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Energy & Fuels 2008, 22, 2504–2511
The Zero Emission Carbon Concept (ZECA): Extents of Reaction with Different Coals in Steam/Hydrogen, Tar Formation and Residual Char Reactivity Lu Gao, Nigel Paterson,* Paul Fennell, Denis Dugwell, and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. ReceiVed March 18, 2008. ReVised Manuscript ReceiVed May 6, 2008
A high pressure wire mesh reactor has been modified to investigate the reactions underlying the zero emission carbon concept (ZECA) process. This is a novel power generation concept that involves producing hydrogen from coal. An earlier paper has described the concept, the wire mesh reactor used for laboratory scale tests, and results of tests with H2 and He. In this paper, results of tests with a range of coals are described, together with tar emission and char reactivity measurements. The tests with a range of coals (from lignite through to bituminous have shown that the performances of the different fuels do vary widely, but not as a direct function of their rank. With lignites, high conversions were achieved in a H2/steam mixture. Lower conversions were apparent with the other coals. However, reaction conditions were not optimized to achieve the highest conversion level. Pittsburgh No. 8 coal (high volatile bituminous) was also found to be reactive, whereas Wyodak (subbituminous) and Daw Mill (bituminous) coals had a similar reactivity to each other, but lower than the other fuels tested. The amount of primary tar was measured, by collection in a cooled trap, immediately above the wire mesh. This is material formed during the release of the volatiles with only a limited time for further reaction. Slightly higher quantities of tar were measured during tests with H2 than with He. Residual char reactivities declined with increasing temperature and hold time at peak temperature, although steam did appear to have an activation effect at low formation hold times.
Introduction On a worldwide basis, the use of fossil fuels (particularly coal and oil) is increasing, with large increases in usage in countries with developing and transitional economies, in addition to projected increases in already industrialized countries. There are serious concerns that this is one of the main causes of increasing CO2 concentrations observed in the earth’s atmosphere. It is suspected this can give rise to an enhanced greenhouse effect, which in turn can influence world climate. There is now broad consensus that it is sensible to control and eventually limit emissions of a range of pollutants (CO2, NOx, SOx) in an attempt to minimize the impact on the environment and the climate. Europe and the USA are independently moving toward stabilizing emission rates (particularly of CO2) at lower than current emission levels. Meanwhile, increasing world populations and expectations of improved standards of living in countries with both developed and developing economies mean that it is not feasible to achieve a step down in overall fossil fuel usage in the short/medium term. With a backdrop of high and rising oil and gas prices, many countries with developed as well as developing economies now foresee a significant future for coal in their energy strategies. However, the immediate environmental and possible climatic considerations require that atmospheric CO2 and other pollutant emissions be reduced to low or near zero values.1 In an ideal scheme, the only releases to atmosphere would be N2 and water vapor. Therefore a range of technology options are * Corresponding author. E-mail:
[email protected]. (1) Our Energy Future - Creating a Low Carbon Economy; HMSO, Cm5761, February 2003.
being considered and encouraged to enable fossil fuel use to continue, while controlling the emissions to the atmosphere at acceptable (low) levels. These include process routes which optimize existing gasification and combustion technologies, coupled to novel technology components. Near zero emissions can be achieved by the gasification of coal to produce an impure fuel gas (i.e., containing N2, H2, H2O, CO, CO2, CH4, H2S, etc.). This gas is further processed to maximize the concentration of H2 and remove the environmentally sensitive gases by a combination of absorption and catalytic decomposition. The H2 may be used to generate power in fuel cells or gas turbines. However, the pollutant capture systems impose a thermal penalty on the overall process and reduce the overall efficiency. When coal gasification is adapted to produce a stream of H2, with the sequestration of CO2, there is an efficiency penalty of about 20%. In other words, more coal has to be used to generate the same amount of power from a zero emission process than for a conventional gasification plant. The climate change debate has also encouraged the development of more novel process concepts, which can potentially raise the efficiency during power generation. Aspects of one potential process have been studied at Imperial College London and the University of Cambridge in the UK. The process is known as the zero emission carbon concept (ZECA) and was first proposed by researchers at the Los Alamos National Laboratory (LANL) and the Louisiana State University in the USA. It is generally referred to as the LANL ZEC technology.2 (2) Solutions for the 21st Century, Zero Emissions technologies for Fossil Fuels; OECD/IEA Technology Status report, May 2002.
10.1021/ef800194k CCC: $40.75 2008 American Chemical Society Published on Web 06/26/2008
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Energy & Fuels, Vol. 22, No. 4, 2008 2505
Figure 1. Basic flow diagram of the ZECA process.
The ZECA concept was described in some detail in a previous paper.3 The process is shown as a block diagram in Figure 1. This process route that has not been studied experimentally, although different parts of the concept have been tried experimentally (e.g., hydrogasification, steam reforming of CH4/ CaO driven shifting). Coal is gasified in H2, at a pressure of approximately 7 MPa to produce CH4 via the methanation reaction. This reaction is exothermic and steam would be added to moderate the temperature to approximately 900 °C. Next, CH4 is steam-reformed to produce H2 and the water gas shift reaction is used to maximize the H2 content of the gas. The carbonation of CaO is used to remove CO2 from the gas phase, which also maximizes the H2 content of the fuel gas. Overall, the H2 producing reactions are endothermic and the heat requirement is provided by the methanation and carbonation reactions. Reaction temperatures may be limited by the melting properties of the coal and its mineral matter, the need to avoid the formation of eutectic liquid phases4 in the solid sorbents used to remove CO2 and the equilibrium controlled inability of CaO to bind CO2 at temperatures above 900 °C at a CO2 partial pressure of less than 1 atm. Overall, the scheme results in the formation of 2 extra moles of H2, for each 2 moles H2 used in the methanation stage. The H2 for the methanation is recycled from the H2 stream remaining after the further processing of the gas. The bound CO2 is subsequently released as a concentrated stream for sequestration by heating the CaCO3. Overall, the same amount of CO2 is formed with both ZEC and conventional technologies, however, the ZEC process potentially has a higher efficiency as losses are avoided by the integration of the various process steps. It also isolates the CO2 as a nearly pure stream for storage, by the use of the carbonation/calcination cycle. The details of the conceptual plant have not yet been developed, pending the collection of fundamental data relating to the reaction processes occurring in the various unit processes. Imperial College is investigating the hydrogasification/pyrolysis/ steam gasification reactions and Cambridge University is studying the reactions in the reforming/shift conversion/ carbonization stage. The current paper presents some key aspects of the missing data. Experimental data on the effects of fuel type and operating conditions on the performance in steam/ hydrogen are described, together with data on tar emissions and residual char reactivity. The findings of the study have been (3) Gao, L.; Paterson, N.; Dugwell, D.; Kandiyoti, R. Energy Fuels 2008, 22, 463–470. (4) Paterson, N.; Elphick, S.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2001, 15, 894–902.
used to recommend a suitable type of gasifier to be incorporated into a ZECA plant. Experimental Details The Wire Mesh Reactor. The experiments described below have been carried out in a high-pressure wire-mesh reactor, modified for H2/steam injection at temperatures up to 1050 °C and 8 MPa. This is a reactor configuration that has proved very versatile and has been used for a variety of pyrolysis, gasification and combustion related experiments.6–15 Figure 2 presents the schematic diagram of the high-pressure wire-mesh reactor (WMR) developed at Imperial College London, as configured more recently. The reactor features water cooled electrodes, a wider span of heating rates and larger mesh than most applications and a stream of sweep gas which carries evolving volatiles away from the heated reaction zone into a cooled tar trap. The present application of the wire-mesh reactor has necessitated a return to the original hydropyrolysis configuration, now modified to accept steam injection. The original steam injection system constructed around the high-pressure wire-mesh reactor16 (also cf. ref. 8) was modified for higher pressure operation to 7 MPa.17 The system is based on the evaporation of a measured flow rate of water in a tube heated to 100 °C above the boiling point of water at the required test pressure (e.g., at 7 MPa the boiling point of water is 280 °C). The whole gas flow path of the steam is preheated, using electrical heating tapes, to avoid recondensation, before it reaches the mesh. In addition, any gas that is to be mixed with the steam is also preheated to avoid condensation on mixing. A flow diagram of the equipment is given in Figure 3. The test program required (5) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122. (6) Kandiyoti, R.; Herod, A. A.; Bartle, K. D. Solid Fuels and HeaVy Hydrocarbon Liquids: Thermal Characterization and Analysis; Elsevier Science Pub.: Amsterdam, Oxford, London, New York, 2006; chapters 3 and 4. (7) Gao, L.; Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Int. J. Oil, Gas Coal Technol. 2008, 1, 152–179. (8) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1988, 2, 505. (9) Gibbins, J. R.; Kandiyoti, R. Energy Fuels 1989, 3, 670. (10) Gu¨ell, A. J.; Kandiyoti, R. Energy Fuels 1993, 7, 943. (11) (a) Messenbo¨ck, R. C.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 1999, 13, 122. (b) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 781. (12) Peralta, D.; Paterson, N.; Dugwell, D.; Kandiyoti, R. Energy Fuels 2005, 19, 532. (13) Wang, B.; Li, X.; Xu, S.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2005, 19, 2006. (14) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2006, 20, 2572. (15) Wu, L.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2007, 19 (4), 2325–2334. (16) Messenbo¨ck, R.; Dugwell, D. R.; Kandiyoti, R. Fuel 1999, 78, 781– 793. (17) Gu¨ell, A. Development of a high pressure wire mesh reactor for the direct determination of tar and total volatile yields: applications to coal and biomass pyrolysis and hydropyrolysis. PhD thesis, University of London, 1993.
2506 Energy & Fuels, Vol. 22, No. 4, 2008
Figure 2. High-pressure wire-mesh reactor:5 [1] gas exit; [2] quartz bell; [3] electrode clamps; [4] mesh support plate; [5] current supply; [6] sinter disk; [7] support plate stands, hollow to allow water flow; [8] copper seal; [9] gas inlet; [10] base plate; [11] throw over sealing ring; [12] flow smoothing cell; [13] spring, hollow to allow water flow; [14] wound corrugated tube; [15] pressure bell; [16] mesh; [17] line to pressure gauge.
the reactor to operate at pressures up to 7 MPa, in atmospheres containing pure He, H2 or steam, or combinations of these gases. Calculations were done to check the pressure/temperature integrity of the pipe-work and rigorous pressure testing was done before any tests were initiated. As an additional safety precaution, a H2 analyzer was used to monitor the atmosphere in the vicinity of the wire-mesh reactor pressure shell. An accurately weighed 5-6 mg sample is used for each test and is evenly distributed in the folded wire-mesh sample-holder. The mesh is held between water-cooled electrodes and serves as a resistance heater. The fuel size is in the range 105-150 µm and is determined by the aperture size of the mesh. The brass support plate under the mesh is lined with a 2 mm thick alumina sheet to prevent electrical contact between the sample-holder and the support plate. A stream of gas is used to sweep the sample holding part of the mesh, carrying evolving volatiles away from the sample holder; this helps to minimize the interactions between volatiles, the char and the mesh. The weight change (called the total volatile yield) of the mesh and sample during a test is the measure of the extent of reaction. Total volatile yields can be determined with a repeatability of (1%. During experiments using an inert sweep gas, only pyrolysis reactions can occur and the weight loss shows the extent of loss of the fuel volatiles under the conditions used. When a reactive gas is used (H2, CO2, steam, diluted air or mixtures), the weight change is due to the combined weight changes (nearly) successively caused by pyrolysis followed by reaction with the sweep gas. Therefore, to obtain the weight change due to reaction, two experiments must be done: one under inert conditions to measure the weight change by pyrolysis and the other in the reactive atmosphere, under otherwise the same conditions. The pyrolysis result is subtracted from the result in the reactive atmosphere to obtain the weight change due to reaction. The weight loss measured in this way shows the effect of reactive gas (in this study, H2, steam or steam/H2) on the primary pyrolysis process (hydropyrolysis) and on the gasification of char (hydrogasification). The value will not be altered by any reaction between the H2 and the pyrolysis products after they have been released. The occurrence of post mesh reaction of the volatiles in considered to be most unlikely, as the gases cool very quickly after exiting the mesh. The
Gao et al. evolved tar can be estimated by passing the exit gas from the wire mesh through a specially designed cold trap (cooled in liquid N2). The collected material can be weighed and characterized using a variety of methods (e.g., gas chromatography, mass spectrometry and size exclusion chromatography). The difference between the total loss and the weight of tar, gives an indication of the amount of gas that has been produced. The total amount of gas formed is low (because of the low initial sample weight) and the dilution by the sweep gas is high and it has not been possible to measure the composition in this project. A fresh thermocouple pair is used for each experiment (using 0.05 mm wires) and these are tied through mesh apertures to ensure good thermal contact. In this project particular attention had to be given to the preheating of the mesh and inlet lines for work using steam at high pressure. In addition, the mesh width had to be reduced in the part containing the thermocouples, to avoid overheating and mesh melting in parts that were not cooled. Tar Measurement. A stainless steel trap, packed with wire mesh strips was used to collect the tar released from the sample on the heated grid. This was placed immediately above the fuel sample and the released volatiles were swept through the trap by the sweep gas flow. The outlet of the tar trap was connected with the outlet gas flow path from the equipment. The trap itself was 170 mm tall and 34 mm diameter (o.d.). The trap extended outside the reactor pressure shell and therefore was constructed to withstand the high pressures used for the tests. It was cooled by the ambient atmosphere. A test using cooling with liquid N2 showed no differences in tar yield compared with ambient air cooling and was therefore not necessary to ensure a valid result. After a test, the trap was allowed to warm to room temperature and was then carefully washed with chloroform solvent. The solvent was evaporated in a weighed container, under carefully controlled conditions to isolate the tar for weighing. It is noted that the material collected will exclude low boiling point organic material, which are lost during the removal of the solvent. The amount of tar collected for weighing can be sensitive to the solvent removal conditions (especially for biomass). Consequently it is very important that a repeatable test procedure is used. A slightly modified washing procedure was used during tests with steam, because water also collected with the tar in the trap. After washing with chloroform, the tar containing solvent was placed in a separating funnel, where the water collected as a separate layer. The chloroform layer was collected for evaporation to isolate the tar sample. Char Reactivity Measurement. Char reactivity was measured in a Perkin-Elmer TGA 7, using an isothermal method. Char samples were reacted in air at 500 °C (40 mL/min). Initial char sample masses were 1.5 mg ( 0.2 mg. The maximum rate of reaction (Rmax) is used as the measure of reactivity and is defined as Rmax ) -
( )( )| 1 dW Wo dt
max
(1)
where, Wo ) the initial weight if the char sample, dry, ash free (daf) basis and dW/dt ) rate of weight loss, obtained from the first derivative of the weight loss curve. Fuel Samples. The analyses of the coals used in this part of the study are given in Table 1. The coals were chosen to represent lignite, sub bituminous and bituminous coals. Illinois No. 6, Pittsburgh No. 8, Wyodak, and Beulah Zap coals were obtained from the Argonne Premium Coal Sample Bank,18 and the analyses shown above for these coals are the certified values. The Illinois No. 6 coal analysis is consistent with it being classed as a subbituminous coal; however, the coal sample bank reference manual notes that its microscopic appearance is more similar to that of a high volatile bituminous coal. Baag Noor lignite was provided by a visting academic from the Mongolian Academy of Sciences. There are large reserves of this coal in Mongolia. Daw Mill coal is a (18) Vorres, K. S. Energy Fuels 1990, 4, 420.
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Figure 3. Flow diagram of the steam injection system: (1) water reservoir; (2) displacement pump; (3) steam generator; (4), (5) on/off valves; (6) steam bypass collector; (7) gas cylinder; (8) pressure relief valve; (9) flow control valve; (10) gas heater; (11) nonreturn valve; (12) mixing point of steam and gas; (13) pressure transducer; (14) transformer. P pressure, T temperature, C control. The full lines represent tubing, and the dash-dot line is for for thermocouple lines. Table 1. Fuel Analyses Daw Mill bituminous
analysis moisture, % as received ash, % as received volatile matter, % db C H N S O (by difference)
%, db
%, dmmf
4.1 6.9 33.7 72.7 4.7 1.1 1.1 na
Illinois No. 6 sub-bituminous
Pittsburgh No. 8 bituminous
Wyodak subbituminous
8.0 14.3 40.1 65.7 4.2 1.2 4.8 10.1
1.7 9.1 37.8 75.5 4.8 1.5 2.2 6.9
28.1 6.3 44.7 68.4 4.9 1.0 0.63 16.9
typical UK bituminous coal and is used for power generation. The ash contents of the coals were used to convert the results obtained with the WMR to the daf basis.
Results and Discussion An earlier paper has described the equipment validation and initial test results. Pyrolysis weight loss data as a function of temperature, pressure and residence time (in He) were obtained. These data were used to remove the effect of pyrolytic weight loss from data obtained during tests in H2, steam and H2/steam mixtures. Tests in pure H2 with a UK bituminous coal showed extents of hydrogasification (hydropyrolysis weight-loss minus pyrolysis weight-loss) between 15 and 25%, in the temperature range 750-1050 °C. The extent of conversion in steam was slightly lower than in H2 under similar reaction conditions. Tests with a mixture of H2/steam with Daw Mill coal, however, indicated that the conversion with the mixture was higher than that calculated from the results of tests with the individual gases. The scope of the experiments has now been expanded to include a suite of coals of different ranks, tar yield measurements, and residual char reactivities. Effect of Pressure on the Extent of Hydrogasification. The conceptual operating pressure for the ZECA gasifier is 7 MPa. This high pressure is favored because the concentration of CH4 at equilibrium will be greater (on the basis of Le Chatelier’s Principle). However, operating at such a high pressure, although enabling high throughputs for a particular plant diameter, will increase plant complexity and result in high capital costs. If
Baag Noor coal lignite 6.8 11.0 40.0 56.8 4.6 0.27 0.39 na
Beulah Zap lignite 32.2 6.6 44.9 65.9 4.4 1.0 0.80 19.1
Table 2. Effect of Total Pressure on the Extent of Reactiona total pressure, MPa 2.0 5.0 7.0
total volatile yield, %, daf
pyrolysis yield (in He), %, daf
extent of conversion by H2%, daf
52.3 56.2 55.8
41.3 38.2 35.8
11 18 20
a Temperature: 950°C. Heating rate: 1000 °C s-1. Hold time at peak temperature at peak temperature: 10 s. Coal: Daw Mill. Sweep gas: 100% H2.
equilibrium is not approached in the gasifer, then there may be the possibility of considering a lower operating pressure. Several tests have therefore been done to measure the impact of total pressure (in the range 2-7 MPa) and the results are shown in Table 2. The total volatile yield column gives the total weight change of the starting sample, which includes the changes due to both pyrolysis (including tar release) and the reaction of coal with the gasification gas. The next column shows the pyrolysis contribution which is subtracted from the total volatile yield to give the extent of conversion by H2. Within errors, the data show no difference in the total volatile yields between 5 and 7 MPa, with a small decrease at 2 MPa. However, as the test pressure was decreased from 7 to 2 MPa, the extent of pyrolysis rises by approximately 7% and consequently, the extent of hydrogasification (last column) shows a larger decrease over the pressure range. This observation provides a useful insight into the effect of pressure and may have implications for the design of a ZECA process. It suggests a lower operating pressure (down to 5 MPa) may not have too
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Table 3. Data Obtained using Lignites in He and Hydrogena
sample Baag Noor
Beulah Zap
total volatile range of yield, individual temp, flow hold average, values, no. of °C gas time, s (%, daf) %, daf determinations 950
H2
700 950
He He
950
H2 He
10 0 10 0 10 10 10
75 37 35 33.9 35.2 81.3 43.2
70-87 33-39 34.8-35.5 33.9 35.2 62-90 42.6-44.0
20 4 3 1 1 10 3
a Pressure: 7 MPa. Heating rate: 1000 °C s-1. Gas composition: 100% of named component.
great an impact on the performance. However, further work on this would be necessary before a firm recommendation could be made. Hydropyrolysis and Hydrogasification of Lignite. Two samples of lignite have been tested: one from Mongolia, Baag Noor coal (a weathered, partially oxidized deposit not far from the country’s capital), and another from the USA (from the Argonne set), Beulah Zap, North Dakota lignite. The data obtained are shown in Table 3. It is apparent that the scatter in the data is greater than seen at lower levels of conversion with bituminous coals. Results with a bituminous coal have been given in the previous paper3 where the scatter was reported as (1.5%. This is to be expected. In the absence of any softening of sample particles, even under rapid heating conditions, the reduction in particle size makes loss from the wire mesh more likely than with samples that soften during heat-up. The scatter in the data was greater for the Mongolian lignite and it was noted that both sample and resultant char particles were more needlelike in appearance. These would be more prone to fracture and slipping through the mesh at high conversion. The Beulah Zap particles were more rounded in nature, and showedsalbeit marginallys evidence of softening. The values for the total volatile yield represent the weight losses caused by complete pyrolysis of the sample and by the impact of H2. Hydrogen has two effects: it can alter the pyrolysis pathways, so that the sample is hydropyrolysed and it can hydrogasify the char. Pyrolysis and hydropyrolysis are rapid processes and are essentially complete in the WMR within a couple of seconds (including the heating time). Hydrogasification of residual char (from hydropyrolysis) is slower and the longer the hold time at peak temperature the greater the extent of this reaction. The impact of the presence of H2 on the test results can be assessed from the data shown in Table 3. The data obtained in He with the Mongolian lignite shows that pyrolysis is completed after a 10 s holding time at 700 °C. The similarity of the 0 and 10 s data at 950 °C data shows that the volatile release is virtually complete during the heating ramp, i.e. it is very rapid and that only a minor further increase occurs between 0 and 10 s. In H2, the result with the Mongolian lignite for the 0 s hold time at peak temperature test is about 4% (actual) higher than the pyrolysis result. This indicates the extent of hydropyrolysis occurring during the heating time to 950 °C. However, after a 10 s hold time, the extent of hydrogasification is significant and 38% of the char has been hydrogasified (difference between the 0 and 10 s weight-loss data in H2). The difference between the pyrolysis result in He and the result in H2 with a 0 s hold time is lower than that noted for the bituminous coal3 (where a 7% difference measured at 950 °C). The difference in behavior may be caused by the different stages in maturity of the lignite and bituminous coal.
Table 4. Data Obtained with Beulah Zap Lignitea sweep gas H2/He H2/steam
total volatile yield, %, daf
temperature, °C 850 na 74.4
950 71.7 86.2
a Hold time at peak temperature: 10 s. Heating rate: 1000 °C s-1. Pressure: 7 MPa. Mixture composition: 50/50 (by volume).
Pyrolysis data in He has been obtained for Beulah Zap lignite together with data in H2 at 7.0 MPa and 950 °C. The difference in the values shows that reaction with H2 increased the conversion by 38%. This is similar to the extent of hydrogasification observed with the Mongolian Lignite. The extent of conversion due to H2 for the bituminous coal was 20% at 950 °C and 7 MPa, which compares with a value of 38% for the lignites. This appreciably higher conversion reflects the more reactive nature of the lower rank coal. The tests with the lignites showed high levels of overall conversion in H2 under the conditions used. A limited number of tests have also been done using the Beulah Zap lignite in a steam/H2 mixture. The data obtained is shown in Table 4 below. The test with 50/50 H2/He showed a lower conversion than measured with 100% H2 (81.3 reduced to 71.7%), under otherwise the same conditions. Replacing the He in the sweep gas mixture with steam raised the conversion to 86.2% at 950 °C. A test at 850 °C, using the H2/steam mixture showed a lower conversion (74.4%). The data shows that lignite is a reactive fuel under the simulated ZECA conditions. This is an encouraging result, however, it must be remembered that the particle size in these tests was only in the range 150-200 µm, whereas larger particles, possibly up to 6 mm might be used at the larger scale. The impact of particle size on the conversion levels has not been investigated in this work. Performance of Some Different Coals. In the previous paper, a set of data was presented for Daw Mill bituminous coal using a 10 s hold time at peak temperature in the reactor. The total extent of conversion (i.e., including pyrolysis) in the presence of H2 and steam was approximately 70%. It was considered that more reactive coals would approach complete conversion at this hold time at peak temperature. Therefore, the hold time was reduced to 5 s for the current tests, so that the levels of conversion were lower and differences in reactivity should be more apparent. The results are shown in Table 5. For each coal, the experimentally determined total volatile yield and extent of reaction are shown. The extent of reaction has been calculated by subtracting the total volatile yield in He, from the total volatile yield for a test in either steam/He, H2/ He or H2/steam. The following orders of reactivity are apparent: In steam, Pittsburgh > Illinois > Wyodak ≈ Daw Mill In H2, Pittsburgh > Illinois > Daw Mill ≈ Wyodak In steam/H2, Pittsburgh > Illinois > Daw Mill ≈ Wyodak That is, the order is the same for the different sweep gases. “Theoretical” conversions for the four coals in H2/steam have been calculated by adding the extents of reaction measured in steam and H2 separately. These added values are lower than the experimental conversions. This implies that the reactions of steam and H2 with C do not occur on the same active sites. Otherwise, some competition for active sites might be expected, with a potential decrease in the experimental conversion compared with the calculated value. This is the same observation as was made for the 10 s hold time at peak temperature experiment with Daw Mill coal. The above extents of reaction within 5 s appear to indicate faster rates of reaction than have been reported for a study done
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Energy & Fuels, Vol. 22, No. 4, 2008 2509 Table 5. Performance of Different Coalsa
Illinois No. 6 sweep gas He H2/He steam/He H2/steam,experimental H2/steam, calculated a
Pittsburgh No. 8
Wyodak
Daw Mill
total volatile extent of total volatile extent of total volatile extent of total volatile extent of yield, %, daf reaction, %, daf yield, %, daf reaction, %, daf yield, %, daf reaction, %, daf yield, %, daf reaction, %, daf 42.6 59.3 55.7 77.6 72.4
16.7 13.1 35.0 29.8
28.5 49.3 44.8 75.1 65.6
39.4 52.6 51.9 68.5 65.1
20.8 16.3 46.6 37.1
35.0 48.8 47.1 64.5 60.9
13.2 12.5 29.1 25.7
13.8 12.1 29.5 25.9
Temperature: 850 °C. Pressure: 7 MPa. Hold time at peak temperature: 5 s. Heating rate: 1000 °C s-1. Gas mixtures: 50/50 (by vol).
for the ZECA corporation at the Gas Technology Institute (GTI).19 This used a pressurized thermogravimetric balance (PTGA) at similar temperatures and pressures to those used in the WMR. However, different feed samples were used and two of the three samples were charred under unspecified conditions prior to the actual PTGA test. In the GTI study, the order of reactivity for the three coals was lignite > sub-bituminous > bituminous. The lignite reached 50% conversion (of the C) in approximately 7 min in pure H2, whereas in our study 38% conversion was reached in 10 s. Similarly, the sub-bituminous and bituminous coals required approximately 8 and 45 min, respectively, to reach the 50% conversion level in a 50/50 steam/ H2 mixture in the GTI study. In our work the same types of coal required only 5 s to reach a conversion of 26% (Daw Mill, bituminous) and 37% (Pittsburgh, sub-bituminous). This substantial difference in behavior between the two sets of tests may be explained in terms of several factors. As already detailed elsewhere,20 thermogravimetric balances are not suitable for many experiments involving a pyrolytic step. In addition, ill defined hydrodynamics around the sample pan is likely to introduce external (bulk gas to particle surface) mass transfer resistances, that are particularly acute in high pressure applications. Furthermore, it is known that heating rates in a TGA are very low compared with the rate used in the WMR. This provides time for char to deactivate before the required test temperature is reached. We have previously shown that chars lose nearly three-quarters of their reactivity in about 10 s at 1000 °C. In addition, two of the fuels used in the GTI study were “charred” (under unspecified conditions) prior to the TGA test and this would have enhanced the degree of deactivation. These two factors, which would have reduced the reactivity of the char, may account for the low char reaction rates measured during the GTI study. Tar Formation. In this study, the quantity of tar released has been measured during selected tests. Tars are formed as part of the pyrolytic process and the weight of this material is included in the pyrolysis total volatile yield. The balance of the pyrolysis total volatile yield is gas, which cannot be easily measured directly in the WMR, due to the low actual volume released and the comparatively large volume of carrier gas. Tars formed in the WMR are regarded as “primary” tars, as the system is designed to sweep the volatiles rapidly out of the high temperature reaction zone of the mesh. This is thought to largely suppress secondary thermal tar modification and destruction reactions, after tar molecules have been released from external particle surfaces. In these experiments, the weight of tar collected is relatively low and shows some dependency on the conditions used to (19) Ziock, H-J.; Anthony, E. J.; Brosha, E. L.; Garzon, F. H.; Guthrie, G. D.; Johnson, A. A.; Kramer, A.; Lackner, K. S.; Lau, F.; Mukundan, R.; Nawaz, M.; Robison, T. W.; Roop, B.; Ruby, J.; Smith, B. F.; Wang, J. Progress in the development of zero emission coal technologies. Proceedings of the 28 th International Technical Conference on Coal Utilisation and Fuel Systems, Clearwater, FL, 2003. (20) Kandiyoti, R. Fuel 2002, 81, 975.
Table 6. Effect of Temperature on the Emission of Tar, in He and H2a Tar Release, % daf individual values temperature, °C 750 850 950 1050
average
He
H2
He
H2
18.1, 18.9 16.5, 16.2 15.4, 14.7 14.4, 13.9
20.5, 19.1 19.3, 18.7 18.6, 17.4 17.4
18.5 16.4 15.1 14.2
19.8 19.0 18.0 17.4
a Pressure: 7 MPa. Heating rate: 1000 °C s-1. Hold time at peak temperature: 0 s. Coal: Daw Mill.
Table 7. Effect of Hold Time on the Emission of Tar in H2a tar yield %, daf
char yield, %, daf
gas yield, % daf, by difference
temperature, °C
0s
10 s
0s
10 s
0s
10 s
750 850 950 1050
19.8 19.0 18.0 17.4
24.2 17.7 13.4 12.2
65.1 60.2 54.8 54.4
49.4 47.7 44.2 39.2
15.1 20.8 27.2 28.2
26.4 34.6 42.4 48.6
a
Pressure: 7 MPa. Heating rate: 1000 K s-1. Sweep gas: H2.
remove the solvent, which was used to wash the collected tarry material from the tar trap. It is therefore important that a rigorous and repeatable procedure is used to collect and weigh the tar; otherwise trends in the data can be lost through experimental error. Table 6 shows tar values measured during experiments at different mesh temperatures, in the range 750-1050 °C. Comparison of individual experimental results and average values show good repeatability of the measurements. For 0 s hold time, Table 6 shows that tar yields were somewhat higher in H2 than in He. This is consistent with the partial hydropyrolysis of the more reactive portions of tar precursors accumulating on external particle surfaces. In He, some of these materials are not released as tars. It is likely that in the absence of H2, less of the tar precursor free-radicals are quenched and remain more reactive. More of this material would then repolymerize and char, rather than evaporate from external particle surfaces. This effect, however, was relatively minor. The effect of peak temperature on tar release, with a 0 s hold, in both He and H2 can also be seen in Table 6. In both atmospheres, the data show that the tar emission decreased slightly as the temperature was raised from 750 to 1050 °C. Once again the effect is relatively minor, but apparently greater in He than H2. At atmospheric pressure, tar release is largely completed after 1 s at 700 °C. Table 7 shows that completion of tar release from coal particles takes longer under increasing pressure at 750 °C. The slow but gradual decrease in tar yield with increasing temperature shown in Table 6 is consistent with increased repolymerization on external particle surfacessmore intense under helium compared to hydrogensand also as the temperature moves toward 1050 °C. We will see in the next paragraph and Table 7 that the affect appears to be more pronounced for longer hold times at peak temperature.
2510 Energy & Fuels, Vol. 22, No. 4, 2008
Gao et al.
Table 8. Effect of Steam on the Emission of Tar in the Presence of H2a
Table 9. Combustion Reactivity of Hydrogasification Chars Formed under a Range of Conditionsa
average tar yield, %, daf hold time at peak temperature, s 0 10
H2/steam (50/50)
H2 (100%)
19.7 17.1
18.0 13.4
temperature, °C
carrier gas
Rmax (% min-1, daf) 0 s
Rmax (% min-1, daf) 10 s
850
H2 steam H2 steam
1.35 3.69 0.88 3.37
1.09 2.57 0.82 1.08
a Pressure: 7 MPa. Heating rate: 1000 K s-1. Temperature: 950 °C. Coal: Daw Mill.
950
Tar release has also been determined after 10 s hold time at peak temperature; the data are compared with 0 s data in Table 7. It is noted that 10 s is the time that the mesh is held at the test temperature. It does not represent the actual tar residence time at temperature, because the system is designed to ensure the rapid removal of the primary tar from the heated mesh. The volatiles are released rapidly during the heating period and initial part of the hold period. The repeatability of the tar measurement is (1.5% (actual), and the differences between the 0 and 10 s data shown in Table 7 are thought to be experimentally significant. At 750 °C, it appears that pyrolysis was not complete within the heating time (0 s hold) and consequently, after 10 s, the tar yield was higher. The actual evolution of tar will have been completed well within this time. At higher temperatures, the tar yield after 10 s was lower than that measured with a 0 s hold time at peak temperature and the difference increased with temperature. It is also likely that under H2 some of the tar precursors have a slightly longer time at temperatureson external particle surfacessto react with H2 to form CH4 and other lighter volatiles. This behavior was not observed (Table 6) with a 0 s hold time. It does indicate that tar/H2 contact time at temperature is an important factor in maximizing the tar destruction. It is likely that the pathway followed by the released volatiles in a H2 rich environment will depend on their molecular structure. Low molecular weight material could be hydropyrolysed to gas, whereas larger material could be partly converted to gas, leaving a residue that could react further to form even larger mass material. Table 7 also shows the char yields (100 - total volatile yield) and the gas yield (100 - tar and char yield). The measured char values decrease with both increased hold time and temperature and this must be reflecting the impact of H2 on the hydropyrolysis and hydrogasification reactions and the removal of carbon from the solid phase. The amount of gas produced was not determined directly, but can be estimated from the tar and char yields. The data shows that the gas yield (which includes permanent gases and volatile organic material) increased with hold time and with temperature (at each hold time) and shows the extent of gas formation from both char and tar. Tar release has also been measured in a 50:50 H2/steam mixture at 7 MPa. The results are compared with data obtained in H2 (100%) in Table 8. The tar release in H2/steam was greater than that in H2 alone, at both hold times, with the difference being larger at the longer hold time at peak temperature. The tests were done at 950 °C, and the data must be reflecting the lower concentration of H2, when steam is present, i.e. less tar was being destroyed at the lower partial pressure of H2. There may be ways of reducing the extent of release of tars, e.g. by the use of higher temperatures and longer tar/char/H2/ steam contact times. Testing this hypothesis requires tests in a different type of reactor (e.g., fluidized bed or entrained flow), since contact times at temperature are deliberately restricted in wire-mesh reactors. However, complete tar removal appears likely to be difficult and hence the impact of its presence must be considered in the design of a viable ZECA process.
a
Rmax (% min-1, daf) 30 s 0.90 na 0.73 na
Coal: Daw Mill. Heating rate: 1000 °C s-1. Pressure: 7 MPa.
Reactivity of Chars. The relative combustion reactivity21 of the residual char from selected WMR tests was assessed using a TGA, which measured the rate of reaction with air at 500 °C. Table 9 shows the effect of the char formation conditions in the WMR (hold time at peak temperature, temperature and gaseous environment) on the TGA reactivity. The data shows how the reactivity of the char declined with the increased severity of the conditions of its formation. The chars formed in steam had a higher reactivity than those formed in H2. It is likely, this is due to the increased plasticity and attendant blocking of pore structures observed with most coals in the presence of high-pressure hydrogen. However, the char reactivity under steam showed a more marked decline with increasing hold time at peak temperature. The decline of char reactivity at and above about 1000 °C within about 10 s or less has been graphically shown in previous work.22 Some Thoughts on the Design of the ZECA Gasifier. The results of the project have been used to develop several lines of thought on the type of gasifier that would be most suited to the ZECA process. The results have shown that temperatures around 900-1000 °C are appropriate for the process and that a reasonably long residence time will be needed to achieve a high degree of conversion. Ten seconds were needed with the relatively fine fuel particle size used in the WMR. Entrained flow reactors seem to be the most widely applied type of gasifier recommended for commercial scale power generation applications, although the actual number of plants built is rather low. They are O2/steam blown and operate at higher temperatures (>1400 °C) and with shorter residence times (
