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EU clean coal technology—co-combustion of coal and biomass
Fuel Processing Technology 54 Ž1998. 159–169
EU clean coal technology—co-combustion of coal and biomass K.R.G. Hein a
a,)
, J.M. Bemtgen
b
Institut fur Verfahrenstechnik und Dampfkesselwesen, UniÕersity of Stuttgart, Stuttgart, Germany b European Commission, DG XII Science, Research and DeÕelopment, Brussels, Belgium
Abstract Apart from a more economical use of fossil fuels, the application of regenerative energy sources should be advanced in order to reduce CO 2 emissions. One of the alternatives considered to decrease the net emissions of CO 2 are the cultivation and combustion of solid biomass, or the thermal utilization of sewage sludge. There are different types of biomass which could be used as energy source in power production: Ža. surplus and by-products from agricultural activities within the European Union ŽEU., e.g., straw, Žb. fast growing energy plants from reutilization of areas which become available by a necessary reduction of agricultural overproduction within Europe, and Žc. wood waste from forestry or wood processing. In order to achieve a noticeable CO 2 reduction, as well as fossil fuel substitution, it is desirable to use fairly large quantities of biomass for energy production. However, an exclusive biomass utilization would consequently lead to the construction of many decentralized plants, which is time-consuming and would require high financial investments as well as large storage capacities due to the seasonal fuel availability. Co-combustion, in contrast, is considered to be a cheap option for utilizing the existing biomass resources. For these reasons, an EU-project, ‘Combined Combustion of BiomassrSewage Sludge and Coals of High and Low Rank in Different Systems of Semi-industrial and Industrial Scale’ was launched in 1993. Under the coordinatorship of the Institut fur Verfahrenstechnik und Dampfkesselwesen ŽIVD., University of Stuttgart, partners from eight European countries investigated the effects of burning sewage sludge, agricultural residuals, such as straw and manure, as well as especially cultivated energy plants in combination with coals of various ranks and origin. Both the pulverized fuel ŽPF. and the fluidized bed ŽFB. mode were tested, ranging from laboratory rigs to large scale utility boilers. This paper provides an overview of the activities of the various partners involved and will, in particular, show the synergetic cooperation towards a common aim. The results of the 2-yr project will be summarized. q 1998 Elsevier Science B.V. Keywords: Co-combustion; Biomass; Coal; Sewage sludge
)
Corresponding author.
0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 7 . 0 0 0 6 7 - 2
160
K.R.G. Hein, J.M. Bemtgenr Fuel Processing Technology 54 (1998) 159–169
1. Introduction The observed increase of the CO 2 concentration in the atmosphere, with its negative effect on the global climate, is predominantly caused by the combustion of fossil fuels. Among these fuels, coal is burnt in power plants and industrial installations in large quantities and with increasing tendency. In order to alleviate the problems associated with the CO 2 emission, control methods and processes are under investigation worldwide. In Europe, this area was—and still is—subject to various research and development projects initiated and substantially co-financed by the European Union ŽEU. through the Directorates General XII ŽScience, Research and Development. and XVII ŽEnergy.. One approach, the increase of plant efficiency, i.e., was initiated within the JOULE ŽJoint Opportunities for Unconventional or Long-Term Energy Supply. programmes in 1989, which also dealt with the removal of CO 2 from flue gases and the various alternatives of CO 2 disposal. Likewise, the thermal use of regenerative energy sources as one of the alternatives, e.g., the combustion of cultivated solid biomass, of biomass by-products, and wastes including sewage sludge was—and still is—under investigation within the same European programme. The application of such fuels in industry offers a wide range of ecological and, in many cases, economical advantages like: conservation of fossil fuel resources, reduction of the dependence on fuel imports, utilization of agricultural and forest residues, reduction of emission of harmful species from fossil fuel combustion, recultivation of non-utilized farming areas, minimization of waste disposal. In principle, there are two possibilities for biomass and waste utilization in the power industry: biofuels can either be burnt as single fuel in combined heat and power plants of limited capacity or they can be co-utilized in existing coal fired power stations. The latter of the two alternatives was subject of a more recent EU programme under the abbreviation APAS ŽActivite de Promotion, D’Accompagnement et de Suivi. because of the advantage that in coal-fired power stations, the seasonal and regional fluctuations in biomass and waste composition and in their availability can be compensated by varying the coalrbiofuel firing ratio. Also, several studies show that lower investment and operational costs of co-combustion compared with exclusively biomassfired units can be expected.
2. Objectives With the background of the above mentioned advantages, the European Commission launched this research project of 2-yr duration Ž1993–1994. in which the co-combustion in laboratory, pilot and full-scale units was jointly investigated by partners from industry, research organizations and universities. For each of the technical
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alternatives—pulverized fuel ŽPF. and fluidized bed ŽFB. application—the following main areas had to be considered: Ža. best suitable fuel preparation and feed and the subsequent effects of the combustion process, Žb. full fuel utilization—complete burnout —under minimization of envisaged operational problems like fouling, slagging, and corrosion, Žc. emission and its control of gaseous species ŽSO x , NO x , PAH, PCDDrF, halides, trace metals etc.. and of solid by-products Žashes., Žd. scale considerations and scale-up criteria for industrialrutility application including retrofit aspects, Že. large scale demonstration and the determination of special fuelrprocess dependent requirements and restrictions, and Žf. economics, also in relation to single fuel firing systems. Within the above areas of investigation, careful attention had to be given to certain fuel properties of biofuels and wastes, like volatile matter, inorganic impurities and the strong variety of chemical composition and physical characteristics. The results were intended to provide a profound base in knowledge resulting in a wide spread of utilization of biofuelrwaste in co-combustion mode within Europe and beyond.
3. Technical programme 3.1. Partners Under the coordinatorship of the Institut fur Verfahrenstechnik und Dampfkesselwesen ŽIVD., University of Stuttgart, Germany, 25 partners from eight European countries representing industry, national and international research organizations, and universities participated in the project. 3.2. Fuels During this project the following additional fuels were used: biomass residuals from agriculture and forestry Žstraw, wood, bark, Cynara cardunculus ., biomass cultivated from fast growing energy plants Ž Miscanthus sinenesis, Arundo donax ., waste paper of domestic and industrial origin and residuals from sewage sludge processing. Both the biomass and the sewage sludge investigations were carried out in various scale pilot plants as well as in large-scale power stations. The installations can be conveniently grouped into different combustion techniques like FB and PF, and further classified according to their thermal input ranging from pilot scale to large scale ŽFig. 1.. 3.3. Installations The programme of the project was separated into two parts according to the different biogenic fuels.
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Fig. 1. EU programme for co-combustion, participants and facilities.
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3.3.1. Biomass co-combustion Taking the above given considerations into account, biomass was fired jointly with coal in the following research combustion rigs: Ža. drop tube reactor, 10 kWth , KTH, University Stockholm, S; Žb. pulverized coal combustion, 0.35 MWth , IVD University Stuttgart, D; Žc. pulverized coal combustion, 1 MWth , RWE Energie Žfor brown coal., D; Žd. pulverized coal combustion, 1 MWth , KEMA, Žmainly used to study the influence of co-combustion on desulphurization behaviour.; Že. pulverized coal combustion, 2.5 MWth , IFRF; Žf. atmospheric bubbling FB combustion, 0.3 MWth , ECN, NL Žfuel pellets mainly.; Žg. atmospheric bubbling FB combustor, 1 MWth , CIEMAT, E; Žh. circulating FB combustion, 0.3 MWth , INETI, P, Ži. circulating atmospheric FB reactor, 1 MWth , CIEMAT, E; Žj. circulating atmospheric FB combustion, 1 MWth , RWE Energie, D; and Žk. pressurized FB combustion, 1 MWth , Delft University of Technology, NL. Apart from these research rigs three large-scale boilers were part of the programme: pulverized coal combustion, 131 MWel at Esbjerg, ELSAMrVestkraft, DK Žhigh rank coal., pulverized coal combustion, 100 MWel at Lubbenau, VEAG, D Žlow rank coal. and ¨ circulating FB combustion, 80 MWth at Grenaa, ELSAMrMidtkraft, DK. 3.3.2. Sewage sludge co-combustion The programme for the co-combustion of sewage sludge concentrated on pulverized coal and on FB combustion. In addition, the utilization of sewage sludge in metal processing was investigated Žsee Fig. 1.. The tests for the co-combustion of sewage sludge were carried out in the following pilot and large-scale facilities: pulverized coal combustion, 0.15 MWth , Imperial College London, GB, pulverized coal combustion, 0.35 MWth , IVD University Stuttgart, D Žfor bituminous coal., pulverized coal combustion power plant, 150 MWel at Weiher, Saarbergwerke, D Žhigh rank coal. FB combustion, 2 MWth , Stadtwerke SaarbruckenrThyssen, D and ¨ FB combustion, 10.7 MWth , Fechner, D. In the pilot-scale facilities the impacts of sewage sludge addition Žup to 30%. on combustion behaviour and emissions ŽSO 2rNO x . were studied. In addition, trace species like dioxinsrfurans and heavy metal emissions were determined.
4. Results In this section, only major results are highlighted; details are presented in the final report of the project w1x. 4.1. Fuel characterization Biomass offers important advantages as a combustion feedstock due to the high volatility of the fuel and the high reactivity of both the fuel and the resulting char.
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The generally low sulphur content is also positive; e.g., for wood, straw and energy crops, values found were below 0.15 wt.% of the dry material. Both municipal solid waste and sewage sludge have sulphur contents similar to that of low sulphur coals. However, it should be noticed that in comparison with solid fossil fuels, biomass contains much less carbon and more oxygen and—as a consequence—has a low heating value. Also, chlorine contents of certain biofuels, like straw can exceed the level of coal. With regard to the ash content, values for biomass are much lower than for coal, e.g., for wood about 0.2 wt.% of the dry material. In contrast, sewage sludge can have very high ash content. In general, the ash content of different sludges vary between 30–60 wt.% of the dry material, with unfortunately low fluid temperatures as low as 9008C which may lead to problematic fouling of the boiler heating surfaces. 4.2. Fuel preparation The type and extent of the necessary preparation of the biomass fuels depend on the combustion system and the type of biomass. In general, bituminous coal mills are not suitable for biomass. Therefore, biomass co-combustion requires separate milling and feeding devices. As a consequence within this project biomass like straw and Miscanthus were crushed or chopped. Due to the fibrous structure of Arundo donax, Cynara, Miscanthus, straw and wood, it was difficult to determine an exact particle size distribution. For co-combustion of PF facilities, straw and Miscanthus, maximum particle sizes in the range of 4–8 mm in diameter and 1–2 cm in length were required on pilot-scale for a complete burnout. The pilot-scale combustion system presented the worst case regarding the necessary particle size because of the limitations in residence time. Therefore, for the large-scale co-combustion tests chopped straw with a maximum length of about 20 cm was sufficiently burnt in boilers due to the long residence time of large-scale PF combustion facilities. With wood the experience has shown that the particles had to be milled to less than 1 mm in order to burn satisfactorily within typical boiler residence times. However, milling may cause problems if the wood is not dried to a moisture below 8%. In circulated FB ŽCFB. combustion systems the experience with straw-firing showed neither the need to mill nor to pre-dry the straw. A pneumatic feeding of straw into the CFB unit operated very satisfactorily. Also, CFBs can be designed to handle the size of the hogged wood and wood chips. For the majority of the tests the sewage sludge was delivered pre-dried in form of granules with a maximum size up to 10 mm. With such fuel preparation it was possible to use milling systems typical for coal resulting in comparable particle size distributions. Unfortunately, excessive fines and inorganic fuel impurities may give rise to problems during the milling process. 4.3. Co-combustion of biomass in pulÕerized fuel mode Ignition and combustion tests were carried out in a number of installations. This includes laboratory equipment ŽRWE, ICSTM, KEMA., pilot plants ranging from 0.5–2.5 MWth ŽRWE, IVD, ICSTM, KEMA, IFRF. and full-scale boilers of 100 and
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Fig. 2. Combustion efficiency for various facilities and fuels.
120 MWel ŽELSAM, VEAG.. Ignition appeared not to cause any problems in all cases because the high volatile content of biomass, compared with coal, favours a rapid ignition. The combustion efficiency was measured by burnout and remaining CO and C x H y . In general the results of the various projects showed complete combustion for the biofuel ŽFig. 2.. The absolute values, however, may differ dependent on fuel type and biomass percentage added for known reasons. The burner configuration for the combined combustion of coal and biomass is an important factor in the combustion process. Within the project, both the injection via the coal burner and an addition at separate locations is investigated. At test facilities of different scales, investigations on the combustion of biomass, i.e., straw, additionally to the main fuel, i.e., bituminous coal, via the burner were carried out using different burner configurations. This has a great impact on the formation of nitrogen oxides ŽNO x .. If the fuel is added by the central gun, arranged on the inside, it enters the substoichiometric inner recirculation zone resulting in low NO x emissions. Fuel added by the annular clearance devolatilizes in high oxygen conditions therefore leading to an increased conversion to NO x . Therefore, the fuel with a higher N-content should be injected at the centre. Hence, for the case of a fuel blend coalrstraw, coal injection via the central gun and biomass injection via the annular clearance causes lower NO x emissions in contrast to a pre-blended fuel injection via the annular clearance. The results ŽFig. 3. of the test facilities with different capacities Ž0.15 MW, 0.5 MW, 2.5 MW. show a good correspondence with regard to the burner configuration under consideration and agree with the data from a utility boiler Ž300 MWth .. With an increasing biomass share, the NO x emissions decrease in all burner arrangements. Because of the content of volatiles, biomass is excellently suited for the application of NO x-reducing procedures such as air staging and reburning. In contrast to the situation in pure coal flames, with coalrbiomass blends and applying air staging in the
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Fig. 3. NO x data from various facilities with several biomass fuels.
furnace low NO x emissions are attained already under fairly air-rich conditions. In reburning, biomass as reduction fuel is superior to bituminous coal with regard to both emissions and burnout. Also the production of SO 2 emissions is positively affected by an increasing percentage of biomass because of the low sulphur content of the added fuel. In addition, SO 2 can be partly retained in the ash due to the alkaline-content of the biomass ash. Chlorine, which is found in certain biomass types, such as straw or Miscanthus, may affect operation by corrosion. In an utility boiler corrosion tests with two different materials ŽFig. 4. show in case of a straw share of roughly 10% of the thermal input that the corrosion rate increases in comparison with pure coal operation but are still tolerable.
Fig. 4. Corrosion rates for two different fuels and tube materials.
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With regard to deposit formation on heat transfer surfaces the investigations showed that fouling rates rises only negligibly. In the case of small biomass shares, the characteristics of pure coal ash are dominant. If, however, large, incompletely burnt straw particles remain slagging may occur because of the low melting point of the biomass ash. 4.4. Co-combustion of biomass in fluidized bed combustion mode One major research objective was the effect of biomass addition on the gaseous emissions. For SO 2 all partners confirmed that biomass co-combustion has a significant positive effect ŽFig. 5.. Reductions of up to 75% were observed. This can be mainly attributed to the low sulphur content of biomass fuels. In addition, increased sulphur retention in the ash was detected. At the facilities of RWE, TU Delft, and Grenaa the low NO x emissions of about 200 mgrm3 with biomass addition remained more or less unchanged. ECN, CIEMAT, and INETI found decreasing NO x concentrations which is due to the low fuel-N content of added fuel Žwood.. N2 O emissions where measured by INETI, TU Delft, and Grenaa. TU Delft basically did not find any influence of biomass addition on the emission. INETI discovered a slight decrease while the emissions at the 88 MWth plant at Grenaa decreased considerably with increased wood input. Emissions of chlorine were investigated by RWE, ECN, and Grenaa. Especially in co-firing straw with its high content of Cl, high concentrations of chlorine in the flue gas were found. At Grenaa the chlorine content in the flue gas for a 60% biomass share was 20 times higher than for coal only. One should note, however, that in this case a very low chlorine coal was used.
Fig. 5. SO 2 emissions as a function of facility and biomass fuel.
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4.5. Co-combustion of sewage sludge in pulÕerized fuel mode At the facilities with a capacity of 0.15 MW ŽIC., 0.5 MW ŽIVD., and 2.5 MW ŽIFRF., respectively, a wide range of different flame configurations, injection methods, as well as air and fuel staging were investigated bringing about clear differences between biomass co-firing and sewage sludge co-firing. In general, sufficiently high burnout and low CO emissions were observed in all coalrsewage sludge tests at all facilities. Depending on the burner injection modes the burnout can be improved or reduced. The NO x emission levels were found to be extremely sensitive with respect to co-firing ratio, injection mode, i.e., central or annular, primary and reburn zone stoichiometry, and flame ignition. An increase in NO x emission levels was observed when increasing the sewage sludge under unstaged combustion. However, NO x reductions of 70–80% were obtained under staged conditions but these values were extremely critical with respect to burner operation and injection mode. As an example, at the 0.5 MW facility ŽIVD., for all the different injection modes investigated, NO x emissions varied from 2800 mgrm3 at 50% unstaged co-firing to approximately 350 mgrm3 under low primary zone stoichiometry Ž0.7–0.9. in 25% co-firing mode. SO 2 emissions were strongly related with the fuel-S input and they are lower compared with biomass co-firing. In general, the conversion ratio of fuel-S into SO 2 was about 90%. Heavy metal measurements in the flue gases did not show problematical emissions and the ash remained of acceptable quality for utilization in, e.g., the building industry. As an example for large scale application at the power station Weiher 2 of the Saarbergwerke dried sewage sludge was added to the coal before milling, and the coalrsewage sludge mixture was fed to a high temperature slag tap unit. The investigations focused on the influence of the milling system, the emissions and by-products quality like granules, fly ash and gypsum stages. The impact of co-firing sewage sludge on boiler and FGD system was hardly detectable. Temperatures inside the boiler combustion chamber and the frequency of soot blowing were similar to the standard operation on coal. In co-combustion of sewage sludge with coal, the amounts of Zn, P, Cr, Pb, Cu, Co, and Ni determined in the granules were higher compared with pure coal firing. However, these values remained within the limits of legislation. In the fly ash, P and Zn were observed to be in proportion to the quantity of sewage sludge used and these concentrations were found to be higher when compared with pure coal firing. Trace element emissions were relatively low and far below the strong limits in, e.g., Germany. Therefore, except for the more difficult handling and feeding, co-combustion of coal with sewage sludge was observed to be an available option for the application in utility boilers. 4.6. Co-combustion of sewage sludge in fluidized bed combustion mode Industrial fuels consisting of blends of coal and different sewage sludge qualities were burnt in a 10.8 MWth FN combustion plant of the Fechner. The flue gas pollutant
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emission levels measured during the co-combustion of blends were fully in compliance with the emission standards of the tight pollution control acts and directives in Germany. However, due to the special type of shallow-bed fluidized stationary FBC, continuous co-combustion of dry sewage sludgercoal blends caused operational problems in terms of fly ash deposition and adhesions in the furnace toprhot gas duct areas. Major technical modifications of the FBC furnace such as increased freeboard height and installation of a hot gas cyclone for fly ash separation required for a continuous co-combustion of sewage sludge, were proposed. Stadtwerke Saarbrhcken verified in comprehensive tests at a 2 MWth FB installation that a wide range of mixtures of coal and sewage sludge can be incinerated without problem. The concentrations of the major components in the exhaust gas depend on the composition of the material used. In all tests, the values for both CO and NO x were well below the legislature limits. The FB installation was equipped with a filtration system for halogenated organic pollutants. In the clean gas after efficient dust separation the dioxin content was less than 0.1 ng TErm3.
5. Conclusions From the results of the APAS project it became obvious that both modes of combustion—the PF combustion and the FB application—are well suited for co-combustion of biomass and sewage sludge provided that a fuel dependent feed and preparation system is installed. With site dependent biofuelrmain fuel firing ratios no major negative effects on fuel conversion were found. Care has to be taken if high chlorine and high alkaline biofuels are added because of the known negative effects on operation like corrosion andror slagging of the heat transfer surfaces. For both mechanisms more detailed investigations, in particular, long term tests are desired. With regard to the emission of hazardous gaseous compounds no increase of the concentrations in the flue gases was experienced; in many cases substantial reductions of the emission were found which are due to the biofuel composition and also dependent on the biofuel injection mode. This favourable effect of injection of the biofuels was particularity obvious for the NO x-emission; a further optimization is expected from more detailed data and, thus, future investigations in this area are recommended. While the change of the flue gas composition in co-combustion does not disturb the operation and the final product of flue gas desulphurization processes a slide deactivation of catalysts for NO x-reduction was reported.
References w1x J.M. Bemtgen, K.R.G. Hein, A.J. Minchener, Combined Combustion of BiomassrSewage Sludge and Coals, Clean Coal Technology Programme 1992–1994, European Commission, DGXII: Science, Research and Development, ISBN 3-928123-16-5.
EU clean coal technology—co-combustion of coal and biomass K.R.G. Hein a
a,)
, J.M. Bemtgen
b
Institut fur Verfahrenstechnik und Dampfkesselwesen, UniÕersity of Stuttgart, Stuttgart, Germany b European Commission, DG XII Science, Research and DeÕelopment, Brussels, Belgium
Abstract Apart from a more economical use of fossil fuels, the application of regenerative energy sources should be advanced in order to reduce CO 2 emissions. One of the alternatives considered to decrease the net emissions of CO 2 are the cultivation and combustion of solid biomass, or the thermal utilization of sewage sludge. There are different types of biomass which could be used as energy source in power production: Ža. surplus and by-products from agricultural activities within the European Union ŽEU., e.g., straw, Žb. fast growing energy plants from reutilization of areas which become available by a necessary reduction of agricultural overproduction within Europe, and Žc. wood waste from forestry or wood processing. In order to achieve a noticeable CO 2 reduction, as well as fossil fuel substitution, it is desirable to use fairly large quantities of biomass for energy production. However, an exclusive biomass utilization would consequently lead to the construction of many decentralized plants, which is time-consuming and would require high financial investments as well as large storage capacities due to the seasonal fuel availability. Co-combustion, in contrast, is considered to be a cheap option for utilizing the existing biomass resources. For these reasons, an EU-project, ‘Combined Combustion of BiomassrSewage Sludge and Coals of High and Low Rank in Different Systems of Semi-industrial and Industrial Scale’ was launched in 1993. Under the coordinatorship of the Institut fur Verfahrenstechnik und Dampfkesselwesen ŽIVD., University of Stuttgart, partners from eight European countries investigated the effects of burning sewage sludge, agricultural residuals, such as straw and manure, as well as especially cultivated energy plants in combination with coals of various ranks and origin. Both the pulverized fuel ŽPF. and the fluidized bed ŽFB. mode were tested, ranging from laboratory rigs to large scale utility boilers. This paper provides an overview of the activities of the various partners involved and will, in particular, show the synergetic cooperation towards a common aim. The results of the 2-yr project will be summarized. q 1998 Elsevier Science B.V. Keywords: Co-combustion; Biomass; Coal; Sewage sludge
)
Corresponding author.
0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 7 . 0 0 0 6 7 - 2
160
K.R.G. Hein, J.M. Bemtgenr Fuel Processing Technology 54 (1998) 159–169
1. Introduction The observed increase of the CO 2 concentration in the atmosphere, with its negative effect on the global climate, is predominantly caused by the combustion of fossil fuels. Among these fuels, coal is burnt in power plants and industrial installations in large quantities and with increasing tendency. In order to alleviate the problems associated with the CO 2 emission, control methods and processes are under investigation worldwide. In Europe, this area was—and still is—subject to various research and development projects initiated and substantially co-financed by the European Union ŽEU. through the Directorates General XII ŽScience, Research and Development. and XVII ŽEnergy.. One approach, the increase of plant efficiency, i.e., was initiated within the JOULE ŽJoint Opportunities for Unconventional or Long-Term Energy Supply. programmes in 1989, which also dealt with the removal of CO 2 from flue gases and the various alternatives of CO 2 disposal. Likewise, the thermal use of regenerative energy sources as one of the alternatives, e.g., the combustion of cultivated solid biomass, of biomass by-products, and wastes including sewage sludge was—and still is—under investigation within the same European programme. The application of such fuels in industry offers a wide range of ecological and, in many cases, economical advantages like: conservation of fossil fuel resources, reduction of the dependence on fuel imports, utilization of agricultural and forest residues, reduction of emission of harmful species from fossil fuel combustion, recultivation of non-utilized farming areas, minimization of waste disposal. In principle, there are two possibilities for biomass and waste utilization in the power industry: biofuels can either be burnt as single fuel in combined heat and power plants of limited capacity or they can be co-utilized in existing coal fired power stations. The latter of the two alternatives was subject of a more recent EU programme under the abbreviation APAS ŽActivite de Promotion, D’Accompagnement et de Suivi. because of the advantage that in coal-fired power stations, the seasonal and regional fluctuations in biomass and waste composition and in their availability can be compensated by varying the coalrbiofuel firing ratio. Also, several studies show that lower investment and operational costs of co-combustion compared with exclusively biomassfired units can be expected.
2. Objectives With the background of the above mentioned advantages, the European Commission launched this research project of 2-yr duration Ž1993–1994. in which the co-combustion in laboratory, pilot and full-scale units was jointly investigated by partners from industry, research organizations and universities. For each of the technical
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alternatives—pulverized fuel ŽPF. and fluidized bed ŽFB. application—the following main areas had to be considered: Ža. best suitable fuel preparation and feed and the subsequent effects of the combustion process, Žb. full fuel utilization—complete burnout —under minimization of envisaged operational problems like fouling, slagging, and corrosion, Žc. emission and its control of gaseous species ŽSO x , NO x , PAH, PCDDrF, halides, trace metals etc.. and of solid by-products Žashes., Žd. scale considerations and scale-up criteria for industrialrutility application including retrofit aspects, Že. large scale demonstration and the determination of special fuelrprocess dependent requirements and restrictions, and Žf. economics, also in relation to single fuel firing systems. Within the above areas of investigation, careful attention had to be given to certain fuel properties of biofuels and wastes, like volatile matter, inorganic impurities and the strong variety of chemical composition and physical characteristics. The results were intended to provide a profound base in knowledge resulting in a wide spread of utilization of biofuelrwaste in co-combustion mode within Europe and beyond.
3. Technical programme 3.1. Partners Under the coordinatorship of the Institut fur Verfahrenstechnik und Dampfkesselwesen ŽIVD., University of Stuttgart, Germany, 25 partners from eight European countries representing industry, national and international research organizations, and universities participated in the project. 3.2. Fuels During this project the following additional fuels were used: biomass residuals from agriculture and forestry Žstraw, wood, bark, Cynara cardunculus ., biomass cultivated from fast growing energy plants Ž Miscanthus sinenesis, Arundo donax ., waste paper of domestic and industrial origin and residuals from sewage sludge processing. Both the biomass and the sewage sludge investigations were carried out in various scale pilot plants as well as in large-scale power stations. The installations can be conveniently grouped into different combustion techniques like FB and PF, and further classified according to their thermal input ranging from pilot scale to large scale ŽFig. 1.. 3.3. Installations The programme of the project was separated into two parts according to the different biogenic fuels.
162
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Fig. 1. EU programme for co-combustion, participants and facilities.
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3.3.1. Biomass co-combustion Taking the above given considerations into account, biomass was fired jointly with coal in the following research combustion rigs: Ža. drop tube reactor, 10 kWth , KTH, University Stockholm, S; Žb. pulverized coal combustion, 0.35 MWth , IVD University Stuttgart, D; Žc. pulverized coal combustion, 1 MWth , RWE Energie Žfor brown coal., D; Žd. pulverized coal combustion, 1 MWth , KEMA, Žmainly used to study the influence of co-combustion on desulphurization behaviour.; Že. pulverized coal combustion, 2.5 MWth , IFRF; Žf. atmospheric bubbling FB combustion, 0.3 MWth , ECN, NL Žfuel pellets mainly.; Žg. atmospheric bubbling FB combustor, 1 MWth , CIEMAT, E; Žh. circulating FB combustion, 0.3 MWth , INETI, P, Ži. circulating atmospheric FB reactor, 1 MWth , CIEMAT, E; Žj. circulating atmospheric FB combustion, 1 MWth , RWE Energie, D; and Žk. pressurized FB combustion, 1 MWth , Delft University of Technology, NL. Apart from these research rigs three large-scale boilers were part of the programme: pulverized coal combustion, 131 MWel at Esbjerg, ELSAMrVestkraft, DK Žhigh rank coal., pulverized coal combustion, 100 MWel at Lubbenau, VEAG, D Žlow rank coal. and ¨ circulating FB combustion, 80 MWth at Grenaa, ELSAMrMidtkraft, DK. 3.3.2. Sewage sludge co-combustion The programme for the co-combustion of sewage sludge concentrated on pulverized coal and on FB combustion. In addition, the utilization of sewage sludge in metal processing was investigated Žsee Fig. 1.. The tests for the co-combustion of sewage sludge were carried out in the following pilot and large-scale facilities: pulverized coal combustion, 0.15 MWth , Imperial College London, GB, pulverized coal combustion, 0.35 MWth , IVD University Stuttgart, D Žfor bituminous coal., pulverized coal combustion power plant, 150 MWel at Weiher, Saarbergwerke, D Žhigh rank coal. FB combustion, 2 MWth , Stadtwerke SaarbruckenrThyssen, D and ¨ FB combustion, 10.7 MWth , Fechner, D. In the pilot-scale facilities the impacts of sewage sludge addition Žup to 30%. on combustion behaviour and emissions ŽSO 2rNO x . were studied. In addition, trace species like dioxinsrfurans and heavy metal emissions were determined.
4. Results In this section, only major results are highlighted; details are presented in the final report of the project w1x. 4.1. Fuel characterization Biomass offers important advantages as a combustion feedstock due to the high volatility of the fuel and the high reactivity of both the fuel and the resulting char.
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The generally low sulphur content is also positive; e.g., for wood, straw and energy crops, values found were below 0.15 wt.% of the dry material. Both municipal solid waste and sewage sludge have sulphur contents similar to that of low sulphur coals. However, it should be noticed that in comparison with solid fossil fuels, biomass contains much less carbon and more oxygen and—as a consequence—has a low heating value. Also, chlorine contents of certain biofuels, like straw can exceed the level of coal. With regard to the ash content, values for biomass are much lower than for coal, e.g., for wood about 0.2 wt.% of the dry material. In contrast, sewage sludge can have very high ash content. In general, the ash content of different sludges vary between 30–60 wt.% of the dry material, with unfortunately low fluid temperatures as low as 9008C which may lead to problematic fouling of the boiler heating surfaces. 4.2. Fuel preparation The type and extent of the necessary preparation of the biomass fuels depend on the combustion system and the type of biomass. In general, bituminous coal mills are not suitable for biomass. Therefore, biomass co-combustion requires separate milling and feeding devices. As a consequence within this project biomass like straw and Miscanthus were crushed or chopped. Due to the fibrous structure of Arundo donax, Cynara, Miscanthus, straw and wood, it was difficult to determine an exact particle size distribution. For co-combustion of PF facilities, straw and Miscanthus, maximum particle sizes in the range of 4–8 mm in diameter and 1–2 cm in length were required on pilot-scale for a complete burnout. The pilot-scale combustion system presented the worst case regarding the necessary particle size because of the limitations in residence time. Therefore, for the large-scale co-combustion tests chopped straw with a maximum length of about 20 cm was sufficiently burnt in boilers due to the long residence time of large-scale PF combustion facilities. With wood the experience has shown that the particles had to be milled to less than 1 mm in order to burn satisfactorily within typical boiler residence times. However, milling may cause problems if the wood is not dried to a moisture below 8%. In circulated FB ŽCFB. combustion systems the experience with straw-firing showed neither the need to mill nor to pre-dry the straw. A pneumatic feeding of straw into the CFB unit operated very satisfactorily. Also, CFBs can be designed to handle the size of the hogged wood and wood chips. For the majority of the tests the sewage sludge was delivered pre-dried in form of granules with a maximum size up to 10 mm. With such fuel preparation it was possible to use milling systems typical for coal resulting in comparable particle size distributions. Unfortunately, excessive fines and inorganic fuel impurities may give rise to problems during the milling process. 4.3. Co-combustion of biomass in pulÕerized fuel mode Ignition and combustion tests were carried out in a number of installations. This includes laboratory equipment ŽRWE, ICSTM, KEMA., pilot plants ranging from 0.5–2.5 MWth ŽRWE, IVD, ICSTM, KEMA, IFRF. and full-scale boilers of 100 and
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Fig. 2. Combustion efficiency for various facilities and fuels.
120 MWel ŽELSAM, VEAG.. Ignition appeared not to cause any problems in all cases because the high volatile content of biomass, compared with coal, favours a rapid ignition. The combustion efficiency was measured by burnout and remaining CO and C x H y . In general the results of the various projects showed complete combustion for the biofuel ŽFig. 2.. The absolute values, however, may differ dependent on fuel type and biomass percentage added for known reasons. The burner configuration for the combined combustion of coal and biomass is an important factor in the combustion process. Within the project, both the injection via the coal burner and an addition at separate locations is investigated. At test facilities of different scales, investigations on the combustion of biomass, i.e., straw, additionally to the main fuel, i.e., bituminous coal, via the burner were carried out using different burner configurations. This has a great impact on the formation of nitrogen oxides ŽNO x .. If the fuel is added by the central gun, arranged on the inside, it enters the substoichiometric inner recirculation zone resulting in low NO x emissions. Fuel added by the annular clearance devolatilizes in high oxygen conditions therefore leading to an increased conversion to NO x . Therefore, the fuel with a higher N-content should be injected at the centre. Hence, for the case of a fuel blend coalrstraw, coal injection via the central gun and biomass injection via the annular clearance causes lower NO x emissions in contrast to a pre-blended fuel injection via the annular clearance. The results ŽFig. 3. of the test facilities with different capacities Ž0.15 MW, 0.5 MW, 2.5 MW. show a good correspondence with regard to the burner configuration under consideration and agree with the data from a utility boiler Ž300 MWth .. With an increasing biomass share, the NO x emissions decrease in all burner arrangements. Because of the content of volatiles, biomass is excellently suited for the application of NO x-reducing procedures such as air staging and reburning. In contrast to the situation in pure coal flames, with coalrbiomass blends and applying air staging in the
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Fig. 3. NO x data from various facilities with several biomass fuels.
furnace low NO x emissions are attained already under fairly air-rich conditions. In reburning, biomass as reduction fuel is superior to bituminous coal with regard to both emissions and burnout. Also the production of SO 2 emissions is positively affected by an increasing percentage of biomass because of the low sulphur content of the added fuel. In addition, SO 2 can be partly retained in the ash due to the alkaline-content of the biomass ash. Chlorine, which is found in certain biomass types, such as straw or Miscanthus, may affect operation by corrosion. In an utility boiler corrosion tests with two different materials ŽFig. 4. show in case of a straw share of roughly 10% of the thermal input that the corrosion rate increases in comparison with pure coal operation but are still tolerable.
Fig. 4. Corrosion rates for two different fuels and tube materials.
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With regard to deposit formation on heat transfer surfaces the investigations showed that fouling rates rises only negligibly. In the case of small biomass shares, the characteristics of pure coal ash are dominant. If, however, large, incompletely burnt straw particles remain slagging may occur because of the low melting point of the biomass ash. 4.4. Co-combustion of biomass in fluidized bed combustion mode One major research objective was the effect of biomass addition on the gaseous emissions. For SO 2 all partners confirmed that biomass co-combustion has a significant positive effect ŽFig. 5.. Reductions of up to 75% were observed. This can be mainly attributed to the low sulphur content of biomass fuels. In addition, increased sulphur retention in the ash was detected. At the facilities of RWE, TU Delft, and Grenaa the low NO x emissions of about 200 mgrm3 with biomass addition remained more or less unchanged. ECN, CIEMAT, and INETI found decreasing NO x concentrations which is due to the low fuel-N content of added fuel Žwood.. N2 O emissions where measured by INETI, TU Delft, and Grenaa. TU Delft basically did not find any influence of biomass addition on the emission. INETI discovered a slight decrease while the emissions at the 88 MWth plant at Grenaa decreased considerably with increased wood input. Emissions of chlorine were investigated by RWE, ECN, and Grenaa. Especially in co-firing straw with its high content of Cl, high concentrations of chlorine in the flue gas were found. At Grenaa the chlorine content in the flue gas for a 60% biomass share was 20 times higher than for coal only. One should note, however, that in this case a very low chlorine coal was used.
Fig. 5. SO 2 emissions as a function of facility and biomass fuel.
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4.5. Co-combustion of sewage sludge in pulÕerized fuel mode At the facilities with a capacity of 0.15 MW ŽIC., 0.5 MW ŽIVD., and 2.5 MW ŽIFRF., respectively, a wide range of different flame configurations, injection methods, as well as air and fuel staging were investigated bringing about clear differences between biomass co-firing and sewage sludge co-firing. In general, sufficiently high burnout and low CO emissions were observed in all coalrsewage sludge tests at all facilities. Depending on the burner injection modes the burnout can be improved or reduced. The NO x emission levels were found to be extremely sensitive with respect to co-firing ratio, injection mode, i.e., central or annular, primary and reburn zone stoichiometry, and flame ignition. An increase in NO x emission levels was observed when increasing the sewage sludge under unstaged combustion. However, NO x reductions of 70–80% were obtained under staged conditions but these values were extremely critical with respect to burner operation and injection mode. As an example, at the 0.5 MW facility ŽIVD., for all the different injection modes investigated, NO x emissions varied from 2800 mgrm3 at 50% unstaged co-firing to approximately 350 mgrm3 under low primary zone stoichiometry Ž0.7–0.9. in 25% co-firing mode. SO 2 emissions were strongly related with the fuel-S input and they are lower compared with biomass co-firing. In general, the conversion ratio of fuel-S into SO 2 was about 90%. Heavy metal measurements in the flue gases did not show problematical emissions and the ash remained of acceptable quality for utilization in, e.g., the building industry. As an example for large scale application at the power station Weiher 2 of the Saarbergwerke dried sewage sludge was added to the coal before milling, and the coalrsewage sludge mixture was fed to a high temperature slag tap unit. The investigations focused on the influence of the milling system, the emissions and by-products quality like granules, fly ash and gypsum stages. The impact of co-firing sewage sludge on boiler and FGD system was hardly detectable. Temperatures inside the boiler combustion chamber and the frequency of soot blowing were similar to the standard operation on coal. In co-combustion of sewage sludge with coal, the amounts of Zn, P, Cr, Pb, Cu, Co, and Ni determined in the granules were higher compared with pure coal firing. However, these values remained within the limits of legislation. In the fly ash, P and Zn were observed to be in proportion to the quantity of sewage sludge used and these concentrations were found to be higher when compared with pure coal firing. Trace element emissions were relatively low and far below the strong limits in, e.g., Germany. Therefore, except for the more difficult handling and feeding, co-combustion of coal with sewage sludge was observed to be an available option for the application in utility boilers. 4.6. Co-combustion of sewage sludge in fluidized bed combustion mode Industrial fuels consisting of blends of coal and different sewage sludge qualities were burnt in a 10.8 MWth FN combustion plant of the Fechner. The flue gas pollutant
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emission levels measured during the co-combustion of blends were fully in compliance with the emission standards of the tight pollution control acts and directives in Germany. However, due to the special type of shallow-bed fluidized stationary FBC, continuous co-combustion of dry sewage sludgercoal blends caused operational problems in terms of fly ash deposition and adhesions in the furnace toprhot gas duct areas. Major technical modifications of the FBC furnace such as increased freeboard height and installation of a hot gas cyclone for fly ash separation required for a continuous co-combustion of sewage sludge, were proposed. Stadtwerke Saarbrhcken verified in comprehensive tests at a 2 MWth FB installation that a wide range of mixtures of coal and sewage sludge can be incinerated without problem. The concentrations of the major components in the exhaust gas depend on the composition of the material used. In all tests, the values for both CO and NO x were well below the legislature limits. The FB installation was equipped with a filtration system for halogenated organic pollutants. In the clean gas after efficient dust separation the dioxin content was less than 0.1 ng TErm3.
5. Conclusions From the results of the APAS project it became obvious that both modes of combustion—the PF combustion and the FB application—are well suited for co-combustion of biomass and sewage sludge provided that a fuel dependent feed and preparation system is installed. With site dependent biofuelrmain fuel firing ratios no major negative effects on fuel conversion were found. Care has to be taken if high chlorine and high alkaline biofuels are added because of the known negative effects on operation like corrosion andror slagging of the heat transfer surfaces. For both mechanisms more detailed investigations, in particular, long term tests are desired. With regard to the emission of hazardous gaseous compounds no increase of the concentrations in the flue gases was experienced; in many cases substantial reductions of the emission were found which are due to the biofuel composition and also dependent on the biofuel injection mode. This favourable effect of injection of the biofuels was particularity obvious for the NO x-emission; a further optimization is expected from more detailed data and, thus, future investigations in this area are recommended. While the change of the flue gas composition in co-combustion does not disturb the operation and the final product of flue gas desulphurization processes a slide deactivation of catalysts for NO x-reduction was reported.
References w1x J.M. Bemtgen, K.R.G. Hein, A.J. Minchener, Combined Combustion of BiomassrSewage Sludge and Coals, Clean Coal Technology Programme 1992–1994, European Commission, DGXII: Science, Research and Development, ISBN 3-928123-16-5.