Energy from gasification of solid wastes - ResearchGate

Energy from gasification of solid wastes

V. Belgiorno, G. De Feo*, C. Della Rocca, R.M.A. Napoli

Department of Civil Engineering, University of Salerno, via Ponte Don Melillo, 84084, Fisciano (SA), Italy

Accepted 11 September 2002

Abstract

Gasification technology is by no means new: in the 1850s, most of the city of London was illuminated by ‘‘town gas’’ producedfrom the gasification of coal. Nowadays, gasification is the main technology for biomass conversion to energy and an attractivealternative for the thermal treatment of solid waste. The number of different uses of gas shows the flexibility of gasification andtherefore allows it to be integrated with several industrial processes, as well as power generation systems. The use of a waste–bio-

mass energy production system in a rural community is very interesting too. This paper describes the current state of gasificationtechnology, energy recovery systems, pre-treatments and prospective in syngas use with particular attention to the different processcycles and environmental impacts of solid wastes gasification.

# 2002 Elsevier Science Ltd. All rights reserved.

1. Introduction

Today, the world demand for renewable energy sour-ces is the key factor in the revival of the use of gasifica-tion systems, which was in strong decline after theadvent of petroleum (Cuzzola et al., 2000). Gasificationsystems are successfully applied to the production ofenergy from biomass. They also represent an attractivealternative to the well-established thermal treatmentsystems for the recovery of energy from solid wastes.Gasification is particularly suitable to treat industrialwastes but there are some problems with municipalsolid wastes related to their heterogeneity.In this paper, the relative complexity of technology

needed for feasible gasification process cycles is dis-cussed with particular reference to the different reactors,energy recovery systems and gas clean up systems.The aim of this paper is not to determine or to

demonstrate whether gasification is the best process forthe thermal treatment of solid wastes or not. The con-cept of ‘‘best’’ is valid solely in the context of localvalues, limits and problems, such as characteristics ofwaste, environmental regulation or communities

dimension. Nevertheless modern incineration is de-factothe standard for comparison of the gasification perfor-mance (Juniper, 2000).

1.1. Gasification

Combustion, gasification and pyrolysis are the ther-mal conversion processes available for the thermaltreatment of solid wastes. As shown in Fig. 1, differentproducts are gained from the application of these pro-cesses and different energy and matter recovery systemscan be used to treat these.Gasification can be broadly defined as the thermo-

chemical conversion of a solid or liquid carbon-basedmaterial (feedstock) into a combustible gaseous product(combustible gas) by the supply of a gasification agent(another gaseous compound).The thermochemical conversion changes the chemical

structure of the biomass by means of high temperature.The gasification agent allows the feedstock to be quicklyconverted into gas by means of different heterogeneousreactions (Di Blasi, 2000; Hauserman et al., 1997; Bar-ducci, 1992; Baykara and Bilgen, 1981). The combus-tible gas contains CO2, CO, H2, CH4, H2O, traceamounts of higher hydrocarbons, inert gases present inthe gasification agent, various contaminants such assmall char particles, ash and tars (Bridgwater, 1994a).

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Waste Management 23 (2003) 1–15

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E-mail address: [email protected] (G. De Feo).

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Direct gasification occurs when an oxidant gasifica-tion agent is used to partially oxidise the feedstock. Theoxidation reactions supply the energy to keep the tem-perature of the process up. If the process does not occurwith an oxidising agent, it is called indirect gasificationand needs an external energy source (Figs. 2 and 3)(Hauserman et al., 1997; Staniewski, 1995). Steam isthe most commonly used indirect gasification agent,because it is easily produced and increases the hydrogencontent of the combustible gas (Hauserman et al., 1997).Pyrolysis is an indirect gasification process with inert

gases as the gasification agent. As shown in Fig. 2,resulting from the gasification process and varying withthe temperature at which the process is carried out, thethree major output fractions are (De Feo et al., 2000):

1. a combustible gas;2. a liquid fraction (tars and oils); and3. a char, consisting of almost pure carbon plus

inert material originally present in the feedstock.

As shown in Table 1, the heating value of the gas issignificantly affected by the presence of nitrogen. Due tothe absence of nitrogen in the gasification agent, theindirect gasification process increases the volumetricefficiency and produces a gas with a higher heatingvalue (De Feo et al., 2000; Paisley, 1998). The loweringof gas production rate, typical of indirect gasification,reduces the cost of energy recovery and gas cleanupsystems but is still complex and increases investmentcosts (Hauserman et al., 1997).

Fig. 1. Thermal conversion process and products (Bridgwater, 1994a).

Fig. 2. Gasification and pyrolysis processes.

2 V. Belgiorno et al. /Waste Management 23 (2003) 1–15

Direct gasification with pure oxygen has the sameadvantages as the indirect gasification process. How-ever, the cost of oxygen production is estimated to bemore than 20% of the overall electricity production(Della Rocca, 2001).Typically, a gasification system is made up of three

fundamental elements: (1) the gasifier, useful to producethe combustible gas; (2) the gas cleanup system, neces-sary to remove harmful compounds from the combus-tible gas; (3) the energy recovery system. The system iscompleted with suitable sub-systems useful to controlenvironmental impacts (air pollution, solid wastes pro-duction, wastewater).

2. Solid waste and biomass

For a correct and efficient gasification process, a suf-ficiently homogeneous carbon-based material isrequired. Therefore many kinds of waste cannot betreated in the gasification process and for certain types

an extensive pre-treatment is required (refuse derivedfuel) (Fig. 4). Instead there are several types of wastethat are directly suitable for the process; they are: papermills waste, mixed plastic waste, forest industry wasteand agricultural residues (Juniper, 2000).

3. Gasifiers

The gasifier is the reactor in which the conversion of afeedstock into fuel gas takes place. There are three fun-damental types of gasifier: (1) fixed bed, (2) fluidised bedand (3) indirect gasifier. In Table 2, the main advantagesof the different type of gasifiers are summarised. Pres-surised reactors, not discussed in this paper, are onlysuitable for coal and oil gasification.

Fig. 3. Direct and indirect gasification processes.

Table 1

Gasification processes

Process Gasification

agent

Producer gas

heating value

(MJ/Nm3)

Direct gasification Air 4–7

Pure oxygen gasification Oxygen 10–12

Indirect gasification Steam 15–20

Fig. 4. Wastes suitable for gasification.

V. Belgiorno et al. /Waste Management 23 (2003) 1–15 3

A key factor of the reactor is the capacity to producea gas with low tar content (condensable bituminouscompounds). A high tar concentration causes a lot ofproblems to energy recovery systems because of its cor-rosive characteristics.

3.1. Fixed bed

Vertical fixed bed reactors (VFB) are the most com-petitive fixed bed gasifiers. As shown in Fig. 5, they aresubdivided into updraft and downdraft gasifiers.Updraft is a counter-current gasifier, where the feed-stock is loaded from the top while air is introduced fromthe bottom of the reactor. In the reactor the solidmaterial is converted into combustible gas during itsdownward path (Quaak et al., 1999; Bridgwater, 1994a).Feedstock is treated in the following sequence starting

from the top: drying, pyrolysis, reduction and combus-tion (Juniper, 2000; Quaak et al., 1999; Hauserman etal., 1997; Bridgwater, 1994a). In the combustion zone,the highest temperature of the reactor is greater than1200 �C. As a consequence of the updraft configuration,the tar coming from the pyrolysis zone is carriedupward by the flowing hot gas: the result is the produc-tion of a gas with a high tar content. Typically, thesensible heat of gas is recovered by means of a directheat exchange with feedstock (Bridgwater, 1994a).In a downdraft reactor, co-current, the carbonaceous

material is fed in from the top, the air is introduced atthe sides above the grate while the combustible gas iswithdrawn under the grate (Juniper, 2000; Quaak et al.,1999; Hauserman et al., 1997; Bridgwater, 1994a). As aconsequence of the downdraft configuration, pyrolysis

vapours allow an effective tar thermal cracking. How-ever, the internal heat exchange is not as efficient as inthe updraft gasifier (Quaak et al., 1999; Bridgwater,1994a).

3.2. Fluidised bed

Fluidisation is the term applied to the processwhereby a fixed bed of fine solids, typically silica sand,is transformed into a liquid-like state by contact with anupward flowing gas (gasification agent) (Juniper, 2000).Fluidised bed gasification was originally developed tosolve the operational problems of fixed bed gasificationrelated to feedstocks with a high ash content and, prin-cipally, to increase the efficiency (Quaak et al., 1999).The efficiency of a fluidised bed gasifier is about fivetimes that of a fixed bed, with a value around 2000kg/(m2 h) (Quaak et al., 1999; Bingyan et al., 1994).Fluidised bed reactors are gasifier types without dif-

ferent reaction zones. They have an isothermal bedoperating at temperatures usually around 700–900 �C,lower than maximum fixed bed gasifiers temperatures.The bubbling fluidised bed (BFB) and circulating flui-dised bed (CFB) gasifiers are schematically presented inFig. 6.In a BFB reactor, the velocity of the upward flowing

gasification agent is around 1–3 m/s and the expansionof the inert bed regards only the lower part of the gasi-fier. Bed sand and char do not come out of the reactorbecause of the low velocity (CITEC, 2000; Ghezzi, 2000;Quaak et al., 1999).The velocity of the upward flowing gasification agent

in a CFB reactor is around 5–10 m/s (CITEC, 2000;

Table 2

Comparison of different gasifier [modified by (Juniper, 2000; Bridgwater, 1994 a)]

Characteristicsa Fixed bed Fluidised bed Indirect gasifier

Updraft Downdraft Bubbling Circulating Char Gas

Carbon conversion **** **** ** **** ***** **

Thermal efficiency ***** **** *** **** *** ***

CGE ***** *** *** **** *** ***

Turndown ratio *** ** **** **** **** ****

Start-up facility * * *** *** ***** *****

Management facility **** **** ** ** * *

Control facility ** ** **** **** ***** *****

Scale-up potential *** * *** ***** *** ***

Sized feed elasticity **** * ** ** ** **

Moisture feed elasticity **** ** *** *** * *

Ash feed elasticity * * **** **** *** ****

Fluffy feed elasticity **** ** * *** *** *

Sintering safety * * *** ***** ***** ***

Mixing * * **** ***** ***** ****

Cost safety ***** **** ** ** * *

Tar content * ***** ** *** ** **

Particulate content ***** *** *** ** ** ****

LHV * * * ** ***** *****

a * poor, ** fair, *** good, **** very good, ***** excellent.


Ghezzi, 2000). Consequently, the expanded bed occu-pies the entire reactor and a fraction of sand and char iscarried out of the reactor together with the gas stream(De Feo et al., 2000). This fraction is captured andrecycled in the reactor using an air cyclone that inter-cepts the gas stream (Niessen et al., 1996).

3.3. Indirect gasifier

Indirect gasifiers are the reactors used for the steamindirect gasification and are grouped as char indirectgasifiers and gas indirect gasifiers depending on the typeof internal energy source (Fig. 7).

Fig. 6. Fluidised bed gasifiers.

Fig. 5. Fixed bed gasifiers (Quaak et al., 1999).


A char indirect gasifier consists of two separate reac-tors: a CFB steam gasifier that converts feedstock intoproduced gas and a CFB combustor that burns residualchar to provide the necessary heat to gasify the feed-stock. Sand is circulated between the two reactors totransfer heat. Energy is provided by combustion of resi-dual char, reserving all gaseous and condensable pro-ducts for gas production (Hauserman et al., 1997; Craiget al., 1995; Staniewski, 1995). This process is also called‘‘fast fluidised process’’ because it has the highestthroughputs and yields of gas (Farris et al., 1998; Hau-serman et al., 1997; Niessen et al., 1996; Staniewski, 1995).Gas indirect gasifiers use a steam fluidised bed gasifier

within bed heat exchange tubes (Hauserman et al., 1997;Niessen et al., 1996). A fraction of combustible gas isburned with air in a pulse combustor and the hot com-bustion products provide heat to gasify the feed (Hau-serman et al., 1997; Niessen et al., 1996; Staniewski,1995). Gas indirect gasification is extremely versatilewith a wide range of feeds (Hauserman et al., 1997).The main advantage of indirect gasification is the high

quality of the combustible gas produced in contrast withgreater investment and maintenance cost of the reactor.Therefore it is necessary to improve the quality of gaswith the adoption of a highly efficient energy recoverysystem.

4. Energy recovery systems (ERS)

4.1. Steam cycle

The steam cycle is the simplest option for energyrecovery. It does not need gas pre-treatment, becausetar is burned in the combustor and cannot damage the

boiler (Quaak et al., 1999). The maximum net electricalefficiency of a gasification–steam cycle plant is about23%, which is comparable with the efficiency of a typi-cal solid waste incinerator (Consonni, 2000).A limitation in the traditional waste incineration and

the gasification–steam cycle boiler is the maximummetal temperature of the superheater tubes, normallylimited to less than 450 �C to prevent excessive corro-sion of the tubes by the HCl that may be present in theflue gas. This limitation results in a lower steam tem-perature to the steam turbine and thus a low overallplant electrical efficiency (Rensfelt and Everard, 1998).In a gasification–steam cycle plant, this limitation

could be overcome by gas pre-treatment or by integrationwith a thermoelectric power plant (Della Rocca, 2001).Pre-treatment of the gas can remove the HCl before it

goes into the burner, thus the firing of the clean gas in amodern boiler combination would allow a steam tem-perature of 520 �C, with a 6% improvement in electricalefficiency (Rensfelt and Everard, 1998).The integration with conventional power plants is

called ‘‘co-firing’’: it allows to increase the performancetaking advantage of the high efficiency steam cycle ofthe thermoelectric power plant. Usually a co-firing sys-tem is performed in two possible configurations (Con-sonni, 2000; Nieminen et al., 1999): adopting a gasburner in a separate boiler only for the water evapora-tion phase, as shown in Fig. 8, or adopting a gas burnerin the same boiler as the primary fuel, as shown in Fig. 9.

4.2. Engine

Spark ignition engines, normally used with petrol orkerosene, can be run on gas alone. Diesel engines can beconverted to full gas operation by lowering the com-

Fig. 7. Indirect gasifiers.


pression ratio and by installing a spark ignition system(Quaak et al., 1999; FAO, 1993).Because of the low lower heating value (LHV), engines

converted to gas are less efficient than those not converted;nevertheless a modern engine correctly modified can reachover 25% of net electricity output (FAO, 1993).The engines have the advantage of being robust and

having a higher tolerance to contaminants than gasturbines (Bridgwater, 1994a). Nevertheless if the gas iscompressed into a turbocharger the same condition asin the gas turbine will result (Bridgwater, 1994a; FAO,1993).

The main disadvantages of gas engines are the lowincrease in efficiency obtained using the combined-cyclemode and the poor economy of scale (Bridgwater,1994a).

4.3. Gas turbine

The power plants based on advanced combined cyclegas turbine could allow an efficiency-rate of around 60%(Najjar, 1999). The effective net electrical output is lowerthan 40% because of the consumption for gas pre-treat-ment (De Lange and Barducci, 2000; Van Ree et al.,

Fig. 8. A possible configuration of cofiring system with two different boilers.

Fig. 9. A possible configuration of cofiring system with one boiler.


1997). In fact gas turbines are very sensitive to the qualityof gas, only extremely low levels of contaminants, prin-cipally tar, alkali metals, sulphur and chlorine com-pounds, can be tolerated (Bridgwater, 1994a).The chemical recovery cycle is a new and very inter-

esting option. In this case, the energy content in the tur-bine exhaust gas is used to feed the pre-treatment processof gas, such as catalytic cracking of tar or steam reform-ing process (Della Rocca, 2001; Happenstall, 1998).Typical gas turbines must be adapted to the low LHV:

for an easier start-up phase, the burners must allow dualfuel operation and longer combustion chambers arenecessary to improve the control of CO emissions(Zanforlin, 1995; Becker and Schetter, 1992).

5. Future alternative use of syngas

Future alternative uses of syngas include molten car-bonate fuel cells and methanol production. Molten car-bonate fuel cells are very interesting because of the highefficiency-rate (more than 50%) and the easy integrationwith different energy recovery systems (Iacobazzi, 1995).Methanol, well known as a clean fuel, can be synthe-sised by a gas containing H2, CO and CO2 using a cop-per catalyser (Jung, 1999; Nowell et al., 1999).Methanol production by conversion of a homogeneouswaste could be an interesting alternative.

6. Pre-treatment of gas

Pre-treatments of gas can be used to avoid environ-mental pollution and dangerous components, such astar and particulate, for the energy recovery system or toincrease heating value and hydrogen contents. The

design of pre-treatment systems principally depends onthe energy recovery technologies in use (Quaak et al.,1999; Bridgwater, 1994a).In Table 3, gas properties related to pre-treatments

are briefly described. While the required gas propertiesfor different energy recovery systems are given inTable 4. Finally, Table 5 shows issues and cleanup pro-cesses related to flue gas contaminants.

6.1. Thermal cracking

Biomass and waste-derived tars are very stable andrefractory to cracking by thermal treatment (Depnerand Jess, 1999; Bridgwater, 1994b). Temperaturesrequired are around 1000–1300 �C (Depner and Jess,1999; Quaak et al., 1999; Bridgwater, 1994a).Two competitive different approaches are used in

fixed bed gasifiers to obtain thermal cracking: use oftemperatures of hearth zone and/or increase of gas resi-dence time.Some advanced applications of modified downdraft

gasifiers with internal recycle of gas, proposed for theautomotive gasifier application, can obtain a tar levellower than 50 mg/Nm3 (Susanto and Beerackers, 1996).

Table 3

Contaminant presence in the gas and relative problems

Contaminant Presence Problems

Particulates Derive from ash, char, condensing compounds and bed material

for the fluidised bed reactor

Cause erosion of metallic components

and environmental pollution

Alkali metals Alkali metals compounds, specially sodium and potassium,

exist in vapour phase

Alkali metals cause high-temperature

corrosion of metal, because of the stripping

off of their protective oxide layer

Fuel-bound nitrogen Cause potential emissions problems by forming NOx during

combustion

NOx pollution

Sulphur and chlorine Usual sulphur and chlorine content of biomass and waste is

not considered to be a problem

Could cause dangerous pollutants and acid

corrosion of metals

Tar It is bituminous oil constituted by a complex mixture of

oxygenated hydrocarbons existing in vapour phase in the

producer gas, it is difficult to remove by simple condensation

Clog filters and valves and produce metallic

corrosion

Table 4

Gas quality requirements/energy recovery system

Boiler Engine Gas turbine

Stand alone Cofiring

LHV (MJ/Nm3) >4 None >4 >4

Particulate (mg/Nm3) None None <5–50 <5–7

Tars (g/Nm3) None None <0.5 <0.1–0.5

Alkali metals (ppm) None None <1–2 <0.2–1


6.2. Catalytic cracking

Catalytic processes for the conversion of tars needreaction temperatures of around 800–900 �C. Tarremoval efficiency is 90–95% and dolomite is an effec-tive and inexpensive tar cracking catalyst (Rensfelt andEverard, 1998; Delgado et al., 1996; Orio et al., 1996;Bridgwater, 1994b; Mudge et al., 1987). Dolomitedemand is around 0.03 Kg/Nm3 of raw gas (De Langeand Barducci, 2000; Rensfelt and Everard, 1998).

The process can be carried out both in a fluidised bedgasifier with catalysts added to the bed or in a specialreactor below the gasifier (Bridgwater, 1994b; Mudge etal., 1987). The first solution uses the temperature of thereactor but the catalyst life is not very long. With asecondary reactor, the catalyst is protected by deactiva-tors but requires added oxygen to oxidise gas andincrease the temperature.Fig. 10 shows catalytic processes in gasification

systems.

6.3. Steam reforming and CO-shift

Shift and reverse methanation reactions allow anincrease of up to 10% of the gas volume of hydrogencontent by conversion of methane and steam (Aznar etal., 1998; Caballero et al., 1997). A commercial catalystis used for steam reforming and for CO shift; catalystsare activated at a low temperature.Steam reforming and CO-shift need a preliminary

abatement of tar because catalysts are easily deactivatedwhen the tar content is greater than 2 g/Nm3 (Aznar etal., 1998).

Fig. 10. Catalytic processes in gasification systems (Bridgwater, 1994b).

Table 6

Advantages and disadvantages of tar removal systems

System Advantages Disadvantages

Thermal cracking Simple control

Low cost

LHV losses

Low efficiency

Catalytic cracking LHV unchanged

Upgrade too

No gas cooled

Catalyzer cost

Difficult control

Scrubber Easy control

Air pollution control

LHV losses

Gas cooled

Wastewater production

Table 5

Fuel gas contaminants: problems and cleanup processes

Contaminant Range Examples Problems Cleanup method

Particulate (g/Nm3) 3–70 Ash, char, fluid bed material Erosion, emission Filtration, scrubbing

Alkali metals (g/Nm3) Sodium and potassium compounds Hot corrosion Condensation and filtration

Fuel nitrogen (g/Nm3) 1.5–3.0 Mainly NH3 and HCN NOx formation Scrubbing, SCR

Tars (g/Nm3) 10–100 Refractory aromatics Clog filters, deposit internally Tar cracking, scrubbing

Sulphur, chlorine (g/Nm3) 2.5–3.5 H2S, HCl Corrosion, emission Lime scrubbing


6.4. Scrubber or saturator

Modified scrubbers called saturators are adopted as atar control system (Larson, 2000).A saturator has two separate towers. In the first tower

the gas is saturated by water droplets at a temperature of40–80 �C (Quaak et al., 1999; Bridgwater, 1994a). Tem-perature and water saturation allow tar condensation onthe droplet (Larson, 2000; Cernuschi, 2000). In the sec-ond tower a scrubbing process eliminates suspendeddroplets and condensed tar. A humidified packed bed isusually applied to increase the contact surface betweengas and wash water (Quaak et al., 1999; Cernuschi, 2000).In Table 6, the main advantages and disadvantages of

tar removal systems are described.

6.5. Baghouse

A baghouse is a very effective and proven technologythat allows the removal of particulate matter larger than0.1 mm with an efficiency-rate of around 99% (Contiand Lombardi, 2000; Urbini, 2000).

6.6. Alkali condenser

Alkali metals condense at 550 �C on the particulate.Consequently if the gas reaches 550 �C and is treated bya baghouse, alkali metals are removed with the particu-late (Quaak et al., 1999; Craig et al., 1995).

7. Process cycles

Several elements of a gasification system can be com-bined to follow both the wastes characteristics and

energy recovery requirements. In the following para-graphs four different process cycles adopted in differentsituations are presented.

7.1. Gasification/steam cycle, stand-alone configuration

This effective and reliable configuration can be easilyintegrated with industrial processes for onsite use ofheating and electricity and for the recycling of gasifierinert residues (SAFI, 1995).Gas is burned directly to give heat at a stand-alone

steam cycle. Absence of a cleanup system simplifies theprocess and reduces the plant cost, nevertheless an airpollution control system may be necessary to meetemission limits (SAFI, 1995; Barducci, 1992). Theoverall electricity output is around 20% and it can beincreased by 6% if an acid gas control systems isadopted (De Feo et al., 2000; Rensfelt and Everard,1998).Fig. 11 shows a synthetic scheme of TPS installation

in Greve in Chianti (Italy).

7.2. Gasification/steam cycle, co-firing configuration

A co-firing plant uses gas to give energy to a steamcycle power generation plant. Gas can be used in thesame fuel boiler or in a secondary one which producessteam, later superheated in the first boiler (Figs. 8 and9). Efficiency of this process cycle can allow an electricaloutput of over 30%.In the first configuration acid gases are diluted,

whereas in the secondary boiler the temperature of thesteam tubes remains below 180 �C and corrosive actionis not effective (Della Rocca et al., 2001; Consonni,2000). Moreover, a co-firing plant allows the use of highmoisture content feedstock because of the superheatedtemperatures, which do not depend on the heating valueof the combustible gas (Consonni, 2000; Nieminen etal., 1999).Fig. 12 shows a scheme of an installation in Lahti

(Finland).

7.3. Gasification/engine

The gasification of coal and carbon containing fuelsand the use of gas as fuel in internal combustionengines is a technology that has been utilised for morethan a century (FAO, 1993). The main innovation forwaste and biomass gasification/engine process are clean-up systems used for removing dust, tar, and alkalimetals from the raw gas (Quaak et al., 1999). Experi-ence leads to a use of a modified downdraft gasifier fortar thermal cracking and a hot gas filter for dustremoval. Electrical efficiency is around 25%, because ofthe low heating value of gas (Bridgwater, 1994a; FAO,1993).

Fig. 11. TPS gasification plant in Greve in Chianti (Italy) (De Feo et

al., 2000).


7.4. Integrate gasification combined cycle (IGCC)

IGCC could be competitive in a few years (Larson,2000). An IGCC plant uses a clean gas in a gas turbinecombined cycle to produce energy. Net electrical output

is over 30% and can increase up to 40% if total thermalpower is over 50 MW (Morris and Waldheim, 1998).The main disadvantage of this plant is the need for a

cleanup system for the control of corrosive gas phasecompounds such as tar, acid gas and alkali metals(Quaak et al., 1999; Bridgwater, 1994a; Larson, 1992).

Fig. 12. Foster Wheeler gasification plant in Lahti (Finland) (Nieminen et al., 1999).

Fig. 13. TPS/ARBRE gasification plant in Eggborough (UK) (Van Ree et al., 1997).


The most interesting element is the system of tar controlthrough catalytic tar cracking or wet scrubbing. Thefirst solution is implemented in the Termiska ProcesserARable Biomass Renewable Energy (TPS/ARBRE)installation in Eggborough (UK), as shown in Fig. 13.Fig. 14 shows the second solution as adopted in theThermie Energy Farm (TEF) installation in Cascina(Italy).

8. Environmental impacts

8.1. Air pollution

Atmospheric emissions of gasification systems dependon the installed air pollution control equipment andenergy recovery systems in use. Therefore it is difficultto compare the air pollution impact of gasification sys-tems with a conventional combustion process.However, the use of gas allows for a successful com-

bustion control, better than solid combustion resultingin an effective reduction in the emission of CO, NOx,dioxins and unburned compounds (Giugliano, 2000;Tchobanoglous et al., 1993). Moreover gas pre-treat-ment can be performed to remove pollution precursorssuch as nitrogen and chlorine compounds and improve

emissions (SAFI, 1995; Staniewski, 1995; Tchoba-noglous et al., 1993).

8.2. Solid waste production

Solid waste production is related to char and ashextracted from the gasifier, the particulate controlequipment and the possible boiler, with a production of2–9, 5–10 and 3–6% of treated feedstock, respectively(SAFI, 1995).Gasifier residues could be used to fertilise the

ground (rarely if the feedstock is not agriculturalwaste) or disposed in a sanitary landfill. Instead, solidresidues of gas pre-treatment and air pollution con-trol systems are typically disposed in landfills, becauseof their high heavy metal concentration level. Some-times, solid residues can be used in industrial pro-cesses, such as cement mills, for a completeintegration between gasification and industrial pro-cesses (SAFI, 1995).

8.3. Wastewater

In the gasification process, wastewater may be pro-duced by the gas cooler and the wet scrubber containingmany soluble and insoluble pollutants, such as acetic

Fig. 14. TEF gasification plant in Cascina (Italy) (De Lange and Barducci, 2000).


acid, sulphur, phenols and other oxygenated organiccompounds (Bridgwater, 1994a). The insoluble fractionof the wastewater consists mainly of tars.Wet scrubber effluent production is around 0.5 kg/

Nm3 of treated gas (Barducci et al., 1997). If a scrubberis used for tar removal the effluent needs expensivetreatment, otherwise the usual problems are a low pHand a high salt content, which can be easily controlledby neutralisation and chemical precipitation, respec-tively (Cernuschi, 2000).In the gasification plant Thermie Energy Farm, one of

the three IGCC projects selected for funding by theEuropean Union, the sequence of treatment for tar-richwastewater is (Barducci et al., 1997; Barducci and Neri,1997):

� precipitation of sulphur by iron sulphate addition;� recovery of sulphur and dust by filtering;� disposal of filter cake;� stripping off gases dissolved in the water and the

major part of the hydrocarbons;� partial evaporation of water and usage of con-

densate as scrubber make-up;� discharge of evaporator blow down to conven-

tional bio-treatment.

The salt recovered has a very low polluting potential,and is conveyed to a sanitary landfill. Hydrocarbons

and other stripper gases are recycled in the combustorfor destruction, so that the tar energy content is recov-ered (Barducci et al., 1997; Barducci and Neri, 1997).Because of the complexity of treatment and disposal,

the present trend is to develop a system that does notproduce a liquid effluent (Bridgwater, 1994a), never-theless this is possible only for wastes that do not con-tain many contaminant precursors.

9. Gasification and waste management

Gasification represents a future alternative to thewaste incinerator for the thermal treatment of homo-geneous carbon-based waste and for pre-treated hetero-geneous waste. As shown in Fig. 15, gasification shouldbe considered as an option for the thermal treatment ofwastes in an integrated waste management system. Forexample, co-firing and co-gasification (gasification ofsolid waste with coal or biomass in the same gasifyer) areinteresting solutions for both decentralised energy systemsand waste management systems in rural communities.It is difficult to compare the costs of gasification pro-

cesses with conventional combustion on a direct basis.This is due to the fact that the costs available refer todifferent specifications of plant, for example to meetdifferent emission standards or have a varying ash con-tent or water treatment requirements (Altmann and

Fig. 15. Integrated waste management system.


Kellett, 1999). Moreover it is incorrect to compare thecost of new technology with the cost of old technology,because the former also includes the R&D cost.

10. Conclusions

The gasification process offers considerable energyrecovery and reduces the emission of potential pollu-tants. It is considered an interesting alternative to theconventional technology for the thermal treatment ofsolid wastes.The principal difficulties of solid waste gasification,

especially for municipal solid waste (MSW), are relatedto the heterogeneity of wastes. A possible solution is theproduction of a refuse derived fuel (RDF) with homo-geneous and controlled characteristics. In any case, gasi-fication is particularly suitable for many homogeneousagricultural and industrial wastes (waste tyres, paper andcardboard wastes, wood wastes, food wastes, etc.).Gasification plants could be integrated with pre-

existing industrial and thermoelectric plants, because oftheir flexibility and compactness. The most significantchoices of design are the reactor type and process cycle,which can be conveniently adopted according to wastecharacteristics.The main components of gasification plants for the

recovery of energy from solid wastes are discussed whiledescribing and comparing the possible technologicaloptions and their environmental impacts. Severalindustrial scale applications are discussed and futureperspectives of gasification are introduced.

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