598
Energy & Fuels 2005, 19, 598-605
Pilot-Plant Gasification of Olive Stone: a Technical Assessment A. Go´mez-Barea,* R. Arjona, and P. Ollero Chemical and Environmental Engineering Department, Escuela Superior de Ingenieros (University of Seville), Camino de los Descubrimientos s/n, 41092 Seville, Spain Received July 7, 2004. Revised Manuscript Received November 2, 2004
A 150 kWth bubbling-fluidized-bed gasifier pilot plant was designed to study the gasification performance of different biomasses and blends. This paper presents the results of pilot-plant gasification tests carried out at atmospheric pressure and temperatures within the range of 700820 °C in order to assess the technical viability of gasifying untreated olive stone, also called “orujillo”, a byproduct of the olive oil industry that comprises both olive stone and pulp. Different air flow rates and air-to-biomass ratios were used, giving an equivalence ratio (ER) within the range of 0.17-0.31. A series of parameters, such as gasification efficiency, thermal efficiency, gas yield, overall carbon conversion, gas quality, and composition, were measured as a function of ER. The results indicate that the operation temperature attained in the gasifier (controlled by the air-to-biomass ratio) has a strong impact on these parameters. These results reveal that the gasification process at higher temperatures is better as far as the carbon conversion and gas yield are concerned. However, the low heating value of the gas decreased with increasing ER and, thus, a maximum of the gasification efficiency was found around ER ) 0.26. In addition, the high potassium content and low ash fusion temperature of the orujillo ashes were found to be responsible for the bed agglomeration problems. Thus, two inert materials (sand and ofite) were used, and their impact on fluidization quality was tested. The use of ofite turned out to be critical for a properly fluidized bed operation and for achieving long-term tests.
1. Introduction The use of a renewable energy source such as a residual biomass for heat and power production has become an interesting matter because of a growing concern for future energy supplies and for limiting CO2 emissions. The gasification of solid fuels such as biomass (residual and energetic crops) is one of the more promising technologies for thermochemical conversion.1 This process leads to a fuel gas suitable for combustion in a furnace or for feeding efficient gas engines and gas turbines. Biomass fuels are characterized by high and variable moisture content, low ash content, low density, and fibrous structure. In comparison with other fuels, they are regarded as low quality, despite the low ash content and very low sulfur content.2 Atmospheric air gasification of biomass/waste in a bubbling-fluidized-bed (BFB) reactor is an attractive simple process to convert a solid material to a gaseous fuel. The main advantages of this process are its simplicity and its modest capital costs. However, the industrial use of produced gas is limited because of its LHV and the specific gas-cleaning requirements of each application. In the olive oil mill industry, after drying of the “Orujo” (the direct byproduct from the mills), the * Telephone: +34 95 4487223. Fax: +34 95 4461775. E-mail:
[email protected]. (1) Bridgewater, A. V. Fuel 1995, 74, 631-635. (2) Jurado, F.; Cona, A.; Carpio, J. Modelling of combined cycle power plants using biomass. Renewable Energy 2003, 28, 743-753.
remaining oil still present in this byproduct is extracted with hexane, generating a residual solid product with a moisture content of around 10-12%, which is called “orujillo” or wood matter from pressed oil stone. Table 1 shows the main characteristics of this biomass. Traditionally, this residue is sold as fuel for small boilers and furnaces because of its significant calorific value. Approximately 2 Mt/year of “orujillo” is generated in Spain. The generation of this biomass residue is concentrated in Andalusia, a region of southern Spain with a production of 1.2 Mt/year. The solid residue of numerous small- and medium-capacity olive-oil production facilities of a specific region is processed in a few oil extracting plants to recover the residual oil content. The extremely high production of this biomass residue, its heating value of around 18 800 kJ/kg, and the production structure described above make the use of “orujillo” as a fuel for medium-size power plants (12-20 MWe) possible. Nowadays, there are several boilers fueled with orujillo that generate steam for conventional steam turbines. However, a promising, more efficient alternative is the gasification of the olive residue to produce a gas for a gas engine or even for a gas turbine in a combined cycle. Fluidized-bed gasification is considered to be the most advanced method for thermochemical conversion of various biomass fuels to energy. However, ash-related problems such as sintering, agglomeration deposition, erosion, and corrosion, which are due to the low melting point of ash in the agroresidues, are the main obstacles
10.1021/ef0498418 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/31/2004
Pilot-Plant Gasification of Olive Stone
Energy & Fuels, Vol. 19, No. 2, 2005 599
Table 1. Chemical Characterization of the Olive Waste (Orujillo) parameter
as received
dry basis
HHV (MJ/kg) LHV (MJ/kg)
17.7 16.2
19.1 17.7
Proximate Analysis (wt %) moisture 7.6 ash 5.8 volatile matter 66.9 fixed carbon 19.7 C H2 N2 S Na Si as SiO2 Al as Al2O3 Fe as Fe2O3 Ca as CaO Mg as MgO
6.3 72.4 21.3
Ultimate Analysis (wt %) 50.0 K 1.70 6.5 O2 36.3 0.8 Cl2 2000 (ppm) 0.1 others 4.38 0.02 Ash Analysis (wt %) 18.2 Na as Na2O 2.5 K as K2O 2.3 Mn as MnO2 11.9 P2O5 7.3 others
0.4 36.6 0.1 4.7 16.0
ized beds (CFBs). Specifically, refs 7-9 describe pilotplant tests with a 300 kWth atmospheric CFB fed by leached orujillo as the fuel. Finally, other works give information about orujillo and its gasification viability, but their approaches are quite different because they deal with gasification kinetic determinations.10-12 Despite these publications, none is specifically dedicated to the technical feasibility of BFB gasification of an untreated, i.e., direct-from-mill-industry, orujillo. Aims and Scope. Our objective is to contribute to the study of orujillo as a gasifying fuel not only by showing and discussing the results of the preliminary tests carried out in a recently erected pilot plant but also by describing the problem solving resulting from the gasifying experiences with untreated orujillo. The main aim of this work is, therefore, to assess the technical viability of gasifying untreated orujillo in an air-blown BFB gasifier and also to analyze the preliminary test results shown. 2. Material and Methods
Size Distribution (wt %) size (µm)
wt (%)
size (µm)
wt (%)
4760 2830 1410 1000
94.6 51.8 12 6.2
500 350 250
3.1 2.4 1.9
to an economical and viable application of this conversion method for energy exploitation of the specific residues. Thus, a number of remaining problems need to be solved, especially with regard to ash/bed agglomeration.3 Several papers have been published focusing on this important topic.3-6 These are mainly devoted to the study of leached orujillo because leaching offers an approach to avoid extensive ash slagging, deposition, and bed agglomeration associated with the agroresiduederived biomass. With this technique, a reduction of the K2O content from 4.1% in an initial sample to 2.8% in leached orujillo has been described, i.e., a decrease of 32%. There are also several papers devoted to the study of this residue in pilot plants,7-9 but the regime is usually concerned with fast fluidization, i.e., circulating fluid(3) Visser, H. J. M.; Kiel, J. H. A. Ash/Bed agglomeration in fluidized-bed combustion and gasification. ECN Report ECN-B-01-006. (4) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Effect of leaching on the ash behaviour of olive residue during fluidized bed gasification. Biomass Bioenergy 2002, 22 (1), 55-69. (5) Natarajan, E.; O ¨ hman, M.; Gabra, M.; Nordin, A.; Liliedahl, T.; Rao, A. N. Experimental determination of bed agglomeration tendencies of some common agricultural residues in fluidized bed combustion and gasification. Biomass Bioenergy 1998, 15 (2), 163-169. (6) Arvelakis, S.; Gehrmann, H.; Beckmann, M.; Koukios, E. G. Agglomeration problems during fluidized bed gasification of olive-oil residue: evaluation of fractionation and leaching as pre-treatments. Fuel 2003, 82 (10), 1261-1270. (7) Garcı´a-Iban˜ez, P.; Cabanillas, A.; Sa´nchez, J. M. Gasification of leached orujillo (olive oil waste) in a pilot plant circulating fluidised bed reactor. Preliminary results. Biomass Bioenergy 2004, 27 (2), 183194. (8) Garcı´a-Iban˜ez, P.; Cabanillas, A.; Garcı´a-Ybarra, P. L. A pilot scale circulating fluidised bed plant for orujillo gasification. In Progress in thermochemical biomass conversion; Bridgwater, A. V., Ed.; Blackwell Science Ltd.: Oxford, U.K., 2001; Vol. 1, pp 209-220. (9) Garcı´a-Iban˜ez, P.; Cabanillas, A.; Sa´nchez, J. M. The first tests of leached orujillo on a circulating fluidized-bed gasifier. In Pyrolysis and gasification of biomass and waste; Bridgwater, A. V., Ed.; CPL Press: Newbury, U.K., 2003; pp 477-485.
2.1. Feedstock and Inert Material. Two kinds of orujillo were tested, one from the beginning of the olive harvest time and another from the end, which meant that they were kept for more than 5 months in a storage dump. Their different characteristics (mainly volatile and ash content) affect the plant operation because of the energy content and the ash fusibility, but both types were gasified efficiently and the problems found were similar. Thus, despite each type having shown slightly different results, we will only present one of them. The main characteristics of one of the samples used in the gasification tests are shown in Table 1. As can be seen in the table, the potassium content of the ash is extremely high (36.6% as K2O). In all of the valid tests reported here, the inert bed used was ofite, a subvolcanic rock with an average particle size of 380 µm and a particle density of 2620 kg/m3. The ofite is a silicate with formula (Ca, Mg, Fe, Ti, Al)2(SiAl)2O6. Table 2 presents the ultimate and proximate analyses of the bed inert material used as well as its size distribution. Approximately, 15 kg of ofite was used as the bed inert material during each gasification test. 2.2. Test Facility. Gasification trials were conducted at a pilot-scale BFB reactor designed to process up to 30 kg/h of solid biomass and suitable for combustion and gasification studies. Figure 1 and Table 3 present respectively a schematic diagram and the more-relevant operating parameters and data of the pilot plant. 2.2.1. Feeding System. The biomass is injected at the bottom of the gasifier, where a slightly positive pressure exists. To avoid the backflow of hot gases, the feed system consists of two hoppers with a knife valve between them. There are also two screws: the feeder screw placed immediately after the metering hopper and (10) Ollero, P.; Serrera, A.; Arjona, R.; Alcantarilla, S. The CO2 gasification kinetics of olive waste. Biomass Bioenergy 2003, 24, 151161. (11) Ollero, P.; Serrera, A.; Arjona, R.; Alcantarilla, S. Diffusional effects in TGA gasification experiments for kinetics determination. Fuel 2002, 81, 1989-2000. (12) Go´mez-Barea, A.; Ollero, P.; Arjona, R. Reaction-diffusion model of TGA gasification experiments for estimating diffusional effects. Fuel 2003, submitted for publication.
600
Energy & Fuels, Vol. 19, No. 2, 2005
Go´ mez-Barea et al.
Figure 1. Schematic diagram of the pilot plant. Table 2. Characterization of Ofite Si as SiO2 Al as Al2O3 Fe as Fe2O3 Ca as CaO Mg as MgO
Majority Analysis (wt %) 53.93 Na as Na2O 13.61 K as K2O 9.15 moisture at 105 °C 11.15 weight loss at 750 °C 7.90
3.49 0.48 0.47 0.64
Size Distribution (wt %) size (µm)
wt (%)
size (µm)
wt (%)
2500 1900 1410 1000 500
100 98 74.3 45.6 17.9
250 125 100 62
8.54 5.9 5.47 3.8
Table 3. Technical and Operating Data of the Pilot-Plant Facility bed inside diameter (m) bed height (m) freeboard inside diameter (m) freeboard height (m) fluidization velocity (m/s) bed material fuel feed rate (kg/h) gasification agent operation temperature (°C) operation pressure regime of fludisation maximun thermal capacity (kWth)
0.15 1.7 0.25 2.5 0.8-1.4 ofite 18-35 air 700-820 atmospheric bubbling 150
controlled by a variable-frequency driver and the highspeed water-cooled injection screw. This second screw prevents the pyrolysis of the biomass prior to its injection into the reactor. The fuel flow rate is manipulated by the variable-speed motor of the metering screw and is monitored by a system of three load cells connected to the feed hopper. 2.2.2. Air Supply System. The air for combustion or gasification can be preheated in a 7 kW electrical heater and enters the reactor at the plenum located just under the gas distributor. Air flow rate is measured by a thermal dispersion flowmeter. Also, secondary air can be fed to the reactor in the freeboard. This system could be used for increasing the producer gas temperature in
order to crack tars and improve carbon conversion, but it was not used in the tests described in this paper. 2.2.3. Gasifier. The BFB gasifier consists of a refractory stainless steel reactor AISI 310 (150 mm i.d.), fitted with a distributor plate drilled with 37 holes (1 mm diameter) in a square arrangement with 20 mm triangular pitch. It has a total height of 4.2 m and two sections, the bed zone of 150 mm i.d. and the freeboard of 250 mm i.d. Its design allows an easy conversion to CFB with minor modifications. The system is provided with an isolation blanket, which covers completely both the reactor and freeboard. The BFB reactor operates with forced-draft air, which results in a small but significant positive pressure at the bottom of the bed where the biomass is injected. Temperatures and pressures are measured along the bed and freeboard as shown in Figure 2. The gasifier is equipped with eight thermocouple probes (K type). Five of them (probes TT1-TT5) measure the temperature variations within the reaction section: TT1 monitors the temperature at the lower zone (plenum), and probes TT2-TT5 monitor the rest of the bed. Three thermocouple probes are located in the upper zone or freeboard (from TT6 to TT8). Finally, three pressure taps are located along the side of the reactor (PT1-PT3) to monitor the fluidization conditions of the bed. 2.2.4. Gas-Cleaning System. The gas leaving the freeboard section passes through two cyclones in series to collect entrained particles and through a wet scrubbing system to remove the condensable tars. Char, ash, and inert bed material particles are collected in bins placed under the cyclones and under the system to extract ash from the bed bottom. After each test, the two cyclone bins and the extraction ash bin under the reactor were sampled. The three locations were sampled twice and analyzed for each run. Finally, one temperature probe (TT9) is located after the cyclones and one just before the blast enters the scrubber. This protocol of taking samples allowed us to have further details of the ash properties obtained at different operating conditions. The results obtained will be reported in upcoming papers.
Pilot-Plant Gasification of Olive Stone
Energy & Fuels, Vol. 19, No. 2, 2005 601
Figure 2. Schematic diagram of the facility.
2.2.5. Gas Analysis and Sampling Equipment. The gas sampling point is located downstream from the scrubber. A slipstream of the fuel gas is taken out by an Inconel probe with a filter to remove particles. The sampling line is electrically heated (between 300 and 400 °C) to avoid condensation of organic compounds within the probe. The composition of produced gas is measured continuously (CO, CO2, CH4, H2, and O2) by a Fisher-Rosemount analyzer, which uses a nondispersed infrared method for CO, CO2, and CH4 measurements and thermal conductivity and paramagnetic methods for H2 and O2 measurements, respectively. The tar content was not determined in these tests because the new tar sampling system was under evaluation according to the new standard of EC.13 Finally, the producer gas flow rate is measured by a rotameter. 2.2.6. Filters, Induced-Draft Fan, and Produced Gas Postcombustion Chamber. After leaving the analysis section, the producer gas passes through a tar and particle filter system in series, in order to avoid operational problems in the induced fan and the rotameter located downstream. After being blown, the produced gas enters into a postcombustion chamber, which has been included in the system to avoid environmental problems derived from smells and emissions. The postcombustion chamber is a refractory-lined horizontal cylinder, allowing work at temperatures up to 900 °C. Temperatures are measured with two thermocouples (Pt-Pt/Rh). The heat power of the burner is 85 kWth and was designed to process a nominal flow rate of 40 Nm3/h of produced gas with 100 Nm3/h of air and with 3 Nm3/h of propane as a support fuel. 2.2.7. Data System Acquisition and Plant Control. During normal operation, the biomass input is held
constant and the bed temperature is controlled with a PID controller that manipulates the air flow rate. The temperature of the inlet air is controlled by another PID, which manipulates the power input to the electrical preheater (Figure 2). The signals of all of the measuring instruments are transmitted to the computerized data acquisition system, where they are monitored and registered with a sampling period of 0.5 s. 2.3. Start-Up and Test Procedure. The experimental program started with several preliminary tests carried out with silica sand as the inert bed material. However, no valid data were obtained because of bed agglomeration even at low temperatures, caused by the interaction between the high alkali content ashes and the silica sand. As was mentioned, the high potassium content and low ash fusion temperature of the feedstock caused serious partial defluidization and, in the end, defluidization of the whole bed. Nevertheless, this problem was overcome, and stable operational conditions were achieved by using ofite as the bed material, which prevented a low melting point of ash compared to the tests using sand. At the very beginning of each test run, the material in the bed was heated with the preheated air. After about 1 h of heating, the temperature exceeds 400 °C, and then a small amount of biomass is fed. In this oxidizing atmosphere, the biomass is combusted and the reactor rapidly heated to the desired process temperature. The computer-based data acquisition system is activated to monitor and record the temperature, the pressure drop, and the feed rate values. The transition from combustion to gasification is made by increasing the feeding of biomass and thus by decreasing the ratio between the air and biomass flow rates.
(13) Neeft, J. P. A.; Knoef, H. A. M.; Zielke, U.; Sjo¨stro¨m, K.; Hasler, P.; Simell, P. A.; Dorrington, M. A.; Thomas, L.; Abatzoglou, N.; Deutch, S.; Greil, C.; Buffinga, G. J.; Brage, C.; Suomalainen, M. Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases. Version 3.3. Energy Project ERK6-CT1999-20002 (Tar protocol).
3. Theoretical Basis 3.1. Equivalence Ratio (ER). For a nonexternally heated gasifier, given the type of biomass (orujillo) and
602
Energy & Fuels, Vol. 19, No. 2, 2005
the bed material (ofite), there are only two important operating variables that can be independently varied between a limited range: the biomass throughput and the air flow rate (or superficial gas velocity). These two variables determine ER, defined as the operating airto-fuel mass ratio divided by the air-to-fuel mass ratio for stoichiometric combustion. Moreover, because the pilot plant does not have external heating, the necessary heat for the gasification reactions is merely provided by the combustion of part of the carbon present in the biomass. Thus, the ER ratio is one of the most important parameters in direct (also called autothermal) gasification, because it, in turn, establishes the bed temperature and thus the gasifier performance (gas composition, heating value, carbon conversion, tar content, energetic efficiency, and gas yield). In biomass gasification, ER should be set in the range 0.20-0.40. Low values of ER produce a high tar content gas and are therefore just allowed when the produced gas is going to be used as the fuel gas in a furnace or in a boiler without previously cooling it. On the other hand, high values of ER give higher operating temperatures and lower amounts of tar in the gas but at the cost of reducing the produced gas heating value. 3.2. Material and Energy Computations. Gasification can be represented through the overall chemical equation14
CHROβ + aO2 + bH2O + 3.76aN2 S x1C + x2H2 + x3CO + x4H2O + x5CO2 + x6CH4 + 3.76aN2 where it has been assumed that the gas composition is given by CO, CO2, CH4, H2, H2O, and N2, with the latter being assumed as inert. The subscripts R and β are determined from the ultimate analysis of the biomass feedstock, for example, for dry wood14 CH1.4O0.59 and for dry orujillo CH1.56O0.54 (see Table 1). Note also that the coefficients a and b are known once ER and the humidity of the biomass (and steam if any) are set, respectively. The equation above is just a proposal, and other alternatives may be set.15 In any case, the validity of the proposal should be proven by comparison against experimental tests. The present model has been favorably compared with real operation, and therefore it has been useful for our calculation purposes, as shown in the following. The procedure carried out here has been (1) to compute the mass balances of the four main ingoing elements (carbon, hydrogen, oxygen, and nitrogen), (2) to compute the energy balance over the system, and (3) to impose some restrictions from the measured magnitudes, for example, setting the temperature equal to the measured temperature or the CO-to-H2 ratio also equal to the measured one. Step 3 is necessary for closing the system because the number of unknowns is seven (CO2, CO, H2, CH4, H2O, and N2 flow rates and T), and we only are provided with five equations (C, H, O, and N balances as well as energy balance). Note that, as usual, it has been (14) Wang, Y.; Kinoshita, C. M. Kinetic model of biomass gasification. Sol. Energy 1993, 51, 19-25. (15) de Souza-Santos, M. L. Comprehensive modelling and simulation of fluidized-bed boilers and gasifiers. Fuel 1989, 68, 1507-1521.
Go´ mez-Barea et al. Table 4. Main Operation Conditions and Test Resultsa run number air/orujillo air flow rate (Nm3/h) orujillo flow rate (kg/h) Tbed (°C) Tfreeboard (°C) produced gas flow rate (Nm3/h) % v/vb CH4 % v/vb CO % v/vb CO2 % v/vb H2 ER gas LHV (MJ/Nm3) produced gas flow rate (Nm3/h) (cfb) gas yield (kg/kg) carbon conversion gasification efficiency
1 0.57 20 35 700 530 na
2 0.66 18 27 740 540 na
3 0.74 19 26 750 550 32
4 0.76 18 24 760 600 31
5 0.81 20 25 785 600 na
6 0.98 18 18 780 590 na
7 1.02 18 18 820 650 na
6 14 18 12 0.17 5.21 45.2
6 12 17 11 0.20 4.85 36.8
7 12 20 12 0.22 5.31 37.4
7 9 17 12 0.23 4.93 33.2
7 10 17 8 0.24 4.63 39.2
5 13 16 10 0.29 4.51 31.3
5 14 16 9 0.31 4.52 30.5
1.56 1.62 1.75 1.68 1.90 2.05 2.09 74.3 73.4 82.7 79.2 86.1 85.1 85.7 52.7 52.4 65.0 59.8 60.4 58.6 57.7
a cfb ) calculated from balances; na ) not available. bDry basis.
assumed that O2 is absent in the produced gas and that N2 acts as an inert. Other authors16,17 have used other relationships in order to close the system. For example, a classical and rather accepted way to add one equation is to impose the relationship given by the equilibrium of the water gas shift reaction16 together with further assuming that the amount of methane in the producer gas per kilogram of dry fuel is constant (as is more or less the case of gasifiers under normal operating conditions17). However, in the present work, this closure was not realistic because the water gas shift reaction is still too far from the equilibrium for all of the tests, as is shown in section 4.6. Therefore, step 3 provides the system with two inputs from experiments. Because we have measured in each test the temperature (1), composition (5), and gas flow rate (only available in two tests) of the produced gas, we had various alternatives for providing these two necessary inputs. In practice, we used different inputs for each test (H2, CH4, CO, H2/CO, T, and produced gas flow rate, when available), and we obtained good agreement among them. An example of this agreement can be observed in Table 4 by comparing the calculated values of the produced gas flow rates with the experimental ones. This fact confirmed us that this method was much more appropriate than merely the supposition of equilibrium (see section 4.6). Finally, for the energy balance, the enthalpy of formation of the biomass was calculated by using the measured value of the low heating value and by applying the equation
(
0 ∆H f,bio ) LHV + xC
44 18 0 0 ∆H f,CO + xH ∆H f,H 2 2O(v) 12 1
)
(
)
where LHV (kJ/kg) is the low heating value of the 0 0 biomass, ∆H f,CO (kJ/kg), and ∆H f,H (kJ/kg) are the 2 2O(v) standard enthalpies of formation of CO2 and steam, respectively, evaluated at 298 K, and xC and xH are the (16) Schla¨pfter, P.; Tobler, J. Theoretischen und Praktische Untersuchungen uber den Betrieb von Motorfahrzeuge mit Holzgas, Bern, Switzerland, 1937. (17) FAO Forestry Department. Wood gas as engine fuel. Forest Industries Division. http://www.fao.org/DOCREP/T0512E/T0512e00.htm (1986).
Pilot-Plant Gasification of Olive Stone
Energy & Fuels, Vol. 19, No. 2, 2005 603
Figure 4. Bed temperature as a function of ER. Figure 3. Responses of the main measured variables in a typical test.
mass fractions of hydrogen and carbon of the dry fuel. The typical equivalent chemical formula of the dried orujillo is CH1.56O0.54, and its enthalpy of formation (at 298 K and 0.1 MPa) is -146.085 kJ/kmol, which is similar to the values reported for other biomasses.17 4. Results and Discussion In the tests presented in this paper, the effect of ER was analyzed using two air flow rates (18 and 20 Nm3/ h) and several fuel feeding rates ranging from 15 to 30 kg/h, which establishes an air-to-biomass mass ratio within the range 0.57-1.02 (Nm3 of air/kg of biomass). With these air and biomass flow rates, ER ranges from 0.17 to 0.31, which falls within the typical range analyzed by other authors with other biomasses.14-18 This arrangement gives a weight hourly space velocity, expressed as the ratio of the biomass mass flow rate to the weight of the fluidized-bed material, which is kept within the range 0.5-1, also typical of these units.19 Figure 3 shows the temporal evolution of the variables in a valid test. In this case, an initial transitory period of 4 h is followed by a steady-state period of 5 h, which ends with the shutdown of the gasifier. The narrow peaks that appear in each curve at the same instant of time correspond to a short depressurization due to the opening of the upper bin for biomass feeding. Only the operating pressure increases with time because of the progressive clogging of the gas filter located at the mouth of the induced-draft fan. Table 4 shows the main operating conditions and results corresponding to the seven tests that we considered valid based on the stationary input and output variables. Figures 4-9 and Table 4 show the main results of this work. The data included in these figures and table were obtained directly from the experimental tests or indirectly with mass and energy balances based on the experimental results, as was discussed in section 3.2. Figures 5-9 show the effect of ER on the gasification process, namely, on the gas composition (Figure 5), on the LHV of the produced gas (Figure 6), on the gas yield (18) Narva´ez, I.; Orı´o, A.; Corella, J.; Aznar, M. P. Biomass gasification with air in a Fluidized bed: Effect of the in-bed use of dolomite under different operation conditions. Ind. Eng. Chem. Res. 1999, 38 (7), 4226-4235. (19) Rapagna`, S.; Jand, N.; Kiennemann, A.; Foscolo, P. U. Steamgasification of biomass in a fluidized-bed of olivine particles. Biomass Bioenergy 2000, 19, 187-197.
Figure 5. Gas composition as a function of ER.
Figure 6. LHV of the produced gas as a function of ER.
Figure 7. Gas yield as a function of ER.
(Figure 7), on carbon conversion (Figure 8), and on the gasification efficiency (Figure 9). A similar picture could be obtained by representing the above parameters as a
604
Energy & Fuels, Vol. 19, No. 2, 2005
Figure 8. Carbon conversion as a function of ER.
Figure 9. Gasification efficiency as a function of ER.
function of the bed temperature because, as Figure 4 suggests, the bed temperature and ER are related linearly. 4.1. Gas Composition. The effect of ER on the gas composition of the gas produced is displayed in Figure 5. The results show that ER significantly affects the gas composition. Generally, the reported tendencies22 show that increasing ER decreases the concentration of the fuel components (CO, H2, and CH4) and increases the CO2 concentration. However, Figure 5 shows a transfer between the normal CO and CO2 mole fraction behavior because the former increases at the range 0.24-0.31 after having a minimum at 0.24, whereas the latter decreases for the whole range of ER analyzed. This result was possibly due to the low freeboard temperature in these tests, resulting in a drastic increase in the tar yield. At such low temperatures, many researchers experienced lower mole fractions of combustible components and increased tar yield.21-24 Another reason could be the fact that too few tests were carried out to establish clearer tendencies. 4.2. LHV. In Figure 6, the lower heating value of the produced gas is shown as a function of ER. As expected, both the dilution of the gas by the nitrogen contained (20) Olivares, L. A.; Barbosa, E.; Silva, C.; Glauco, A. Bauen. Preliminary tests with a sugarcane bagasse fueled fluidized-bed air gasifier. Energy Convers. Manage. 1999, 40 (2), 205-214. (21) Mansaray, K. G.; Ghaly, A. E.; Al-Taweel, A. M.; Hamdullahpur, F.; Ugurasal, V. I. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy 1999, 17 (4), 315-332. (22) Narva´ez, I.; Orı´o, 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 produced raw gas under different operation conditions. Ind. Eng. Chem. Res. 1996, 35 (7), 21102120
Go´ mez-Barea et al.
in the air used as a gasification agent and the enhancement of the homogeneous water gas shift reaction, which is exothermic, allow the heating value to decrease as ER increases. Consequently, the highest value (5.21 MJ/Nm3) is reached for the lowest ER (0.17) in the interval studied for ER. 4.3. Gas Yield. The produced gas yield expressed as the ratio between the producer gas flow rate and the biomass flow rate is shown in Figure 7. In this figure, the gas yield increases linearly with ER. This finding coincides with the results published by other authors.7,22,25-29 This behavior could be due to the greater production of gas in the initial biomass devolatilization, which is more rapid at higher ER, i.e., at higher temperatures, and also due to the enhancement of the char gasification reactions, which are endothermic. Moreover, as the temperature increases, the tar cracking and steam reforming reactions are favored with the resulting decrease of the same, and the gas yield increases.30 Narva´ez et al.22 reported the decomposition of tar by secondary reactions to be accompanied by a proportional increase in gas yields. From these experimental tests, the gas yield varies from 1.56 at an ER of 0.17 to 2.09 at an ER of 0.31. These values are similar to those reported in refs 20 and 29. The high gas yield of the orujillo obtained in this study can be explained by a possible catalytic effect of the ash; i.e., its high K2O content (see Table 1) enhances the char-steam reaction. Similar conclusions have been reported for the husk rice in ref 30. 4.4. Carbon Conversion. The carbon conversion, defined as the degree to which the carbon in the fuel has been converted into gaseous products, is presented in Figure 8. Carbon conversion is an important parameter in any biomass conversion process.30 Achieving a high carbon conversion is important for the overall plant efficiency and, consequently, for economical operation. At the ER range of 0.17-0.31 studied, the carbon conversion varied between 73.4 and 86.1%, which agrees with those reported in refs 23 and 30-33. Carbon (23) Corella, J.; Herguido, J.; Gonzalez-Saiz, J. Steam gasification of biomass in fluidized bed-effect of the type of feedstock. In Pyrolysis and gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London, 1989; pp 618-623. (24) Liinanki, L.; Lindman, N.; Sjoberg, S. O.; Ekstrom, C. Methane yield from gasification at high temperature and pressure. In Fundamentals of thermochemical biomass conversion; Overend, R. P., Milne, T. A., Mudge, K. L., Eds.; Elsevier Applied Science: London, 1985; pp 923-936. (25) Ekstrom, C.; Lindman, N.; Petterson, R. Catalytic conversion of tars, carbon black and methane from pyrolysis/gasification of biomass. In Fundamentals of thermochemical biomass conversion; Overend, R. P., Milne T. A., Mudge, K. L., Eds.; Elsevier Applied Science: London, 1985; pp 601-618. (26) Kurkela, E.; Ståhlberg, P. Air gasification of peat, wood and brown coal in pressurised fluidized bed Reactor. I. Carbon conversion, gas yield, and tar formation. Fuel Process. Technol. 1992, 31 (1), 1-21. (27) Gil, J.; Corella, J.; Aznar, M. P.; Caballero, M. A. Biomass gasification in atmospheric and bubbling fluidized bed: effect of the type of gasifying agent on the product distribution. Biomass Bioenergy 1999, 17 (5), 389-403. (28) Pinto, F.; Franco, C.; Andre´, R. N.; Gulyurtlu, I.; Cabrita, I. Gasification of waste materials and their co-processing with biomass. In 39th IEA Fluidised Bed Conversion: Fluidised Processing of Unconventional Fuels; Cabanillas, A., Miccio, M., Eds.; Ciemat: Madrid, 1999; pp 123-135. (29) Schoeters, J.; Maniatis, K.; Buekens, A. The fluidized bed gasification of biomass: Experimental studies on bench scale reactor. Biomass 1989, 19 (1), 129-143. (30) Mansaray, K. G.; Ghaly, A. E.; Al-Taweel, A. M.; Hamdullahpur, F.; Ugurasal, V. I. Air gasification of rice husk in a dual distributor type fluidized bed gasifier. Biomass Bioenergy 1999, 17 (4), 315-332.
Pilot-Plant Gasification of Olive Stone
Energy & Fuels, Vol. 19, No. 2, 2005 605
conversion in full-scale plants will be improved to a certain extent by increasing the freeboard height and the residence time of the particles.31 However, conversions close to 100% can be achieved only by efficient recycling of all separated particulates.31,34 4.5. Gasification Efficiency. The gasification efficiency (cold) defined as the ratio of the heat of combustion of the produced gas to the heat of combustion of the feedstock is presented as a function of ER in Figure 9. The gasification efficiency of the product gas was situated between 50 and 65%, in the range of ER studied. The low efficiency found in the tests can be explained by the dimensions of the gasifier,29 which cause the gasification process to be thermally inefficient as a result of the heat losses of the test unit. It could also be possible that the low temperatures make the gasification process less efficient. Additionally, the fitted trend line displayed over the experimental values shows a clear maximum at a ER value of around 0.26. Indeed, this is a common feature of the gasification efficiency because the gas yield increases as ER rises while the LHV decreases with ER.7,29 4.6. Comparison with Equilibrium Prediction. It is well-known that the introduction of the water-gas equilibrium concept provides the opportunity to calculate the gas composition theoretically from a gasifier that has attained equilibrium at a given temperature. This approach for gas composition predictions has successfully been used.17,35-37 However, a high freeboard temperature is usually required in order for this attractive approach to be applied because this leads to higher reaction rates and a shift in the equilibrium value of some gasification reactions, e.g., the homogeneous water gas shift reaction.29 For the tests carried out in this work, the low temperatures reached at the gasifier were insufficient for the reaction to attain equilibrium. We assessed this by calculating the ratio
Kcal ) pCO2pH2/pCOpH2O
(1)
and by comparing this with the equilibrium constant at the given test temperature. Figure 10 shows these differences for all of the tests presented in this work. In this figure, the equilibrium constant was computed with the expression15 (31) Maniatis, K.; Bridgwater, A. V.; Buekens, A. Fluidized bed gasification of wood. In Research in thermochemical biomass conversion; Bridgwater, A. W., Kluester, J. L., Eds.; Elsevier Applied Science: London, 1988; pp 1094-1105. (32) Maniatis, K.; Bridgwater, A. V.; Buekens, A. Fluidized bed gasification of wood: Performance of a demonstration plant. In Pyrolysis and Gasification; Ferrero, G. L., Maniatis, K., Buekens, A., Bridgwater, A. V., Eds.; Elsevier Applied Science: London, 1989; pp 274-281. (33) Schiefelbein, G. F. Biomass thermal gasification research. Biomass 1989, 19 (1), 145-159. (34) Boateng, A. A.; Walawender, W. P.; Fan, L. T.; Chee, C. S. Fluidized bed steam gasification of rice hull. Bioresour. Technol. 1992, 40 (2), 235-239. (35) Li, X. T.; Grace, J. R.; Watkinson, A. P.; Lim, C. J.; Ergudenler, A. E. Equilibrium modeling of gasification: a free energy minimization approach and its application to a circulating fluidized bed coal gasifier. Fuel 2001, 80, 195-207. (36) Watkinson, A. P.; Lucas, J. P.; Lim, C. J. A prediction of performance of commercial coal gasifiers. Fuel 1991, 70, 519-527. (37) Li, X. T.; Grace, J. R.; Lim, C. J.; Watkinson, A. P.; Chen, H. P.; Kim, J. R. Biomass gasification in a circulating fluidized bed. Biomass Bioenergy 2004, 26, 171-193. (38) Zainal, Z. A.; Ali, R.; Lean, C. H.; Seetharamu, K. N. Prediction of performance of a downdraft gasifier using equilibrium modeling for different biomass materials. Energy Convers. Manage. 2001, 42, 14991515.
Figure 10. Equilibrium constant, Keq (eq 2), and calculated ratio, Kcal (eq 1), as a function of the temperature.
Keq ) 0.0265 exp(3958/T)
(2)
In all of the tests, the experimental value calculated with eq 1 was well under the equilibrium constant value calculated with eq 2. More specifically, the values of Kcal were approximately between 45 and 80% of the equilibrium values Keq, being more similar at higher temperatures. This trend is coherent because the calculated ratio, Kcal, and the equilibrium constant, Keq, become respectively higher and lower as the temperatures increases, which is coherent with trends found in the literature.29 5. Conclusions The test series presented in this work demonstrated the technical viability of gasifying untreated olive stone in an air-blown BFB gasifier. The BFB test rig operated quite well, and it was possible to carry out long-term gasification experiments using untreated olive stone as the fuel. The well-known ash-related problems of olive stone, such as sintering and agglomeration deposition, were overcome by using ofite as an inert bed material together with a correct operation of the test rig. The influence of ER was studied through its effect on a series of parameters, such as the gasification efficiency, thermal efficiency, gas yield, carbon conversion, and gas composition. ER ranged between 0.17 and 0.31, which correspond to bed temperatures ranging between 700 and 820 °C. The results showed quite clear general trends and agreed well with those found in the literature for similar arrangements and operating conditions. This study reveals that the gasification process at higher temperatures is better as far as the carbon conversion and gas yield are concerned. However, the heating value of the gas decreased with increasing ER and, thus, a maximum of the gasification efficiency was found. Further tests could be useful for confirming the optimum thermal efficiency of the process in a broader range of operating conditions. Acknowledgment. The authors are thankful for the financial support of the European Commission and the Commission of Science and Technology (CICYT) of Spain. Also they express their appreciation to Ernesto Garcı´a and Antonio Cresis for their assistance in performing the experimental work. EF0498418
