李岩

Biomass and BioenergyVolume 26, Issue 4, April 2004, Pages 377-388--------------------------------------------------------------------------------doi:10.1016/j.biombioe.2003.08.003 | How to Cite or Link Using DOIPublished by Elsevier Science Ltd.Permissions & ReprintsHigh-pressure co-gasification of coal and biomass in a fluidized bedT. R. McLendon, , a, A. P. Luib, R. L. Pineaulta, S. K. Beera and S. W. Richardsonaa National Energy Technology Laboratory, 3610 Collins Ferry Road, Morgantown WV 26507, USAb Parsons Infrastructure and Technology Group, 3610 Collins Ferry Road, Morgantown WV 26507, USAReceived 14 May 2003; revised 29 July 2003; accepted 5 August 2003. ; Available online 25 September 2003.AbstractMixtures of coal and biomass were co-gasified in a jetting, ash-agglomerating, fluidized-bed, pilot scale-sized gasifier to provide steady-state operating data for numerical simulation verification. Biomass used was sanding waste from furniture manufacture. Powder River Basin subbituminous and Pittsburgh No. 8 bituminous coals (screened from −1.2 to +0.25 mm) were mixed with sawdust (screened to −1.2 mm) and pneumatically conveyed into the gasifier at an operating pressure of 3.03 MPa. Feed mixtures ranged up to 35% by weight biomass. The results of gasification tests of subbituminous coal/sawdust mixtures showed few differences in operations compared to subbituminous coal only tests. The bituminous coal mixture had marked differences. Transport properties of coal/biomass mixtures were greatly improved compared to coal only.Author Keywords: Biomass gasification; Co-gasification; Coal and biomass gasification; Biomass solids fluidization; Jetting fluidized bed gasificationArticle Outline1. Introduction2. Experimental3. Results/discussion4. ConclusionsAcknowledgementsReferences1. IntroductionDisposal costs of waste biomass and increasing environmental penalties of mining coal have suggested to many that trying to utilize waste biomass as a partial fuel substitute for coal was potentially beneficial. Also, the possibility of using less desirable or waste coals in conjunction with biomass to explore whether synergies would exist is appealing. Considerable excellent work in co-gasification from bench scale to large pilot scale has been undertaken during the last decade [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12 and 13]. Davidson [14] provides a very good summary of pertinent efforts prior to 1997. The first step is simply to determine whether it is realistic (or even possible) to use the intended resource(s) in whichever of the many gasifier types are available. Whether synergies occur is dependent upon the test conditions such as: feedstock type, direct particle contact, pressure, reactor type, temperature, etc. Tars are a significant problem for some of the systems reported and some researchers report that biomass with coal reduces them. The salient issue for consideration is that there are numerous varieties of both gasifiers and feedstocks and the possible permutations of gasifiers with feedstocks are considerable. Only actual tests can determine if synergies or insurmountable obstacles will occur. Numerical simulation is most valid for screening studies, extrapolation of experimentally determined data, and evaluation of the possible applications.Madsen and Christensen [1] report on a series of air-blown fluidized bed and entrained bed co-gasification tests with coal and straw. Pressures in the larger unit (based on U-Gas design) were up to 14.2 bar and feed rates of the feedstock were a maximum of 720 kg/h. Feeding presented problems, but some synergies were noted. Sjostrom et al. [2 and 3] also report synergies in fluidized bed co-gasification of wood and coal mixtures at small particle sizes with maximum feed rates of 5.2 kg/h and maximum pressures of 15 bar. Reinoso et al. [4] report on a comprehensive experimental program including a series of tests in larger pilot scale air-blown circulating fluid bed gasifiers at near atmospheric pressures. Feedstocks were waste coal, lignite, and pine chips at solids feed rates up to 800 kg/h. Results indicated that using waste coals in circulating fluid beds of the type tested would not be viable because of economic, operational and design constraints. Modeling comparisons indicated synergies existed between coal and biomass. Kurkela et al. [5] describe evaluations of a variety of biomasses and coals (and mixtures) in a fluidized bed gasifier with maximum feed rates of 80 kg/h at pressures up to 5 bar. de Jong et al. [6] report synergies with co-gasification of coal, miscanthus and straw in an air-blown fluidized bed gasifier operating at up to 5 bar. Pan et al. [7] co-gasified coals with pine chips at atmospheric pressure in an air-blown, bench scale fluidized bed gasifier. Some synergies were noted.2. ExperimentalThe purpose of the experimental program was to provide highly instrumented, steady-state operating data for numerical simulation matching and verification. The gasifier used was the National Energy Technology Laboratory's (NETL) pilot scale-sized Fluidized Bed Gasifier (FBG) located in Morgantown WV. Since many of the operational and technical details of the present system have never been described completely in the open literature [15] key specific information will be given here. With a good understanding of the phenomena occurring within the gasifier results are better understood. A simplified picture of the FBG is shown in Fig. 1 with reactor dimensions. The reactor is dense-refractory lined and has no internal heat-loss compensation mechanism. The coal for these tests was screened from −1.2 to +0.25 mm, and the sawdust(sanding waste from furniture manufacture) was screened to −1.2 mm. Solid feed was premixed and conveyed to the Batch Hopper from the Silo with N2. The Batch Hopper pressure was lowered to receive the feed and raised to match system pressure when dumping to the Feed Hopper directly below. For coal only feed, the Feed Hopper is constantly fluidized to prevent bridging and rat-holing. The feeder is directly below the Feed Hopper and the solids fall into one of the “pockets” in a rotating shaft where it is expelled by air pressure at the upside down position. The feed is then pneumatically conveyed with cool air into the jet in the axial center of the FBG bottom.Full-size image (33K)Fig. 1.Fluidized bed gasifier.View Within ArticleThe FBG in the jetting mode is a dry-bottom, ash-agglomerating gasifier with the solids/air mixture being surrounded by an annular stream of heated air and steam. In order to prevent excessive agglomeration of the char particles during the rapid pyrolysis conditions just above the jet, particle velocities are kept high. The minimum desirable velocity is 15 m/s. The jet area is where the free radical oxidation reactions occur and it is the hottest place in the gasifier. Once through the initial pass inside the jet, the coal particles (thus the biomass also) are assumed to be essentially completely pyrolyzed. Analyses of solids removed from the reactor verify this. Considerable problems with clinkering have been experienced with this type of gasifier [16]. A delicate balance must be maintained where some ash agglomeration occurs, but at a slow controlled rate, if possible. This is not easy. Regardless of the issues associated with this type of gasifier, it has many positive features leading to its use in the Pinon Pine IGCC demonstration project [17].The pyrolyzed char particles are jetted upward into the upper parts of the gasifier where circulation patterns develop. The general direction of the particles in the center is upward and along the walls it is down. Reentrainment of the particles occurs at the jet, aided by auxiliary jets inside the refractory insert at the main jet level. Modeling videos and analysis of conditions have indicated that the most significant particle mixing occurs at the level of the jets. More detail is provided in an unpublished report [18]. At nominal conditions, average coal char particle residence times are on the order of 1.5–2 h, and gas residence times are less than 1 min. At the gas flow rates in these tests, the fine particles are entrained in the gas flows and probably have residence times inside the gasifier on the order of a few minutes. Therefore, any catalytic effect of wood ash may not be as pronounced in the FBG as in other gasifier types. Differential pressure gauges are used to determine particulate densities in the FBG. Solids are removed in three places: heavier agglomerated particles come out the very bottom (underflow); medium particles comeout at the level of the first freeboard junction (overflow); and fly ash is removed in the Cyclone.The reactor is provided with numerous internal thermocouples as per Fig. 1 locations. Key process gas flows are very accurately measured to within 2% on a cold flow comparison to traceable standards. Process control is by both traditional PID controllers and a redundant distributed control system (APACS/Process Suite) provided by Moore products. The data are captured by APACS on a continuous basis, on about a 1-s interval depending on conditions. Gases for analysis are cleaned up downstream of the cyclone. The primary gas analyzer is a Perkin-Elmer magnetic sector mass spectrometer (MS), with continuous backup by an Ametekquadrupole MS and periodic sampling by a gas chromatograph. Gas analyses are reported on a dry basis. It should be noted that for feeds normally used in the FBG, tar production is not a problem. This gasifier has sufficient high-temperature gas residence time and enough char suspended throughout the reactor to provide ample opportunity for tar degradation. Downstream equipment is not designed for tars and problems with tars do not occur.Reactor temperature is not controlled. The modeling effort is stand-alone work and each steady-state operational period can be used as is, but for the series of tests reported here, only one parameter at a time was varied (where possible) to prevent second order interactions. However, when changing coal feed rate, there is an associated effect of changing char particle buildup in the gasifier. This is significant because the suspended char reacts with gases to gasify the char. The diffusion control constraint is lessened by having more char as reactant. More char particles suspended gives more surface area.The issue of when steady state has been achieved is not a trivial consideration. There are considerable fluctuations in gas analyses, bed temperatures, and reactor internal differential pressures even when the system has been operating with no set point changes for many hours. Of course, a constant set point does not preclude movement within an instrument's dead-band range. There are short-term changes, intermediate-term changes and long-term drifts. Feedstocks vary slightly from drum to drum. In addition, when the Feed Hopper is fluidized the feed particles segregate as per density and size; so the feed at the bottom will be different from the feed at the top. Therefore, for the biomass tests the fluidization feature was turned off. In addition, the issue of reactor temperature is important. It is significant that any thermocouple inside the gasifier reads the local temperature and there is considerable variation between the readings from the bed thermocouples. Reporting one temperature as the temperature of gasification is problematic. We simply report temperature at location as per time. To avoid the oxymoron condition of transient steady state, considerable judgement must be used.The strategy of the experimental effort was to establish a base case of operation about which one variable at a time could be changed. The base case was Powder River Basin subbituminous coal at a feed rate of 31.8 kg/h, with a reactor internal pressure of 3.03 MPa. The bituminous coal was Pittsburgh No. 8. Feedstock analyses are given in Table 1. The coal/biomass mixtures were as given in Table 2. For all tests: reactor pressure was 3.03 MPa, convey air was 42.9 m3/h at standard conditions, reactor air was 27.5 m3/h at standard conditions and steam was 27.2 kg/h. It should be noted that the coal feed rate was not constant for all tests. This introduced morethan one variable change for each test.--------------------------------------------------------------------------------Table 1. Analyses of feeds used in biomass testingFull-size table (<1K)View Within Article--------------------------------------------------------------------------------Table 2. Steady-state operating conditionsFull-size table (<1K)View Within Article3. Results/discussionFig. 2 shows the comparison of CO for all coal/biomass tests reported compared to the base case. It should be noted that the time period shown for the bituminous coal mixture includes an earlier part of the test when steady state had not yet been reached. This unsteady-state operation was included because the run was terminated early due to a malfunction. At the beginning of the bituminous/biomass run, the bed was mostly subbituminous coal char but the feed had been bituminous coal for at least 30 min. The CO level at time zero is nearly the same for all tests, but CO begins to drift down for the bituminous coal as the bed eventually fills with its char. Of significance is that the CO for the subbituminous coals, regardless of addition of biomass, is nearly the same. Fig. 2 shows another consistent feature of results from this gasifier. There is considerable fluctuation of readings from the gas analyzers with time. This is typical and is not an artifact of the instruments. The thermocouples and the differential pressure instruments show the same transient behavior. In fact, all figures presented in this paper are time averaged because all data points plotted show a wide band. Since the system is so well controlled, the fluctuations are obviously the result of feed differences with time. However, in the case of the bituminous coal mixture, adjustments of some control features were causing additional variations. We were trying to lower temperatures in the jet area (to prevent clinkering) without compromising the test.--------------------------------------------------------------------------------Full-size image (17K)Fig. 2.CO comparison.View Within ArticleFig. 3 shows that CO2 production is slightly higher for all biomass tests compared to the base case. Fig. 4 shows that the H2 production was within a range of 2 or 3% for most of the test periods. The bituminous mixture H2 could have been lower during the latter part of its run because of the efforts to lower temperatures. Regardless of the causes, the inherent characteristic of fluctuations must be taken into account by persons using such data for evaluation and design. From the perspective of considerable operational experience, it could not be concluded that there is a significant difference in H2 or CO production from any of the runs using subbituminous coal, with or without biomass included.--------------------------------------------------------------------------------Full-size image (15K)Fig. 3.CO2 comparison.View Within Article--------------------------------------------------------------------------------Full-size image (15K)Fig. 4.H2 comparison.View Within ArticleFig. 5 shows the comparison of readings for all runs from TE-733 (just above the jet less than 1 cm inside the bed). TE 733 is not in a thermowell, therefore, it is more sensitive than some of the others. But because it is near the wall it will normally read lower than a thermocouple sticking out further. Anything sticking out into the bed near the jet must be in a thermowell in order to survive for any significant time. All biomass tests show higher temperatures than the base case and the bituminous test shows about twice the temperature reading of the subbituminous tests. Because of its location near the wall, TE-733 is within the zone of descending char particles. Note that when the bituminous test began, TE-733 read about the same as it did for the other runs. As the bed began to contain more bituminous char, TE-733 began to show increased temperatures. Fig. 6 shows temperatures from TE-700 (inside a thermowell nearly 4 cm inside the bed at the same elevation as TE-733). Fig. 7 shows temperatures from TE-701 (inside a thermowell about 7.6 cm inside the bed, same elevation). The further inside the bed, the closer the temperatures become (differences are less). This is because the char from the bituminous coal is nearly absent from the lower part of the bed when the bed becomes nearly all bituminous in origin (to be discussed later). At the temperatures near the jet, heat transfer is predominantly by radiation. The bituminous coal has so little char that TE-733 can see the heat source from the jet. With the subbituminous coal char present, radiant energy from the jet must be absorbed and re-radiated to get to the wall. The further up the reactor, the closer all temperatures get and at the top they are essentially the same for all tests. It is of interest to note that TE-701 shows temperatures that approach ash fusion temperatures for the Pittsburgh No. 8 bituminous coal we used.--------------------------------------------------------------------------------Full-size image (17K)Fig. parison of TE-733.View Within Article--------------------------------------------------------------------------------Full-size image (18K)Fig. parison of TE-700.View Within Article--------------------------------------------------------------------------------Full-size image (17K)Fig. parison of TE-701.View Within ArticleThat the lower bed was nearly absent of char during the bituminous run is shown by Fig. 8. It is the differential pressure transducer located across the section just above the jet, PDT-707. Fig. 8 compares only the base case to the bituminous test but all subbituminous runs are about the same and all are several times the values recorded during the bituminous test. Fig. 9, PDT-708, the next differential pressure transducer up from PDT-707, shows that the difference is decreasing and at the top of the reactor the values are nearly the same. Fig. 10, from PDT-709 (the next pressure transducer up from PDT-708), shows another interesting feature discussed earlier. The 65/35 subbituminous test had significantly less coal feed than the base case and all other biomass tests. Note that the char particulate loading for this test decreases in the PDT-709 zone. The bed was being depleted of coal char and this behavior is consistent with other tests where th e coal feed was insufficient to “build the bed” with char. Its equilibrium value is probably about the value indicated after the 200 min interval shown in Fig. 10.--------------------------------------------------------------------------------Full-size image (12K)Fig. parison of PDT-707.View Within Article--------------------------------------------------------------------------------Full-size image (14K)Fig. parison of PDT-708.View Within Article--------------------------------------------------------------------------------Full-size image (16K)Fig. parison of PDT-709.View Within ArticleTable 4 shows the partitioning of the solids removed from the FBG for the various test periods (i.e. which of the ports from which the char is removed as shown in Fig. 1). Toward the end of the bituminous test, essentially no solids were removed from the underflow. Table 3 shows why. It is a comparison of overflow solids for the base case and the 75/25 biomass experiments. The bituminous coal char has about half the bulk density as the subbituminous char and the screen analyses show dramatic differences. This is consistent for a coal with a free swelling index of 7.5.--------------------------------------------------------------------------------Table 3. Overflow solids comparisonFull-size table (<1K)View Within Article--------------------------------------------------------------------------------Table 4. Carbon consumption/solids partitioningFull-size table (<1K)View Within ArticleThe data do not show much in the way of synergies for the subbituminous tests. One can hardly see the difference in the respective tests. However, there is one very significant synergy present for the bituminous coal/biomass mixture. Without the biomass present, it is not possible to use Pittsburgh No. 8 in the FBG [15]. It has always clinkered almost immediately when used alone. It is speculated that the presence of the biomass somehow deposits a coating on the bituminous char at the exit of the jet to prevent the typical uncontrollable agglomeration.There was no opportunity to try to optimize the operation of the FBG during the bituminous coal/biomass mixture test. Since we had never run such a coal in the gasifier before, we were trying various tweaks to keep temperatures low to keep it from clinkering. We discovered that frequent dumping of the overflow was necessary due to it filling up quickly. We had to eventually dump every 15 min in order to keep up. This precluded focusing on other issues. The run was terminated when an unscreened piece of sawdust pellet plugged the feed tube and we had to shut down to unplug the tube.Table 4 shows carbon consumption and solids partitioning for the base case and biomass tests. It should be noted that the conclusion that more biomass, hence less coal, leads to more efficient use of carbon could be misleading. The general experimental evidence is that biomass is easy to gasify. Coal char is not. With less coal char, there is more reactive gas available to consume the coal char, since the gas flow rates were unchanged. Table 5 compares carbon utilization for several fluidized bed coal/biomass operations. Care should be taken when comparing results such as these. Experiments are not necessarily designed to optimize conditions. They are conducted to evaluate whatever parameters are of most interest, while attempting to reduce experimental uncertainty.--------------------------------------------------------------------------------Table 5. Carbon conversion comparison to other research in fluidized bed coal/biomass utilization Full-size table (4K)View Within ArticleAnother unexpected synergy existed for the greatly improved transport and handling properties of the coal/biomass mixtures compared to coal alone. It was not necessary to fluidize the Feed Hopper with the biomass present. In addition, when calibrating the Feeder about 5% of the coalis lost to the dust collection system when coal alone is used. With all mixtures this was reduced to about 3%.The most significant alteration of flow properties occurred when we were calibrating the Feeder with mixtures. The Feed Hopper is normally kept at less than 20 kPa above the pressure in the reactor in order to assure no back flow of hot gas up the feed tube. During one calibration with biomass mixtures the differential pressure controller was inadvertently set at several times that value. The calibration showed an increase of several times the expected flow rate. Had that occurred with coal only, the feeder would have instantly plugged. Also, we were not fluidizing the Feed Hopper. Because of the mechanical configuration and the constant rotational speed, the only way for the coal/biomass mixture to have had such a remarkable increase in flow rate was for the feed to be undergoing dense packing in the pocket. Why this behavior occurs is not readily apparent. The speculation is that the fine biomass dust may act like miniature roller bearings or it will fill in depressions in the coal particles to make them smoother and rounder. Perhaps, the biomass affects static electric charges on the quite dry feeds. An extensive literature survey was conducted and much work has been done in the area of copier toner materials wherea few weight percent of various materials are added to significantly affect solids flow properties[19]. Considerable additional work is being done in the pharmaceutical industry with deliveries of inhalent powders [20]. However, nothing was revealed in the search that indicated anyone had published any work with materials like coal and biomass.4. Conclusions•Synergies with subbituminous coal/biomass mixtures are not readily apparent in gasification. •A most significant synergy exists with gasification of highly caking coals and biomass in the FBG since without the biomass, such coal cannot be processed at all.•The transport (rheological) properties of all the coal/biomass mixtures are greatly improved relative to coal only for our system. Plugging is greatly reduced and handling is much easier. •The particulate flow patterns in the FBG for swelling bituminous coals are greatly altered compared to particulate flow patterns for subbituminous coals.•Carbon utilization for the FBG is about the same as similar fluidized bed gasifiers using biomass.AcknowledgementsWe greatly appreciate the support of John Rockey and David Wildman of the US Dept of Energy, NETL. Their foresight and efforts made this work possible and valuable.References1. Madsen M, Christensen E. Combined gasification of coal and straw coal. 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