Countercurrent Fixed-Bed Gasification of Biomass at Laboratory Scale

Ind. Eng. Chem. Res. 1999, 38, 2571-2581

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Countercurrent Fixed-Bed Gasification of Biomass at Laboratory Scale Colomba Di Blasi,* Gabriella Signorelli, and Giuseppe Portoricco Dipartimento di Ingegneria Chimica, Universita´ degli Studi di Napoli “Federico II”, P.le V. Tecchio, 80125 Napoli, Italy

A laboratory-scale countercurrent fixed-bed gasification plant has been designed and constructed to produce data for process modeling and to compare the gasification characteristics of several biomasses (beechwood, nutshells, olive husks, and grape residues). The composition of producer gas and spatial temperature profiles have been measured for biomass gasification at different air flow rates. The gas-heating value always attains a maximum as a function of this operating variable, associated with a decrease of the air-to-fuel ratio. Optimal gasification conditions of wood and agricultural residues give rise to comparable gas-heating values, comprised in the range 5-5.5 MJ/Nm3 with 28-30% CO, 5-7% CO2, 6-8% H2, 1-2% CH4, and small amounts of C2- hydrocarbons (apart from nitrogen). However, gasification of agricultural residues is more difficult because of bed transport, partial ash sintering, nonuniform flow distribution, and the presence of a muddy phase in the effluents, so that proper pretreatments are needed for largescale applications. Introduction Air gasification of biomass is a process where solid conversion is made to occur in the presence of reduced oxygen, so that temperatures are lower than those encountered in combustion.1-3 The limited oxygen supplied is used to burn only a small part of the fuel, either solid or gas, to provide the heat needed for char gasification, biomass pyrolysis, drying, and of course, preheating. The main product is then a gaseous combustible which can be processed in a second step. In direct (grate-fired) combustion the supply of underfire air, whose role is to gasify the fuel, is coupled with the addition of overfire air to burn the gaseous products (for instance, see ref 4). In general, the advantages of gasification over combustion coincide with those of a gas fuel over a condensed-phase fuel and include higher rates of heat release, higher burning efficiencies, easily controlled and adjustable energy output, simpler burner construction, no particle emissions, less air pollution (especially NOx emissions which are reduced by a factor of about 44), less fouling in the heat-exchange equipment, direct gas burning in internal combustion engines and application in combined cycles, and easy distribution of gases over short distances. Furthermore, ash agglomeration and sintering are partially prevented and noxious substances are left in the solid phase and easily removed. Gasification produces a gaseous mixture of hydrogen, carbon monoxide, steam, methane, and light hydrocarbons with other undesired effluents, such as organic aerosols, inorganic particulates, condensable organic vapors (tars), sodium, potassium and chlorine compounds, ammonia, and hydrocyanic acid. Particularly problematic is the behavior of tars, whose content6 in the gas varies much from one process to another, from about 1 to 180 g/Nm3. Unless the gasifier is closecoupled with an external combustion chamber, this concentration has to be lowered to only 50-500 mg/Nm3, * Corresponding author: Tel.: 39-081-7682232. Fax: 39081-2391800. E-mail: [email protected].

depending on the applications, or even brought to zero for downstream fuel cells. The energy content of the gas produced through gasification depends on numerous factors, such as the oxidizing agent, reactor type, fuel type and form, etc. The oxidizing agent can be chosen as air, oxygen, steam, or a mixture of these.1-3 When air is used, the resulting gas has a low calorific value (3.8-5.6 versus 38 MJ/Nm3 of natural gas). This can be increased (1018 MJ/Nm3) by using oxygen or steam but in the latter case sufficient heat should be provided because steam gasification is an endothermic process. In some cases, steam is added to air to increase the level of hydrogen in the producer gas. Two main classes of chemical reactors, fixed-bed and fluid-bed reactors, are applied for biomass gasification.1-3 Fixed-bed, countercurrent (updraft), and concurrent (downdraft) reactors are, in general, of very simple construction and operation. They also present high carbon conversion, long solid residence times, and low ash carry-over. On one hand, the updraft process is more thermally efficient than the downdraft process but the tar content of the gas is very high. On the other hand, scaling problems are reported1-2 for the classical throated design, and though the open-core version7 is claimed to overcome such drawbacks, no large-scale downdraft plant (above 0.5 t/h) is currently in operation.3 The more complicated and more expensive technology of fluidized(bubbling-, circulating-, and entrained-) bed reactors allows very high capacities, very good solid-gas contact, and easy scalability. Feedstock requirements (size, moisture, and ash content) depend upon the gasification unit and should be carefully met as inadequate fuel preparation is the cause of serious technical drawbacks. In this study, updraft gasification of biomass is experimentally investigated, in that though several industrial updraft fixed-bed plants are in operation in Northern Europe for peat and wood chips gasification,8-10 detailed information is not available for the gasification characteristics of different feedstocks, the process optimization, the implementation of effective gas cleaning

10.1021/ie980753i CCC: $18.00 © 1999 American Chemical Society Published on Web 05/29/1999

2572 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999

technologies, and the process modeling. Indeed, though the commercial applications of fixed-bed gasification of coal and wood date back to the first years of 1800,1,11 the process fundamentals are largely unknown. In particular, no experimental analysis is currently available on updraft gasification with simultaneous measurements of temperature profiles and gas composition for different operating conditions even for an “easy” fuel such as wood. To be economically competitive, a gasification plant should be operated with different, locally available feedstocks, whose nature and condition, presumably, change in the course of the year. This aspect has been examined to a certain extent by means of fluidbed reactors,12,13 but the results are not applicable for fixed-bed gasification units. Only the gasification of bundled jute sticks, through a laboratory-scale updraft reactor, should be mentioned.14 Also, despite the high condensate production, there is interest in the cleaning of the updraft gas for electricity production, as lowtemperature tars are more reactive and thus easier to be removed, than the high-temperature tars produced in much lower amounts by downdraft and fluid-bed gasifiers.1 Finally, though mathematical modeling of gasification reactors is a powerful tool for process design, optimization, and scale-up, before confident use of numerical simulations, validation has to be carried out and data are needed on temperature profiles, product distribution, and gas composition. Gasification System A laboratory-scale gasification plant has been designed and constructed. The main components are schematically represented in Figure 1A. The core of the plant is an updraft gasifier (1), where pressure and temperature profiles are measured by a pressure transducer (2) and a set of thermocouples (3), respectively. The data are collected by an acquisition board and converted into digital data, which are then elaborated by a PC (4). Biomass is fed at the top of the gasifier by means of a double-slide valve (5). Air is fed at the bottom and its rate is measured by a rotameter (6). A safety valve (7) is located on the gas exit tube. Here, the gas stream enters two condensers (8), where steam and tars are condensed and collected at the bottom. The gas is further purged through a closed water tank (9) and a packed-bed filter (10). Before sampling (11) for gaschromatographic (GC) analysis, the gas also encounters two demisters (12). Finally, the exit gas flow rate is again measured by a rotameter (13). The gasifier (Figure 1B) is a cylindrical steel shaft 0.63 m high above the grate and 0.10 m large (internal diameter). A thick layer of refractory material, built interior to the reactor (0.019 m), and external ceramic fibers (0.08 m) create the thermal insulation of the reactor. The bottom zone, under the grate, has a truncate conical shape to collect and then discharge the ash produced in the process. The grate, used as an air distributor, is a hexagon constructed using square section tubes with small holes in the bottom. The air, fed to the reactor, flows along the channels and exits through the small holes along the grate, so it is distributed across the whole section of the gasifier. At the bottom, a small tube allows the use of a premixed flame to ignite the bed at the beginning of the process. Temperature profiles along the gasifier axis are measured by a set of thermocouples (chromel-alumel, 1 mm diameter) placed within a steel protective tube.

Figure 1. (A) Schematic representation of the updraft gasification plant: (1) gasifier, (2) pressure transducer, (3) thermocouple set, (4) acquisition data system, (5) double-slide valve, (6) rotameter, (7) safety valve, (8) condensers, (9) closed water tank (gas scrubbing), (10) packed-bed filter, (11) gas-sampling system, (12) demisters, and (13) rotameter. (B) Details of the gasifier (sizes in mm).

The feeding system is constituted by a chamber closed by two sliding valves (total height 0.96 m), one separating the container from the chamber and the other at the bottom of the chamber, just above the gasifier. By opening of the first valve, the chamber is filled up with biomass, and then the first valve is closed and the second valve is opened. The two condensers are shell and tube units (20 tubes with i.d. 0.014 m and a total exchange surface of 0.45 m2 for each unit), where the condensed liquids flow down the tubes and also capture the solid particles entrained by the gas. At the bottom, the liquid phase is discharged and collected for analysis. Wet scrubbing takes place through a stainless steel closed tank (0.16 m i.d. and 0.30 m length). The filter is a packed bed of ashes (0.15 m i.d. and 0.20 m length). The demisters

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2573

consist of two glass cylinders filled up with cotton wool. Condensers, wet scrubbing, packed bed, and cotton wool filters constitute the gas-cleaning system, which though not optimized, guarantees a gas sufficiently clean for gas-chromatographic analysis (GC equipped with thermal conductivity detector (TCD) and a packed column (Supelco Carboxen 1000)). Gas sampling and analysis are carried out at selected times during the whole duration of the tests. The first step in the gasification process is the ignition of the bed. This is caused by a pilot flame and combustion is favored by the spreading of methyl alcohol along a layer about 0.01 m thick and the addition of small quantities of a highly flammable material. Initially, the bed height is only 0.30 m and the first 0.03 m is made by a granular refractory material, to minimize the effects of high combustion temperatures and ash behavior on the grate life. There are two possible operation modes of the gasifier, corresponding to a constant or a variable bed height. In the first modality, after ignition, the bed height is brought to the desired value (0.55 m) and maintained constant ((0.025 m). This is achieved by feeding biomass at proper time intervals. Therefore, as a consequence of variations in the air flow rate, the air-to-fuel ratio will also vary, given that the fuel feed rate is the adjustable variable to control the (constant) bed height. The feeding process is an important aspect in the operation of fixed-bed gasifiers.15 The rate of biomass consumption is essentially dependent on the intrinsic reactivity and the rate of air supply. Sufficient biomass can be added to keep the bed height at a constant value. However, as the rate of biomass consumption increases with the air flow rate, the feeding frequency should also be properly adjusted. Therefore, biomass is fed at regular time intervals, whose duration varies with the air flow rate. In particular, a limit is expected at very high flow rates, when the feeding frequency becomes so high that a semicontinuous procedure is no longer possible. The rate of biomass consumption can also be adjusted by choosing a proper rate of solid discharge at the grate, but this may be problematic for small-scale systems. Indeed, frequent solid discharge causes significant heat loss (the discharged solid is at a high temperature), with the introduction of instabilities in the gasification process. In the second modality, gasification tests can be made for different air-to-fuel ratios, thus allowing the bed height to vary. For instance, after the selection of the air flow rate, the fuel feed rate can be varied and, consequently, the bed height will also vary. Tests have not been made with this modality; however, it can be understood that there is again a limit at very low fuel feed rates, when the continuous operation approaches the behavior of a batch system and the processes of drying/ devolatilization, on one side, and gasification/combustion, on the other side, tend to become uncoupled. Results As in the countercurrent fixed-bed gasification of biomass the different physical and chemical processes are stratified along the reactor height1 in the order of combustion, gasification, devolatilization, and drying (from the bottom), a first set of experiments has been carried out for fixed-bed pyrolysis of beechwood, through a bench-scale reactor. The main purpose of this activity is the determination of product yields and gas composition to establish the role of devolatilization in the

Table 1. A. Elemental Composition (% mass) of Wood and Agricultural Residues (Oxygen by Difference) and Air-to-Fuel Weight Stoichiometric Ratio, Rs biomass

C%

H%

N%

S%

O%

Rs [kg/kg]

beechwood nutshells olive husks grape residues straw pellets

46.23 46.85 50.90 47.89 43.60

6.49 6.30 6.32 6.22 6.20

0.07 0.29 1.37 2.11 0.30

0.03 0.02 0.03 0.09 0.08

47.18 46.53 41.37 43.69 49.82

5.5 5.6 6.3 5.9 5.0

B. Chemical Composition and Ash Content (% of Dry Weight) of Wood and Agricultural Residues biomass beechwood nutshells olive husks grape residues straw pellets

extractives hemicellulose cellulose lignin ash % % % % % 3 8 8.7 15.6 7.4

30 25 18.5 17 27

45 37 31 35 46

22 33 28 30.4 16.4

0.5 0.9 2.5 5.1 5.5

C. Physical Characteristics of the Biomass Beds biomass

density [kg/m3]

moisture %

size [mm]


350 390 515 325 256

5-6 5-6 5-6 5-6 5-6

3(2.5)-5(95%) 1(80%)-2(15%) 1(90%)-3(5%) 1(5%)-3(90%) 20

gasification characteristics. Numerous tests have been carried out on biomass gasification for the optimization of the plant in relation to gasifier insulation, gascleaning procedures, operation modalities (bed height and feeding process), and biomass pretreatments (particle sizes, presence of fines, and predrying). In this study only the results obtained after this preliminary phase are presented. All the tests have been carried out with a constant bed height (0.55 m, unless otherwise specified). For beechwood, the air flow rates have been varied in the range 1.250-2.340 kg/h, with corresponding feeding frequencies varying from about 12 to 5 min, where the lower value is a limit for semicontinuous feeding. Axial temperature profiles, the gas composition, and amount of condensate have been determined. Also, gasification of some agricultural residues (wheat straw pellets, nutshells, olive husks, and grape residues) has been carried out (air flow rates always below 2 kg/h, this limit deriving from the physical properties of the bed) and the results compared with wood. Feedstocks Characteristics. The main information on the chemical composition of feedstocks is reported in Table 1, parts A and B. An important factor in the gasification process is the hydrogen/carbon (H/C) atomic ratio of feedstocks. For biomass it is roughly 1.5 and is between the value reported11 for coals (about 1) and for light fuel oil (about 2); however, the oxygen content is higher (about 50% weight versus 10-20% coals). Consequently, the heating value is also lower (roughly 20 versus 30 MJ/kg for coals and 40 MJ/kg for oil). The high oxygen content also reduces the values of the stoichiometric weight air-to-fuel ratio, Rs. From the ultimate analysis of biomasses a formula can be derived by converting the weight % to moles and then dividing the number of moles of the other elements (H, O, N, S) by the moles of carbon. Such a formula is then used to obtain Rs (Table 1A), assuming complete conversion to water vapors and carbon dioxide. The values are between 5 (straw) and 6.3 (olive husks) (versus values of roughly 10 for coals and 15 for oil). The characteristics


of beechwood and nutshells are very similar, while the carbon content is higher for olive and grape residues and lower for straw, with consequent variations in Rs. Another interesting feature is that the nitrogen content is much higher for agricultural residues, especially grape residues. The contents of extractives and inorganics (ash) are also higher for agricultural residues, whereas the lignin content is lower for straw and higher for the other residues. The ash content has been determined through biomass combustion at a temperature of 850 K for 30 min, holocellulose determined according to the Kurschener-Hoffer method (Soxthec HT2 extraction), and lignin determined according to the Klason method. Particle size is an important parameter in biomass gasification because it determines the bed porosity (not easy to be computed for the highly heterogeneous agricultural residues) and thus the fluid-dynamic characteristics of the bed. However, the effects of this parameter have not been investigated in this study and biomass pretreatments, prior to gasification, have been kept to a minimum. Wood has been cut in parallelepipeds, 5 mm thick (90%, the largest dimension was in the range 5-20 mm), resulting in bed densities of 350 kg/m3, which given a density of beechwood equal to 640 kg/m3, correspond to bed porosities of 0.55. Highdensity 780 kg/m3 straw pellets have been cut into smaller cubic pieces roughly 20 mm thick, which however, partly lose the original consistency and give rise to a packed-bed density of 256 kg/m3. Fines have been eliminated from other agricultural residues, but the resulting material still presents rather small particle sizes. Olive husks consist of stones (about 90%) and fruit skin, pulp, leaves, etc. (the remaining 10%), with roughly spherical particles 1 mm thick and very high bed densities (515 kg/m3). Grape residues are those left after wine production and consist of a highly heterogeneous material (fruit skin and stones and grape branches) with particle sizes of about 3 mm and packed-bed densities of 325 kg/m3. Nutshells present bed densities comparable to wood, but the void fraction of the bed is smaller as they consist mainly of particles 1 mm thick. Finally, though only for grape residues, which present an initial moisture content above 50% (moisture-free (mf) basis) as received, drying prior to gasification is necessary; this pretreatment has also been applied, for comparison purposes, to wood and other residues. Thus, in all cases, the initial moisture content is about 5% (dry basis). Fixed-Bed Pyrolysis of Beechwood. To evaluate the role of the devolatilization stage on the heating value of the producer gas and the dynamic features of beechwood gasification, some pyrolysis tests have been carried out (characteristics as in Table 1, parts A-C). The experimental conditions are those of a packed bed with a diameter of 0.04 m, exposed along its external surface to a uniform and constant radiative heat flux in an inert environment (a continuous nitrogen flow is applied which also makes secondary extra-bed reaction rates negligible). The process temperature, which is associated with the steady value achieved below (2 mm) the heat-exposed surface, Tb, is in the range 580-850 K. As shown in the following, these temperatures correspond to those attained in the upper section of the gasifier, where biomass devolatilization takes place. The yields of the main classes of pyrolysis products, char (the solid residue collected at the completion of the process),

Figure 2. (A) Product yields (% on moisture-free (mf) initial mass) from fixed-bed pyrolysis (bench scale) of beechwood and bedheating rates. (B) Gas yields (% on moisture-free (mf) initial mass) from fixed-bed pyrolysis (bench scale) of beechwood.

liquids (all the condensable organic products and the water formed, collected through a condenser train and weighed), and gas (the light volatile components, determined through sampling and analysis at selected times) have been determined. Their dependence on temperature is shown in Figure 2, parts A and B (yields are expressed as percentages of the initial moisture-free (mf) mass). Figure 2A also reports the average bedheating rates, evaluated by means of the time needed by the bed centerline temperature to achieve the maximum value (dependent on the applied radiation intensity). Product yields show the same qualitative dependence on temperature as already reported for flash pyrolysis conditions,16 with volatiles (tars and gases) increasing at the expense of char, as the reaction conditions are made more severe. However, the temperature dependence is significantly weaker and the char yields higher (with corresponding lower liquid yields). Indeed, the heating rates are much slower (about 0.2-1.5 versus 1000 K/s and above) and the characteristic thermal length, coincident with the bed radius, much larger (20 versus 0.5-3 mm of the fluid-bed particle sizes). Given the existence of intrabed temperature gradients, primary degradation is only weakly affected by the external heating conditions.16 Also, secondary reactions are barely active because of both the relatively low temperatures and the reduced extra-bed residence time of tar vapors. Consequently, product yields show only small variations for temperatures above 650-700 K. Significant wood conversion is achieved for temperatures above 620 K with char yields in the range 45-25% and liquid yields in the range 40-55% and the balance is gas (1520%). This consists essentially of carbon dioxide (512%), carbon monoxide (4-6%), and small quantities of hydrogen, methane, and C2- hydrocarbons. It is worth noting that the amounts of carbon monoxide and carbon dioxide tend to become constant for temperatures above


Figure 3. (A) Temperature-time profiles at several locations along the gasifier axis for beechwood gasification and Wa ) 1.560 kg/h: thermocouple recording (solid lines) and average values (dashed lines). (B) Temperature-time profiles at several locations along the gasifier axis for olive husks gasification and Wa ) 1.560 kg/h: thermocouple recording (solid lines) and average values (dashed lines). (C) Temperature-time profiles at several locations along the gasifier axis for straw pellets gasification (average bed height 0.30 m) and Wa ) 1.560 kg/h: thermocouple recording (solid lines) and average values (dashed lines). (D) Axial temperature profiles (average values) before and after ash discharge (grape residues, Wa ) 1.560 kg/h).

650-700 K, so that the slight increase in the total gas yields for higher temperatures is due to the other components. These are presumably products of lignin degradation, which is displaced toward higher temperatures in comparison with holocelluloseis. Dynamic Behavior. Examples of temperatures measured at several positions along the gasifier axis for an air flow rate of Wa ) 1.560 kg/h are reported in Figure 3, parts A-C, where the solid lines are the thermocouple recordings and the dashed lines the average values. At first glance, it appears that the process becomes successively less regular in the order of beechwood, olive husks, and straw pellets. In particular, in the last case, steady conditions are not achieved and the gasification process is interrupted because of bed transport. In reality, the gasification of beechwood particles always results in a highly reproducible process with a uniform bed structure. The situation is more complicated for agricultural residues and, in particular, for straw pellets which cannot be gasified even at the small scale considered in this study. Thermocouple recordings reflect the cyclic feeding of wood particles (Figure 3A), sometimes associated with bed stirring to prevent biomass agglomeration, caused by vapor condensation in the upper part of the bed. The temperature decrease, measured in correspondence of biomass addition, is mainly due to bed shrinkage with the displacement of colder solids toward the grate. It is worth noting that the amplitude of temperature oscillations is wider at the bottom of the gasifiers (up to 100 K), where the enthalpy variation associated with the conversion process (in particular, combustion) is also higher. However, the average temperature values (dashed lines) are a clear indication of the progressive and regular heating of the bed. Because of forced ignition, a rapid temperature rise is seen in the vicinity of the

grate, while the remaining part and the reactor walls are heated more slowly. Consequently, the first two thermocouples approach steady values in a relatively short time, but about 3 h are needed for the attainment of steady profiles along the whole bed. As temperatures are measured with sheathed thermocouples, the recording is representative of average values between the gas and the solid. However, the prompt variation, recorded when feeding biomass, suggests that measurements are closer to the solid temperature. Also, it is unlikely19 that the gas, fed at ambient conditions, can attain close to the grid temperatures as high as those measured. The gasification of agricultural residues is more complicated for different reasons. Before all, because of the smaller particle sizes, bed transport becomes a serious drawback especially at high flow rates, which may lead to line blockage. Particles transported consist of unreacted biomass and mainly char, which presents a much lower density. Tests carried out with a shorter bed height (0.30 m) do not result in significant improvements because of the fast devolatilization rates of the small biomass particles and the prompt production of light char. The residues behave differently with conditions successively more critical in the order of grape residues (very low transport), nutshells, olive husks, and straw pellets (very high transport), as a consequence of the different sizes (Table 1C) and structural properties of char. Notwithstanding the use of pellets, straw devolatilization rates are very fast and the pellets lose their integrity in a very short time. The resulting char particles present a very weak structure, which is destroyed by fresh biomass addition and bed stirring, so that large amounts of fines are transported out from the bed, thus impeding their gasification. The temperature profiles reported in Figure 3C, obtained with an average bed height of 0.30 m and almost continuous


stirring, are very irregular and associated with very high oscillations in the gas composition. After a few tests without the attainment of steady conditions, this biomass has not been considered for further investigation. It is expected that the use of pellets, which retain the original structure during the devolatilization process, can also allow straw to be gasified. The very high bed densities established by olive husks result in a poor flow distribution with amplitudes in the temperature oscillations up to 200 K (Figure 3B). Nonuniform bed properties are also associated with the heterogeneity of the material (grape residues) and give rise to the formation of bridges and large voids, which have been prevented by periodic bed stirring. However, it has also been observed (nutshells) that the oscillations, due to the cyclic feeding process, tend to disappear at the bottom of the gasifier because of the low void fraction of the bed. Ash agglomeration has been observed in several cases, thus making air distribution during gasification and the subsequent reactor cleaning difficult. Particularly critical in this regard is the behavior of nutshells and olive husks, probably because of the high chlorine and potassium contents in their ashes. For wood and agricultural residues, vapor condensation occurs through the formation of an aqueous phase, as observed downstream of the condensing units, except for grape residues. In this case, a muddy phase also appears. This creates deposits along the condenser tubes and pipelines, which can obstruct gas flow, with consequent interruption of the gasification process. Despite the more difficult behavior in comparison with wood particles, steady gasification conditions have been achieved for grape residues, olive husks, and nutshells and it has been possible to determine their gasification characteristics. An important feature in the dynamic behavior of the gasifier is due to the formation of an ash layer. The amount of ash produced for each test (about 8 kg of biomass gasified) is low for wood and nutshells (about 40 and 70 g, respectively), but the quantities become significant for olive husks (about 200 g) and mainly grape residues (400 g). Therefore ash discharge has been made for the last two biomasses. This operation is usually accomplished when a flattening of the temperature profile at the bottom of the gasifier or a displacement of the combustion front well above the grate are observed. Temperature profiles (average values) are shown in Figure 3D for grape residues (Wa ) 1.560 kg/ h) before and after ash discharge. After ash discharge (about 0.2 kg) a very rapid temperature rise is again observed near the grate and, as expected, a slight reduction in the upper part, due to bed movement. These profiles are considered for the examination of the steady behavior of the gasifier in the next section. An example of the dry gas evolution during beechwood gasification (Wa ) 1.560 kg/h, temperature-time curves reported in Figure 3A) is shown in Figure 4. The dry gas consists, apart from nitrogen, of carbon monoxide, carbon dioxide, hydrogen, methane, and small amounts of C2- hydrocarbons (C2H4 and C2H6). Gas sampling and analysis are usually carried out at median times with respect to biomass feeding, but some oscillations are present in the measurements, associated with temperature variations, actual bed height, and activity of devolatilization reactions. Oscillations disappear as the feeding frequency is increased or steady conditions are approached. After a rapid increase (for

Figure 4. Composition of the dry gas and corresponding heating value as functions of time for beechwood gasification and Wa ) 1.560 kg/h.

a time of about 25 min), concentrations undergo a very slow stage, which leads to constant values. The main difference is between carbon dioxide concentration, which in the second stage decreases, and other species. To explain the gas species behavior, it should be kept in mind that all of them are products of the devolatilization reactions, while the main products of heterogeneous reactions in updraft atmospheric air gasification are essentially carbon dioxide (char combustion) and carbon monoxide (char combustion and gasification). It is plausible that the first very fast stage is essentially due to combustion and devolatilization. Successively, associated with the progressive heating of the bed, the enlargement of the reaction zone and the formation of a char layer, char gasification reactions also become active. Therefore, in the second slow stage, carbon dioxide is consumed by the gasification reactions, with a corresponding increase in the concentration of carbon monoxide and in the heating value of the gas. A positive contribution for the increase in the gas-heating value also comes from methane and hydrogen, indicating that the devolatilization region becomes thicker and/or the average reaction temperature increases. However, the variations in these species are smaller in comparison with carbon monoxide and carbon dioxide. Indeed, pyrolysis species concentrations present only small variations after 120 min, whereas temperatures attain steady values only after 180 min. Hence, the slow heating of the upper part of the bed plays only a secondary role on the gas characteristics, that is, a large part of wood undergoes pyrolysis at bed temperatures that are always higher than the steady values measured close to the feeding section. The dynamics of gas evolution during the gasification of agricultural residues are qualitatively similar to that of wood, but the faster pyrolysis rates of the smaller particles give rise to larger oscillations in the gas composition. This effect can be seen for nutshells and Wa ) 1.950 kg/h (profiles of temperatures reported in Figure 5A and concentrations in Figure 5B, where also data concerning beechwood gasification are enclosed for comparison purposes). On one hand, the prompt devolatilization of biomass particles causes large amounts of pyrolysis gas to be released in a relatively short time with peaks in the carbon dioxide, methane, and hydrogen concentrations. The time profile of carbon monoxide is, on the other hand, more regular because it is for a large part determined by the gasification reactions whose evolution is less influenced by the feeding process. However, though gas composition is affected by the time chosen for sampling and analysis, the deviations in the


Figure 7. Steady axial profiles of temperature (solid lines) and spatial temperature derivative for several air flow rates and beechwood gasification. Table 2. Air-to-Fuel Weight Ratio, R ) Wa/Wb, Specific Throughput, Ws, Size of the Drying/Devolatilization Region, zp, Maximum Devolatilization Temperature, Tp, Bed-Heating Rate during Drying/Devolatilization, h, and Total Liquid Yields, Yc, for Different Air Flow Rates, Wa, and Beechwood Gasification

Figure 5. (A) Temperature-time profiles at several locations along the gasifier axis for nutshells gasification and Wa ) 1.950 kg/h: thermocouple recording (solid lines) and average values (dashed lines). (B) Composition of the dry gas and corresponding heating value as functions of time for nutshells gasification and Wa ) 1.950 kg/h (data of beechwood gasification are also enclosed for comparison purposes).

Figure 6. Temperature-time profiles (average values) at several locations along the gasifier axis for beechwood gasification and Wa ) 1.950 kg/h (solid lines) and Wa ) 1.250 kg/h (dashed lines).

gas-heating value (with respect to the mean value) tend to decay as steady conditions are approached (deviations of about 4% for the conditions of Figure 5B). Consequently the measurements carried out for agricultural residues can still be considered representative of a continuous process. Associated with the increase in the air flow rate, there are two counteracting effects on the temperature profile: (1) the amounts of both biomass burned and heat released increase, thus favoring the attainment of successively higher temperatures, and (2) the amount of biomass and air to be brought at a high temperature also increases, which would lead to lower temperatures. The temperature profiles (average values), shown in Figure 6 for beechwood gasification, clearly show that the first effect predominates, with progressively higher values. Steady Gasification Characteristics. In the countercurrent fixed-bed gasification, temperature profiles are representative of the different processes stratified

Wa [kg/h] R [kg/kg] Ws [kg/(m2 h)] zp [m] Tp [K] h [K/s] Yc [wt %, mf]

1.250 1.267 126 0.31 725 0.43 34.5

1.560 1.131 176 0.29 788 0.74 37.2

1.950 1.112 223 0.29 831 1 42.6

2.340 1.055 282 0.29 852 1.35 52.6

along the reactor axis. In general, overlapping occurs and it is not possible, by means of temperature measurements, to individuate exactly the boundaries between the different zones. Some considerations can, however, be made through the introduction of characteristic process temperatures and the observation of the spatial temperature gradients. Therefore, the steady axial temperature (average values) profiles measured for beechwood gasification and air flow rates in the range 1.250-2.340 kg/h are examined through Figure 7. The shape of the profiles is not significantly affected by the air flow rate, but for values above 1.560 kg/h the rate of increase is significantly reduced, especially near the grate. Indeed, temperatures become sufficiently high for gasification and if, on one hand, the improved thermal conditions and the higher concentration of carbon dioxide produced from combustion enhance the gasification process, on the other hand, the higher endothermicity of this process also tends to keep the temperature low. The space derivative of temperature, also reported in Figure 7 for the two limit values of the air flow rates, allows two main regions to be individuated (apart from the high-temperature variation observed close to the grate). These roughly correspond to combustion/gasification (the first region with increasing values of dT/dz) and devolatilization/drying (the second region with decreasing values of dT/dz). The boundary between the two regions can be assumed coincident with the position where the spatial temperature gradient approaches its maximum, that is, -500/900 K/m (Table 2). This position can be considered indicative of the quenching of the gasification reactions because their high endothermicity is mainly responsible for the temperature decrease. The temperature profile in the second zone is due mainly to preheating, as fixed-bed pyrolysis of wood is a neutral or weakly endothermic process. Indeed, though the pyrolysis of lignocellulosic materials18 can be exothermic (essentially because of lignin decomposition and char


formation) or endothermic (essentially because of holocellulose decomposition and volatile formation), for the heating rates and temperatures of interest in this study, component degradation is observed to take place simultaneously and the enthalpy variations are small. Despite insulation, it is also likely that wall heat losses are not negligible, owing to the small size of the gasifier. The spatial temperature gradients along the first region are higher for the slowest air flow rate because of both the lower convective transport and the lower combustion temperatures. Consequently, the extension of the combustion/gasification region is also slightly smaller (conversely, the size of the drying/devolatilization zone, zp, is larger (Table 2)) compared with the other profiles. Differences are more evident in the temperatures measured at the boundary between the two regions, Tp, with values increasing with the air flow rate from 725 to 852 K (Table 2). According to ref 19, oxygen consumption takes place only along a very thin region, usually 4-5 times the characteristic particle size. Thus, this region corresponds only to the first 0.020-0.025 m of the bed and is characterized by maximum (average) temperatures between 1180 and 1260 K, which for beechwood do not give rise to ash-related problems. Associated with the highly exothermic combustion reactions, the temperature rise near the bottom of the bed is very steep, corresponding to gradients of 5 × 104 K/m, which are of the same order of magnitude as observed in the updraft gasification of anthracite.19 Again, in agreement with this study, the ash temperature at the grate is nearly equal to the ambient temperature. Thus, the heat losses at the bottom of the bed are small and the convective transport, in the opposite direction, is very effective. Char gasification, which takes place essentially through carbon dioxide produced during the combustion stage, is mainly responsible for the rapid temperature decrease, though the boundary temperatures, Tp, are lower than those usually reported for the gasification reactions to become active (about 900 K1). Hence, it is likely that the thin combustion and gasification zones overlap and are positioned close to the grate. At the reactor inlet (x ) 0.55 m), the biomass temperature is at the ambient value, but a rapid rise is observed. In particular, temperatures above 550 K are attained over a distance becoming successively shorter as the air flow rate is increased. Though intraparticle heat-transfer resistances are not negligible (thick particles), this size is always below 0.05 m and it is likely that the small moisture content, for the highest flow rate, evaporates almost instantaneously. The absence of steep gradients in the temperature profiles in the second zone excludes the existence of a well-defined devolatilization front, as would be the case for a sudden decrease in the bed density by a factor of at least 2, deriving from volatile release. The main effect of the air flow rate on the devolatilization process is to cause successively faster heating rates, so that the reaction temperature is displaced toward higher values. Given the regular structure of the bed during beechwood gasification, an average solid residence time for the devolatilization region can be evaluated for a bed density halved with respect to the initial value (Table 1C). On the basis of this assumption, it turns out that the solid heating rates for the drying/devolatilization region vary from about 0.4 to 1.3 K/s (Table 2), which are comparable with those established in the pyrolysis

Figure 8. Composition and heating value of the dry gas at steady conditions as functions of the air flow rate for beechwood gasification.

tests (Figure 2, parts A and B). Therefore, the devolatilization stage of countercurrent updraft gasification of wood takes place under conditions indicated as slow or conventional pyrolysis, where variations in the heating rate exert only a small influence on products distribution. Table 2 also reports the total condensate (tars + water) yields, evaluated by weighing (first and after the tests) the water and ash filters and collecting liquids at the exit of the condensers. Total yields are in the range 34-53% (by weight of the mf wood), values very close to those determined through the pyrolysis tests (Figure 2, parts A and B) and reported for industrialscale updraft wood gasification.20 The energy content of the tars is significant and is completely exploited when the hot raw gas is burned in an external combustion chamber. As an alternative, reinjection of tar/water mixtures in the hot char zone of the gasifier or separate combustion in entrained flow reactors are applied. Given the same liquid yields obtained for the pyrolysis and gasification tests, it can be inferred that the char burned/gasified roughly varies from 50 to 25% (about 15-20% pyrolysis gas). On one hand, the amount of condensed liquid increases with the air flow rate, i.e., the reaction temperature, because the amount of both water and organic compounds (tars) increases.16,21 On the other hand, the water content and chemical composition of the condensate are expected to vary with the reaction conditions (this aspect is currently under investigation). The composition of the gas and the corresponding heating value at steady conditions, measured for beechwood gasification, are reported in Figure 8 as functions of the air flow rate. As the steady temperature profiles and the gas composition are strictly related, the influence of the air flow rate on the gas composition can be derived directly from the corresponding changes in the thermal conditions. At low temperatures, the devolatilization products consist of, in large part, liquids and char (Figure 2A). Also, the temperature attained by the bed is low and hardly sufficient for char gasification; therefore, only a small part of carbon dioxide, produced from char combustion, is actually consumed by char gasification for carbon monoxide production. At these low temperatures, the air-to-fuel weight ratio, R ) Wa/ Wb where Wb is the rate of fuel supply, is high (the average value computed for the whole time of the test is 1.267 with a specific throughput, Ws, of 126 kg/(m2 h) (Table 2)) and the heating value of the gas is low (4.67 MJ/Nm3 or 125.5 Btu/SCF). For the lower limit of a selfsustaining process, the heat developed from char combustion is barely sufficient to preheat the wood particles to cause pyrolysis and drying.


The rate of fuel consumption increases with the air flow rate and the air-to-fuel ratio slightly decreases (to an average value of 1.05 for the highest flow rate (Table 2)). As the bed temperature increases (higher air flow rates), the amount of gas produced from wood pyrolysis also slightly increases (Figure 2B), mainly at the expenses of char, while the gasification process is improved because of both the more favorable thermal conditions and the larger amounts of carbon dioxide, produced from combustion, with an improvement in the heating value of the producer gas. In other words, as the air flow rate is increased, the air-to-fuel ratio decreases because gasification becomes successively more favored. Indeed, the amount of fuel burned also increases and results in higher temperatures, which increase the rate of heat transfer to the fuel making faster before all the rate of char gasification (possible only for temperatures sufficiently high) and also the drying and pyrolysis rates. It is worth noting that the same behavior has also been reported for the fixed-bed combustion of wood.4 In practice, for air flow rates of 1.560 kg/h and above, the gas-heating value only varies in the range 5.355.5 MJ/Nm3 (143.8-147.8 Btu/SCF), while the air-tofuel ratio varies from 1.131 to 1.05 and Ws from 175 to 282 kg/(m2 h). In accordance, the molar gas composition also presents small variations, with 28.6-30% carbon monoxide, 1.6-1.8% methane, 6-7% hydrogen, 7-5.5% carbon dioxide, 0.06-0.11% C2- hydrocarbons, and about 55% nitrogen. The slight decrease in the carbon dioxide concentration, which for the devolatilization stage remains almost constant (Figure 2B), is indicative of further improvement in the gasification efficiency. Because of stratification of the different processes along the reactor axis, for optimal gasification conditions, the carbon dioxide concentration in the gas should only be due to wood devolatilization (according to Figure 2B, this minimum should be about 10-11%, by weight mf wood). It is worth noting that the gas composition is very close to that measured for a larger scale plant (1.5 MW updraft wood gasifier8). Also, the gas-heating value is the same as that reported for industrial-scale reactors.1-3 These findings together with the comparable values of the liquid yields obtained for industrial scale20 and this laboratory-scale plant indicate that the gross energy efficiency (the ratio of the total energy of the hot, raw producer gas divided by the total input energy) is also the same, that is, 60-80%.1 Unfortunately, apart from gas composition and heating value and liquid yields, no other data are available about updraft gasification to be used for comparison purposes. In this study the gasification process is investigated as the air flow rate is varied, but as a consequence of the variations in this parameter, the air-to-fuel weight ratio (or the equivalent ratio) also varies, given a constant bed height. The equivalent ratio, ER, defined as the air-to-fuel weight ratio used, R, divided by the corresponding stoichiometric value, Rs, varies from 0.2 to 0.4 for fluid-bed gasification of biomass.22 For a fixedbed gasifier, slightly lower values, up to 0.19, are reported,20 resulting in a higher quality of the producer gas. For the beechwood gasification tests presented here, the ER varies from 0.23 (for the lowest air flow rate, with R ) 1.267) to 0.19 (for the highest air flow rate, with R ) 1.055) which corresponds to excess air (with respect to the stoichiometric value) varying from -77% to -81%, respectively. Thus, it appears that variations

Figure 9. Heating value of the dry gas at steady conditions as a function of the air-to-fuel weight ratio for beechwood gasification.

Figure 10. Steady axial temperature profiles for beechwood and agricultural residues gasification and Wa ) 1.560 kg/h.

in the air flow rates up to factors of roughly 100% cause only small variations in the ER (or R). However, they still exert a significant influence on the producer gas composition (and heating value) as it appears from Figure 9. Here, data are plotted which include those already discussed and obtained with an average bed height (Lb) of 0.55 m and others obtained for a lower feeding frequency, resulting in shorter average bed heights (0.4 m). The higher the air-to-fuel ratio, the lower the gas-heating value is. There are two different conditions which may result in a high air-to-fuel ratio. The first is that already discussed, observed for low air flow rates. Furthermore, for air feed rates higher than the lower limit, the lower the temperature of the exit gas, the more thermally efficient the process is. Therefore, gasification carried out with a lower bed height causes that part of the heat generated by combustion to be lost, resulting in a higher air-to-fuel ratio and a lower gas quality. For the feeding procedure adopted here, the process may also become discontinuous and the oscillations in both temperature and gas composition very high. A quantitative comparison between the different thermal treatments undergone by wood and agricultural residues, for a chosen air flow rate (Wa ) 1.560 kg/h), can be made through Figure 10, which reports the steady temperature (average values) profiles along the reactor axis. Other process characteristics are reported in Table 3. From the thermal point of view, the behavior of wood and nutshells is very similar, whereas lower temperatures, especially in the upper part of the bed, are measured for the other two residues. This can be attributed to a higher thermal capacity (olive husks), lower effective thermal conductivity (large void fraction, grape residues), and bed displacement due to ash discharge (olive husks and grape residues). Given the less regular structure of the bed and the influence of ash discharge on the temperature profile, it is difficult

2580 Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 Table 3. Air-to-Fuel Weight Ratio, R, Specific Throughput, Ws, and Total Liquid Yields, Yc, for Beechwood and Agricultural Residues Gasification (Wa ) 1.560 kg/h) biomass

beechwood

nutshells

olive husks

grape residues

R [kg/kg] Ws [kg/h m2] Yc [wt %, mf]

1.131 176 37.2

1.159 171 34.8

1.38 144 23.2

1.55 128 12.7

to make correct estimates of the size of the different regions and particle heating rates, though it can be seen that devolatilization takes place at lower temperatures (olive and grape residues). On the average, the air-tofuel ratio is larger and the specific throughput lower for agricultural residues, as a consequence of different physical bed characteristics and char reactivities. In particular, it has been observed that reactivity in the air of grape residue chars is significantly lower with respect to wood and other biomasses.23 The same procedure applied for beechwood has also allowed the condensate yields to be evaluated for agricultural residues. However, owing to some particle transport and the presence of a muddy phase (grape residues), only rough figures are obtained (Table 3). In general, condensate yields are lower for agricultural residues, especially olive husks and grape residues. This is attributed to the higher lignin content, whose degradation produces mainly char, and the presence of ashes, which again catalyze the charring reactions.21 It should be noted that this is a positive aspect in the updraft gasification of residues, as liquid products are undesired effluents. The differences in the gas composition at steady conditions between wood and agricultural residues (Table 4) can be explained on the basis of the temperature profile. Thus, similar temperature profiles for wood and nutshells (Wa ) 1.560 kg/h) result in about the same gas characteristics, whereas lower gas-heating values are obtained for the other two residues with lower concentrations of carbon monoxide and methane. However, as the gas flow rate is increased, the heating value of the producer gas becomes about the same (5.5 MJ/Nm3 (148 Btu/SCF) for wood, nutshells, and olive husks and 5 MJ/Nm3 (134 Btu/SCF) for grape residues). Indeed, only small variations are observed in the species concentrations, with contents of carbon monoxide of 2830%, carbon dioxide of 5.5-7%, and hydrogen of 7-8%. Larger differences are seen for methane with very low values for grape residues (0.75% versus values of 1.61.9 for other biomasses). Also, the amounts of C2 components (not reported) are about the same for all the feedstocks. These findings indicate that optimal gasification conditions for grape and olive residues are established at higher flow rates than than those for wood. There are several reasons for this behavior: (1) the higher bed densities that require higher air flow rates to attain the same temperature conditions (nutshells and olive husks), (2) the different reactivity of the residues, especially in relation to the heterogeneous reactions of char combustion and gasification (low values for grape residues), (3) different chemical com-

position which results in higher air-to-fuel stoichiometric ratios (Table 1A), and (4) the higher char yields generated by the pyrolysis process, as Table 5 shows for bench-scale tests carried out for a surface temperature of 730 K (a wide temperature range has been investigated in ref 24). As the physical behavior of these biomasses becomes progressively more problematic with the air flow rate, adequate pretreatments appear to be necessary for their exploitation on a large scale. Conclusions and Further Developments Countercurrent gasification of wood and agricultural residues (nutshells, olive husks, grape residues, and straw pellets) at laboratory scale has been carried out and some fundamental data have been obtained on process dynamics and gasification characteristics (steady temperature profiles, product yields, and gas composition, not available from the literature). Gasification of wood particles results in a very regular process, but the gasification of agricultural residues is difficult, owing to bed transport (straw pellets, olive husks, and nutshells), nonuniform flow distribution across the highdensity bed (olive husks), partial ash agglomeration (nutshells and olive husks), and presence of a muddy phase in the gasification effluents (grape residues). The very fast pyrolysis rates and the consequent high transport of very small sized char particles have hindered the gasification of straw pellets, whereas it has been possible to achieve steady gasification conditions for the other residues. However, it is believed that extensive pretreatments are required at industrial scale. These include drying, to improve the thermal behavior of the process and to reduce the effects of vapor condensation on particle agglomeration and pipeline blockage, pelletization, to reduce particle transport and bed density (with improvements in the flow distribution), and leaching, to reduce the chlorine and potassium components in the ashes, thus improving their hightemperature behavior. For the optimal conditions achieved by the laboratoryscale plant developed in this study and with pretreatment on biomass maintained at a minimum (essentially predrying and elimination of fines from residues) the molar composition of the producer gas consists of 28-30% CO, 5-7% CO2, 6-8% H2, 1-2% CH4, and minor fractions of C2 species (apart from nitrogen). The gas-heating value is comparable for wood and residues (5-5.5 MJ/Nm3 or 134-148 Btu/SCF) and in the range of values reported for industrial gasification. From the qualitative point of view, the effects of the air flow rate on the gasification process are the same for all the feedstocks. As it is increased, the air-to-fuel ratio decreases with a corresponding increase in the gasheating value. This is due to an improvement in both char gasification, with an increase in the carbon monoxide formation (and a reduction in the carbon dioxide), and biomass devolatilization, with a slight increase in the hydrogen and hydrocarbon production. On the average, for agricultural residues, optimal gasification conditions are displaced toward higher air flow rates,

Table 4. Composition of the Dry Gas for Beechwood and Agricultural Residues biomass

Wa [kg/h]

CO [vol %]

CO2 [vol %]

H2 [vol %]

CH4 [vol %]

HHV [MJ/Nm3]

beechwood nutshells olive husks grape residues

1.560-2.340 1.560-1.950 1.560-1.716 1.560-1.716

28.6-30 28.4-30 26-28.5 26-28

7-5.5 7-6.7 7.5-6.2 8-6

7-7 7.3-6 6.4-8 7-7.7

1.8-1.8 1.7-1.9 1.4-1.6 0.7-0.75

5.35-5.5 5.3-5.6 4.8-5.5 4.6-4.9

Ind. Eng. Chem. Res., Vol. 38, No. 7, 1999 2581 Table 5. Product Yields (% on Moisture-Free (mf) Initial Mass) from Fixed-Bed Pyrolysis (Bench Scale) of Beechwood and Agricultural Residues for Tb ) 730 K biomass

char [wt %]

gas [wt %]

liquids [wt %]


29.00 36.82 34.13 44.84 34.63

16.03 17.91 19.60 17.44 20.10

55.52 41.66 46.01 35.72 41.50

compared to wood, because of both different physical and chemical characteristics of the bed and/or low char reactivity. Temperature profiles indicate that pyrolysis take place over a wide portion of the bed with heating rates corresponding to conventional pyrolysis. Condensate yields roughly correspond to 50% (of initial dry mass) for wood and nutshells whereas olive husks and grape residues present lower values. The combustion/gasification zones overlap and extend only for a limited portion of the bed. Though the present study has allowed some important aspects of the countercurrent gasification of biomass to be clarified and some fundamental data to be measured for model development and validation, further experimental analysis is needed in relation to the effects of feedstock properties, such as particle size and moisture content, and operational variables. In particular, a wider range of air-to-fuel ratios should be investigated with a view of applications for grate-fired combustors, as the proper selection of the underfire to the overfire air for these units is a critical point for process efficiency and reduced emissions. The formation of harmful components and the exploitation of the energy content of the high condensate yields produced in the updraft gasification of biomass are additional topics worthy of future study. Of high importance, especially from the practical point of view, is also the analysis about the feasibility and the possibile synergies of cofiring biomass/coal or biomass/waste. Special emphasis should be given to process simulation which, despite the numerous models proposed for the fixed-bed countercurrent gasification of coals (for instance, the study of ref 19 also provides a comparison with experimental measurements), has not yet applied for wood or biomass. On the basis of the experimental measurements, which show the complexity of the process, the basic conservation equations have been coupled with chemical reactions and efforts are currently underway in this laboratory for further understanding and interpretation of experimental data. Successive applications of model results are planned for process design and optimization. Acknowledgment The research was funded in part by the European Commission in the framework of the Non Nuclear Energy Programme (JOULE III), Contract JOR3-CT950021. Thanks are also due to some undergraduate students of chemical engineering at the University of Napoli “Federico II” for their help in carrying out the tests and to the anonymous reviewers for their useful comments and suggestions about the manuscript. Literature Cited (1) Buekens, A. G.; Bridgwater, A. V.; Ferrero, G. L.; Maniatis, K. Commercial and marketing aspects of gasifiers; EUR 12736; Commission of the European Communities: Bruxelles, Belgium, 1990.

(2) Bridgwater, A. V. The technical and economic feasibility of biomass gasification for power generation. Fuel 1995, 74, 631. (3) Beenackers, A. A. C. M.; Maniatis, K. Gasification technologies for heat and power from biomass. In Proceedings of the 9th European Bioenergy Conference; Chartier, P., Ferrero, G. L., Henius, U. M., Hultberg, J., Sachau, J., Wiinblad, M., Eds.; Pergamon Press: New York, 1996; p 228. (4) Bryden, K. M.; Ragland, K. Numerical modeling of a deep, fixed-bed combustor. Energy Fuels 1996, 10, 269. (5) Larson, L. E. Development of a downdraft wood gasification system for electric utility boiler applications. Energy Biomass Wastes 1989, 12, 805. (6) Delgado, J.; Aznar, M. P.; Corella, J. Biomass gasification with steam in fluidized bed: effectiveness of CaO, Mgo, and CaOMgO for hot raw gas cleaning. Ind. Eng. Chem. Res. 1997, 36, 1535. (7) Reed, T. B.; Markson, M. A predictive model for stratified downdraft gasification of biomass. In Progress in Biomass Conversion; Tillman, D. A., Jahn, E. C., Eds.; Academic Press:: New York, 1983; Vol. 4, p 217. (8) Kurkela, E.; Stahlberg, P.; Simell, P.; Leppalahti, J. Updraft gasification of peat and biomass. Biomass 1989, 19, 37. (9) Kristensen, O. Combined heat and power production based on gasification of straw and wood chips. In Proceedings of the 9th European Bioenergy Conference; Chartier, P., Ferrero, G. L., Henius, U. M., Hultberg, J., Sachau, J., Wiinblad, M., Eds.; Pergamon Press: New York, 1996; p 272. (10) Haavisto, I. Fixed-bed gasification for heat production. In Proceedings of the International Conference on Gasification and Pyrolysis of Biomass; Kaltschmitt, M., Bridgwater, A.-V., Eds.; Cpl Press, Newbury Berkshire, U.K., 1997; p 99. (11) Littlewood, K. Gasification: theory and application; Prog. Energy Combust. Sci. 1977, 3, 35. (12) Prasad, B. V. R. K.; Kuester, J. Process analysis of a dual fluidized bed biomass gasification system. Ind. Eng. Chem. Res. 1988, 27, 304. (13) Herguido, J.; Corella, J.; Gonzalez-Saiz, J. Steam gasification of lignocellulosic residues in a fluidized bed at a small pilot scale. Effect of the type of feedstocks. Ind. Eng. Chem. Res. 1992, 31, 1274. (14) Kayal, T. M.; Chakravarty, M. Mathematical modelling of continuous updraft gasification of bundled jute sticksa low ash content of woody biomass. Biores. Technol. 1994, 49, 61. (15) Kosky, P. G.; Floess, J. K. Global model of countercurrent coal gasifier. Ind. Chem. Eng. Process Des. Dev. 1980, 19, 586. (16) Scott, D. S.; Piskorz, J.; Bergougnou, M. A.; Graham, R.; Overend, R. P. The role of temperature in the fast pyrolysis of cellulose and wood. Ind. Eng. Chem. Res. 1988, 27, 8. (17) Di Blasi, C. Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Ind. Eng. Chem. Res. 1996, 35, 37. (18) Di Blasi, C. Modeling and simulation of combustion processes of charring and non-charring solid fuels. Prog. Energy Combust. Sci. 1993, 19, 71. (19) Goldman, J.; Xieu, D.; Oko, A.; Milne, R.; Essenhigh, R. H. A comparison of prediction and experiments in the gasification of anthracite in air and oxygen enriched/steam mixtures. Proceedings of the Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; p 1365. (20) Esplin, G. J.; Fung, D. P. C.; Hsu, C. C. A comparison of the energy and product distribution from biomass gasifiers. Can. J. Chem. Eng. 1986, 64, 651. (21) Scott, D. S.; Piskorz, J.; Radlein, D. Liquid products from the continuous flash pyrolysis of biomass. Ind. Eng. Process Des. Dev. 1985, 24, 581. (22) Narvaez, I.; Orio, A.; Aznar, M. P.; Corella, J. Biomass gasification with air in an atmospheric bubbling fluidized bed. Effect of six operational variables on the quality of the raw producer gas. Ind. Eng. Chem. Res. 1996, 35, 2110. (23) Di Blasi, C.; Buonanno, F.; Branca, C. Reactivity of some biomass chars in air. Carbon 1999, in press. (24) Di Blasi, C.; Signorelli, G.; Di Russo, C.; Rea, G. Product distribution from pyrolysis of wood and agricultural residues. Ind. Eng. Chem. Res. 1999, in press.

Received for review November 30, 1998 Revised manuscript received March 22, 1999 Accepted April 5, 1999 IE980753I

Countercurrent Fixed-Bed Gasification of Biomass at Laboratory Scale

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