300
1400
1500
1600
GastemperatureTg[oC]
Measurement port: R3
Air: q1
OF 21: q1
OF 27: q1
Air: Tg
OF 21: Tg
OF 27: Tg
33
The results show that the gascomposition of the flame envelope ofthe OF 21 case is clearly different fromto the two other cases. The amount ofunburned hydrocarbons is significantlyhigher for the OF 21 case and, and as aconsequence, oxygen is still present inthe flame core (i.e. oxygen from therecirculation motion of the flame). Thesuppressed development of the OF 21flame is further seen in the carbonmonoxide profiles that give significant-ly lower concentration levels comparedto the air and OF 27 cases. The declinein combustion stability and per-formance of the OF 21 case can to alarge extent be attributed to thereduced temperature levels comparedto the other two cases, as shown infigure 8, and is caused by the highspecific heat of carbon dioxide. Itshould be noted that the gas con-centration and temperature data fromintrusive measurement instrumentssuch as the ones used here are rathercollected from a measurement volumethan an exact position. This applies tothe gas suction probes and suctionpyrometers employed here, especiallythe latter probe due to the rather highsuction velocity as described above.
This is especially important to keepin mind for regions with sharpgradients, for example when com-paring the hydrocarbon profiles andthe temperature profiles in one of thefigures above, where the highconcentration of unburned fuel at 50mm from the centerline can explainedby the fact that it corresponds totemperatures represented within adistance from 50 to 100 mm from thecenterline, that is somewhere in therange between 700 to 1200ºC, rather
than the exact same position as thetemperature measurement at 50 mm.Compared to the OF 21 case the fuelburn-out of the OF 27 case is favouredby the increased temperature level andimproved mixing conditions betweenfuel and oxygen, due to the reducedflue gas recycle. The hydrocarbonlevels are low and the oxygen profile atthe centerline exhibit a similar distinctzero-level as for air-fired conditions.
Radiation intensityThe radial distributions (from thecenterline to the probe wall) ofunidirectional total radiation intensityhave been measured for the three casesusing the narrow angle radiometer.The maximum measured radiationintensity 100 mm from the centerline isrecorded close to the flame boundaryas indicated by the temperatureprofiles. Moving towards the probewall to the right the intensity drops, asthe absorption becomes higher thanthe emission.
Difference in emissivityThe information from the temperaturedistributions shown previously togetherwith the radial profiles of radiationsuggests a change in emissivity for anoxy-fuel environment compared toreference conditions in air. As seen theradiation intensity is higher in the OF27 case than in the air case despite thefact that the radial temperature levelsare generally lower or similar to thoseof air and, in spite of the overall lowertemperature levels, the radiationintensity profiles of the OF 21 casehave a close resemblance to those ofair. The obvious physical differencebetween the air case and oxy-fuel
conditions is the increase in carbondioxide partial pressure (approximately8 times the concentration level ofnormal air-fired conditions), fromwhich an increase in gas band radiationis anticipated. The difference in totalemissivity and in gas emissivity willtherefore be compared between theoxy-fuel and the air case in order toOF 27 evaluate if the increase inoverall emissivity can be attributed toan increase in oxygen band radiationonly. The oxy-fuel environment isrepresented by the OF 27 case; sincethe experimental radiation data isrestricted for the OF 21 case (i.e. nomeasurement data is available on q2 –see appendix).
It should be noted that differences inlocal emissivity may exist between OF21 and OF 27 conditions since the sootformation may change from one oxy-fuel atmosphere to another. In fact, bycomparing the shape of the radiationintensity profiles it is seen that thepeak intensity 100 mm from thecenterline is clearly accentuated for theOF 27 condition compared to the twoother cases. This may indicate localdifferences in emissivity between thetests conditions, which could be causedby changes in soot volume fraction.Still, a local increase in temperature(between 50 and 100 mm) for the OF27 case would cause the same effectand, hence, the spatial distribution ofthe measurements needs to beimproved to further evaluate the localproperties of the flames.
Part 2
The equations and parameters are
defined in appendix.
34
Figure 10. Radiation intensity, mean flame emissivity and average flame radiation temperature
a) Comparison of radiation intensity profiles, air and OF 27 conditions, measured against both
cold black body target (q2) and hot refractory furnace wall (q1) in port R3.
b) Tmr and εm based on the measurements given in Figure a and calculated as a function of different furnace
wall temperatures (Air: 655±20ºC and OF 27: 680±20ºC).
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Rad
iatio
nin
tens
ityq
[kW
/m2 s
r]
Meas Port: R3Air - q1
OF 27 - q1
Air - q2
OF 27 - q2
The left figure above shows acomparison of radiation intensityprofiles for air and OF 27 conditions inport R3, measured against both a coldblack body target, which represents theterm q2 in Eq. (9), and the hotrefractory lined furnace wall, that is q1in Eq. (9). These data together withthe radiation intensity reading of thebackground wall alone, q3 in Eq. (9), isused to determine the mean flameemissivity, εm, and an average flameradiation temperature, Tmr , for the OF27 and air-fired conditions, respectively.(The equations are presented in theappendix ”Theory”.) The results arepresented in the right figure above,where em and Tmr are plotted as a
function of the background furnacewall temperature. For the calculationthe radiation intensities at 400 mmfrom the centerline are used, since thefurnace cross sectional emissivity is ofinterest. As seen, the accuracy of thesurface wall temperature prediction iscritical, but given that the wall surfacetemperature prediction is correct forboth cases, that is 655ºC for air thecase and 680ºC for the OF 27 case, thetotal emissivity becomes 0.41 and themean radiation temperature becomesabout 930ºC, for the air-fired case andfor the OF 27 case the total emissivitybecomes 0.52 and the mean radiationtemperature becomes approximately950ºC.
Wall intensityThere was a difficulty to obtain a stableradiation intensity signal from thefurnace wall alone (i.e. q3) since thewater-cooling of the probe cool downthe wall as soon as the probe ispositioned close to the wall. Therefore,the measurements of wall radiationintensity was repeated twice for theair-fired test condition in port R3 witha measured intensity of 13,4±0,4kW/m2sr which together with εb = 1corresponds to an inner wall surfacetemperature of 655ºC with an errormargin of about ±10ºC. This repetitionwas however not carried out for theOF 27 condition, but instead the sametemperature difference has been
Part 2
0 100 200 300 400
Distance from centreline [mm]
5
10
15
20
25
30
35
40
45
Radiationintensityq
[ kW/m
2 sr]
Meas Port: R3Air - q1OF 27 - q1Air - q2OF 27 - q2
630 640 650 660 670 680
Air conditions - Ts [oC]
0.3
0.4
0.5
0.6
0.7
Meanemissivity
m=f(Ts)
660 670 680 690 700
OF 27 conditions - Ts [oC]
800
850
900
950
1000
1050
Meanradiationtemperature,Tmr=f(Ts)
Measurement port: R3Air: m
OF 27: mAir:TmrOF 27:Tmr
35
assumed as for air-fired conditions,based on temperature measurements,over the 20 mm of refractory lining.The temperature level correspondingto probe port R3, in principle constantin time, is approximately 25ºC higherfor OF 27 conditions compared tothose of air from which the inner wallsurface temperature Ts has beenestimated to 680±10ºC for the OF 27condition. Including the thermocouplemeasurement error this results in atotal error margin of about ±20ºC forthe assumed wall temperatures.
Input parameters for the gasemissivity modelEqs (12) to (18) (presented in theappendix “Theory”) are used to de-termine the gas emissivity with theinput parameters listed in the table 6,where the partial pressures of the tworespective gases are assumed constantwithin the measurement volume.Compared to measurement data thisassumption does not introduce anysignificant error. As previouslydescribed, the gas concentrationmeasurements were performed on drybasis, and so the water vapourconcentration is obtained fromstoichiometric calculations withconditions as specified in Table 2–3and fuel properties according to Table4. The background surface temperatu-res are the same as in the right figureabove, although the error margin hasbeen disregarded, since in this case ithas a negligible influence on theresults. The path length of the gases isthe same as the inner diameter of thefurnace.
Table 6. Input parameters, partialpressure (pa), background surfaceradiation temperature (Ts) andpath length (L) to the gas emissivity model.
Test case pCO2 pH2O Ts [K] L[m]Air 0,10 0,14 928 0,80OF 27 0,82 0,18 953 0,80
Gas emissivity modellingGas emissivity modeling with Eqs (12)to (18) shows that the emissivity of thewater vapour is significant in bothenvironments although dry flue gasrecycling is applied, with air and OF 27conditions. In fact, at lower gas tempe-ratures εH2O is still dominating overεCO2 also in the OF 27 case althoughthe temperature dependence of εH2Oreduces its importance at higherradiation temperatures such as in thenear flame region. The measured gastemperatures in port R3 (see figure 8)show a close resemblance for the twocases as does the calculation of themean radiation temperature in theright figure 10 above.
Increased gas emissivityFor a similar mean gas temperature asthe air case the OF 27 total gasemissivity is increased with about 20 to30 percent for the relevant gas tempe-rature interval, where the difference isaccentuated at higher temperatures dueto the stronger temperature depend-ence of εH2O compared εCO2 for theconditions specified in Table 4. Forexample, with the mean radiationtemperatures given in the right figureabove, i.e. in the range between 930 to950ºC, εg becomes 0.23 in the air-firedcase and 0.29 in the OF 27 case. Whencomparing the absolute values of thechange in εm and εg it is suggested that
for similar gas temperatures, thedifference between the two combustionenvironments in terms of increasedradiation intensity of the OF 27 casecan partly, but not solely, be explainedby an increased gas emissivity. Errorsintroduced in the calculation of εm,mainly due to the wall temperatureprediction, as shown in the right figureabove, and the grey gas approximation,as will be discussed below, can how-ever not be neglected when comparingεm and εg. Still, the conclusion that thetotal emissivity of the OF 27 flameincreases more than that suggested byan increased emissivity due to CO2, isconsistent with the measured radiationintensities for the OF 27 environmentcompared to air, where the netincident wall intensity (q2 in the leftfigure 10 above) increases with morethan 30% despite similar temperatureprofiles for air and OF 27 conditions(Figure 8).
Another possible reason for theincreased radiation intensity in the OF27 case is the possibility that local hightemperatures at the flame front maynot have been detected due to therestricted spatial resolution of themeasurement positions. Yet anotherreason is that the rate of sootproduction may increase from air tothe OF 27 atmosphere.
Reduced sooting tendencyThe figures below display thedifference in flame luminosity betweenthe air-fired and the OF 21 case. Asseen, the observations of the OF 21flame to the right with its blueemission suggest that the sootingtendency is clearly reduced (sootcreates the yellow light emission in aflame) compared to reference condi-tions. In fact, the background probeopening is clearly visible (the annular
Part 2
36
opening in the background) in theimage of the OF 21 flame since theyellow light is only concentrated intothin sheets of the burning soot.
The background to theseobservations is supported byexperimental findings reported in theliterature suggesting a reduced sootproduction tendency when addingcarbon dioxide on either fuel or
oxidizer side to a hydrocarbondiffusion flame. Soot production isdependent on the temperature andconcentrations of soot-precursors andthe soot reducing effects by carbondioxide addition are reported to be dueto both dilution and thermal as well aschemical effects. Hence, besides thedirect effect with an increased gas bandabsorption by the carbon dioxide, the
CO2 may also have significant effectson the soot formation during oxy-fuelconditions, hence causing secondaryeffects on the change in the radiativeproperties of the flame. Furthermore,visual observations in this work suggestthat the black body radiation againincreases from the OF 21 to the OF 27case, but that the difference betweenair and OF 27 conditions are less clear.
a) Propane/air fired flame, 3 percent oxygen excess
b) Propane/OF 21 fired flame, 3 percent oxygen excess
Images showing the luminosity of propane fired flames in port No 2 (about 0.3 meters from theburner inlet). Figure 14a) shows the air-fired flames with an oxygen excess of 3 volume-percent inthe flue gas and 14b) shows the OF 21 flame with an oxygen excess of 3 volume-percent.
Further experimental workThe modeling shows that the maxi-mum difference between the total gasemissivity and absorptivity for thereference condition in air isapproximately 10 percent when the gastemperature varies between 700ºC and1300ºC with two different backgroundtemperatures: the background temp-eratures equal to the furnace walltemperature or to that of a cold blackbody target. It should be noted that thetemperature of the cold black body is
below the strict applicable minimumtemperature of the model. That is lessthan 400K, which may affect theaccuracy of the results. The differenceincreases for the OF27 case with amaximum difference of about 25percent between emissivity andabsorptivity for the corresponding gasand background temperatures. Still, theresults obtained from Eqs (9) and (10)in the appendix are consistent with themeasurement data. The meanemissivity as determined by Eqs (9)
and (10) therefore serve as a usefulcomparison of the two combustionenvironments, i.e. air and OF 27 con-ditions, in relation to the total gasemissivities determined according toEqs (12) to (18). In conclusion, in orderto facilitate the use of more precisemethods to locally determine thechange in emissivity, measurements oftemperature and radiation intensitieswith a higher spatial resolution arerequired. Further experimental work istherefore planned with this aim.
Part 2
37
Part 2
On-going work on oxy-coal combustionThe work on gas-fired combustiontests is currently being followed up bytests with lignite as fuel. The focus ofthe lignite fired tests is partly the sameas for the gas fired tests, that is theradiation properties of the flame andcombustion gases, which will changedrastically from those during the gas-fired tests, due to the properties oflignite as a fuel. The emissioncharacteristics will however also be stu-died in detail in terms of for examplethe nitrogen chemistry as well as thebuild up of acidic components (sulfurdioxide and sulfur trioxide) in the recy-cle loop.
Two images of the lignite flameThe figures (right) shows two imagesof the lignite flame for two differenttest environments with a significantdifference in the amount of oxygenavailable in the combustion zone. Thefigure to the left shows an image of anoxygen rich oxy-fuel flame, with 10volume-percent oxygen excess in theflue gas. The figure to the rightdemonstrates an air-fired flame with 3volume-percent oxygen excess in the
flue gas. The image quality of the air-flame is affected due to a coal dustladen quartz window, but there is aclear difference in the light emissionfrom the two flames, where the oxygenrich flame to the left is almost entirelywhite and much brighter than the air-fired flame. This is typical for oxygenrich flames, both gas and coal fired,and the colour of the flame has asignificant importance on the radiativeproperties of the flame.
The color itself is due to the amountof soot, which is formed and thereafterburns, in the flame envelope as well asthe corresponding combustionintensity and peak temperatures in theflame zone. The images in the figurecan for example be compared with thephotos in the previous section showingthe flame emission from gas-fired oxy-fuel and air-flames. The OF 21 flame isdominated by the blue backgroundemission and the correspondingtemperature levels are significantlylower in this flame compared to the airfired flame. The latter flame is as seeninstead dominated by the yellow lightfrom the soot.
a) Lignite/oxy-fuel flame with 10volume-percent oxygen excess
b) Lignite/air flame with 3 volume-percent oxygen excess
Side view of the flame at theburner inlet a) combustion oflignite in oxy-fuel flame with ahigh oxygen excess of 10volume-percent and b) com-bustion of lignite in air with an oxygen excess at 3 volume-percent.
38
Part 2
Two side view images of the air-fired flameThe images (right) show two sideviews of the air-fired flame, from portno 2, which is about 0.3 meters fromthe burner inlet. The image to the leftshows the flame during measurementwith the suction probe, which deliversthe gas samples from the combustionchamber to the gas analysis system foranalysis of oxygen, carbon monoxide,carbon dioxide, total hydrocarbons,nitrogen oxide, nitrogen dioxide andsulfur dioxide in the standard set-up.As seen in the image to the left theflame structure is obviously affected bythe probe compared to flame in theother image, where the probe has notyet been inserted. This underlines theimportance of trying to minimize thedifferent measurement systems effecton the test conditions, for example interms of improved designs of the
sampling probes used for gas analysis,temperature measurements, radiationintensity measurements and particleconcentration measurements etc. A prerequisite for such improvementsis that the experiments are continuous-ly followed up with modeling activities.Such modeling activities are beingperformed on the Chalmers test unit ina number of parallel projects. Thiswork is carried out at Chalmers, butalso at the University of Stuttgart,Fluent Europe Ltd and VattenfallUtveckling AB. These activities aim toimprove the knowledge on combustionand fluid dynamics modeling of oxy-fuel combustion to reach today’smodeling standard of a normal air-firedflame. This, since the modelling toolsare necessary for both predicting andevaluating the performance of a futurefull scale oxy-fuel power plant.
a) Lignite/air flame with the suction probe inserted
b) Lignite/air flame without the suction probe
Side view of the flame at port no 2 (approximately 0.3 m from the burner inlet) duringcombustion of lignite in air withan oxygen excess at 3.0volume-percent. a) shows thedisturbance of the gas suctionprobe on the flow field compared to the unaffected flow pattern in b).
39
To improve the knowledgeThe overall aim of the experimentalwork presented in this paper is toimprove the knowledge on the oxy-fuelcombustion technique with respect tothe flame characteristics with anemphasis put on the radiative heattransfer and burnout behavior.Recently performed tests with propaneas fuel are presented, which comprisethree different test cases: reference testsin air and two oxy-fuel test cases withdifferent recycled feed gas concentra-tions of oxygen (21 and 27 volume-percent) and carbon dioxide (79 and 73volumepercent) A reduction of therecycled flue gas media is a trivial wayto increase reactor temperature levelsin order to improve the combustionintensity, in this case from the OF 21to the OF 27 case.
The temperature levels The temperature levels of the OF 21flame are significantly lower than inthe reference conditions in air due tothe cooling by the recycled carbondioxide, which results in a suppresseddevelopment of the OF 21 flame,which is seen from the profiles ofoxygen, carbon monoxide and totalhydrocarbon. Compared to the OF 21case the fuel burn-out of the OF 27
case is favored by the increasedtemperature level and improved mixingconditions between fuel and oxygenand in general, the OF 27 case exhibitssimilar overall combustion behavior asthe air-fired reference case in terms ofgas concentration and temperatureprofiles. The systematic picture of thetemperature and radiation profilessuggests a significant change inemissivity for the oxy-fuel environmentcompared to reference conditions inair. For example, the radiation intensityincreases for the OF 27 case comparedto air conditions despite the fact thatthe radial temperature levels aregenerally lower or similar to those ofair. The difference in total meanemissivity compared to the differencein gas emissivity at similar meanradiation temperatures for the air andthe OF 27 case suggest that the higherradiation intensity in the OF 27 casecan partly, but not solely be explainedby an increased gas emissivity.
More detailed information isrequiredMoreover, in order to improve theinterpretation of the changes betweenthe different combustion environments,more detailed information on thespatial resolution of temperature and
radiation intensities are required inorder to locally determine the changein emissivity. Further experimentalwork is planned with this aim for bothgas and coal fired tests. The on-goingexperiments oxy-coal combustion isbriefly summarized in this report. Inthese tests, the aim is to characterizethe performance of Lausitz lignite(German brown coal) during oxy-fuelfired conditions compared to referencetests in air. A strong focus will be puton the radiation properties of the flameand combustion gases, just as for thepreviously performed tests with gas.The emission characteristics willhowever also be studied in detail, forexample in terms of the nitrogen andsulphur chemistry. The experimentscarried out at the Chalmers oxy-fueltest unit are carried out in parallel withmodeling work in order to improve theoverall understanding of the com-bustion process, and to develop designtools required in the process ofbuilding a 30-50 MWth pilot plant, andin the end, the demonstration of a fullscale 600–1000 MWe oxy-fuel powerplant with CO2 capture.
ConclusionsPart 2
40
Appendix
For the interpretation, measurements with the narrow angle probe were carried out both with thehot refractory lined furnace wall and a cold non-reflecting plate as backgrounds. The radiationintensity at any given distance d from the hot furnace wall to the probe end represents the sum of:
i radiation from the flame and combustion gases received along the viewing path of the probe,
ii background radiation from the hot refractory furnace wall, partly absorbed by the gas layeralong the viewing path d of the probe, and
iii in the background reflected radiation from the flame and combustion gases (i) partly absorbedwithin the interval d.
Thus, the total radiation intensity that reaches the probe can be expressed as:
(1)
where q1 is the sum of contributions (i) to (iii) and the emissivity of the flame and combustiongases at a certain wavelength el is treated as a general function of wavelength λ whereas the back-ground wall emissivity εβ is assumed constant. The function RλT gives, for a temperature T [K] anda wavelength interval λ to λ + dλ, the black body radiation RλT dλ. The temperatures, Tg and Tsare the gas and background wall temperatures, respectively.
At the measurement levels employed, the furnace wall temperature typically varies ±50ºC (aroundan average of 600ºC) in all three cases (as will be discussed below, the inner surface temperatureincreases slightly compared to the wall temperature measurements). This results in an emissivity ofabout 0.9 of the refractory lining material. However, with the low conductivity and considerablethickness of the furnace walls the conduction losses through the lining are negligible at thermalsteady state conditions. Furthermore, on the wall surface, the convective term is small comparedwith the dominating radiative term. Thus, since the heat exchange is in principle only due toradiation the energy balance in Eqs (2) to (6) gives that the effective emissivity εb is equal to unity.With Ts being the temperature and Q– the total incident heat flux of the wall surface in the energybalance the total outgoing heat flux Q+ can be written:
(2)
According to the assumption above, all heat is exchanged by means of radiation and hence the rateof emission must equal the rate of absorption, from which follows,
APPENDIXTheory
The following describes the interpretation of theradiation data obtained with the previouslydescribed narrow angle radiometer probe (in part 2) and the method to characterize theflame in terms of flame emissivity together withthe gas emissivity model applied.
λεεελεελε λλλλλλλ dRdRdRq TgbTsbTg ∫∫∫∞∞∞
−−+−+=000
1 )1()1()1(
−+ −+= QTQ bsb )1(4 εσε
41
(3)
(4)
(5)
(6)
Thus, it is clear that the effective emissivity εb is 1 regardless of the actual emissivity of the materialof the wall surface. Therefore, the reflected radiation (iii) can be left out without any significanterror, and Eq. (1) can instead be written as:
(7)
Eqs (1) and (7) take into consideration the wavelength dependence of the emissivity of theflame/gas layer over which the total radiation measurement is carried out. However, the measureddata do not give information on the spectral resolution of the radiation. Therefore a simplifiedtreatment of the data is required in order to evaluate the difference in total emissivity between theair and the O2/CO2-environment, the latter being represented by the OF 27 case, due to restrictedmeasurement data for the OF 21 case. Hence, for the air and the OF 27 case the total meanemissivity of the gas layer is determined, based on three separate measurements of the radiationintensity. The general basis of the method used to determine the mean emissivity is that the flameor gas layer is considered grey, so that the emissivity becomes independent of wavelength andequal to the absorptivity, independent of the relation between the gas and the backgroundtemperature. The scattering is neglected due to the small size of the soot particles and hence theemissivity is assumed equal to the absorption. These assumptions result in a flame emissivityindependent of wavelength and, integrated over the spectrum, Eq. (7) can be written:
(8)
where εm represents the mean emissivity and Tmr the average radiation temperature of the gaslayer concerned, i.e. along the viewing path of the probe. In order to determine εm and Tmr,separate measurements of the radiation from the gas layer σεmTmr
4 and the background furnacewall σTs
4 in Eq. (8), are required in addition to measurements of q1, a procedure which is followedin the present work. Hence, the term q2 is represented by the first term in Eq. (8). Together withthe net contribution of the background wall, q3, the mean emissivity and the mean radiationtemperature of the gas layer can be calculated according to
−= QT bsb εσε 4
4sTQ σ=−
++ −+= QTQ bsb )1(4 εσε
4sTQ σ=+
λελε λλλλ dRdRq TsTg )1(00
1 ∫∫∞∞
−+=
441 )1( smmrm TTq εσσε −+=
Appendix
42
Appendix
(9)
(10)
Obviously, with Tmr and εm from Eqs (9) and (10) drastic simplifications are introduced comparedto real conditions for which there is a spectrally dependant emissivity ελ, and non-uniformconcentration and temperature profiles within the measurement volume. For example, the averageradiation temperature of the furnace (Tmr) should not be confused with the real average gastemperature. The grey approximation is also more questionable in the case of a gas-fired flamecompared to a highly luminous oil flame. This since the gas band radiation will have a strongerimpact on the total emissivity in a moderately sooting flame and cannot be neglected incomparison to the continuous radiation due to soot and especially so for oxy-fuel conditions. This implication on the results will be further discussed below.
Determination of total gas emissivityAs discussed above εm will include emission from both gas and soot. Therefore εm can beexpressed as:
(11)
where eg and es are the gas and soot emissivities across the gas layer and the product eges repre-sents the band overlap of soot and gas. A reduction factor M should be introduced, which dependsmainly on the gas temperature and to less extent on the soot volume fraction. This, since theemission bands of CO2 and H2O are not randomly located in the spectrum and since the mono-chromatic soot emissivity increases with decreasing wavelength. A possible change in band overlapbetween soot and gas will however not be treated in this work, due to its small effect on the overallemissivity. There are no experimental data available on the soot volume fraction from the flamesinvestigated in this work. Instead, in order to see if the difference in mean emissivity between thetwo combustion environments corresponds to the difference in gas emissivity, including H2O andCO2 as radiating gases, the gas emissivity is determined using an existing model. This model waschosen since it accommodates a high degree of generality for which it has been recognised. This,since the CO2 and H2O emissivities are treated separately and the total emissivity of the mixture isdetermined by taking a total band overlap correction term into consideration. Hence, any arbitrarypartial pressures of CO2 and H2O can be applied in the model. The polynomial correlations of themodel produce a maximum error of 5 percentfor water vapour and 10 percentfor CO2 emissivitiesat a temperature above 400 K, compared to the spectral integration data upon which the model isbased. The total emissivity for a mixture of CO2 and H2O is obtained from:
(12)
3
211q
qqm
−−=ε
42
σεπ
m
mr
qT =
sgsgm M εεεεε −+=
( )OHCOOHCOg 2222 +Δ−+= εεεε
43
In the model a zero partial pressure emissivity is given by the correlation:
(13)
where Tg is the gas temperature, T0 = 1000K, p is the total pressure, pa is the partial pressure and Lis the path length of the gas, either CO2 or H2O, (paL)0 = 1 bar cm and Cji are correlation con-stants. The emissivity for higher partial pressures, i.e. if pa > 0, can then be calculated according to:
(14)
where a, b, c and (paL)m are correlation parameters and PE is an equivalent pressure. Forcalculation of the total gas emissivity the last term in Eq. (12) needs to be considered, which is due to the band overlap of the two gases. This correction term is written
(15)
with a partial pressure ratio _ defined as
(16)
The absorptivity is determined by multiplying a factor (Tg /Ts)0,5 to the emissivity evaluated at the
external surface temperature Ts and a temperature scaled pressure path length according to:
(17)
and, finally, the total absorptivity of the gas mixture can be written:
(18)
( ) ⎥⎥
⎦
⎤
⎢⎢
⎣
⎡
⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛== ∑∑
= =
M
i
N
j
i
a
a
j
gjiga Lp
Lp
T
TcTbarpLp
0 0 0
100
0 logexp),1,(ε
( )( )
( )( ) ( )⎟⎟
⎠
⎞
⎜⎜
⎝
⎛⎥⎦
⎤⎢⎣
⎡−
+−+
−−−=
=
2
100
logexp1
111
,1,
,,
Lp
Lpc
Pba
Pa
TbarpLp
TpLp
a
ma
E
E
ga
ga
ε
ε
( )( )
( )
76,2
0
210
4,10 2
22log0089,0
1017,10 ⎟⎟⎠
⎞⎜⎜⎝
⎛ +⎥⎦
⎤⎢⎣
⎡−
+=Δ +
Lp
Lpp
a
OHCOOHCO ζ
ζζ
ε
22
2
COOH
OH
pp
p
+=ζ
( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛⎟⎟⎠
⎞⎜⎜⎝
⎛≈ s
g
sa
s
gsga Tp
T
TLp
T
TTTpLp ,,,,,
5,0
εα
⎟⎟⎠
⎞⎜⎜⎝
⎛Δ−+=
g
sCO
g
sOHOHCOg T
TLp
T
TLp
2222,εααα
Appendix
44
Nomenclature
a, b, c Correlation parameters
cji Correlation constants
Hi Heating value, inferior [MJ/kg]
L Path length [cm]
M Reduction factor
p Pressure [bar]
pa Partial pressure [bar]
PE Equivalent pressure [bar]
(paL)m Correlation parameter
pCO2 CO2 partial pressure [bar]
pH2O H2O partial pressure [bar]
q Radiation intensity [W/m2,sr]
q1 Radiation intensity fromcombustion gases and furnace wall [W/m2sr]
q2 Radiation intensity from combustion gases [W/m2sr]
q3 Radiation intensity from furnacewall [W/m2sr]
Q+ Emitted heat flux [W/m2]
Q– Incident heat flux [W/m2]
RλT Gives, for a temperature T [K] and a wavelength interval λ to λ+ dλ, the black body radiation RλΤ dλ
TBL Measured furnace wall temperature LHS of the probe ports [ºC]
TBR Measured furnace wall temperature RHS of the probe ports [ºC]
Tg Gas temperature [ºC]
Tgmin Minimum gas temperature [ºC]
Tgmax Maximum gas temperature [ºC]
Tmr Mean radiation temperature [ºC]
Ts Furnace wall temperature [ºC]
Greek letters
α Absorptivity
αCO2 CO2 absorptivity
αH2O H2O absorptivity
αg Gas absorptivity
εb Furnace wall emissivity
εm Mean emissivity
εg Total gas emissivity
εs Soot emissivity
ελ Spectral emissivity
ε0 Zero partial pressure emissivity
_ Partial pressure ratio:pH2O/(pCO2+pH2O)
s Bolzmann constant [W/m2K4]
l Stoichiometric ratio
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Pathways to sustainable European energy systemsThe European pathways project is a five year project withthe overall aim to evaluate and propose robust pathwaystowards a sustainable energy system with respect toenvironmental, technical, economic and social issues. Thefocus is on the stationary energy system (power and heat)in the European setting. Evaluations will be based on adetailed description of the present energy system andfollow how this can be developed into the future under arange of environmental, economic and infrastructureconstraints. The proposed project is a response to the needfor a large and long-term research project on Europeanenergy pathways, which can produce independent resultsto support decision makers in industry and in governmentalorganizations. Stakeholders for this project are: theEuropean utility industry and other energy relatedindustries, the European Commission, EU-Member Stategovernments and their energy related boards and oil andgas companies. The overall question to be answered by theproject is:
How can pathways to a sustainable energy system becharacterized and visualized and what are the consequences ofthese pathways with respect to the characteristics of the energysystem as such (types of technologies, technical and economicbarriers) and for society in general (security of supply,competitiveness and required policies)?
This question is addressed on three levels; by means ofenergy systems analysis (technology assessment andtechnical-economic analysis), a multi-disciplinary analysisand an extended multi-disciplinary policy analysis. From adialogue with stakeholders, the above question has beendivided into sub-questions such as:
• What is the critical timing for decisions to ensure that apathway to a sustainable energy system can be followed?
• What are ”key” technologies and systems for theidentified ”pathways” - including identification ofuncertainties and risks for technology lock-in effects?
• What requirements and consequences are imposed onthe energy system in case of a high penetration ofrenewables?
• What are the consequences of a strong increase in theuse of natural gas?
• What if efforts to develop CO2 capture and storage fail?
• Where should the biomass be used – in the transport-ation sector or in the stationary energy system?
• Are the deregulated energy markets suitable to facilitatea development towards a sustainable energy system?
• Will energy efficiency be achieved through free marketforces or regulatory action?
• What are the requirements of financing the energyinfrastructure for the different pathways identified?
In order to address the sub-questions in an efficient andfocussed way the project is structured into 10 workpackages addressing topics such as description of theenergy infrastructure, energy systems modelling, tech-nology assessment of best available and future technologiesand international fuel markets. In planning of the projectsignificant efforts have been put into ensuring that theproject should not only be strong in research but also inmanagement, communication and fundraising.
The global dimension will be ensured through integrationwith the other three regional AGS pathway projects in theAmericas, East Asia, and India and Africa.
More information at Pathways website:www.ags.chalmers.se/pathways
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The Alliance for Global Sustainability The Alliance for Global Sustainability (AGS) bringstogether four of the world’s leading technical universities –the Massachusetts Institute of Technology, The Universityof Tokyo, Chalmers University of Technology and theSwiss Federal Institute of Technology – to conduct researchin collaboration with government and industry on some ofsociety´s greatest challenges.
The AGS represent a new synthesis of multidisciplinaryand multi-geographical research that draws on the diverse
and complementary skills of the AGS partners. In additionto academic collaborations each of the universities hasextensive experience in working with stakeholders,particularly a growing number of visionary leaders fromindustry who recognise their fundamental role in achievingsustainable development.
More information at AGS website: globalsustainability.org
Photo: Måns Ahnlund
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