Biomass Gasification with Air in an Atmospheric ... - ACS Publications

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Ind. Eng. Chem. Res. 1996, 35, 2110-2120

Biomass Gasification with Air in an Atmospheric Bubbling Fluidized Bed. Effect of Six Operational Variables on the Quality of the Produced Raw Gas Ian Narva´ ez,† Alberto Orıó,† Maria P. Aznar,‡ and Jose´ Corella*,† Department of Chemical Engineering, Faculty of Chemistry, University “Complutense” of Madrid, 28040 Madrid, Spain, and Department of Chemical Engineering, University of Saragossa, 50009 Saragossa, Spain

Biomass gasification with air in a bubbling fluidized bed is studied in a small pilot plant. Variables analyzed are equivalence ratio (from 0.20 to 0.45), temperatures of the gasifier bed (750-850 °C) and of its freeboard (500-600 °C), H/C ratio in the feed, use of secondary air (10% of the overall) in the freeboard, and addition (2-5 wt %) of a calcined dolomite mixed with the biomass used as the feedstock. Using advanced tar and gas sampling and analysis methods, the gas composition and tar content in the gas are determined and their variation with the operation parameters is given. A statistical analysis of the effects of the gasification variables is also here presented. Introduction Biomass is a renewable energy source whose advantages and drawbacks, compared to fossil fuels, are periodically analyzed (Chartier and Beenackers, 1995). An advantage of the biomass as a fuel is that, if the biomass as a feedstock is assumed to be derived from dedicated energy crops, in all the biomass-based technologies there is not a net production of CO2. So, biomass is a CO2 neutral energy. There is also enough biomass in some scenarios to set up power plants from it. For instance, for 100 and 1000 DTE (dry tone equivalent biomass)/day power plants, situated in the middle of a forest and assuming a yield of 10 DTE/ha/ yr, if 100% of the area surrounding the power plant was forested and available for harvesting each year, the radii for harvesting and transporting are 2.4 and 7.6 km, respectively (McMullan et al., 1995) which can be fulfilled in a lot of geographical zones (although 1000 DTE/day is not realistic nowadays for a biomass gasifier). K. Sipila¨ has recently shown (Sipila¨, 1995) the thermochemical conversion routes, technologies, and products from biomass. Gasification is one of them, and it has a good future. For instance, for a biomassintegrated gasification combined cycle (IGCC), using a high-temperature ceramic filter for the raw gas cleaning, an overall efficiency to electricity of 42.4% has been calculated (McMullan et al., 1995), which is quite bigger than the efficiencies obtained with combustion technologies. Thermochemical gasification is a well-known technology which can be classified depending on the reactor type (bubbling or circulating fluidized beds, moving (updraft or downdraft) beds, ...), its pressure (atmospheric or pressurized), gasifying agent (air, steam, steam + oxygen, air + steam, O2-enriched air, etc.), downstream gas cleaning (wet, hot filtration, catalytic, etc.), etc. Between these technologies and/or processes for gasification, the atmospheric biomass gasification with air in a fluidized bed seems to have a feasible application in some scenarios (Katofsky, 1993). It will be the only objective of this paper. * Author to whom correspondence is addressed. Fax: +34 1 394 4164. † University “Complutense” of Madrid. ‡ University of Saragossa.

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Quite a lot of scientific and technical information has already appeared on biomass gasification with air and in fluidized beds. An exhaustive list of authors and institutions having published in this field would be very long, and it is out of scope in this paper. Only as an example we cite some pioneer works in Studsvik (now TPS) AB in Nykoping, Sweden (Rensfelt, 1978), at the Texas Technological College in Lubbock, TX (Beck and Wang, 1980), at the Texas A&M University in College Station, TX (Hiler, 1982), and at Twente University in Enschede, The Netherlands (van der Aarsen et al., 1982). Besides the unpublished work on biomass gasification made by some companies, like the Swedish TPS AB, there are several published studies in this process from the Technical Research Centre of Finland or VTT (Kurkela and Sta¨hlberg, 1992, Kurkela et al., 1992; Simell et al., 1992, ...), Free University of Brussels (Buekens and Schoeters, 1985, Maniatis et al., 1988), University of Hawaii at Manoa (Wang and Kinoshita, 1992; Kinoshita et al., 1994), etc. Although there is thus much information on biomass gasification with air in a fluidized bed, we considered there was still something to be studied in this area. For instance, the raw gas from biomass gasifiers is dirty and it has to be cleaned for most applications. The best technology for IGCC is the hot and catalytic downstream purification of the raw gas. The European Union (EU) is financing a project along such lines in which University “Complutense” of Madrid is the Coordinator. A big effort is being made thus in EU in studying the catalytic downstream gas cleaning. In such a study and for good operation of the downstream catalyst, the coming gas should be “as good as possible”. Of course, the quality of the gas coming to the downstream catalytic reactor depends on a good operation of the gasifier. That is to say, a good and feasible gasification process must operate not only with good end-of-the-pipe solutions (like catalytic cleaning) but also with the best possible operation of the gasifier to produce a gas with the best quality. Cleaning this good raw gas afterward, will be easier than processing a very dirty (high tar and ash content) gas. This work will only focus on the biomass gasifier, bubbling fluidized-bed type, and gasifying with air. The catalytic and hot cleaning of the gas coming from this gasifier will be studied in forthcoming papers (i.e., Narvaéz et al., 1996; Orıó et al., 1996). © 1996 American Chemical Society

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2111

Figure 1. Experimental facility for advanced biomass gasification used in this work.

The gas entering the downstream reactors has to be thus well characterized and as clean as possible. This paper will only focus on the operation variables in the gasifier that can have some influence on the dirtiness of the raw gas at the gasifier exit. We present here thus the effects on the product distribution and gas quality of six different operational variables: equivalent ratio (ER), H/C ratio in the feeding, temperatures in the bed and in the freeboard, fine calcinated dolomite added to the gasifier bed, and secondary air injection at the freeboard zone. Two other important operational variables such as location of the feeding point to the gasifier and type of biomass (feedstock) were already previously studied and published (Corella et al., 1988a,b, respectively). Experimental Facility The experiments have been made in the bench (or small pilot) scale facility shown in Figure 1. It is new, and it has been built up 100% for this project. This installation is based on a fluidized-bed gasifier with a biomass throughput of 200-300 kg/h‚m2. It is a low value, but higher superficial gas velocities of air might not be fed to the gasifier because of its small height. The gasification agent is air, but steam or oxygen can also be used. Feeding System. The feeding system used here was developed after several years of working on “feeding biomass”. It works very well, and its good performance has been an important key factor in this research. Some details of it are shown in Figure 2. It has three screw feeders in series. The third screw (which enters 1 cm into the fluidized bed) has a high speed and is externally refrigerated to avoid pyrolysis (with tar production) in it (which would plug the screw, cut the feeding, and stop the process). The diameter of the last two screws is essential for a good flow of the biomass. The diameters of the particle of the solid fed and of the screw are related. The minimum diameter recommended for the screw, for

small or laboratory units, is 3.0 cm, but such a small diameter leads to many problems, even for pine sawdust. So, screws of 4.5 cm of diameter were used. There is a big difference between feeding at the top or at the bottom of the gasifier (Corella et al., 1988a). These two feeding points generate different problems and product distributions. Biomass has a density 2-5 times lower than silica sand, used as the fluidizing medium in the bed. This difference of density is 10 (or more) times higher than when sand is compared with the char or ash formed from the biomass. Therefore, there is a big tendency (of the biomass, char and ash) to flow upward in the bed and to segregate at the top of the bubbling bed. If the feeding is at the top, the biomass fed does not really enter into the fluidized bed. Its heating rate is low, and the contact with the fluidizing gas is very poor. A big amount of subproducts like tars and char is formed. So, a feeding near the gas distributor plate was selected, although (i) it is mechanically more difficult to operate than feeding at the top and (ii) there is more carryover of fines from the bed due to the important erosion/abrasion of the sand by the screw. Gasifier. This reactor is a bubbling fluidized bed of 6 cm i.d. The biomass is fed at the bottom of the bed near the distributor plate. Operating temperatures are 700-850 °C. The expansion of the gas from the bottom to the top of the bed is important, and it has to account for modeling, carryover, etc. The expansion factor () is between 0.3 and 0.7. The particle size for the silica sand in the bed is important. We used -500 to +320 µm in most experiments. In the bed there is no biomass but only its char or ash. For the bed under operation, and according to our previous work (Aznar et al., 1991), we considered:

(umf)mixture in the bed ) 1.2-1.3(umf)sand

(1)

The superficial gas velocities in the gasifier bed were close to twice the umf of the mixture in the bed. Under

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Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996

Figure 2. Details of the feeding system used. Table 1. Feedstock (Pine Sawdust) Analysis (Dry Basis) particle size, mm proximate analysis (wt %) volatile matter fixed carbon ash ultimate elemental analysis (wt %) carbon hydrogen oxygen nitrogen sulfur LHV (MJ/kg of daf fuel)

-4.0 to +0.80 81-83 16-17 0.5-1.2 50.0 5.7 44.1 0.1-0.3 0.03 18.0-18.4

this condition backmixing is not very important and the flow in the bed (both for the gas and for the biomasschar) is more like piston flow than perfect mixing, but a good isothermicity was always detected in the gasifier bed. Feedstock. The biomass fed at the gasifier was pine (Pinus pinaster) sawdust (C4.2O2.7H5.8N0.02 dry basis). It has a particle diameter between -4 and +0.8 mm and 10-25 wt % moisture. Some of its properties are indicated in Table 1. Particulate Gas Cleaning. The carryover of char, ash, etc., from the gasifier is important and it is necessary to use particulate gas cleaning devices. A hot (450-550 °C) metallic filter was used. Secondary or Guard Bed. After the hot gas filter the gas passes through the guard bed (i.d. 6 cm). This reactor can operate as a fixed, bubbling fluidized, or moving bed, depending on the experiment. Solids used in this bed are calcined dolomites, magnesites, and calcites from different quarries (Corella et al., 1992; Delgado et al., 1996). Temperatures used in this bed were mainly 800-900 °C. Exhaustive Tar Destruction Bed, or Third Bed, or Catalytic Bed. It is a fixed bed, but sometimes it was fluidized, depending on the experiment. Several commercial Ni catalysts and also monoliths not commercialized yet for this process are being used here at temperatures between 700 and 860 °C (Narvaéz et al., 1996). Its objective is the exhaustive elimination by

steam reforming of the light hydrocarbons and tars present in the raw gas from the gasifier exit (Aznar et al., 1992). This paper will be concerned only with the gasifier. The experience gained in the secondary or guard bed and in the catalytic bed will be shown in forthcoming papers (i.e., Orıó et al., 1996; Narvaéz et al., 1996). Methodology Each experiment has two different and well-defined parts: the gasification process and the analytical procedures. The gasification process comprises three steps: startup, operation, and shutdown. Before each experiment the hopper is filled with the selected type and amount of biomass. The three reactors are filled with the selected solids. Then, the different reactors are heated to the desired temperature (i.e., 800 °C in the gasifier; 850 °C in the guard bed; and 800 °C in the catalytic reactor). Some nitrogen is flushed through the pipes to avoid cold zones. When temperatures have reached the set points, biomass is fed to the gasifier and air is introduced according to the previously selected ER. After about half an hour of gasification, the system reaches a stationary state. Then, the tar sampling system (Figure 3) is turned on. Temperatures are measured in the center and at the wall (inside) of each reactor at different heights. The gasifier is always a fluidized bed (isothermal), but the secondary or guard bed and the catalytic one are often fixed beds (not isothermal ones). Tar samples are collected every 30 min at the three different points, after each reactor. All variables in the rig (temperatures, pressures, flows, and on-line gas analysis) are measured and stored each minute by our data acquisition system and software. After some hours (2-8 h) of operation under a stationary state, the plant is shut down. The ingoing air is immediately stopped to prevent combustion of char and coke inside the reactor and on the catalysts. Some (cold) nitrogen is passed then through the pilot plant for a fast cooling.


Figure 3. Tar and gas sampling system. Table 2. Typical Operation Parameters and Results for Some Gasification Experiments run number [time on stream (h)] 26 [5.0]

27 [5.0]

28 [2.7]

moisture (wt %) dp (mm) mB (g/min) throughput (kg/h m2) Qair (sL/min) ER H/C O/C (including the air)

23.5 -4 to +0.8 9.6 204 12.0 0.32 2.2 1.6

Feedstock ) Pine Sawdust 21.0 23.0 -4 to +0.8 -4 to +0.8 9.2 6.5 195 138 12.0 12.0 0.37 0.47 2.1 2.2 1.6 1.9

T1,c (°C) T1,f (°C) solid in bed, dp (µm) L (fixed, bulk) (cm) umf (cm/s) u1,o (cm/s) u1,s (cm/s) τ1 (s)

800 540

800 550

20 10 19 25 0.9

20 10 19 26 0.9

29 [4.0]

30 [5.0]

31 [5.2]

22.0 -4 to +0.8 11.4 242 12.0 0.26 2.1 1.5

25.0 -4 to +0.8 10.9 231 14.0 0.36 2.3 1.7

19.0 -4 to +0.8 11.3 240 14.0 0.32 2.0 1.5

790 560

800 530

20 10 22 30 0.8

19 10 22 39 0.6

9.5 13.0 15.0 2.7 1.6 58.3 2011 24.0 2.5 2.0 4.6 1.23 2.4

9.5 18.0 13.5 4.5 2.3 45.0 9981 24.0 2.3 1.7 6.3 11.53 2.1

Gasifier 810 800 500 600 silica sand (-500 to +320) 20 20 10 10 19 19 23 26 1.0 0.9 Gasifier Exit

gas composition (vol % dry) H2 CO CO2 CH4 C2H4 N2 Ctar (mg/N m3) yH2O (vol %) H/C O/C LHV (MJ/N m3) Ytar (g/kg of daf fuel) Ygas (N m3/kg of daf fuel)

7.0 14.0 13.5 3.0 1.2 61.3 3733 8.0 1.4 1.5 4.3 1.76 2.3

9.5 13.0 15.0 2.7 1.6 58.3 7163 7.7 1.5 1.5 4.6 4.21 2.5

Examples of the operation parameters for some runs are shown in Table 2, together with some typical results for such runs. Tar and Gas Sampling and Analysis After each reactor, there is a special tar and gas sampling device to know the tar amount and gas composition after each reactor. A scheme of it is shown in Figure 3. This tar sampling system is similar to the one used in VTT (Simell and Bredenberg, 1990). It consists of five impinger flasks or traps of 200 cm3 each. The first trap is a flask impinger containing 150 cm3 of an emulsifier solution. The last four traps were placed

8.0 10.0 12.0 2.4 1.1 66.5 2987 27.0 3.2 2.3 3.7 1.35 2.5

9.5 13.0 15.0 2.7 1.6 58.3 2011 21.6 2.3 1.9 4.6 0.97 2.1

in an ice + salt bath (-5 °C) to condense tar and water. Sometimes the last two traps are placed in a cold ice + acetone solution (80 °C). About 0.3 N L/min of gas are drawn through this sampling system. The overall sample of condensates (coming from the condensates in the five flasks, the first one with the emulsifier) was diluted with enough pure water until a homogeneous phase is obtained. The total organic matter in this sample is then analyzed by a Total Organic Carbon Dohrmann Analyser DC90, which is based on the measurement of the amount of CO2 produced during the catalytic combustion of the tar in the liquid sample.

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Figure 4. Gas composition of the raw gas produced at different ERs (T1,b ) 800 °C). Figure 6. Low heating value of the raw gas at the gasifier exit vs ER (T1,b ) 800 ( 20 °C; T1,f ) 550 ( 20 °C; H/C ) 2.1).

Figure 5. Tar content in the raw gas at different equivalence ratios (T1,b ) 800 ( 20 °C; T1,f ) 550 ( 20 °C).

Once the tar and water have been removed from the gas in the cold traps, gas samples after the three reactors were drawn in two different ways: (i) on-line gas analysis with an apparatus from Fisher-Rosemount, including a Model 400A flame ionization detector, to analyze the content of hydrocarbons in the gas and a Binos 100 thermal conductivity detector, for the CO and CO2 concentration in it; (ii) off-line gas analysis, to check and complete the previous gas analysis, on a 6890 HP gas chromatograph with a TCD. Two packed columns, molecular sieve 13X 60/80 mesh and Porapak Q, were used. The GC operating conditions were as follows: detector temperature, 200 °C; carrier gas (He) flow, 20 mL/min; column temperature, 60 °C. Effect of the Equivalence Ratio The equivalence ratio (ER) is one of the most important operational variables in biomass gasification with air. It is defined as the air-to-fuel weight ratio used divided by the air-to-fuel weight ratio of stoichiometric combustion. In biomass gasification it varies from 0.20 to 0.40. When the raw gas is going to be burnt in downstream furnaces, without previously cooling it, the gasifier can be operated at the minimum ER (about 0.20) because tars are not then a problem and the gas should have the maximum possible heating value. When tars cannot be present in large amounts, the ER to be used depends on the temperatures of the gasifier bed and freeboard. When the gasifier operates at high temperatures (900 °C) and with dolomite in the bed, there is some tar cracking in it and the tar yield is low. Low values of ER (of about 0.25) are then allowed as in the VTT gasifier (Simell et al., 1992, Kurkela et

al., 1992) and in the Chinese Academy of Science (Xu et al., 1994). If the gasifier operates at temperatures lower than 850 °C, the tar yield is high; the ER has then to be increased until about 0.30-0.40 to compensate for such an effect. The gasifiers at the University of Sherbrooke (Czernik et al., 1994), Power Combustion Co. (Guillory and Goldbach, 1986), or the pilot of TPS AB in Sweden (Rensfelt, 1991) operate with these high ERs. Values for ER lower than 0.18 are not practical because much tar is produced (process close then to pyrolysis). Values for ER higher than 0.45 produce a not useful gas. Gas Composition. The effect of ER on the gas composition of the produced or raw gas is shown in Figure 4 both for our results and for several ones found in bibliography. In the interval studied for ER (from 0.20 to 0.45), on increasing ER, the amount of fuel gases (H2, CO, CH4, and C2H2) decreases. The maximum amount of H2 (10 vol %), for instance, is obtained at the minimum ER used (0.26), for T1,b ) 800 °C, T1,f ) 600 °C, and H/C (fed to the gasifier) ) 2.3. The results obtained in our gasifier agree with the results and trend obtained by other authors, as Figure 4 shows. Some minor differences in the H2 and CO concentrations can be explained by the different design of the different gasifiers involved in Figure 4. Tar Amount in the Gas. The effect of ER on the tar yield or tar amount in the raw gas produced is shown in Figure 5 for two H/C ratios in the gasifier. As was already well-known, as ER increases, the tar content decreases. As a reference, gasifying at 800 °C with an ER ) 0.30, the tar content is between 4 and 18 g/N m3. The H/C ratio is very important in this amount. If H/C increases, the tar content decreases. If the ER is increased until 0.45, the tar content decreases until 2-7 g/N m3 but the heating value of the produced gas is going to be very low then. Low Heating Value of the Gas. The effect of ER on the LHV of the gas is indicated in Figure 6. Two references are as follows: at ER ) 0.25, the LHV is 5.27.0 MJ/N m3; at ER ) 0.45 the LHV ) 3.5-4.5 MJ/N m3, very low values. They give the limit of a useful (for combustion) gas. Gas Yield. On increasing ER, the gas yield increases, as Figure 7 shows. Our results agree again with the published ones by other authors. So, when ER increases, the gas yield increases and the tar yield decreases but the heating value of the gas also decreases.


Figure 7. Gas yield at different equivalence ratios (T1,b ) 800 ( 20 °C; T1,f ) 550 ( 20 °C).

Figure 8. Gas composition of the raw gas produced at different temperatures in the gasifier bed (ER ) 0.30; T1,f ) 550 °C).

Figure 9. Hydrogen content in the raw gas (ER ) 0.30; T1,f ) 550 °C).

Effect of the Temperature of the Gasifier Bed The temperature of the gasifier bed, bubbling fluidized-bed type (T1,b), affects all the chemical reactions involved in the gasification network. It will depend on a lot of variables such as the type of feedstock (moisture, heating value, ash content), ER, gasifying agent (air, air + steam, ...), loss of heat to surroundings, external heating, etc. In this work, the effect of T1,b on product distribution has been evaluated in the interval of 700850 °C, space time in the bed) for the gas of 1.2 ( 0.2 s, ER ) 0.30, and H/C ) 2.1. Gas Composition. The effect of T1,b on the gas composition is shown in Figure 8. In the interval

Figure 10. Tar amount in the raw gas at different temperatures of the gasifier (ER ) 0.35 ( 0.05; H/C ) 2.1).

Figure 11. Variation of the LHV gas with the temperature of the gasifier (ER ) 0.30 ( 0.02; T1,f ) 550 ( 50 °C).

studied, the H2 content increases from 5 to 10 vol %; the CO varies a lot, from 12 to 18 vol %; the CH4 and C2Hn do not vary very much; and the CO2 decreases with T1,b. To compare our results with the ones previously published by other authors, let us see the H2 content in the gas. As Figure 9 shows, our results follow the same trend as the ones from other authors. Simell et al. (1992) get a higher H2 content because their gasifier freeboard is at higher temperatures than ours and also because they use dolomite in bed with a secondary air injection, and both factors could increase the H2 content in the fuel gas. Tar Content in the Gas. The tar content in the gas decreases with the temperature of the gasifier, as is already known. Our results and some other previous ones are shown in Figure 10. This result is due to tar cracking and steam reforming reactions of the type:

CnHx a nC + (x/2)H2

(2)

CnHx + mH2O a nCO + (m + x/2)H2

(3)

Low Heating Value of the Produced Gas. The effect of T1,b on the LHV of the gas is indicated in Figure 11. The LHV increases a little with T1,b due to the increase on the yields of C2H2, H2, and CO (Figure 8). The species C2Hm are formed by the (steam) cracking of the aromatic components (PAHs) of the tars, as Figure 10 indicated and as Taralas et al. (1991), for instance, already noticed.

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Figure 12. Effect of the temperature of the gasifier freeboard on the gas composition and tar content (ER ) 0.30; T1,b ) 880 °C; H/C ) 2.1).

Figure 13. Gas composition and heating value vs H/C (T1,b ) 800 °C; 550 ( 50 °C; ER ) 0.30 ( 0.03).

Effect of the Temperature of the Gasifier Freeboard The effect of the temperature of the gasifier freeboard (T1,f) has been evaluated between 500 and 600 °C. These temperatures are very low compared with the ones in gasifiers with a secondary air injection. The tar content in our raw gas is thus something higher than the one obtained in gasifiers with higher freeboard temperatures (and bigger freeboard volume too). The gas composition does not vary very much with T1,f in the interval studied, as Figure 12 shows. The tar content in the gas seems to decrease from 6 to 2 g/N m3 on increasing 100 °C T1,f. Effect of the H/C Ratio Fed to the Gasifier The H/C ratio is the total number of the atom-g of H divided by the total number of atom-g of C fed to the gasifier. It is calculated considering the H and C coming with the biomass fed, with its moisture, and with the steam present in the gasifying air fed. H/C can easily be varied (in the interval of 1.6-2.3) by modifying the moisture of the biomass fed to the gasifier. So, the H/C ratio is an index of the H2O fed to the gasifier with the biomass and with the air. A high H/C ratio can be obtained with a biomass with a high moisture content (>30%), but such a high content of steam in the gasifier affects very much the energy balance and the gas composition (Buekens and Schoeters, 1985). Nevertheless, some amount of H2O in the feed seems to improve the heating value of the gas because it affects the

Figure 14. Effect of the H/C ratio on the tar content in the raw gas (T1,b ) 800 °C; T1,f ) 550 ( 50 °C; ER = 0.30 ( 0.03).

Figure 15. Tar content in the exit gas and freeboard temperature, decreasing first ER and then introducing a secondary air flow into the freeboard.

reactions of steam reforming (of the tars), char gasification, water-gas, and water-gas shift. The effect of the H/C ratio has been studied at T1,b ) 800 °C, T1,f ) 550 ( 50 °C and ER ) 0.31 ( 0.03. On increasing H/C to 2.3 (increasing the moisture in the biomass to 25%), the H2 content in the raw produced gas increases quite a lot (Figure 13). The LHV of the gas also increases from 4.0 to 6.0 MJ/N m3. More important is the effect of H/C on the tar amount present in the raw gas. On increasing H/C from 1.6 to 2.2, the tar content decreases from 18 to 2.0 g/N m3 (Figure 14). The decrease in tars and the increase in H2 and CO are due to the reaction given by eq 3. On increasing the partial pressure of the steam in the gasifier, this reaction (eq 3) is favored with important benefits (tar decrease and H2 + CO increase). Effect of a Secondary Air Injection in the Freeboard One experiment was made for testing the effect of the injection of a secondary air flow in the freeboard. This gasification test was started at T1,b ) 800 °C, H/C ) 2.2, and T1,f ) 560-580 °C using an ER of 0.29, without secondary air injection. The tar content in the soproduced exit gas was 12-14 g/N m3, as Figure 15 shows. After 5 h on stream under a stationary state, the ER was lowered to 0.24, without using still secondary air. The tar amount increased then to 27-28 g/N m3 (Figure 15). Two hours later, a secondary air flow with a relative ER of 0.05 was introduced in the


Figure 16. Predicted hydrogen content in the raw gas vs H/C and ER (T1,b ) 800 °C; T1,f ) 600 °C).

Figure 17. Predicted tar content in the raw gas vs H/C and T1,b (T1,f ) 600 °C; ER ) 0.30).

Table 3. Exit Gas Composition with and without Secondary Air (ERoverall ) 0.29; H/C ) 2.2)

shown in Table 2, was repeated in the same experimental conditions except for the feedstock. It contained now 3 wt % of a previously calcinated dolomite (Chilches II) whose characterization and properties are shown in detail in a companion paper (Orıó et al., 1996). The biomass (97 wt %) and the calcined dolomite (of -2.0 to +1.0 mm, 3 wt %) were mixed by hand in an external device and then carefully introduced in the feeding hopper to avoid segregation of the dolomite in it. The tar content in the exit gas for this experiment was lowered by 40% until 4.0 g/N m3. It needed some improvement, but the experiment had to be stopped because of mechanical problems in the screws. Also the carryover of fines to the filter increased a lot because of the softness of the calcined dolomite in the fluidized bed. So, positive and negative effects were found with in-bed dolomite. It is interesting to study this aspect or variable with more detail in future research work.

H2

CO

CO2 CH4 C2Hn

(2ary/1ary) air ) 0 8.7 17.3 15.7 (2ary/1ary) air ) 1/6 9.6 18.0 15.2 with ER ) 0.24 9.9 13.2 16.3 and no 2ary air

4.5 5.0 4.5

1.6 2.0 2.2

N2

LHV (MJ/N m3)

52.2 50.2 53.9

5.4 6.0 6.2

freeboard (using a good bubble cap as air distributor there). An increase of about 70 °C and a tar content in the gas of about 16 g/N m3 were then obtained (Figure 15). The gas composition at the gasifier exit with and without secondary air, maintaining in both cases an overall ER ) 0.29, is shown in Table 3. Not much modification in the exit gas composition is obtained. Splitting the air flow (0.29) in two (0.24 + 0.05), the exit gas becomes perhaps a little more reducing, with a little increase of the LHV, not very significative. This small positive effect of the secondary air (maintaining constant the overall ER) is nevertheless compensated by the increase of the tar content in the gas so produced: from 12-14 to 16 g/N m3. With bigger gasifiers the increase of T in the freeboard could be higher (to 1000 °C), obtaining thus a lower tar content in the gas. To check these results, more experimentation using secondary air will be made in the future. Effect of the Addition of Calcined Dolomite to the Gasifier Bed It is well-known how the addition of limestones or dolomites to the bed changes the product distribution in processes of combustion, incineration, gasification, and pyrolysis of coals, wastes, biomass, etc. These calcinated solids (CaO/MgO mixtures) mainly react with some contaminants like HCl, SO2, PAHs, etc., and eliminate them in some extent from the fuel gas. In biomass gasification in a fluidized bed, these solids have already been used in-bed by Walawender et al. (1985) in USA, Corella et al. (1988b) in Spain, and Kurkela et al. (1992) in Finland. The in-bed use of dolomite in biomass gasification seems also to have found commercial application in Finland and in Sweden (Karlsson et al., 1995). Both Walawender et al. and Corella et al. used in-bed limestone and dolomite, respectively, for biomass gasification with steam, and Kurkela et al. used dolomite for biomass gasification with air but under pressure. Since there was some lack of knowledge in biomass gasification with air at atmospheric pressure, one experiment was made for this purpose. Run no. 31,

Statistical Analysis To predict variations in the dependent variables of the gasification process from the independent ones and from their interactions, each variable in the produced raw gas has been adjusted to the following equation:

ith variable in the raw gas ) R + β(H/C) + γT1,c + λT1,f + σER (4) We have not introduced as independent variables the addition of dolomite to the gasifier bed and secondary air injection because we did not have many results with these two variables. So, the following analysis is only valid for a biomass gasification process without in-bed dolomite and without secondary air injection. The experimental results seem to adjust well to eq 4. The values of the parameters of such equations, or correlation coefficients, are shown in Table 4. As in Table 4. Values of the Different Parameters of Equation 4 dependent variable H2 (vol %) CO (vol %) CH4 (vol %) C2H2 (vol %) CO2 (vol %) Ygas (Nm3/kg) LHV (MJ/N m3) tar (mg/N m3)

R 0.827 -7.24 5.3 0.909 19.3 0.12 9.55

β 3.136 2.8 0.88 0.478 -4.39 0.419 1.58

parameters γ 5.12 × 10-3 0.03 -3.7 × 0-3 -5.2 × 0-4 5.53 × 0-3 2.69 × 0-3 4.12 × 0-3

80600 -11060 -68.58

λ

σ

5.26 × 0-4 -6.88 × 0-3 1.417 × 0-3 2.1 × 0-3 -2.7 × 0-3 -2.66 × 0-3 1.63 × 0-3

-9.35 -15.93 -4.423 -2.91 1.78 2.33 -6.76

1.094

-7425.8

2118


Figure 18. Predicted tar content vs T1,b and ER (T1,f ) 600 °C; ER ) 0.30).

most of the mathematical models, these equations will only be valid in the intervals studied. Their extrapolation to new circumstances is uncertain. Let us show some results using this fitting. H2 Content in the Raw Gas. The H2 content in the gas vs the H/C and the equivalence ratio is shown in Figure 16 (for T1,b ) 800 °C and T1,f ) 600 °C). On increasing H/C (AB line), the H2 content increases. On increasing ER (BC line), the H2 decreases. The maximum of H2 is obtained for H/C ) 2.3 and ER ) 0.20 (B point). Tar Content in the Raw Gas. The tar content in the gas depends on the H/C ratio and T1,b, as Figure 17 indicates (for T1,f ) 600 °C and ER ) 0.30). On increasing T1,b (for H/C ) 1.6, AB line), the tar content decreases. On increasing the H/C ratio (BC line), the tar content also decreases. The minimum tar content is obtained at T1,b ) 850 °C and H/C ) 2.3 (C point). The dependence of the tar content on the temperature of the gasifier bed (T1,b) and on ER (for T1,f and H/C

constants) is shown in Figure 18. On increasing T1,b (for ER ) 0.20, AB line), the tar content decreases from 15 to 4 g/N m3. On increasing ER to 0.40 (at 850 °C, BC line), the tar content in the raw gas becomes lower than 3 g/N m3. When the experimental values are compared with the predicted ones (with eq 4 together with Table 4), there is only a small difference, as Figure 19 shows for the contents in H2 and in CH4 in the fuel or exit gas. So, this model is significative for a level confidence of 95%. The standardized Pareto’s diagram for the H2 content in the raw gas, as an example and for a level of confidence of 95%, is shown in Figure 20. It is deduced from it how the ER and H/C are very important and how T1,b and T1,f are not so important operational parameters. The sign (+ or -) of the effects in Figure 20 indicates that on increasing the independent variable (ER, H/C, T1,b, or T1,f) the answer or effect is possible or negative. Conclusions 1. The equivalent ratio (ER) is perhaps the most important factor in biomass gasification with air. In practice, it defines the temperatures of the bed and of the freeboard, the tar yield, and the composition and calorific value of the fuel gas. On increasing ER from 0.20 to 0.45, the heating value decreses about 2 MJ/N m3 (negative effect) but the tar yield also decreases about 50 wt % (positive effect). 2. The H/C ratio is a measure of the steam content in the gas inside the gasifier and is a very important variable in biomass gasification. It improves the quality of the raw gas produced. On increasing H/C from 1.6 to 2.2, the tar decreases by about 75 wt % and the heating value of the gas increases by about 1 MJ/N m3. 3. To obtain a raw gas with a good quality (maximum heating value and minimum tar content), we recommend to gasify, feeding the biomass near the bed bottom, in the following conditions: ER around 0.250.30; H/C around 2.2; T1,b > 800 °C; T1,f > 600 °C

Figure 19. Checking for the hydrogen (left) and methane (right) contents in the raw gas (T1,b ) 800 °C; T1,f ) 600 °C; ER ) 0.30).

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2119 umf ) minimum fluidization velocity (at the temperature and gas composition inside the gasifier) of the solid in the bed, cm/s W ) weight of the silica sand in the gasifier bed, g Ytar ) tar yield, g of tar/g of daf fuel Ygas ) gas yield, sm3/kg of daf fuel yH2O ) fraction of H2O in the fuel gas, dimensionless Greek Symbols R, β, σ, λ, δ ) parameters of eq 4 ) expansion factor of the gas flow in the gasifier, dimensionless τ1 ) space time of the gas in the gasifer, defined as L/u, s

Literature Cited Figure 20. Pareto’s standardized chart. Effect of the operating conditions on the hydrogen content in the raw gas.

4. A small secondary air injection (10-20% of the primary air) in the freeboard and an addition (1-5 wt % respect to the biomass feed) of a calcined dolomite to the bed, mixed with the biomass, also improves the quality of the raw gas produced. Nevertheless, in this last case the elutriation of fines from the bed (to the downstream cyclones or filters) increases, and it has to be taken into account in the design. 5. To get tar contents in the raw gas below 1.0-2.0 g/N m3 is quite difficult and perhaps not possible. A secondary (hot, catalytic) cleaning becomes necessary to go below such small levels of tar. 6. Having here studied the gasifier performance, the gas quality, composition, and dirtiness at the gasifier exit are now known. Such gas quality is the one at the inlet of the secondary or downstream catalytic reactor. The detailed effects of a catalyst there placed can thus now be studied under a good, known, and even optimized gas composition. Acknowledgment This work has been done thanks to the DGXII EC/ UE Project, Agroindustry Programme No. AIR2-CT931436. We also thank student Ms. Itziar Martıńez for her help in the laboratory work. Nomenclature Ctar ) tar concentration in the raw gas at the gasifier exit, mg or g/N m3 daf ) dry, ash free (for the biomass) dp ) particle diameter, mm ER ) equivalence ratio, dimensionless H/C ) hydrogen to carbon ratio (atom-g in feedstock), dimensionless L ) length of the bed (fixed, bulk), cm LHV ) low heating value of the fuel gas, MJ/N m3 mB ) flow rate of biomass fed to the gasifier, g (as received)/ min N m3 ) normal cubic meter (at T ) 0, P ) 1 atm) O/C ) oxygen to carbon ratio (in feedstock), dimensionless Qair ) flow rate of air entering the gasifier, standard (20 °C, atmospheric pressure) L/min t ) time on stream, h T1,b, T1,f ) temperatures in the gasifier bed and in its freeboard, °C u1,o, u1,e, u ) superficial gas velocity at the inlet, exit and averaged in the gasifier, respectively, cm/s

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Received for review December 14, 1995 Revised manuscript received April 26, 1996 Accepted April 29, 1996X IE9507540

X Abstract published in Advance ACS Abstracts, June 15, 1996.

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