Energy & Fuels 1996, 10, 789-796
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Investigations in Combined Combustion of Biomass and Coal in Power Plant Technology Helmut Ru¨diger,* Andreas Kicherer, Ulrich Greul, Hartmut Spliethoff, and Klaus R. G. Hein Institut fu¨ r Verfahrenstechnik und Dampfkesselwesen, University of Stuttgart, Pfaffenwaldring 23, D-70569 Stuttgart, Germany Received November 3, 1995. Revised Manuscript Received February 23, 1996X
The possibility of a combined application of coal and biomass using two different co-combustion technologies has been investigated. A blending of pulverized biomass with coal showed a high burnout up to 20% thermal input of biomass for all particle sizes of the biofuels tested. CO emissions were generally lower than 150 mg/m3 (6% O2) and remain below 100 mg/m3 in the most cases. Reburn investigations with three pulverized biomasses resulted in NOx emissions of approximately 300 mg/m3 (6% O2). With pyrolysis gas as reburn fuel, minimum NOx emissions of 200 mg/m3 (100 ppm) at 6% O2 in the flue gas are possible. The main parameters are pyrolysis gas composition, stoichiometry, and residence time in the reduction zone. Best minimizing results have been achieved with pyrolysis gas produced at about 800 °C using coal as raw material; using biomass as feedstock, the influence of the pyrolysis temperature is only small. The nitrogen concentration, especially in the tar components of the pyrolysis gas, appears to have a positive effect on NOx reduction in the reburn zone of the combustion reactor.
Introduction Biomass is attracting increasing interest in power plant technology. To lower CO2 emissions in power plant technology, biomass is suitable as a nearly carbon dioxide neutral fuel. Beside co-combustion of coal/ biomass mixtures, it seems also attractive to run a combined fired boiler using biomass in a pregasification or prepyrolysis to produce a gas that can be applied as an additional fuel in a secondary combustion zone. This technology promises advantages in NOx emission control. Detailed studies have been carried out to lower NOx emissions in coal-fired boilers using pyrolysis gas as reburn fuel. A research project has been initiated together with an industrial partner.1 With the experience of coal-based reburn fuels produced by pyrolysis, first test runs have been carried out with pyrolysis gas from biomass as a reburn fuel in coal combustion. Besides good reburn results, a prepyrolysis of biomass in a divided process of co-combustion results in separated ashes of both fuels (biomass and coal), which may have a positive influence on slagging and boiler corrosion and the further application of the ashes. Experimental Section The BTS (Brennstofftrennstufung/“fuel splitting and reburning”) test facility is a joint development by the Institut fu¨r Verfahrenstechnik und Dampfkesselwesen (IVD, University of Stuttgart, Germany) and the Saarbergwerke AG, Saarbru¨cken, a German coal mining company and energy supplier. Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Spliethoff, H.; Ru¨diger, H.; Greul, U. Combined Minimizing of NOx-Production and Reduction of Formed NOx During Combustion of Coal Dust (in German). Final report, Bundesministerium fu¨r Forschung und Technologie, BMFT (Kennz.: 0 326 535 C), Bonn, Germany, 1993. X
0887-0624/96/2510-0789$12.00/0
Figure 1 shows the whole test rig. The test facility has been built to investigate the possibility of minimizing NOx formation and reducing NOx formed during the combustion of pulverized hard coal. The process separates coal into volatiles with their own share of nitrogen and residual coal (char) with the remaining nitrogen share. Char or coal and volatiles are burned in a fuel-staged combustion with char or coal as primary fuel and volatiles as reburn fuel. Results of these investigations are published elsewhere. 1,2 Pyrolysis Reactor. The pyrolysis unit consists of a fuel feeding system with an inert gas preparation, the pyrolysis reactor, and a hot gas filtration. A flow schematic is shown in Figure 2.3 The entrained flow reactor is an electrically heated furnace (approximately 30 kWel, total height 2400 mm) with three regulated heating zones for a 2000 mm long reaction tube. The tube diameter is between 50 and 100 mm. The maximum furnace wall temperature is 1200-1300 °C depending on the material of the reaction tube. The experiments outlined in this paper were carried out at a maximum temperature of 1200 °C. Fuel is fed using a gravimetric feeding device and blown into the reactor with a small cold N2 gas stream (injection gas). The main stream of the gas (carrier gas) is preheated (p.h. 1-3 and 0) to the reaction temperature to ensure high heating rates to the feedstock. The inert gas stream (carrier gas and injection gas) was equal in all test runs (approximately 4.4 normal m3/h). The residence time in the reaction zone was between 2 and 5 s. Calculations showed that devolatilization had finished in less time (approximately 150 ms).3 The hot gas filter consists of a vessel (stainless steel, diameter 273 mm, length 500 mm) with external electrical heating and insulation to reduce temperature losses. The filtration temperature must be higher than approximately 500 (2) Greul, U.; Ru¨diger, H.; Spliethoff, H.; Hein, K. R. G. Use of Pyrolysis Gas as Reburn Fuel. Presented at the 3rd European Conference on Industrial Furnaces and Boilers, Lisbon, April 1995. (3) Ru¨diger, H.; Greul, U.; Spliethoff, H.; Hein K. R. G. Co-Pyrolysis of Coal/Biomass- and Coal/Sewage Sludge-Mixtures in an Entrained Flow Reactor. Final report, APAS Clean Coal Technology Programme CT92-0001, Commission of the European Communities, Bruxelles, 1994.
© 1996 American Chemical Society
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Figure 1. Arrangement of the BTS facility.
Figure 3. Scheme of the small-scale combustion reactor (BTS test facility).
Figure 2. Flow schematic of the pyrolysis unit (BTS test facility). °C to avoid tar condensation and filter plugging. In the vessel the solid fraction of the pyrolysis products is separated gravimetrically through flow deflection and filter cartridges (sintered ceramic or ceramic fiber).3 Small-Scale Combustion Reactor. The combustion unit of the BTS facility (see Figure 3) is an electrically heated flow reactor, too.2 Wall temperature can be varied in five regulated heating zones up to 1350 °C. Compared to a conventional combustion chamber, this arrangement makes it possible to investigate the influence of temperature on formation and decomposition of NOx in a combustion process. The reaction tube is made of sintered ceramic; the heated length is 2500 mm (with an inner diameter of 200 mm). Air staging at the burner and swirled secondary air are possible. A gravimetric screw conveyor (0.5-5 kg/h) supplies a constant coal feeding rate. Fuel staging in the furnace can be investigated with different residence times in each zone. The flue gas composition (NO, NO2, CO, CO2, SO2, O2) in the different combustion zones can be measured with a cooled gas probe.2 0.5 MW Test Rig. For experimental research of the combustion process in pulverized coal flames, a vertical, rooffired combustion chamber with a maximal thermal input of 500 kW and a length of 7 m has been installed at the IVD. Figure 4 shows the installation with the possibilities of air staging, fuel staging, and flue gas recirculation. The chamber consisting of six round segments was constructed in a way that the temperature and residence time conditions during the combustion of the pulverized coal particles correspond to the characteristics of power plants. The first section of 4 m, refractory lined and water cooled, simulates the furnace of a boiler, whereas the second section, which is only water cooled, simulates convective characteristics. The diameter of the furnace is 0.75 m in the refractory-lined part and 0.85 m in the non-refractory-lined part. Three rows of access at the
Figure 4. 0.5 MWth test rig. vertical combustion chamber, each staggered for about 90°, allow the use of probes and optical measurement techniques. The entrance of staged air and fuel into the reactor is possible by additional nozzles. Sampling System of the Pyrolysis Unit. Pyrolysis gases were constantly connected on-line in all test runs. The sampling point is located laterally in the hot gas filter, which is placed after the pyrolysis reactor. A small amount of pyrolysis gas is taken, cleaned by a small ceramic filter, and cooled (4 °C), to carry out on-line analyses for CO, CO2, O2, and CHtot using different gas analyzers. Further, the gas species H2 (TCD) and CmHn (FID) are analyzed and quantified
Combined Combustion of Biomass and Coal with a GC. To determine the different pyrolysis tar components, different tar sampling systems were checked for precision and handling. Detailed information about the sampling systems tested is published elsewhere.3 The sample equipment used for this paper was placed after a short heated and insulated stainless steel line, which is connected to the hot gas filter. It consists of a small ceramic fiber filter followed by an activated carbon cartridge. Both devices were weighed before and after sampling and dissolved in CH2Cl2 to determine tar components using a GC/MSD. Char samples are taken in the hot gas filter at the end of the reactor. When the sample was taken out, attention was paid to cover the hot sample with cold nitrogen to avoid oxidation, which would have caused errors in proximate or ultimate analyses. Sampling System of the Small-Scale Combustion Unit. Gas analyses have been carried out on-line for NO, NO2, CO, CO2, O2, and SO2 using different gas analyzers. Normally, the sampling point is placed at the outlet of the combustion reactor. The gas is cleaned by a small ceramic fiber filter and cooled like the pyrolysis gas described above. For measurements regarding the influence of residence time on NOx emissions, a water-cooled probe, which can be placed along the center line of the reaction tube, has been used. Sampling System of the 0.5 MW Test Rig. Components of the flue gas, such as O2, CO, CO2, SO2, and NOx, are recorded continuously. Others, e.g., hydrocarbons, can be determined if necessary. The emission probe for the flue gas analysis is installed at the end of the hot part of the furnace at a distance of about 5.5 m from the burner outlet. The gas sample is filtered and cooled. Different gas analyzers are used. Special flame adjustments are further investigated by in-flame measurements of gas components and temperature at several points of the upper furnace part with suction probes. Additionally, a burnout analysis of the fly ash was taken.
Aims of Research Within the experiments different aims of research as follows have been investigated: to compare different technologies of a combined combustion of biomass and coal; to find the optimum composition of the fuel mixtures in co-combustion; to achieve high burnout and low emissions; to find the main reductive components in the reburn fuels; to achieve low NOx emissions without using SCR. CO2-Minimizing Potential through Biomass. Biomasses such as wood, straw, and miscanthus are commonly called CO2-neutral energy sources. Neutral with regard to CO2 means that the combustion of biological material releases the same amount of CO2 as the plant has extracted from the atmosphere during its growing period. This perspective disregards that the cultivation of energy plants itself needs energy, too, and causes additional CO2 emissions. A balance of CO2 has been made taking as an example the cultivation of Miscanthus sinensis, covering all of the processes from the production of the plants to their utilization in a pulverized fuel rig. For every production step the energy consumption or the resulting CO2 emissions, respectively, have been summarized and compared with the CO2 emissions caused by the supply with and utilization of hard coal. Table 1 shows the results of the balance.4 The provision of coal for combustion consumes less energy and has therefore a lower specific CO2 emission compared with biomass. But coal (4) Lewandowski, I. CO2 and Energy Balance for the Production of Biomass and the Combustion in a Power Plant. Proceedings, 8th European Conference on Biomass for Energy, Environment, Agriculture and Industry; Ademe: Paris, 1994; p 255.
Energy & Fuels, Vol. 10, No. 3, 1996 791 Table 1. Comparison between CO2 Emissions by Utilization of Biomass and Hard Coal
supply combustion total
hard coal (Go¨ttelborn)
biomass (miscanthus)
kg of CO2/ metric ton
kg of CO2/GJ
kg of CO2/ metric ton
kg of CO2/GJ
100 2730 2830
3.4 93.2 96.6
110.3
6.03
110.3
6.03
combustion emits approximately 93.2 kg of CO2/GJ of heat energy, while biomass is CO2-neutral in combustion. In total, 96.6 kg of CO2/GJ was emitted by hard coal utilization and about 6 kg of CO2/GJ by miscanthus utilization. The result is an avoidance of greenhouse gases of about 93%.4 A special cultivation of energy plants (basis of Table 1) represents the most unfavorable case from the aspect of energy saving. Using residual material such as straw and wood, the additional energy needed for the supply with this fuel is considerably lower. With the technically available biomass potential in Germany (346-426 PJ/year) and the data from Table 1, a calculation shows that 31 × 106 up to 38 × 106 metric tons of CO2 could be avoided. NOx Formation and Decomposition. Nitrogen oxide can be created by three different mechanisms (Zeldovich, Fenimore, fuel nitrogen), which are described in manifold publications. In the test facility used, the main NOx emissions are caused by fuel nitrogen. In the combustion zone the primary fuel coal is pyrolyzed. Depending on the coal type and the temperatures, a part of the fuel nitrogen is released with the volatiles. The combustion air causes a partial conversion to NO. The conversion of the char nitrogen is approximately 20-30%.5 The conversion of volatile nitrogen can be about 60% for unstaged combustion;6 however, for staged combustion the conversion can be lowered to a range comparable with the conversion of char nitrogen. In the fuel staged combustion the addition of the reburn fuel causes a more or less effective reduction of NO to N2, depending on the reburn fuel composition, the temperature, and the available residence time. After the reduction zone, still existing N species are oxidized with the addition of burnout air. Detailed information about NOx formation and reduction in staged combustion is published by different authors. 7-9 Results and Discussion Feedstocks. Different fuels have been used in the pyrolysis and combustion test runs (see Table 2). A detailed description of feedstock analyses of all fuels applied in biomass and coal pyrolysis experiments is published elsewhere.3 Table 2 contains basic informa(5) Schulz , W. Formation of Nitrogen Oxides in Pulverized Coal Combustion and Its Avoidance (in German). VGB Kraftwerkstech. 1986, 66, 541-550. (6) Song, Y. H.; Pohl, J. H.; Beer, J. M.; Sarofim, A. F. Nitric Oxide Formation During Pulverized Coal Combustion. Combust. Sci. Technol. 1982, 28, 31-39. (7) Pohl, J. H.; Sarofim, A. F. Devolatilization and Oxidation of Coal Nitrogen. 16th (International) Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1976; pp 491-501. (8) Mechenbier, R.; Kremer, H. Fuel Staging of Coal Dust/Methane to Reduce Fuel Originated NOx-Emission (in German). VDI-Bericht No. 645; Du¨sseldorf, 1987; pp 87-98. (9) Fenimore, C. P. Studies of Fuel-Nitrogen Species in Rich Flame Gases. 17th (International) Symposium on Combustion; The Combustion Institute: Pittsburgh, PA, 1978; pp 661-670.
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Table 2. Feedstock Analyses fuel hard coal straw miscanthus wood proximate analyses (%) fixed C (daf) volatile matter (daf) ash (dry) ultimate analyses (% daf) C H N S O Cl particle size d50 (µm) sieve (mm) a
nd, not determined. combustion 1.5-6 mm.
b In
63.4 36.5 10.3
18.9 81.1 6.0
18.8 81.2 4.9
16.9 83.1 0.6
81.4 5.6 1.5 1.0 10.6 0.24
50.5 4.8 0.8 0.1 43.0 0.8
53.3 4.6 0.5 0.3 41.1 0.2
50.8 6.5 0.2 0.1 42.4 nda
58.8 1.5b
1.5c
4.0
co-combustion 0.75-4 mm. c In co-
Figure 5. Burnout of coal/biomass mixtures, preblended, unstaged.
tion about the feedstock used in the reburn experiments reported. Fuel moisture has been between 2.5% and 10% for all feedstocks. All fuels have been dried before use in pyrolysis to avoid reactions at higher temperatures, which could influence the mass balances. Co-combustion of Coal and Pulverized Biomass. The results reported have been achieved by combustion test runs in the 0.5 MWth test rig. Preblended flames have been compared with fuel staged combustion. In preblended combustion mixed flames and pure biomass firing have been investigated with regard to burnout and CO emissions. Most interesting in co-combustion using fuel staging are burnout of the reduction fuel and NOx reduction capability. Further investigations (e.g., air staging, preblended flames) are published elsewhere. 10 Preblended Flames. Different kinds of biomass (straw, miscanthus, wood) with different particle size distributions were preblended with various amounts of pulverized coals.11 The air ratio λ after the combustion zone was approximately 1.1; the temperatures in the combustion zone differed only little between pure coal flames and preblended flames.10 In Figure 5 the burnout of these flames is shown versus the biomass portion.10 (10) Kicherer, A.; Gerhardt, T.; Spliethoff, H.; Hein, K. R. G. CoCombustion of Biomass/Sewage Sludge with Hard Coal in a Pulverized Fuel Semi-Industrial Test Rig. Final Report, APAS Clean Coal Technology Programme CT92-0002, Commission of the European Communities, Bruxelles, 1994. (11) Kicherer, A.; Gerhardt, T.; Go¨rres, J.; Spliethoff, H.; Hein, K. R. G. Investigations and Calculations onto Biomass Co-Combustion in Pulverized Fuel Units. Presented at the 3rd European Conference on Industrial Furnaces and Boilers, Lisbon, 1995.
Figure 6. CO emissions for different biomass portions in coal flames, preblended, unstaged.
With coarse biomass (miscanthus, 4 mm; straw, 6 mm) a slight decrease of burnout in preblended flames is measured with increasing biomass portion. This indicates that the residence time in the hot zones (approximately 1.3-1.5 s) is too short for the large particles, but the overall burnout for all flames (except one) is more than 99% and therefore sufficient in preblended flames. If, however, the technically feasible biomass portion (0-20% of thermal input) is regarded, it can be observed that the biomass portion hardly influences the burnout. Thus, the main coal flame affects the burnout of the coarse biomass particles in a positive way. A more finely ground biomass (0.75 or 1.5 mm) even results in an increasing burnout of the blendings with an increasing biomass portion. The high reactivity and the high amount of volatiles combined with the large surface of the biofuel improve the combustion behavior of the coal flame. If the biomass should be used as single fuel, however, the grinding fineness should be less than 1.5 mm for stable ignition and burnout. Thus, the reaction course of the blends can be controlled by the particle size distributions of the biofuels. It can be summarized that the co-combustion with the technically relevant portion of biomass does not influence the burnout of the flames even with coarse particle diameters. The carbon monoxide emissions of all tested cocombustion flames are shown in Figure 6.10 As a lot of variations of injection modes and air-staging techniques have been used, the CO emissions are spread in a wide range. It should be noted that all values are lower than the German limit of 250 mg/normal m3 at 6% O2. Only a small number of flames are emitting more than 150 mg/normal m3, and most others were under 100 mg/ normal m3. That result agrees well with the burnout measurements, which show the miscanthus co-flames to be worse than the straw co-flames. On the basis of the results of the CO and burnout measurements, the reactivity of the biofuels may be estimated. The most important influence on reactivity is represented by the particle size distribution. With smaller mean particle diameter the burnout increases and the CO emissions decrease. The second parameter influencing the reactivity is the kind of biomass. The particle density of miscanthus and wood is higher than that of straw; therefore, these fuels may react worse than straw. This effect can be shown by the higher CO
Combined Combustion of Biomass and Coal
Figure 7. NOx emissions using different reburn fuels (0.5 MWth test rig).
emissions and lower burnout of miscanthus and wood co-flames compared to straw co-flames. Apart from small differences, it may be summarized that the burnout of co-combustion is very high and the CO emissions are low and clearly below the German limit. Reburning with Pulverized Fuels. Another possibility to co-fire biomass in a coal furnace is its utilization as reburn fuel (shown in Figure 7).12 In the primary zone where the coal is fired, there exists an air ratio >1. To create a reducing secondary zone with air ratios
