Turndown Ratio of a Gasifier-Combustor Predicted by a Simulation Model

Transcription

1 Turndown Ratio of a - Predicted by a Siulation Model p. K. Chandra, F. A. Payne MEMBER ASAE MEMBER ASAE ABSTRACT A atheatical odel of a gasifier-cobustor was developed and validated with experiental data. The observed and predicted theral efficiencies differed by an average of.8%. The odel was used to analyze the effects of bioass oisture, unit size, and degree of insulation on turndown ratio. Turndown ratio was defined as the ratio of design to iniu theral output and was calculated for a specific updraft bioass gasifier-cobustor configuration. Turndown ratio decreased fro approxiately 8 to 2 as oisture content increased fro 10 to 0% (w.b.). The effect of insulation was ore significant for saller gasifier-cobustors than for larger ones. INTRODUCTION Bioass gasifier-cobustors, which gasify bioass in the first stage and burn the gas in the second stage, have been found to be efficient, controllable, and clean burning systes if properly designed and operated (Richey et. al., 198; Morey et. al., 1984; Payne, 1980). One of the prie characteristics of a bioass cobustion syste is its "turndown ratio." Turndown ratio was defined as the ratio of design to iniu theral output. Miniu theral output is the lowest gasifier-cobustor output at which the particular syste can be operated and still aintain the iniu level of teperature and oxygen in the secondary cobustor. The design output is less tangible and for this work was defined as the theral output of the syste when it is operating at a preselected axiu gasification rate. Design of two-stage bioass cobustion systes requires ore attention to detail than conventional single-stage systes because of the increased deands for a clean exhaust and, for soe systes, a controllable theral output. Design is further coplicated by both the high oisture content of the typical bioass fuel and the relatively large quantity of heat loss for typical sallsized bioass burners. Both conditions reduce the operating range over which satisifactory cobustion is possible. Matheatical odeling of both the physical processes involved and echanical design of a two-stage cobustion syste would allow the quantitative deterination of the effects of bioass oisture content and design on turndown ratio. Reed (1981) discussed the need for turndown ratio to be included in easureents and specifications of gasifiers because this property is required for loadvarying applications. He also reported that fixed-bed and fluidized bed gasifiers have typical turndown ratios of and 2, respectively, and that the field of gasification is so new that very little reliable data on turndown ratio of established systes is available. Several configurations of two-stage cobustion systes designed specifically for bioass have been studied by Payne (1980); Bozdech (1980); Lepori et al. (1983); Morey et al. (1984); and Richey et al. (198). The turndown ratio was reported by Richey et al. (198) to be 3 for dry corncobs for a downdraft channel gasifier. Payne et al. (198b) tested a 0.3 GJ/h updraft gasifiercobustor and easured a turndown ratio of approxiately 2. No inforation was found in the literature quantifying the cobined effect of fuel oisture content and gasifier-cobustor insulation on turndown ratio. The objectives of this study were to: (a) forulate a steady state atheatical odel of a specific updraft gasifier-cobustor design, (b) validate the odel using experiental data, and (c) deterine the effects of insulation, unit size, and bioass oisture content on turndown ratio for the specific gasifier-cobustor design being considered. MATHEMATICAL MODEL DEVELOPMENT A scheatic of the updraft gasifier-cobustor fabricated and odelled is shown in Fig. 1. It consisted of an insulated and fire brick lined gasifier where the bioass was gasified and a cyclonic secondary cobustion chaber (SCC) where the cobustible gases and tar vapors were burned. The SCC design shown in Fig, 1 was constructed to iniize heat loss. The secondary air fro the centrifugal blower entered the Article has been reviewed and approved for publication by the Electric Power and Processing Div. of ASAE. Presented as ASAE Paper No The investigation reported in this paper (No. 242) is in connection with a project of the South Carolina Agricuhural Experient Station and is published with approval of the Director. The paper was prepared with partial support of the USDA (Grant No. 82-CRSR ) and the Southeast Bioass Energy Progra, DOE, Adinistered by TVA (Grant No. TV-64320A). The authors are: P. K. CHANDRA, Research Specialist, Agricultural Engineering Dept., Cleson University, Cleson, SC; and F. A. PAYNE, Associate Professor, Agricultural Engineering Dept., University of Kentucky, Lexington. DILUTION TUBE Fig, 1 Scheatic of updraft qasifier-cobustoi- design showing easureent points, heat losses, ass flows, and insulating aterials Aerican Society of Agricultural Engineers /86/ $02.00 Vol. 29(6):Noveber-Deceber, 1986

2 annular section around the SCC such that it oved with a spiral otion picking up heat as it proceeded through the chaber. A venturi tube with ultiple nozzles was used to draw the gas fro the gasifier. Secondary air was injected through these jets and iparted with both an angular and forward velocity. The forward velocity coponent created a suction on the gasifier. The gas was drawn fro the gasifier, ixed with the secondary air, and ignited in the SCC. A detailed description of the venturi design was presented by Payne et al. (198a). Physical diensions of the laboratory gasifier were given by Payne and Chandra (198) and of the cobustor by Payne and Chandra (1986). These publications also describe the procedure and equipent used to collect data for a coplete ass and energy balance on the gasifier-cobustor. The atheatical odel was organized such that three paraeters: (a) net heat output, (b) SCC teperature, and (c) fuel properties (ultiate analysis, gross heat of cobustion, and oisture content) could be selected and then the ass and energy flows and unit perforance calculated. Net heat output fro the syste, q, was defined as the sensible heat of the exhaust above atospheric teperature. The exhaust flow rate, ^, was then calculated using the operating teperature of the SCC, Tscci atospheric teperature, T^, and the ean specific heat of the exhaust gas, Cp, as follows: e Cp (Tscc -Ta Since the ean specific heat is a function of exhaust coposition and teperature, its value was initially assued and corrected later in the odel in an iterative procedure. The energy lost fro the gasifier cobustor due to conduction and convection through the walls, q^., and radiation through the open end of the SCC, q,., were evaluated using estiated convection coefficients, aterial conductivities supplied by anufacturers, and an assued eissivity of 1.0 for q^.. Conduction heat loss through the gasifier was estiated by assuing the following teperature profile within the gasifier: 1. interior brick surface teperature of 600 C below the grate, 2. gas teperature of 90 C exiting the gasifier, and 3. the fuel bed (and interior brick surface) teperature of 1200 C at the grate decreasing to 90 C at the top of the gasifier. The calculation of the wet bioass gasification rate, ^, was ade by assuing steady state and coplete cobustion of the bioass and dividing the su of heat losses and net heat output (total sensible heat converted) by the net energy content of bioass, AH,^: b _ q + qr + qc AHn The net heat of cobustion AH^^ easures the axiu sensible heat which can be extracted fro a fuel if the water in the fuel and the water generated during cobustion exit the syste as a vapor. The net heat of cobustion as defined by ASTM (1974) corrects the gross heat of cobustion to exclude the heat of vaporization of the water and akes a inor correction fro a constant volue to a constant pressure process. The inor correction was shown to be insignificant for a typical bioass fuel and when neglected perits AH,, to be calculated by the following equation when using positive values for AH (Payne, 1984): AHn = AHo - (X + AHo)X^ kj/wet kg where, AHQ = AH 9AXH = net heat of cobustion at zero fuel oisture content. Units: kj/dry kg. AH = gross heat of cobustion (higher heating value). ASTM (1974) D-201 is a standard ethod of easureent. Units: kj/dry kg. A = 2442 kj/kg, latent heat of vaporization of water at a reference teperature of 2 C. X, = fuel oisture content, wet weight basis. Units: kg water/kg wet fuel. XH = ass fraction of hydrogen in the dry fuel. Units: kg H/kg dry fuel. Excess air,, was calculated at this point since SCC teperature, heat loss per unit fuel, and fuel properties are known (Payne, 1984). With known, the priary and secondary air flow rates were estiated using the equations presented by Payne and Chandra (198): itip =b (1 - Xjn) S 0g s = b (l-x^js^^ (l + e-0g) where: nip,, = priary and secondary air rates, respectively, kg/s = excess air index, i.e., ratio of aount of excess air used to stoichioetric air requireent, diensionless S = stoichioetric air requireent, kg air/kg dry bioass, i.e., the aount of air required for coplete cobustion, and t>g = equivalence ratio for gasification, i.e., the fraction of the stoichioetric air requireent used in the gasification step, diensionless. The equivalence ratio for gasification was estiated using the correlation developed by Payne and Chandra (198) for wood chip gasification: where: f (^g = f = the fraction of fuel gross heat of cobustion which is converted to sensible or latent heat in the gasifier and exits either through the gasifier walls or with the gas. Inclusion of the above relationship akes the odel specific to wood chip gasification with regard to predicting priary air flow rates. The relation for ^^ was slightly different for corncobs as reported by Payne and Vol. 29(6):Noveber-Deceber,

3 Chandra (198). Theral efficiency was calculated by dividing the net energy output by the net energy input. TABLE 2. STATISTICAL ANALYSIS ON MEASURED AND CALCULATED ENTITIES AT LEVEL OF SIGNIFICANCE. Variables Test UQ: b-l=0 b* MODEL VALIDATION The atheatical odel perits the calculation of the ass flows and energy efficiency when the heat output, bioass fuel properties, and SCC teperature are selected. The odel was validated by coparing calculated and easured values for ass flows and theral efficiency. The easured theral efficiency was deterined in two ways: one by calculating the enthalpy of the input and output ass flows; and the other by calculating net energy input and output. The equation used to calculate efficiency using the enthalpy ethod (Payne, 1984) is as follows: (Hp - Hpa) (Hr - Hpa) where = enthalpy of products of cobustion at Ts^c^ H = enthalpy of products of cobustion at T^, and H. = enthalpy of reactants. Theral efficiency was also calculated by coparing net energy input with output using the following equation: (p + b + nis) Cp (Tscc " Ta) nib ^^n An average theral efficiency r]^^ defined as the average of r]^ and r]^ was used to copare with the calculated theral efficiency. Fourteen experients were conducted on the 80 kj/s laboratory gasifiercobustor sketched in Fig. 1. The experiental tests were run with wood chips over a oisture range of 12 to 48% and a specific dry gasification rate of 11 to 38 g/2s (Payne and Chandra, 1986). The easured energy output, fuel properties, and SCC teperature of the experients were used as input to the odel to predict priary and secondary air rates, gasification rate, and b 0.18 ttcrit g 0.2 t tcrit not rejected not rejected * Slope of the least squares line between easured and calculated entities. tcrit ^^ *^^^ ^^^^ ^^s average theral efficiency. Measured and calculated values are copared in Table 1. The easured and calculated ass rates for priary air were close as expected since the relationship for j>g was developed fro the data. Least squares estiation of a linear relation between easured and calculated values of gasification rate and secondary air rate were calculated. A test of the hypothesis that the slopes were not significantly different fro unity was perfored and the results suarized in Table 2. The standard deviation for the difference between calculated and easured values of ^ and ^ was 0.42 and 4.2 g/s, respectively. The easured theral efficiencies ranged between 7 and 9% and are shown plotted against average calculated efficiencies in Fig. 2. The standard deviation between easured and calculated average efficiency v)^^ was.8%. The ability to predict theral efficiency was considered the priary gauge of the odel's accuracy. The.8% standard deviation for a cobustion syste was considered acceptable for validating the atheatical odel. The odel's use to siulate the gasifier-cobustor operation under different conditions was then investigated. SIMULATIONS The validated atheatical odel was used to deterine the effect of bioass oisture, unit size, and insulation on turndown ratio. The laboratory gasifiercobustor described by Payne and Chandra (198) and shown scheatically in Fig. 1 was used as the selected configuration. It was designed for a rated output of 80 kj/s using woodchips at 30 percent oisture content, a specific dry gasification rate of 3g/s, and a residence tie of 0.1s in the SCC. The physical diensions of TABLE 1. COMPARISON BETWEEN MEASURED AND CALCULATED VARIABLES. Measured/calculated values Expt no ^'> g/s 10.3/ / / /9. 3.9/ / / / /. 4.8/4.4.1/ /6.1 3./ / p' g/s 8.8/1 7.0/ / / /.1.0/ /3.9 7./ / /.0 6.3/.2 6.7/ / /4.0 "' g/s 4.3/ / / / / / / / / / / / / /18.0 ^av' % 93.64/ / / / / / / / / / / / Standard deviation of the difference between calculated and easured values. ' PREDICTED THERMAL EFFICIENCY, 7o Fig. 2 Average easured theral efficiency as a function of theral efficiency predicted by the atheatical odel. 170 TRANSACTIONS of the ASAE

4 TAB^E 3. SUMMARY OF PHYSICAL DIMENSIONS OF THE GASIFIER-COMBUSTORS Rated output, kj/s Grate area, height, Gas tube diaeter Inside diaeter, Cyclonic SCC Inside length, three larger gasifier-cobustors having rated output capacities of 200, 800, and 4000 kj/s were specified based on the laboratory odel design. A axiu gasification rate of 3 g/^s was used for all gasifier designs. The inside height between the grate and the top of the gasifier was designed to be equal to the square root of the grate area. An additional length, variable with capacity, was added below the grate for ash reoval. A suary of the gasifier-cobustor diensions is given in Table 3. The average velocity at rated output in the priary air inlet, gas tube, secondary air inlet, and exhaust exit fro the SCC were designed to be 1.0, 2., 6.1, and 3.9 /s, respectively, and were the sae as the gas velocities at rated output for the laboratory odel. The teperatures of the gas entering the SCC and the exhaust exiting the SCC were assued to be 90 and 1100 C, respectively, for all sizes. The cyclonic SCC length and diaeter were estiated as suarized in Table 3. The residence tie at rated output was 0.1 s for all designs except for the 4000 kj/s size which was 0.20 s because the velocity restrictions of 3.9 /s yielded an abnorally short cobustor. The effect of increasing aounts of insulation was tested by increenting the resistance of the insulation while aintaining a constant thickness. The exterior diensions of the gasifier-cobustor thus reained constant for all tests on each size. Six levels of insulation were used in the siulation ranging fro no insulation to very high insulation (theral resistance of 2400 and 000 2s K/kJ for the gasifier and the SCC, respectively). The fire brick in the gasifier and convection coefficients for both the gasifier and SCC provided a base theral resistance which reained constant for all tests. Moisture ass fractions were included in the siulation to show the effect of oisture on turndown ratio. Five levels of oisture considered were 0.1, 0.2, 0.3, 0.4 and 0. kg of oisture per kg of wet bioass. The four levels of gasifier output, five levels of oisture 8 6 /\ 2 SIZE. kj/s: ^^^ ^?oo o ^ MOISTURE LEVEL, X^=0.3 ^^ 0 1 GASIFIER COMBUSTOR THERMAL RESISTANCE OF INSULATION, 2s, K/kJ Fig. 3 Variation of turndown ratio with size of gasifier-cobustor and degree of insulation. content, and six levels of insulation required that turndown ratio be calculated for 120 conditions. The turndown ratio was deterined for each siulation unit by increentally reducing the theral output and adjusting excess air in order to aintain cobustor teperature at 1100 C. The lower liit on theral output was defined at the point where oxygen content in the secondary cobustor fell below 2% (dry gas basis). The oxygen liit was assued to be the iniu level at which coplete cobustion of the gas could be assured and the turndown ratio calculated by dividing the rated output by the lower liit on theral output. RESULTS AND DISCUSSION At 30% bioass oisture content, the turndown ratio was higher for a larger syste at all levels of insulation as shown in Fig. 3 and Table 4. The turndown ratio was 2.28 and 3.90 for the 80 and 4000 kj/s systes, respectively, using no insulation. Using a high level of insulation increased the turndown ratio to.33 and.71 for the respective systes. This difference decreases as the theral resistance increases because of reduced heat losses. It is observed that insulation affects a saller unit ore than a larger unit. Fig. 4 shows the effect of oisture on the turndown ratio for different sizes of gasifier-cobustors at theral resistance of 240 and 00 ^s K/kJ for the gasifier and cobustor, respectively. Turndown ratio is alost tripled when bioass oisture is reduced fro 0. to 0.1 for all sizes. Differences of turndown ratio between the 80 and 4000 kj/s units were greater at lower oisture levels as shown in Table 4 for all six levels of insulation. Table shows the effect of different levels of insulation Q J -^ TABLE 4. VARIATION OF TURNDOWN RATIO WITH SIZE OF GASIFIER-COMBUSTOR HAVING DIFFERENT LEVELS OF INSULATION. Theral resistance of insulation, 2 sk/kj* Turndown ratio (80 kj/s syste)/(40000 kj/s syste) X = 0.1 X = 0.2 X = 0.3 X = 0.4 X, = / / / / / / / / / / / /4.6.33/ / / / / / / / / / / / / / / / / /1.67 Multiply by 0678 to convert to (h ft^ F/Btu). Vol. 29(6):Noveber-Deceber,

5 SIZE, kj/s: I?6 ^ 4 Q z»- 2 XrT,= MOISTURE MASS FRACTION X=O.I X;02 x=03 X= SIZE OF SYSTEM, kj/s ^ 4000 Fig. 4 Turndown ratio as a function of syste size and bioass oisture content for theral resistance of 240 and 00 ^sk/kj for the gasifier and cobustor, respectively. on heat loss fro the gasifier-cobustor for different size units. The calculated total aount of heat loss due to conduction, convection, and radiation was divided by AH to deterine the fraction of heat loss. The heat loss data were calculated for units operating at the rated output and at the iniu output as defined previously. The fraction of heat loss reains alost unchanged for all systes operating at iniu output, indicating that aintaining teperature in the cobustor liits the syste. When operated at the rated output. Table shows that fraction of heat loss is reduced by both insulation and increased size. Fig. shows that the fraction of heat loss is higher for saller 0.1 z o \- o 0.13 CO y 1- LlJ X GASIFIE COMBUSTOR-*' THERMAL RESISTANCE OF INSULATION, ^s K/kJ Fig. Effect of size and insulation on heat losses fro gasifiercobustor at design output for all oisture levels. units with low levels of insulation, and decreases with increasing size and insulation to a iniu. Heat loss can never go to zero because of radiation heat loss through the SCC opening. Radiation heat loss is not related to the level of insulation and depends priarily on the area of the cobustor exit and cobustor teperature. Table 6 shows the rate (kj/s) and proportion of heat loss fro the gasifier and cobustor for the six levels of insulation and two size units. The heat loss fro the cobustor is further divided into conduction and radiation fractions. It is observed that radiation losses are doinant for the large (4000 kj/s) unit. For the sall unit (80 kj/s), insulation greatly alters the proportion of heat losses. Fig. 6 shows the liiting aount of heat which can be lost fro the syste as a function of oisture content. This iniu point of operation is a function of the theral liit placed on SCC teperature and is TABLE. EFFECT OF SIZE AND INSULATION ON HEAT LOSS FROM GASIFIER-COMBUSTOR. Fraction heat loss fro gasifier-cobustor Thera] L resistance sulation, 2 sk/kj$ Size = 80 kj/s Size = 200 kj/s Size = 800 kj/s Size = 4000 kj/s , *The values at rated output vary little with oisture level, f These values are taken at oisture ass fraction of 0.1. : :Multiply by 0678 to convert to (h ft2 F/Btu). TABLE 6. PROPORTION AND RATE OF HEAT LOSSES FROM THE GASIFIER AND COMBUSTOR. Proportion/total heat loss, kj/s Thera J resistance sulation, 2 sk/kj Size = 80 kj/s Size = 4000 kj/s (Conduction) (Conduction)* (Radiation) (Conduction) (Conduction)* (Radiation) / / / / / / /26 311/ / / / / / / / / / /6. 04/1.2 27/ / / / / /90 024/ / / / / / / / / / /320 * These values are taken at bioass oisture of 0.1. All other values are not influenced by oisture. 172 TRANSACTIONS of the ASAE

6 O 0.40 Q:: UJ X LEVEL OF INSULATION: SIZE OF SYSTEM: BIOMASS MOISTURE, X^ ALL ALL Fig. 6 Variation of axiu heat loss fraction with bioass oisture for any size and insulation. independent of the degree of insulation and the syste size. Experience with the 80 kj/s laboratory gasifiercobustor has shown that typical turndown ratio using wood chips is approxiately 2 at 0% oisture and around 4 at 20% oisture. These are slightly lower than the values predicted in Fig. 4. The turndown ratio calculated by the odel and shown in Fig. 4 is for a syste operating in steady state. A practical syste is exposed to fluctuations in feed rate, fuel oisture, and other variables which would yield a turndown ratio lower than that predicted by the above analysis. This analysis is specific to the updraft gasifier-cobustor design selected. CONCLUSIONS The conclusions drawn fro this research are: 1. A atheatical odel of a specific design of an updraft gasifier-cobustor was developed. 2. Model validation was shown by predicting syste theral efficiency fuel feed rate, priary air rate, and secondary air rate with acceptable accuracy for 14 experiental runs on a laboratory gasifier-cobustor. 3. Use of the odel showed that: ( a) Bioass oisture content has a greater effect on turndown ratio than size of insulation of the syste. (b) The turndown ratio increased as the gasifiercobustor size increased for all fuel oisture levels between 0.1 to 0. ass fraction. (c) The turndown ratio was found to increase with increasing insulation. The effect of insulation was ore proinent for saller units than for larger ones. References 1. ASTM, Annual Book of ASTM Standards Aerican Society for Testing and Materials, Philadelphia, PA. 2. Bozdech, S. L Use of corncobs fro seed drying through gasification. DEKALB AgResearch, Inc., Sycaore Road, DeKalb, IL Lepori, W. A., C. B. Parnell, Jr., D. B. Carney, P. P. Ling, and R. Garza-Tobias Sall scale power generation fro bioass-technical potential. ASAE Paper No , ASAE, St. Joseph, MI Morey, R. V., D. P. Thisen, J. P. Lang, and D. J. Hansen A corncob fueled drying syste. TRANSACTIONS of the ASAE 27(2): Payne, F. A The conversion of corncobs into theral energy for drying grain using a gasification and cobustion process. Unpublished PhD Dissertion. Departent of Agricultural Engineering, University of Kentucky, Lexington, Microfil No Payne, F. A Energy and ass flow coputation in bioass cobustion systes. TRANSACTIONS of the ASAE 27(): , Payne, F. A., P. K. Chandra, and B. Merrian. 198a. Cyclonic cobustor for updraft gasifier-cobustor. Proceedings of the th Annual Solar and Bioass Energy Workshop, April 23-2, Atlanta, GA: Payne, F. A., J. L. Dunlap, and P. Caussanel. 198b. Effect of wood chip size on updraft gasifier-cobustor operation. TRANSACTIONS of the ASAE 28(3): , Payne, F. A., and P. K. Chandra Mass balance for bioass gasifier-cobustors. TRANSACTIONS of the ASAE 28(6): Payne, F. A. and P. K. Chandra Predicting ash particulate eission fro updraft bioass gasifier-cobustors. TRANSACTIONS of the ASAE 29(2): Reed, T. B Bioass gasification: Principles and practices. Noyes Data Corporation. 12. Richey, C. B., J. R. Barrett, and R. B. Jacko Downdraft channel gasifier furnace for bioass fuels. TRANSACTIONS of the ASAE 28(2):92-98, 60. f Hpa Hp H. AH AH AHo q qr s Ta Te Te T A ^H i NOMENCLATURE ean specific heat of the exhaust fraction of AH converted to sensible or latent heat in the gasifier, diensionless enthalpy of products of cobustion at T^, kj/kg enthalpy of products of cobustion at Tscc, kj/kg enthalpy of reactants, kj/kg gross heat of cobustion, kj/kg net heat of cobustion, kj/kg net heat of cobution at zero oisture, kj/kg wet bioass gasification rate, kg/s exhaust rate, kg/s priary air rate, kg/s secondary air rate, kg/s net heat output, kj/s conduction and convection heat loss, kj/s radiation heat loss kj/s stoichioetric air requireent, diensionless atospheric teperature, C cyclone air teperature, C process air teperature, C secondary cobustion chaber teperature, C gas teperature, C hydrogen ass fraction oisture ass fraction excess air index, diensionless latent heat of vaporization, kj/kg theral efficiency based on net energy calculation theral efficiency based on enthalpy calculations equivalence ratio for gasification, diensionless Vol. 29(6):Noveber-Deceber,