Fundamental Principles of Incinerator Design

Dev. Chem. Eng. Mineral Process., 7(5/6), pp.623-640, 1999.

Fundamental Principles of Incinerator Design

J. Swithenbank, V. Nassezadeh, R. Goh and R.G. Siddall Shefield University Waste Incineration Centre, Dept of Chemical and Process Engineering, University of Shefield, Shefield SI 3JD, UK

Waste management has become a major concern world-wide and incineration is now being increasingly used to treat waste which cannot be economically recycled. The combustion of conventional well specified fossil fuels is a very complex process since it involves two-phase turbulent reucting flow including radiant heat transfer. Incineration is even more complex since the waste is poorly specified and its composition variesfiom moment to moment. In the past, the design of incinerators has not been based on fundamental understanding and modelling of the process, and empirical rules have had to be used.

Over the last few years, computationaljluid &namics (CFD) has provided a means to model the fieeboard region in a conventional municipal solid waste incinerator but the open literature contains no rigorous fundamentally based model of the bed region. The prediction of the jlow composition emerging fiom this region is particularly important since it provides the "upstream" boundary condition for the jlow calculations in the fieeboard. For example, the calculation of the subsequent history of heavy metals requires knowledge of their emission ratefiom the burning bed.

The processes in the bed include dying, pyrolysis, oxidative burning, and gasifcation of the char. Furthermore, the movement of the grate is designed to mix the waste as it burns. Indeed, the existence of a rigorous bed model would also permit the grate design to be optimised, and, if immediate data on the feed were available, a rational combustion control strategy could be devised. A preliminary model of combustion in the bed is proposed herein based on governing equations for the burning of individual "regions" of waste in the upward gas flow, their motion, and radiant heat transfer within the bed. The emissions of gases fiom the surface of the bed is very non-uniform with oxygen emitted fiom either end of the bed, organic compounds fiom the one-third region, and carbon monoxide fiom the centre. The surface layer of this central part of the bed consists of char with gases coming up @om the oxidising layer below containing NOx derived from the fuel nitrogen. The char region therefore acts as an important reburn zone and it is suggested that this reduces some of the NOx to nitrogen. This is important since the minimisation of NOx by optimising the basic combustion process is likely to be environmentally preferable to subsequent control of NOx by the injection of reactants such as ammonia.

The calculation ofjlow and combustion in the fieeboard using CFD can quantifi the consequences of design concepts, however guidelines are neeakd to devise specifc design concepts which are worthy of investigation. Thus the computer cannot "invent" a design but it can quantifi the results of ideas.

Incinerator design thus requires a judicious combination of fundamental combustion science, ingenious engineering guided by an understanding of the miring process, and last but not least, practical experience ofprevious failures andsuccesses.

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J. Swithenbank et al.

Introduction With up to one tonne per year of municipal waste being generated for every man, woman and child each year across the globe, its safe disposal has become an urgent environmental problem. Once re-use and recycling have been maximised, there still remains a large fraction of municipal, clinical and hazardous wastes which must be disposed of in the best manner possible. Landfill leads to problems of methane leakage, groundwater contamination, and land usage, and is now perceived as environmentally unfriendly. In recognition of these problems, the UK government have recently imposed a landfill tax to discourage burying waste and encourage energy recovery fiom waste. Incineration is now accepted as the most environmentally friendly means of disposing of these forms of waste, and the design of incinerators must be optimised. It is important to note that the recovery of energy fiom waste reduces the net release of carbon dioxide to the atmosphere since it displaces the fossil fuel that would have been used and eliminates the production of methane fiom landfill. Furthermore, the total energy available fiom waste in Europe alone amounts to a staggering 100 million tomes per year of coal equivalent. On the other hand, the emission to the atmosphere of heavy metals and acid gases and perhaps more important in the longer term, the dioxins/furans (PCDDdPCDFs), is one of the key issues facing not only the incineration industry but also society as a whole over the next decade and more. Thus the design of the incinerator plant must include reliable means to control any emissions. Furthermore, there is a growing requirement to eliminate all residues fiom industrial processes and thus incineration is now being used increasingly to convert into inert building material, waste which cannot be economically recycled. The combustion of conventional well specified fossil fuels is a very complex process since it involves two-phase turbulent reacting flow including radiant heat transfer. Incineration is even more complex since the waste is poorly specified and its composition varies h m moment to moment. In the past, the design of incinerators has not been based on fimdamental understanding and modelling of the process, and empirical rules have had to be used. The topic is now receiving increased attention by the engineering research community and the gap between fundamental scientific principles, and plant conshuction and operation is now being bridged. Thus incinerators are evolving rapidly fiom simple covered bonfires, to very sophisticated process plants. However, reliable data and comprehensive, fundamentally based design procedures are not yet available to optimise the design, manufacture, operation and control of incinerators. As a result of our present ignorance, they all fall short of attainable throughput, quality of ash and plant availability. The price of our ignorance can be measured as hundreds of millions of pounds to over the next few years. It is an objective of SUWIC's research programme to help minimise these costs.

The Problem to be Addressed Combustion of waste in an incinerator is arguably one of the most complex combustion problems known The design and control of such a combustion device poses many problems since there is no satisfactory model of the system as a whole. In particular, a process as basic to their operation as the combustion on the grate has not yet been fblly elucidated. Thus at present, a typical design "equation" for the combustion on the grate is merely that the buming rate is about 400 kg/m2hr. Clearly this is a gross over- simplification of the many complex processes taking place simultaneously. This poor

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Fundamental principles of incinerator design

state of knowledge can be contrasted With the dramatic progress which has been made recently in the modelling of reacting; pulverised coal, gas and liquid fuel systems, which has resulted in impressive reductions in the emission of pollutants with little sacrifice in efficiency. The fimdamental analytical approach typically involves iden- the governing differential equations for all of the relevant processes, then solving these equations simultaneously, often using a numerical code in which the equations are converted fiom differential into algebraic form. This generic approach has resulted in fundamentally based commercial codes for fluid dynamics which have developed remarkably rapidly. They have proved to be very versatile in their applicability to a wide range of design problems. The primary objective of our present research programme at S W C is to identify the appropriate set of governing equations for combustion on the grate of an incinerator and to develop and validate a procedure for solving these equations. A key aspect of incinerator bed combustion is the mixing of the solid material on the grate. Experiments are required to characterise and quantify this mixing process so that it can be modelled accurately during the numerical solution of the governing equations. The efficiency of incinerator combustion, and particularly the residual carbon in ash which determines the suitability of the ash for reuse, depends critically on this mixing process. There are several significantly Merent types of grate employing distinct waste mixing strategies that are used in current MSW incinerator design. The three most common are:- 1. The stepped grate, in which the burning waste is mixed as it falls from one level of

the grate to the next, thus giving oxygen access to the unburned material. 2. The reciprocating grate (and its variants), in which the grate bars reciprocate

resulting in local mixing of the waste. 3. The rotating cylinder grate which consists of a series of slotted rotating drums. The

waste mixes as it tumbles from one drum to the next. These experiments are fimdamental to practically all future research on incineration, from understanding of the process to the design and operation of these units.

Experimental Investigations As pointed out above, the design of incinerators with a burning bed of waste material is sti l l being hampered by the lack of an accurate mathematical model of the process. The prediction of the incinerazor gas phase flow is in a more advanced stage of development using computational fluid dynamics (0) analysis, although further experimental data is still required Unforhmately, it is not possible to scale down many aspects of waSte incineration and tests on full scale incinerators are essential. Thanks to a close relationship between SUWIC and Sheffield Heat & Power Ltd, an extended research programme has been carried out at the Bernard Road Incinerator plant in Sheffield. This plant consists of two Municipal Incinerators (35 MW) and two Clinical Waste Incinerators (5 MW). These provide district heating for a large part of the city d e r e the heat is distributed as hot water to commercial, domestic and industrial buildings. To improve the economics, a 6 MW generator is now being added to the system. During the last decade, many investigations have been carried out (please see References. 1 to 10) and a SUWIC laboratory is located at the plant.

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Mathematical Modelling of the Gas and Particle Flow Over the last few years, computational 5uid dynamics (CFD) has provided a means to model the freeboard region in conventional municipal solid waste, clinical waste, sewage sludge, and special waste incinerators. The prediction of the flow composition merging from the bed is particularly important since it provides the "upstream" boundary condition for the flow calculations in the freeboard. For example, the calculation of the subsequent history of heavy metals demands precise knowledge of their emission rate fiom the buxning bed. Computational fluid dynamics is arguably the most important development in modem engineering practice since it is having such a great in5uence on research, design, development and production. Almost all CFD codes were originated by combustion technology research groups as a result of our efforts to umkstand the complexities of the process by solving the set of interacting governing differential equations. Thus combustion technology can claim to be the father of CFD. Developments in CFD have always gone hand-in-hand with developments in digital computer technology and even a basic PC is now a very powerful machine. What of the fuhae of CFD? In a few short years, the range of applications of the codes has increased dramatically. In response to demand, their capabilities are still expanding. For example, the need for the user to be involved in internal details such as grid generation is gradually receding. Nevertheless, engineers must still uuderstand the hdamentals of 5uid dynamics and exercise their ingenuity since CFD can only be used to quantify ideas. Since many industrial processes involve the behaviour of flow in packed beds, with or without mixing of the bed material, an important long-tenn spin-off from this investigation will be a mathematical model which has remarkably wide applicability. The fundamental analytical approach to model a moving grate type incinerator involves identifying the relevant mathematical equations governing all the relevant combustion processes in the burning bed of waste. Due to the heterogeneous nature of the burning waste and the type of waste burned, these processes are often so complex that attempts to model all the process are quite difficult. This section describes the approaches taken in attempts to mathematically link the reduction of bed height and volume with the complex set of inteawing processes occurring in the combustion of solid waste in a systematic modelling procedure. In order to keep this problem manageable whilst producing simulations that are useful for design pu~poses, careful assumptions and model simplifications which retain the main essential characteristics of the combustion process are needed. The main processes of municipal solid waste combustion are drying, pyrolysis or devolatilisation, char gasiscation and gas phase combustion. These four main processes fonn the basis of the present theoretical study of the incinerator bed modelling.

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Figure 1. In the present study, the solid waste is considered to consist of four components: moisture, volatiles, bound ash and fixed carbon as illustrated in Fig. 1. As the solid waste is heated, the moisture is removed by vaporisation at 373 K. The heat transferred to the solid supplies the heat required for the change of phase from liquid fonn into the vapour form. The regressing drying surface is assumed to remain at 373 K until all moisture is evaporated. The volatiles consist of carbon, hydrogen and oxygen and are released during pyrolysis at 533 K. In the absence of 0,, the gaseous volatiles released are assumed to consist of C,H,, CO and water. The moving pyrolysis front is assumed to remain at the pyrolysis temperature until all volatiles are driven out. The pyrolysis products are assumed to be non-reacting until the volatiles are driven off the bed. After the moisture and volatiles have been driven off, the solid surface which consists only of carbon and bound ash begins to char. Any carbon in the solid surface is assumed to be oxidised by any available O2 to form CO and CO,. During the char oxidation process the bound ash crumbles and is removed from the solid surface as free ash. The bound ash and carbon in the solid are assumed to remain in the same proportion relative to each other. Therefore, the rate of ash removal from the solid surface will be in the same proportion as the rate of carbon reacted at the solid surface to CO and CO,. The gas phase consists of 2 volatile species, i.e., CO, and GH,. which oxidises to CO and C02. Since the process of incineration reduces the bulk volume of the municipal waste by approximately 90%, it is apparent that a suitable bed model must also pennit simulations of reduction in the bed volume. Due to the heterogeneous nature of the waste incineration process, it may be impractical to adapt existing models describing the progress of a single particle during gasification of coal, such as the Ash Segregation model and the Shell Progressive model, to the bed. Hence, the major problem to overcome in this area was to establish a physical representation of the volume of the

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solid bed. As moisture and volatiles are liberated during drying and pyrolysis, the solids become more porous. The signScauce of the intemal pore space to the incineration process as a whole is yet unhown. Although the pores fonned can be considered as b e i i occupied by gaseous fluids, the local conditions within the pores in the solid are very merent h m those in the gaseous fluids occupying the voids between the solid. Therefore, it is sensible that the void fiaction and the internal pore space are treated as different elements. An added advantage of this strategy is, if the shape of the solid is allowed to remain relatively unchanged in the incineration process, then the void hction of the bed occupied by the gas phase will be unchanged despite the reduction in the bulk volume of the bed. To represent the void space in the solids left by the drying and pyrolysis process, a seventh component hown as the internal pore space is introduced into the model. This additional component differentiates the solids internal pore space fiom the gas space occupied by the gas phase at the outer surface of the waste. Component 7 therefore, has neither mass nor enthalpy but has a volume to allow changes in the bed volume to be represented. This internal pore space is removed during the char gasification process, hence reducing the bulk volume of the bed. No pore space remains at the end of gasification. If the three processes of drying, pyrolysis and gasification of the solid waste do not occur simultaneously, the change in the bulk volume of the bed with time is as illustrated as in Fig. 2. The first vertical column in Fig. 2 show the initial volume distribution of the six main components in the bed. During the drying and pyrolysis stages, the overall bed volume does not change but the solid becomes less dense due to removal of moisture and volatiles. The reduction in the volume of moisture and volatiles in the solid is replaced by an increase in volumes represented by the internal pore space. In the gasification zone, no additional pore space is being formed but the volume of the pore space is simultaneously reduced as the bound ash becomes fkee ash and is subsequently removed fkom the solid. Thus, as the char gasifies, the bulk volume of the bed reduces whilst maintaining a constant volume ratio of the bound ash to the pore space witbin the solid.

Figare 2. Fig. 3 illustrates the change in the bed bulk volume with time due to the simultaneous OcCllITence of

gasification If the process of drying and pyrolysis do not produce any reduction in the bed volume, the change in the bed volume as shown in Fig. 2 is solely due to the gasification process. However, the net change in the internal pore space in this case is given by the internal pore space fonned during drying and pyrolysis, less the pore space removed during gasification. If the rate of formation of the internal pore space is

drying, pyrolysis and Intnnal pore space

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Figure 4. The static bed can be divided into 4 layers as shown in Fig. 4. Layer A is the layer of material in its initial state, layer B consists of the dried material, layer C is the dried and pyrolysed material and layer D is the layer of free ash (i.e., the dried, pyrolysed and gasified material). The top surface of the bed is exposed to hot radiating gases and the air is supplied from the bottom of the bed. Heat is transferred through the bed by radiation,

greater than the rate of removal of the pore space during gasification, then the volume of internal pore space will continue to increase until drying and pyrolysis ceases.

Radiation Hot gas t and volatiles

Gasification front

/ / I LayerD

Layerc Pyrolysis front Layer B layer^ Drying front

t Airin

Figure 3. A simplified model for the simulation of a steady state moving bed model is an unsteady static bed model. Any variation in the static bed condition with time can be related to changes in the bed at positions at the same time h m e for the moving bed. Variations in the flow rate of the under grate air along the bed can be considered as variation of the under grate air supply with time.

0 assp pace Fixedcarbon Volatiles

Drying, pyolysis and gasification /

Internal pore space

Time I

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Figure 5. If the process fkont above layer L moves a distance AYLF in time At as shown in Fig. 5, the volume of the layer L per unit surface area of bed is decreased by AVu (where, AVU = AYu). The volume of solid removed due to movement of the process fkont is given by:

AVLF,S = (1 - &,)AVLF

w p m e s s f i m t s

--

.-

The vol&e of the component in the process can be expressed as:

Over the same time step At, the volume of the layer L increases due to factors immediately beneath the layer. The addition of solids volume due to movement of the process fkont can be given by the volume of the solids added plus the volume of internal

"W.N = LF.NAvLF,S (3)

pore space generated by the process: AVLB,S = (1 - 0 LF.N ) A ~ L F , S + FN AVLF,N (4)

where FN is the volume hction of component N which leaves internal pores spaces as N is being removed fiom the solid. FN has a value which lies between 0 and 1. Therefore the total volume added due to movement of the process fiont becomes:

1

The net change in volume in any layer L, is given by the sum of AVm - AVu. Based on the above volume change, the enthalpy balance for the solid for the drying, pyrolysis and gasification processes can be written in the general form:

where % is the mass rate of removal of component N &om the solid and 4 is the net heat change associated with the process under consideration. 4 and 4 are parameters which depend on the type of kinetic model or scheme chosen as the process or reaction rate controlling mechanism. (%)N is related AVws by the equation:

A pot burner experiment is currently underway to provide both validation and constants for the model. A closely related study is outlined below-

Proposed Optimisation of NO, Reduction by the Bed

The major source of NO, in municipal waste incinerators is the nitrogen in the fuel (0.4-0.8%). This contrasts with conventional combustion systems where NO, is produced principally by thermal mechanisms. The so-called prompt NO, production

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only accounts for about 10% of the NO, in a conventional combustion system, and this mechanism can be neglected as a significant contributor in incinerators. In the UK, the permissible NO level emitted by incinerators is being gradually

level is determined by the principle of BATNEEC. This principle gives scope for innovation if a superior pollution control technique emerges. At present, it is generally accepted that the reduction of NO, in incinerators should be achieved by either catalytic or non-catalytic d o n with ammonia or urea. The selective non-catalytic reaction (SNCR) takes place in the temperature range 870-950°C. The ammonia injection is therefore in the radiation shaft above the incinerator finnace section. A typical reaction is:-

The quantity of ammonia required is 2 to 4 times stoichiometric, however problems arise due to ammonia slip. Typically, 75 to 90% of the ammonia slip is absorbed in a semi-dry scrubber cake, whilst 100% is absorbed in a wet scrubber. The surplus ammonia thus appears as a compound in the filtrate, filter cake or fly ash and this can lead to an increase in filter M and waste disposal problems. Thus removing NO, in this manner gives rise to other pollution problems. The use of urea (NH92CO instead of ammonia in a SNCR system is effective fiom 540 to 1000°C but leads to the formation of N20. The proportion of N20 formed increases fiom zero at 780°C to about 18% at 1000°C. As an alternative to the SNCR system, selective catalytic reduction (SCR) of NO, takes place at 400 to 450°C using ammonia and a catalyst such as zeolite. The SCR system is often located after the normal scrubber system and the temperature requirement means that the flue gases must be re-heated then cooled to about 140°C before discharge to atmosphere. Inevitably, this is an expensive operation requiring a heat exchanger, supplementary burner and a catalytic reactor, however the SCR system is much more effective than the SNCR system since it can remove 95% of the NO,, whereas the SNCR system only removes about 50% of the NO,. However, the SNCR system tends to be installed if it will meet the required NO, level. Here it is proposed that a degree of NO, reduction can be achieved by opthising the combustion process in the bed. From the forgoing it will be appreciated that this gives a major advantage since it does not incur the inherent problems of ammonia slip, ammonia storage and ammonia supply. The concept involves the complex mechanisms taking place within the burning bed of waste. The following diagram illustrates the different combustion zones.

reduced &om about 400 mgMm r to a value of about 200 mg/Nm3. The acceptable

4NO + 4 w 3 + 0 2 + 4N2 + 6H20

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Figure 6. In Figure 6 can be seen that the waste is progressively

to C02 & H 2 0 dried, pyrolysed, oxidised, then enters into the important reducing region. Notice that there is a thin oxidising layer undemeath the reducing region. This thin region consumes all the oxygen whilst releasing carbon dioxide. In the reducing region, the charred waste bums by converting the carbon dioxide to carbon monoxide and the H20 to CO and H2. Notice that a region can exist where all the C02 has been consumed. As the bed thins fbther along, the reducing layer graduaIly vanishes and eventually only ash remains. This situation is further complicated by movements of the grate which are designed to mix the bed in the vertical direction whilst slowly transporting the waste towards the ash pit. The total transit time of the waste along the grate is typically about 40 minutes.

Figure 7. It is apparent that the gases emitted fiom the bed will vary significantly depending on the axial position along the bed. These gases vary &om excess oxygen, to fuel gases consisting of hydrocarbon pyrolysis products carbon monoxide and hydrogen. Considering the reducing region of the bed, it is proposed that this can be used to reduce the N4( formed fiom nitxogen in the

Equations for these reactions are given in Reference 1 1. These processes are illustrated in the following diagram, Figure 7. Very little information could be located in the literature on the reduction of NOx in a bed of char. However Reference 12 describes some relevant experiments in which air was passed though a buming bed of simulated waste. The results indicated that the NO, was least when the air was passed through the bed rather than added above the bed, and injecting the air well above the bed also minimised the NO, This data can be interpreted as confirming the concept of NO, reduction within the bed, and NO, formation when overfire air reacts with waste at the top of the bed The same reference also reports measurements on a municipal incinerator located at Issy-les-Moulineaux where a 10% reduction in overfire air injection resulted in a 10% reduction in NO,

CO & H2 burn above the bed with secondary air

fuel to nitrogen gas.

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production. Again, this result can be interpreted as reduction of NOx in the reducing region of the bed.

Freeboard Combustion Turning next to the processes above the bed. In present practice in incinerators, secondary air jets are located above the bed to bum out the hydrocarbons and carbon monoxide. The oxygen level in the flue is typically 8- 10% and this represents a great waste of energy as sensible heat of the excess air. Here it is proposed that this level of excess air should be dramatically reduced by the use of recycled flue gases to provide the mixing energy rather than air jets. First we must consider the needs of the "furnace" region immediately above the bed. Here the main requirement is to mix the various gases emitted fiom Merent regions of the bed as explained above. In this case, flue gases can be used since the gases fiom the bed include both fuel and oxygen, However, it is important that these "inert" jets do not impinge on the surface since they would then lead to the removal of many particles fiom the surface, thus increasing both erosiodcorrosion of the boiler and the load on the particle scrubbing system. Following the furnace regioq excess air should be injected at the base of the "radiation shaft" to ensure that the hydrocarbons are completely bumed out. These regions and concepts are illustrated on Figure 8.

Figure 8. The injection of air to mix these gases emerging fiom the bed has two disadvantages. Firstly, the supply of excess air will reduce the efficiency of power generation since there will be an increase in the energy carried away in the flue gas flow. Secondly, if the air jet impinges on the bed surface, this could result in burning of the surface layers of the waste fiom above with a consequent increase in the emission of fuel NOx One solution to this problem is to replace some or all of the air in the jet with recycled flue gases. Furthennore, if the jets impinge on the surface, there will be an undesirable increase in the entrainment of particles into the flow. The criteria goveming the features of the required flow field in the radiation shaft are:- 0

0

0

Little or no recirculation zones since they provide dead space which reduces the residence time and bum-out of the hot gases. Near uniform velocities minimise the cany-over of particulates by high velocities. Flow nearly parallel to the walls to minimise particle impingement and consequent erosiodcorrosion.

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Figure 9. There is a b & m e n M between the completeness of reaction in a jet mixing system and the mixing power available h m the jets. The analysis is summarised below and isillustrated in the diagram:- The key factor to note is that the jets produced either by a bafne or by jets located in a wall produce a turbulent "stit~ed" reactor followed by a plug flow reactor. This is an optimum combination to achieve complete reaction since the stirred reactor initiates the reaction but

Turbulence production and dewy behind a flame itabilber

'

Beffle Plug flow reactor

-

is dissipated in approximately 15 jet diameters.

Figure 10. In the case of the jets in an incinerator, it is interesting to consider the history of the jet mixing power. Figure 10 shows how this power flows &om the fan to the turbulence decay. This energy balance can be expressed mathematically by:-

Jet Mixing Theory

Fan power (Watts) -Pressure and flow

2 - k t kinetic energy (power) - k t velocity 4- =t I

1 kt turbulence kinetic energy (power)

Hence:

Turning now to partially stirred reactor theory; in a partially stirred reactor the oxygen consumption rate is equal the dif€erence between the rate without diffusion and the diffusion rate:

'k 'k 'd Tk = Kinetic time 8 = RTI E = dimensionless temp. C, = Oxygenconc at reactor entry

T,, =Mixing/diffusiontime c k = Average O2 conc in reacto C = O2 conc after mixing

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Where: The dif€erence between the entering oxygen concentration and the exit concentration must be bumed in the stay (residence) time:

2, ’k T~ =Kinetic time 8 = RT/E = dimensionless temp. C, = Oxygen conc. at reactor entry

T~ = Stay (Re sidence) time C, = Average 0, c0nc.h reactor C = 0, conc. after mixing

Where: The rate of mixing is proportional to the concentration difference, hence:

dC - C,-C - - dz =D

t, = Diffusion(Mixin& time Ck =AverageO, conc C = 0, conc after mixing

Where: From these three equations, the completeness of combustion in a partially stirred reactor is:

1

q, = Combustion efficiency (3 =Fraction of O2 untreated

If the kinetic time is very much less than the mixing time, which is true for combustion reactions at high temperature, then unmixedness limits the combustion efficiency in a partially stirred reactor to :-

1

76 T~ = t, /-rd = Ratio of residence time to mixing time q, = Combustion efficiency

To determine the parameters governing the ratio of residence time to mixing lime, we conclude that Tsd = .t / u’, based on dimensional anaiysis, whilst the value of ‘Fd is given by X/U where X is the length of the stirred reactor.

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size and location of the secondary jets of air and recycled flue gases. Once a design has been selected,

the CFD procedures described previously and illustrated in References 1 to 10. The location of the jets must also take into account the macro-mixing in order to ensure that the adjacent micro- mixing eddies contain the two materials to be mixed rather than the same material. This is given by

the flow is then calculated by

Now d , z00 .2h and X z l O h Thus inserting relations fiom

efficiency limited by mixing is approximately:

above, the maximum co~bustiOn Hence rd z 5 and is independent of h

1 Thus : tl, = 1

mhg efficiency factor .,, m = exp[-h/ 01' Size of energy containing eddy = CT

Initial separation of material to be mixed =

0.: r,y,-;- 0 OA 0.8 1.2 1.6 2 &

0

0

1 + 1

- 1 - - 1 50 (u' /U)- 1

50,/m 1+ 1

5 0 d m

' I IC - 1+

Where the dynamic head loss of themkingsystem is E,=APIq

Figure 11.

In an incinerator, the dynamic head of the flow (q) should be determined at the cross- section of the duct into which the jets are being injected, whilst A€' is the pressure loss across the jet. Due to the approximations of the analysis, it is clear that these relations must not be interpreted quantitatively, nevertheless, they do illustrate the parameters governing the maximum efficiency which can be achieved in the secondary combustion

0 Entrainment theory; - An i s o t h d jet entrains its own original mass flow every 3.2 jet diameters.

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0

0

Partially stirred reactor theory; - The completeness of combustion by a partially stirred reactor, followed by a plug flow reactor, is limited by the degree of mixing. Mixing power concept; - The power supplied to the air fan is transfmed progressively to the jet, then to the turbulence kinetic energy, and finally to the turbulence dissipation of the Kolmogorov scale eddies which slightly heats the gas. Molecular mixing; - The dissipation of kinetic energy between eddies takes place by the movement of individual molecules, which simultaneously results in mixing at the molecular level. Macro-mixing; - In order that the species in the adjacent Kolmogorov eddies are merent, thus ensuring good mixing efficiency, the large scale of the energy containing eddies must be greater than the initial separation of the materials to be mixed. The dimensions of the energy containing eddies are comparable to the dimensions of the shear layer at the edge of the jets.

0 Mixing is the product of both micro-scale mixing power and macro-mixing efficiency.

Quantifying the above parameters provides a means to estimate suitable locations and sizes for the various jets installed in an incinerator, including the secondary air jets.

0

0

Energy From Waste

Many industrial wastes such as oil sludges have a significant energy content and in some cases, fuels can be derived from them. One such example, derived from waste organic chemicals, is Cemfuel which is used by the cement industry. Power is also generated in some countries from waste sludges such as those produced by the paper industry. Municipal solid waste typically has an energy content of about 10,000 Wkg. Thus an incinerator burning 10 t/hr is releasing 10 x 10,000/3600 M/s = 27.8 MW of heat. If this heat is used in a steam cycle to generate electricity with 20% efficiency, then 5.56 MW of electricity would be produced. Much of the balance is available as low grade heat at a temperature of say 120°C for district heating. Fortunately, both the heat and the power are produced near the consumer hence the transport of both the heat and the power is convenient. Since electricity is about 8 times more valuable than the same amount of heat as hot water, there is interest in maximising the efficiency of power generation fiom waste. Thermodynamic analysis show that the efficiency of the Rankine steam cycle, consisting of a boiler, superheater, turbine, condenser and pump, depends on the steam temperature and pressure, therefore to increase the efficiency, we must increase the steam temperature.

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Figure 12. Unfortunately, due to the corrosive nature of flue gases, incinerator boilers are prone to corrosion. Figure 12 shows the ranges of flue gas temperature and boiler tube temperature at which corrosion can be expected. The steam operating temperatures and pressures for Euro ean incinerators are typically 400 C at 40 bars. On the other hand Japanese incinerators operate at about 300 OC at less than 30 bars. The reason for this difference stems fiom the European requirement to generate energy fkom the waste, although the extent to which this applies varies from country to country. - The objective towards which we are now working is to integrate the incinerator and power production system into the needs of a sustainable city. The incinerator alone can only provide about 20% of the power needs of the city and the balance must be provided fiom a power unit which is acceptable in the city environment. Although options such as fuel cells are being developed to -1 this need, our research centres on the intercooled, regenerated, reheated, gas turbine. By using the high temperature heat exchanger (up to 1200 OC at high pressure) technology which we have developed (Reference 13), gas turbines with an efficiency of about 60% are feasible. The convenient utilisation .of their 'waste ' heat, to heat or cool the surrounding city buildings raises their overall efficiency to more than 80%. The integration of this CHP concept with the incinerator provides a practical strategy for the introduction of combined heat, power and waste management (CHPWM), leading to the sustainable

t

city.

Conclusions

0 Energy fkom waste is a more environmentally friendly technology than landfill. 0 Incineration technology is maturing rapidly at the present time. 0 Modeling of the burning bed is being developed. 0 CFD can be used to model the gas flows but interpretation requires care. 0 Mixing theory can be used to guide the design 0 The practical experience of plant operators must be combined with fimdamental

studies to guide the f b r e of incinerator design and operation.

Nomenclature

Ap convective heat transfer area 4 heat transfer coe5cient 4 conduction within the solid T, temperature of the solid phase

638


T, V volume El P density 0

Subscripts

2 moisture 3 volatiles 4 bound ash 5 fixed carbon 6 fiee ash 7 internal pore space LB LF S

temperature of the gas phase

volume hction of the gas space

volume hction of component N in the solid (VN Ns)

1 gas Phase

layer immediately behind a process layer in front of a process front solids (including the internal pore space)

References

Three Dimensional Mathematical Modelling Of Sheffield Municipal Solid Waste Incinerator (35 MW) Using Experimental Data and Computational Fluid Dynamics, V. Nasserzadeh, J. Swithenbanlc, B. Jones, Jomal of Institute Of Energy, Vol. 64, September 1991, pp 166-175. Design Optimisation Of a Large Municipal Solid Waste Incinerator, V. Nasserzadeh, J. Swithenbank, D. Scott, B. Jones, Journal of Waste Management,

Three Dimensional Mathematical Modelling of Coventry Municipal Solid Waste Incinerator (65 Mw) Using Computational Fluid Dynamics and Experimental Data', V. Nasserzadeh, J. Swithenbank, C. Schofield, D. Scott and A. Loader, Journal of Process Safety and Environmental Protection, Tramactions of the Institution of Chemical Engineers, Vol. 71, Part B, November 1993, pp 269 - 279. Effect of High Speed Secondary Air Jets on the Overall Performance Of a Large Municipal Incinerator with a Vertical Shaft, V. Nasserzadeh, J. Swithenbank and B. Jones, Journal of Combustion Science and Technology, 92,4 - 6, pp 389 - 422, 1993. Emission Testing and Design Optimisation of Sheffield Clinical Incinerator, V. N a s s d e h , J. Swithenbank, D. Lawrence and N. Garrod, Journal of Process Safety and Environmental Protection, Part B., Institute of Chemical Engineering,

Measurement of Gas Residence Times in Large Municipal Incinerators Using the PRBS Tracer Technique, V. Nasserzadeh, J. Swithenbank., Journal of Institute of Energy, Vol. 11, September 1995, pp 43 - 61. Environmental Advantages and Disadvantages of Modern Landfill versus Waste Incineration, V. N a s s d e h , J Swithenbank, Paper presented at INACAP/SOFOFA Conference Chile, April 1995 Chief Inspector's Process Guidance Note: Techniques for Integrated Pollution Control, Issue Series 2 (S2); Process Sector, Waste Disposal & Recycling, Section

Vol. 11, pp 249-261, 1991.

August 1995, pp 57 - 71.

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.I. Swithenbank et al.

on Computational Fluid Dynamic Simulation of Incinerators, pp 31 - 35 V. N a s s d e h and J. Swithenbank, London: HMSO, December 1996

9 Control of Waste Incinerators, K. Young, M. Vara and J. Swithenbadc, British Flame Days, Paper No. 2.4, L e d , UK, September 1994.

10 A Review of Factors Influencing Boiler Corrosion in Incinerators, J. Swithenbank, B.C.R Ewan, Conference on Waste Incineration, MTG 95, March 1995, Copenhagen, Denmark.

11 An Emissions Model for a Bubbling FBC using Detailed Chemical Kinetics: Significance of Destruction Reactions, Goel S K, Beer J M and S a r o h A F , Journal of the Institute of Energy December 1996 69 pp 201 -21 3.

12 Nox Emissions of Municipal Solid Waste Incineration; Experiments in a Counerflow Fixed Bed Reactor and in a Rotary Kiln Incinerator, Joabouille F, Zhou X, Kerdsuwan S, Bregeon By Goudeau J C, The Third Asian Pacific I@ntemational Symposium on Combustion and Energy Utilisation, Hong Kong, (1 995)

13 New Developments in Power Generation; Technologies for a Cleaner Environment Seminar, Swithenbank J, Smyth R, Lmgston P, Beck S, Cuernavaca, Mexico, July 1995.(available fiom SUWIC)

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Documents

Fundamental Principles of Incinerator Design