Gasification of Olive Tree Pruning in Fluidized ... - ACS Publications

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Gasification of Olive Tree Pruning in Fluidized Bed: Experiments in a Laboratory-Scale Plant and Scale-up to Industrial Operation Susanna Nilsson,a Alberto Gómez-Barea,*,a Diego Fuentes-Cano,a Pedro Haro,a and Guadalupe Pinna-Hernándezb a

Chemical and Environmental Engineering Department Escuela Técnica Superior de Ingeniería (University of Seville) Camino de los Descubrimientos s/n, 41092 Seville, Spain b Foundation Advanced Technological Centre for Renewable Energy (CTAER) Paraje los Retamares S/N. 04200 Tabernas, Almería, Spain ABSTRACT: Olive tree pruning was gasified with air in a laboratory fluidized bed (FB) reactor at 800, 850, and 900 °C and equivalence ratios (ERs) ranging from 0.12 to 0.35. A few additional tests were performed varying the fuel particle size, biomass feed rate, and oxygen enrichment in the air. The composition of the product gas was determined by measuring the light gas, water vapor, tar, and some inorganic contaminants. The solids produced were characterized by sampling from the cyclone and bed, providing approximate information about the char elutriation rate and residence time. The lower heating value of the gas, LHV (excluding benzene and tars), varied between 4.5 and 7.8 MJ/(Nm3) using air, whereas it increased to 9.3 MJ/(Nm3) using enriched air with 40% O2.. Carbon conversion increased with temperature (so did gasification efficiency), reaching 97% at 900 °C, indicating almost complete fuel conversion. Analysis of the results with the assistance of a previously developed FB gasification model indicated that most of the tests were carried out under allothermal conditions (with significant heat added to or removed from the gasifier) and only a few tests were representative of autothermal conditions, i.e., the mode of operation of industrial air-blown FB gasifiers (without heat addition and with small heat losses). The model was also used to scale-up the laboratory-scale results to predict the gas composition of industrial-scale FB gasifiers.

1. INTRODUCTION Olive tree pruning (OTP) is an important biomass resource in Mediterranean countries. Despite having a heating value similar to wood, OTP is still mass-burnt in situ in many farms due to the high costs of collecting and transporting it to large-scale utility plants. A great effort has been made to minimize the cost of collection/transportation, but feasible solutions are still missing. Air-blown moving or fluidized bed gasification (FBG) are relatively simple technologies that could produce high electricity efficiency at small to medium scale by burning the gas, previously cleaned of particles, tars, and inorganics, in an internal combustion engine. Downdraft moving bed gasifiers are difficult to control for heterogeneous fuels such as OTP, containing particles with different sizes and density (branches and leaves), whereas fluidized bed gasification allows flexible operation for this type of fuel. High gasification temperatures are required to produce a gas with low tar content and to achieve high fuel conversion,1 but the operating temperature in an FBG is limited by the melting of the ash. There is some previous experience in the gasification of OTP using steam in a dual fluidized bed at 800 °C2 and air in a fixed bed.3,4 However, to assess the potential of using this fuel in stand-alone autothermal air-blown FBGs, experimental data of its conversion at different temperatures and varying equivalence ratio (ER) are needed. Gasification of different types of biomass and wastes has been studied in FB. A great deal of experience has been acquired using air in laboratory or pilot gasifiers under allothermal conditions,5-8 i.e., providing heat to the gasifier during the © XXXX

operation, by preheating the air or using a furnace surrounding the gasifier, to maintain the operating temperature in small reactors. Some studies have tested the effect of O2 enrichment in the air.9−11 The importance of carrying out tests at pilot scale under near adiabatic conditions to mimic large-scale operation has been highlighted.6,12,13 Some studies have been conducted to achieve autothermal gasification with air, but the small size of the reactors used has prevented operation near adiabatic conditions. Arena and Di Gregorio studied the gasification of solid recovered fuel14 and industrial plastic wastes15 in an autothermal pilot plant gasifier. Although they turned off the oven after the start-up period, operation near adiabatic conditions was not achieved as evidenced by the low temperature of the gas at the freeboard exit. van der Drift et al.12 gasified 10 different types of biomass residues in a 500 kWth CFB gasifier and pointed out that even after the efforts toward minimization of heat losses (increasing the insulation and installing electrically heated walls), they were still losing about 12% of the energy input. Ocampo et al.16 calculated 20% heat losses (based on the heating value of the fuel) in a pilot plant gasifier operating with coal. Campoy et al. conducted some gasification tests with a carefully controlled heating system to mimic autothermal operation,8,11,17 but the analysis of results evidenced that this was not completely achieved.18 Received: August 13, 2016 Revised: October 16, 2016

A

DOI: 10.1021/acs.energyfuels.6b02039 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

Figure 1. Experimental setup.

It is concluded that laboratory or pilot data for scaling-up FBG should be obtained from nearly adiabatic operation, but experimental limitations prevent laboratory/pilot devices from this mode of operation due to the high heat losses during long tests or to the heat provided by a heating element. Conducting allothermal tests is adequate to achieve steady operation in laboratory or pilot reactors because the tests are under controlled conditions, but the results have to be properly interpreted and scaled-up. In spite of this evidence, there are no works showing systematically how to scale-up the laboratory/ pilot results to predict the gas properties under industrial operation. In this work, the gasification of OTP has been carried out allothermally in a small FBG varying the temperature, ER, fuel particle size, biomass flow rate, and oxygen content in the air. Both gas and solid properties were measured to characterize the performance. Then the analysis of the experimental data with a model was conducted to calculate the heat losses in each test, to identify the scalability of the results.

Table 1. Proximate and Elemental Composition of Olive Tree Pruning of Size 0.5−4 mm analysis method LHV (MJ/kg; dry basis) moisture (%; as received) % weight, dry basis ash fixed carbon volatiles C H N O a

18.37±0.04 6.72±0.04

UNE-164001 CEN/TS 14774-1

4.54±0.15 15.86 79.60±0.10 48.84±0.11 6.40±0.09 1.07±0.03 39.15

CEN/TS a CEN/TS CEN/TS CEN/TS CEN/TS a

14775 15148 11510 11510 11510

Calculated by difference

chlorine (below the detection limit) and sulfur (0.05−0.07 mass %, dry basis).4,19 Since the fuel contains a large fraction of small branches and leaves, the ash content is higher than values typically found for wood chips or wood pellets that have an ash content of up to 1%.5,7,8,20,21 The ash composition was not measured in this work, but values measured for a very similar OTP19 are shown in Table 2. To enable stable operation of the biomass feeder, the OTP was ground to a size below 4 mm. Particles with size below 0.5 mm were eliminated, to avoid excessive entrainment of fines from the bed. Most

2. MATERIAL AND METHODS 2.1. Experimental Setup. The experimental setup is represented in Figure 1. The FB reactor consists of a 51 mm bottom bed and an 82 mm freeboard. The reactor is surrounded by an electrical oven that allows control of the temperature in both the bed and freeboard zones. Further details about the reactor can be found elsewhere.19 The biomass feeder consists of a fuel hopper, a feed screw, and a fast rotating screw that introduces the fuel into the lower part of the fluidized bed. The fuel feed rate was adjusted by the rotation speed of the feed screw. The fuel hopper was pressurized to avoid backflow of hot gases from the reactor. The gas feeding system can supply mixtures of air, N2, and O2 with adjustable composition by using three mass flow controllers. At the exit of the reactor there is a cyclone for collecting entrained particles. After the cyclone the gas passes through three scrubbers and a condenser operating at −20 °C to eliminate tars and other contaminants before the gas analyzer. 2.2. Material. The fuel employed was OTP, which is a heterogeneous fuel containing both branches and leaves from olive trees. The proximate and elemental composition of the OTP employed is given in Table 1. During the elemental analysis the contents of C, H, and N were measured, but not those of S and Cl. Typically OTP analyses have shown very low concentration of both

Table 2. Analysis of OTP Ash19

a

B

species

wt % in asha

Fe2O3 MnO2 MgO K2O CaO Na2O SO3 P2O5 Al2O3 SiO2 TiO2

0.90 0.64 3.16 11.54 38.15 0.14 n.d 2.65 1.79 26.38 n.d

n.d., not detected DOI: 10.1021/acs.energyfuels.6b02039 Energy Fuels XXXX, XXX, XXX−XXX

a

C

11.75 50.82 3.85 14.57 15.76 1.64 0.20 0.11 0.24 0.16

8.78

25.10 0.59 3.61 0.99 0.04 22.39 0.35

0.103 40

H2 N2 CH4 CO CO2 C2H4 C2H6 C2H2 C3 C6H6 NH3 HCN·1000 HCl·1000 H2O (wet basis)

gravimetric tar tar class 2 tar class 3 tar class 4 tar class 5 non-GC-MS tarc u (freeboard), m/s

elutriation rate, g/g C in elutriated mat., wt % carbon holdup (wt % C bed)

0.117 42 2.32

0.37

0.043 0.016 n.d

12.56 49.88 3.6 15.49 15.51 1.65 0.20 0.10 0.24

800 0.24 0.5-4 1 air 0.56

2

0.084 28 0.35

9.99 0.85 6.43 1.21 0.03 5.87 0.41

9.97 52.91 3.52 11.97 16.57 1.68 0.18 0.11 0.26 0.26 0.401 0.078 n.d 11.80

800 0.29 0.5-4 1 air 0.69

3

0.128 34 0.85

9.61 0.23 5.79 1.55 0.05 6.17 0.38

14.3 46.83 3.59 16.13 14.31 1.51 0.11 0.07 0.09 0.43 0.053 0.036 0.043 10.17

850 0.24 0.5-4 1 air 0.58

4

0.116 22 0.20

8.3 0.3 5.71 0.95 0.02 5.29 0.43

11.48 51.67 3.64 13.67 15.23 1.58 0.11 0.07 0.11 0.36 0.203 0.418 0.001 7.67

850 0.29 0.5-4 1 air 0.72

5

7

8

9

10

Operating Conditions 850 900 900 900 900 0.35 0.17 0.24 0.24 0.29 0.5-4 0.5-4 0.5-4 0.5-4 0.5-4 1 1 1 1 1 air air air air air 0.86 0.46 0.61 0.61 0.75 Product Gas Composition, vol % (Dry Basis) 8.75 17.56 14.31 14.42 11.16 56.07 38.39 44.63 45.53 50.50 3.21 4.85 4.21 4.22 3.89 11.17 23.68 19.52 19.32 14.50 16.69 11.31 13.53 13.28 14.74 1.4 1.8 1.71 1.63 1.72 0.09 0.06 0.08 0.05 0.04 0.05 0.08 0.10 0.09 0.09 0.12 0.02 0.07 0.03 0.03 0.20 0.42 0.35 0.130 0.074 0.135 0.119 0.084 0.358 n.d 0.041 n.d 8.07 5.98 9.68 Tar, g/(Nm3) (Dry Gas) 6.23 9.11 8.19 0.16 0.13 0.09 5.09 3.88 3.65 1.10 1.58 1.23 0.02 0.04 0.03 3.45 6.11 5.57 0.47 0.34 0.40 0.40 0.43 Solid Streams 0.096 0.043 0.176 0.214 0.055 21 23 24 24 19 0.28 0.45 0.30 0.15

6

e.a., O2-enriched air containing 40 vol % O2. bd.a., N2-diluted air containing 13 vol % O2. cEstimated (see text).

800 0.24 0.5-4 1 air 0.56

1

temperature, °C ER fuel particle size, mm biomass feed rate, kg/h fluidizing agent u (bed), m/s

test

Table 3. List of Gasification Experiments Including Operating Conditions and Some Results

0.103 20

8.84 0.19 4.93 2.06 0.09 4.89 0.46

9.31 55.67 3.32 12.65 15.61 1.37 0.04 0.08 0.03 0.44 0.029 0.039 n.d 17.93

900 0.35 0.5-4 1 air 0.90

11

0.071 24

0.35

11.62 51.01 3.11 12.19 15.72 1.56 0.19 0.09 0.25

800 0.24 2.5-4 1 air 0.56

12

0.027 26 0.36

0.43

11.06 51.84 3.52 13.48 15.68 1.72 0.12 0.08 0.10

850 0.29 2.5-4 1 air 0.72

13

0.106 30

0.52

10.53 50.92 3.83 13.51 16.34 1.69 0.21 0.11 0.30

800 0.23 0.5-4 1.5 air 0.82

14

0.138 22 0.28

0.68

9.34 51.03 3.88 14.04 15.64 1.91 0.06 0.10 0.05

900 0.29 0.5-4 1.5 air 1.12

15

0.059 34

0.27

17.22 27.97 6.44 22.28 20.80 2.87 0.34 0.17 0.35

800 0.23 0.5-4 1 e.a.a 0.30

16

0.041 25 0.63

0.29

15.44 31.16 6.52 21.39 21.65 2.62 0.18 0.16

850 0.29 0.5-4 1 e.a. 0.38

17

0.033 9

0.32

13.65 33.67 5.58 19.12 23.26 2.41 0.09 0.14

900 0.33 0.5-4 1 e.a. 0.48

18

0.042 33

0.36

15.46 43.09 4.98 20.07 11.90 2.40 0.17 0.18 0.14

850 0.12 0.5-4 1 d.a.b 0.46

19

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temperature it is assumed to contain a larger fraction of more heavy PAH compounds (that are too heavy to be detected in GC-MS analysis).6 In any case, the calculated yield of GC-MS undetectable tar is just an approximation, since it is not known exactly to what extent light aromatics are present in the gravimetric tar sample, or evaporated together with the solvent.6 In addition, the gravimetric tar can contain other components different from tar, for example ammonium chloride and other inorganic salts that precipitate during tar and water condensation.23 For sampling of inorganic contaminants, the gas passed through a series of impingers kept at 0°C containing different aqueous solutions. H2SO4 solution was employed for capturing NH3, NaOH for HCN and H2S, and distilled water for HCl. 2.5. Experimental Procedure. First the reactor was heated up to a temperature 100−200 °C below the operating temperature set for the gasification experiment. Then the flow of air was adjusted to give the ER selected for the test, and the feeding of biomass was switched on. When the fuel reached the reactor, the temperature increased rapidly; the setpoint in the oven was then modified to equal the test temperature, and within approximately 10 min, stable temperature was reached. Constant reactor temperature was then maintained during the remaining test time. At this point the sampling of gas and solids commenced. Samples of the solids collected in the cyclone and samples for measuring the light gas composition were taken approximately every 10 min. The values of light gas composition given here are average values of the different samples taken during each test. For each experiment samples for measuring tar and H2O were taken twice, while inorganic contaminants were sampled only once and not for all the tests (see Table 3). The test duration at steady state was between 2 and 3 h, and at the end of the experiments, the air flow was switched to N2 and the reactor was cooled to about 500 °C. At this point the solid material was extracted from the bed and collected for analysis of carbon, which was determined by loss of ignition at 550 °C (assuming that all weight loss was carbon).

tests were carried out using particles of 0.5−4 mm, but also some runs were conducted using particles of size 2.5−4 mm. The bed material employed was bauxite, a material that has been employed as bed material for FBG.22 The particle size of the bauxite employed was 250−500 μm. Some additional physical properties of the bed material have been given elsewhere.19 2.3. Operating Conditions. The gross of tests were carried out using air as fluidization agent, a biomass flow rate of 1 kg/h (throughput of 485 kg/(m2 h)), and particle size distribution of 0.5−4 mm. A batch of 0.5 kg of bauxite was added to the gasifier, and the pressure drop was maintained approximately constant during the operation. The temperature and ER (equivalence or stoichiometric air ratio) were varied (independently) in the range of 800−900 °C and 0.17−0.35, respectively, by adjusting the air flow rate and temperature of the oven. The fluidization velocity (calculated as the rate of air fed divided by the bed cross-section at the temperature and pressure of the bed) varied in the range of 0.46−0.90 m/s. In the following these experiments will be referred to as “base case” experiments. Some additional tests were conducted varying the following parameters with respect to the base case (keeping the rest of the parameters unaltered): (i) biomass feed rate of 1.5 kg/h (throughput of 725 kg/(m2 h)); (ii) particle size of 2.5−4 mm; (iii) volume fraction of O2 in the mixture of 13% and 40%. Just one experiment was carried out using an O2 concentration in the fluidizing gas of 13%, and this was employed to enable operation with low ER (0.12), but maintaining the gas velocity in the bed similar to the other tests without varying the fuel feed rate. In Table 3, all the tests are listed as well as their operating conditions and the most important results. Test pairs 1−2 and 8−9 represent the same operational points and are included in the table to show the repeatability of the measurements. 2.4. Analysis Method. The composition of the exit gas was measured on line with a gas analyzer using a nondispersed infrared method for CO, CO2, and CH4 and thermal conductivity and paramagnetic methods for H2 and O2, respectively. In addition, gas samples were taken with intervals of approximately 5 min, to be analyzed by a micro-GC measuring CO, CO2, CH4, H2, N2, O2, C2H4, C2H2, C2H6, and C3. The concentration of C3 is the sum of the concentrations of C3H6 and C3H8. For analysis of tars, water, and inorganic contaminants, gas samples were taken from a port situated just downstream of the cyclone, using a vacuum pump. The sampling port was heated to approximately 315 °C to avoid condensation of tars and was equipped with a candle filter to eliminate solid particles. For tar and water sampling, the gas passed through a series of impingers filled with isopropanol at −20 °C. The tar sampling line has been presented in more detail elsewhere.23 The water content was determined using Karl−Fischer analysis. The gravimetric tar was determined by evaporating the solvent of the collected samples. The individual tar components were measured using gas chromatography and mass spectrometry (GC-MS). A total of 29 aromatic tar species were measured, including compounds with none or low polarity, from light species such as toluene or phenol to five-ring aromatics such as perylene. A detailed description of the tar analysis method is given in ref 23. Most of the tars measured by GCMS analysis are also present in gravimetric tar except for light compounds such as toluene and xylene that disappear from the tar sample during the solvent evaporation. Gravimetric tar also contains species that cannot be detected by GC-MS analysis, such as oxygenated primary tars with high polarity and heavy PAH compounds.6 Samples for analyzing tar, water, and inorganics were taken for the base case experiments only. To study the composition of tar, it was divided into different classes according to ref 6. The compounds measured by GC-MS are divided into tar classes 2, 3, 4, and 5. The GC-MS undetectable tar fraction was estimated by subtracting the yields of aromatic tars measured by GCMS analysis from the gravimetric tar (toluene and xylene were not considered to be present in the gravimetric tar, as discussed above). At low temperature (800 °C) the GC-MS undetectable tar is expected to contain mostly primary and secondary tar compounds according to Milne and Evans’ classification,24 with high polarity, while at higher

3. RESULTS AND DISCUSSION Table 3 summarizes the operating conditions and some of the most important results obtained in the different tests, including the composition of the gas, the mass yield of particle elutriation, and the mass fraction of carbon in the bed (carbon hold up) at the end of each experiment. 3.1. Composition of the light gas. The concentrations of the main light gas components are shown in Table 3. For ER between 0.24 and 0.35 and temperature between 800 and 900 °C, the measured values for H 2 , CO, CO 2 , and CH 4 concentrations are in the ranges of 8.75−14.37%, 11.17− 19.42%, 13.41−16.69%, and 3.21−4.22%, respectively. As expected, the concentrations of CO, H2, and CH4 decrease with increasing ER (at constant temperature) and increase with increasing temperature (at constant ER), while the tendencies for CO2 and N2 are the opposite. The concentrations of C2 (C2H4, C2H2, and C2H6) and C3 in Table 3 show that C2H4 is by far the most abundant hydrocarbon. The concentrations of C2H6 and C3 decrease with increasing temperature, which is consistent with the conversion of higher hydrocarbons into lighter ones at high temperatures. The variations in the concentrations of C2H4 and C2H2 are smaller, in relative terms. The benzene concentration increases with temperature especially between 800 and 850 °C. The evolution of benzene concentration with ER is less clear. For most operating conditions it decreases with increasing ER at constant temperature, but for some experiments the opposite effect is observed. The C6H6 concentration would be expected to decrease with ER, both because of the dilution with N2 and the higher amount of oxygen available for reaction with D


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Energy & Fuels benzene and other species. More experimental data would be required to explain the increase in benzene concentration observed in some cases, but one possible effect could be the oxidation of tar species yielding lower HC’s that then recombine to give benzene. The gravimetric tar (see Table 3) is high at 800 °C for low ER (0.24) and decreases rapidly with temperature and ER, leveling off at around 8-9 g/(Nm3). The contents of inorganic contaminants such as NH3 and HCl are of the same order of magnitude as results from van der Drift et al.,12 showing maximum concentrations of 0.4% and 0.0004%, respectively. The H2S concentration was below the detection limit for all tests. The gas compositions measured here are quite similar to previous results with wood chips,5,25 but different compared with wood pellets,7,8 the latter giving slightly higher concentrations of CO, H2, and CH4. The yields of the main light gas components at different temperatures are represented as a function of ER in Figures 2−4. Figure 2 shows graphs for the total gas yield (Nm3N2 and moisture free gas/kgdry fuel), and for the yields of CO and CO2. Figure 3 displays the yields of H2, CH4, C2H4, and C6H6 and Figure 4 gives those of C2H2, C2H6, and C3 (open symbols are used for the base case experiments, with lines showing their trends with ER, while filled symbols were employed for the additional tests varying O2 concentration in the gas, particle size, and fuel feed rate, as indicated in the caption to the figures). The total gas yield increases with temperature, especially at low temperature. At low temperature and ER the formation of gas is also enhanced by an increase in ER, probably due to higher char conversion as more oxygen is fed. The decrease in the gas yield at ER above 0.29 is probably due to the higher char entrainment from the bed (the gas velocity increases when increasing ER at a given temperature for air gasification tests). The values of total gas yield are similar to those found for woody biomass in FBG8. The yield of CO2 (Figure 2) increases sharply with increasing ER, while the influence of temperature is small, showing just a slight decrease in CO2 yield with temperature. The yields of CO (Figure 2) and H2 (Figure 3) decrease slightly with ER, while that of CH4 (Figure 3) remains practically unchanged. This is explained by the lower reactivity of CH4 as compared to CO and H2.26 For a given ER, the yields of CO, H2, CH4, C2H4, and C6H6 increase with temperature, while those of C2H6 and C3 drop rapidly. C2H2 is not much affected by temperature. The variation of ER has a smaller effect on C2H6, C2H2, and C3 compared to temperature. The C3 yield seems to increase slightly with increasing ER, while the C2H2 and C2H6 remain practically unchanged. The most significant effect of using O2-enriched air is the higher yield of methane produced, although it is also conducive to higher yields of CO and C2 and C3 hydrocarbons. It is possible that the higher O2 concentration in the gas enhances the conversion of tar species into light gas components. Also the volatiles residence time in the reactor is higher when O2enriched air is employed. To confirm such effect, more experiments would be necessary including thorough analysis of tars. On the other hand, no significant effects on the yields of the light gas components were observed when changing the fuel particle size or biomass feed rate (for some conditions the points in Figures 2−4 overlap), though the tests at 800 °C and

Figure 2. Total gas yield (Nm3 dry and N2 free gas/(kg of dry fuel)) and yields of CO and CO2 (g/g of dry and ash free fuel) as a function of ER for different temperatures: 800 (◊), 850 (Δ), and 900 °C (○). Open symbols are employed for the base case tests (air as fluidization agent, fuel particle size of 0.5-4 mm, and fuel feed rate of 1 kg/h), while filled symbols are employed for tests with O2-enriched air with 40% O2 (solid symbols), fuel particle size of 2.5-4 mm (symbols marked with +), fuel feed rate of 1.5 kg/h (symbols marked with ×), and with N2-diluted air with 13% O2 (symbols marked with ●). The lines show the trends with ER for the base case experiments E


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Figure 3. Yields of H2 (◊), CH4 (○), C2H4 (□), and C6H6 (Δ) (g/g of dry and ash free fuel) as a function of ER for different temperatures: 800, 850 and 900 °C. Open symbols are employed for the base case tests, while filled symbols are employed for tests with O2-enriched air with 40% O2 (solid symbols), fuel particle size of 2.5-4 mm (symbols marked with +), fuel feed rate of 1.5 kg/h (symbols marked with ×), and with N2-diluted air with 13% O2 (symbols marked with ●). The lines show the trends with ER for the base case experiments

Figure 4. Yields of C2H6 (◊), C2H2 (○), and C3 (Δ) as a function of ER for the three temperatures tested, 800, 850 and 900 °C. Open symbols are employed for the base case tests, while filled symbols are employed for tests with O2-enriched air with 40% O2 (solid symbols), fuel particle size of 2.5−4 mm (symbols marked with +), fuel feed rate of 1.5 kg/h (symbols marked with ×), and with N2-diluted air with 13% O2 (symbols marked with ●). The lines show the trends with ER for the base case experiments

ER of 0.23−0.24 (tests 12 and 14) show some deviations, giving unexpectedly low CO yields. F


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Energy & Fuels 3.2. Composition of Tar and Water in the Gas. Figure 5 represents the yields of tar classes 2, 3, 4, and 5 (TC2−TC5) as a function of ER for the three temperatures studied. The figure also includes the yields of GC-MS undetectable tar (named as “1”). The most abundant tar class 2 compounds are phenol (C6H5OH) and benzonitrile (C6H5CN), while toluene (C6H5CH3) is the major tar class 3 species followed by styrene

(C8H8) and indene (C9H8). Naphthalene (C10H8) is by far the most abundant tar class 4 compound, while pyrene (C16H10) and chrysene (C18H12) are the major tar class 5 species. It is observed that the yield of TC2 decreases rapidly with temperature, while TC4 and TC5 increase in agreement with previous works.6,24 TC3 decreases as the temperature is raised from 850 to 900 °C. ER seems to have the most significant effect on TC5 and non-GC-MS tar (1 in Figure 5), increasing ER giving lower tar yields, although this effect is only observed for ER below 0.29. An increase in ER raises the concentration of oxygen available, but it also decreases the residence time of the volatiles in the reactor, as the total gas flow increases. The tar concentrations in the gas measured here differ from those reported in ref 6. The results measured here show lower tar content in the gas, especially for TC4 and TC5. The lower tar concentrations obtained here could be related to the higher ash content of OTP compared to wood and to differences in the hemicellulosic composition of the fuels. Another factor that could partly explain the lower TC4 and TC5 contents measured here are the differences in volatiles residence times in the reactor, 0.82−1.16 s for the base case experiments in this work compared to 1.3−4 s in ref 6. In fact, the results from ref 6 showed that raising the gas (volatiles) residence time from 1.3 to 4 s increased the contents of TC4 and TC5 by more than 50% and 250%, respectively, while it led to a decrease in the concentrations of TC2 and, to a smaller extent, that of TC3. The H2O concentration in the gas for the different tests (see Table 3, expressed as wet basis) ranges between 6 and 18%, in the range of what has been reported in previous work.20 It is difficult to find clear trends of the water content with the temperature and ER, since water takes part in a large number of reactions, including devolatilization, tar reforming, char gasification, and water gas shift reaction, etc., and these reactions are affected differently by temperature and oxygen concentration in the reactor. 3.3. Gas Heating Value. The LHV of the gas is represented in Figure 6. Three different values have been calculated: one that includes only the light gas, one that includes both light gas and benzene, and one that also includes tar (a heating value of 36 MJ/(kg of tar) was assumed for the non-GC/MS tar27). All three LHVs decrease with ER and increase with temperature. The LHVs measured here for all operating conditions are sufficiently high to burn the gas in a gas engine. The LHV of the gas (including only light gas) is about 60% higher when using O2-enriched air, which is directly related to the lower dilution with N2, but also to the higher yields of CH4 and C2 and C3 hydrocarbons produced when using O2-enriched air. In contrast, the variation of the fuel feed rate and the fuel particle size hardly affect the LHV. The data in Figure 6 show that an important part of the heating value of the gas is present as tars, especially for low temperature and ER. This means that gas cleaning methods, where tars are removed rather than converted into light gas, lead to a significant decrease in the heating value of the product gas if the tar is not recycled to the gasifier. 3.4. Carbon Conversion, Char Hold-up, and Elutriation. The carbon conversion (CC) is defined as the carbon measured in the gas, including light gas and tar, divided by the carbon introduced with the fuel. The carbon conversion measured at different temperatures and ER is shown in Figure 7. It is especially favored by high temperature at low ER (below 0.29). At 900 °C the carbon conversion is high (>0.95) and practically independent of ER, the same occurs for 850 °C and

Figure 5. Yields of TC2 (2), TC3 (3), TC4 (4), and TC5 (5) and GC-MS undetectable tar (1), in g/(kg of dry fuel), as a function of ER for three temperatures: 800, 850 and 900°C. Note that the figure shows the TC5 yield multiplied by 10. G


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Figure 7. Carbon conversion for the base case tests as a function of ER for three different temperatures: 800, 850 and 900 °C.

ER of 0.29 and higher. Higher CC than 0.96 is difficult to achieve due to the entrainment of char particles from the bed. The carbon conversion is closely related to the total gas yield (Figure 2) and follows the same trends. The carbon conversion achieved in this work is relatively high, since no material is extracted from the bed during the operation. Large-scale units are likely to need some bed extraction to prevent accumulation of alkaline metals in the bed material, leading to a decrease in char conversion. The mass concentration of carbon in the bed (carbon holdup) after each test is shown in Table 3, where it is seen to decrease with increasing temperature, as the char conversion reactions get faster. The maximum value of 2.32% is found at 800 °C and ER = 0.24, much higher than at any other condition. On the other hand it was observed (by visual inspection of the material extracted from the bed) that the average size of the char particles was significantly higher at the lowest temperature studied, 800 °C. The accumulation of char in the bed is considered negligible as the gas concentrations measured during the tests were stable (once a start-up period of less than 30 min had passed). Only at low temperature (800 °C) and low ER (0.24) the H2 concentration in the gas was observed to increase slightly during the experiments, suggesting that some char accumulation was occurring during these tests. This is also consistent with the low carbon conversion and high carbon concentration in the bed measured under these conditions, since they indicate that the char conversion reactions are slow. If these reactions are slower than the rate at which char is generated from the devolatilization of the fuel fed, accumulation of char will occur and longer time is required before steady state conditions can be achieved. The carbon balance was checked for all the experiments (with tar measurements), and reasonable closure (with error of less than 5%) was found for all the tests except test 1 (800 °C and ER = 0.24), for which the total rate of carbon exiting the reactor (including gas and char) was found to be 14% lower than the rate of carbon fed with the fuel, indicating once again that accumulation of char in the bed probably occurred. The rate of elutriation of solids (g/(g of fuel fed)) from the bed and the mass fraction of carbon in the elutriated solids are

Figure 6. LHV of the gas (MJ/(Nm3)), including the following: (○) only the light gas; (×) light gas and benzene; (◊) light gas, benzene, and tar, as a function of ER for different reactor temperatures. Open symbols are employed for the base case tests, while filled symbols are employed for tests with O2-enriched air with 40% O2 (solid symbols), fuel particle size of 2.5-4 mm (symbols marked with +), and fuel feed rate of 1.5 kg/h (symbols marked with ×), and with N2-diluted air with 13% O2 (symbols marked with ●). The lines show the trends with ER for the base case experiments H


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Energy & Fuels given in Table 3. These values should be considered as estimates due to the practical difficulties of collecting the elutriated solids. Also the gas velocity in the freeboard (including the volatiles) is shown in Table 3. The elutriation is seen favored by high gas velocity, as well as by higher mass of solids in the bed. For example for the experiment carried out at 800 °C with ER of 0.24 (test 2), both the rate of particle elutriation and the carbon fraction in the elutriated solids are high, due to the higher carbon concentration in the bed. It is also noteworthy that increasing the fuel particle size from 0.5−4 to 2.5−4 mm (i.e., eliminating the finest fuel particles) leads to significantly lower char elutriation. The increase in fuel feed rate to 1.5 kg/h led to higher gas velocity and carbon concentration in the bed and thus to higher rate of particle elutriation per mass of fuel fed. For the experiments with O2enriched air (tests 16−18), the gas velocity was lower and so was the rate of particle entrainment. 3.5. Gasification Efficiency. Two gasification efficiencies were defined, the cold gas efficiency (CGE), to represent the case when the gas is cooled before its final application, for example when it is burnt in an internal combustion engine, and the hot gas efficiency (HGE) for applications where the gas is not cooled, as when it is directly burnt in a gas boiler. CGE and HGE were calculated as:

Figure 8. CGE and HGE as a function of ER for the base case tests at different temperatures. 800, 850, and 900 °C. The CGE is represented by open symbols: 800 (Δ), 850 (○), and 900 °C (◊) and solid lines. HGE is represented by solid symbols: 800 (▲), 850 (●), and 900 °C (◆) and dashed lines.

CGE = mgas LHVgas/(m fuel LHVfuel)

operation it is difficult to extrapolate the experimental results discussed above to industrial FBG. The aim of the following discussion is to identify which of the tests presented here, if any, could be directly used to predict the performance of industrial units and how the rest of the tests should be used for scaling-up. This is done by using a previously developed FBG model18 to simulate the laboratoryscale tests conducted here, with the objective of calculating the heat that the reacting system inside the reactor (gas and solids) exchanges with the surroundings (walls and electrical oven), HE. The system is gaining heat in tests with HE > 0, whereas it is losing heat if HE < 0. HE = 0 represents tests under autothermal conditions (without heat losses). A typical industrial FBG operates with heat losses of approximately 2%, so it would be represented by tests conducted with HE = −2%. The gasification process is represented by

HGE = (mgas LHVgas + Hgas)/(mfuel LHVfuel)

with mgas and mfuel being the mass flow rates of product gas and fuel fed, respectively, and Hgas the sensible energy of the gas (the enthalpy of heating the gas from 25°C to the gasification temperature). For calculating the CGE the gas was considered to contain benzene but no tars, whereas both benzene tars and water were considered for the calculation of HGE, consistently to the final application for which these parameters are defined. Since tar and benzene concentrations are needed for the calculations, CGE and HGE could only be evaluated for the base case experiments. CGE and HGE as a function of ER for various temperatures are displayed in Figure 8, where they are both seen to increase with temperature. The results in Figure 8 show that temperature of 850 °C or higher is required to achieve a reasonable CGE (0.70). CGE and HGE decrease with ER at 900 °C, while for lower temperature both efficiencies are observed to increase when raising ER from 0.24 to 0.29. The observed behavior is consistent with the measured gas composition: at low temperature an increase in ER gives more char conversion, compensating the higher extent of gas burning. On the contrary, at 900 °C and at 850 °C for ER above 0.29, the increase in char conversion when raising ER is very limited, and therefore the higher extent of volatiles oxidation is no longer compensated for, leading to a decrease in CGE. 3.6. Scaling-up Results to Autothermal Operation. The gasification tests in this work were conducted setting the bed temperature and the air flow rate (or ER) independently (using a fixed flow rate of OTP). This was achieved by using an external electric oven. In a stand-alone industrial gasifier at steady state operation (without external heat) the temperature depends on the ER employed and the heat losses through the reactor walls are limited (typically in the order of 1−2% of the heating value of the fuel). Due to the different modes of

fueldry + xmois H 2O(l) + xO2O2 + x N2 N2 → yCO CO + yH H 2 + yH O H 2O(g) + yCO CO2 + yCH CH4 2

2

2

4

+ yN N2 + (yLHC LHC + yC H C6H6 + ytar tar) 2

6 6

+ ychar(s)char(s)

where xi are inputs and yi are values predicted by the model and they express the mass of species i in g/(g of dry fuel). Char(s) is the solid exiting the gasifier, including ash and unconverted carbon, whereas tar and LHC represent the tar species (organic compounds heavier than benzene) and light hydrocarbons (heavier than methane and lighter than benzene). In order to reproduce the conditions of the experiments, no extraction of solids from the bed was considered in the model, the ash exits the system as elutriated char. The model in ref 18 calculates the yields of CO, H2, CO2, N2, H2O, CH4, tar, and char, from fuel properties, reactor geometry, and some kinetic data (for char, tar, and the water gas shift reaction), but it included neither the presence of C6H6 I


a

expt

1

J

0.82 0.58 5.85

CC CGEb LHVonly light gas, MJ/(Nm3) heat exchange (%)c

0.92 0.63 6.16 −2.7

15.44 14.98 15.19 12.66 3.66 47.76 0.90 0.63 5.25

11.97 9.97 16.57 13.38 3.52 52.91

expt

3

0.92 0.61 5.33 −7.56

12.40 11.20 16.31 13.80 3.36 53.52

model

2.23 0.26 14.37

800 0.29

3

0.87 0.69 5.95

16.13 14.3 14.31 11.32 3.59 46.83

expt

5

4 model expt model Composition, % (Dry Gas) 16.86 13.67 12.0 15.63 11.48 12.41 14.46 15.23 16.84 10.90 8.31 11.8 3.90 3.64 3.6 46.21 51.67 52.28 Performance Indicators 0.94 0.94 0.95 0.70 0.69 0.63 6.31 5.41 5.30 −0.5 −6.15

5

0.95 0.62 4.52

11.17 8.75 16.69 8.78 3.21 56.07

expt

850 850 0.24 0.29 Measurements Used as Inputs for the Model 1.78 1.87 0.43 0.36 13.76 12.26 Comparison between Model and Measurements

4

Total tar (g/Nm3 dg), bCGE, cold gas efficiency. c% of HHV of fed fuel (losses(−)/added(+)).

14.57 11.75 15.64 9.63 3.85 50.82

CO H2 CO2 H2O CH4 N2

model

2.19 0.16 27.62

C2 + C3, % (dry gas) C6H6, % (dry gas) tara

test

800 0.24

1

T, °C ER

test

Table 4. Comparison of Model with Measurements for Air Gasification Tests

6

0.95 0.58 4.62 −10.7

11.32 9.36 16.42 13.52 3.31 57.16

model

1.66 0.20 9.8

850 0.35

6

0.96 0.76 6.58

19.32 14.42 13.28 6.36 4.22 45.99

expt

9

0.97 0.73 6.64 +1.40

19.26 14.69 13.00 11.08 4.23 45.93

model

1.8 0.42 11.74

900 0.24

9

0.96 0.72 5.56

14.50 11.16 14.74 10.72 3.89 50.50

expt

10

0.97 0.67 5.70 −3.7

14.95 11.60 15.04 12.15 3.43 51.64

model

1.88 0.35 10.55

900 0.29

10

0.97 0.70 4.78

12.65 9.31 15.61 21.84 3.32 55.67

expt

11

0.98 0.60 4.51 −10.1

11.61 7.20 16.21 15.05 3.37 58.68

model

1.52 0.44 12.13

900 0.35

11

Energy & Fuels Article


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Energy & Fuels

Figure 9. (a) Cold gas efficiency (taking into account benzene but not tars) and carbon conversion (CC) as a function of temperature calculated by the model (lines) and the corresponding experimental values resulting from the identified “autothermal” tests (1 (800°C), 4 (850°C), and 9 and 10 (900°C)) represented by circles (CC) and squares (CGE). (b) Low heating value of the gas (calculated by accounting for light gas only (CO, H2, CH4, C2H4, and C3), light gas + benzene, and light gas + benzene + tars as a function of temperature. Circles are experimental LHVs (without tar and benzene) for the aforementioned tests. The upper horizontal axis shows the ER corresponding to each temperature for autothermal operation with heat losses of 2%.

accumulation of char in the bed in test 1, as discussed above. The latter is also supported by the large disagreement detected in the carbon conversion, for which the measured value was much lower than that predicted by the model (10% in test 1 compared to lower than 2% for the rest of the tests). Overall it is concluded that the model gave good predictions of the experimental results so that the value of the heat exchanged (HE) calculated with the model is considered reliable enough. It is seen that the HE calculated by the model is negative for 7 of the 8 tests in Table 4, meaning that for these 7 tests the system was losing heat. Inspection of Table 4 allows identification of three approximate autothermal tests, one at each temperature: 1 (ER = 0.24) at 800 °C, 4 (ER = 0.24) at 850 °C, and 10 (ER = 0.29) at 900 °C. Only test 4 at 850 °C presents heat losses below 2% and could be considered truly representative of industrial operation. Test 1 has heat losses of 2.7%, but, as discussed above, there are doubts about the char accumulation in this test. For 900 °C the actual autothermal test would be between 9 (ER = 0.24) and 10 (ER = 0.29). Figure 9 displays the computed carbon conversion (CC), cold gasification efficiency (CGE), and heating value of the gas (LHV) for autothermal operation (for heat losses of 2%) at different temperatures. For these simulations the model was run under semipredictive mode; i.e., C6H6, C2H4, and total tar were calculated by fitting the values of the experiments with temperature between 800 and 900 °C assuming linear behavior for C6H6, and C2H4 and exponential behavior for total tar. Extrapolation was conducted for simulations at temperature above 900 °C. Three different LHVs of the dry gas have been displayed in Figure 9b: (i) considering only the light gas species (CO, H2, CH4, C2H4, and C3), (ii) light gas + benzene, and (iii) light gas + benzene + tars. For each temperature read on the lower horizontal axis in Figure 9 the corresponding ER necessary to maintain that operating temperature (for autothermal operation and heat losses of 2%) can be read on the upper horizontal axis.

nor LHC species. As a result, the model was expanded to consider the presence of these compounds in such a way that the model enforced these species to be present in the product gas at specified values, provided as input. Therefore, these species are not predicted by the model. Instead they are inputs taken from measurements. C2H4 was taken as a model compound to represent the various species measured as LHC (C2 and C3). This is a good approximation since the concentrations of LHC other than C2H4 are comparably small.26 Due to the uncertainty of the predicted values of tar in the original model, the tar is also provided as input, taken from the measurements (calculated as the sum of all tar classes, from 2 to 5 plus non-GC-MS tar). These two decisions were consistent with the main purpose of using the model to accurately calculate the heat losses. Therefore, the model was used more for data reconciliation rather than as a prediction tool. Table 4 presents the comparison of the model results with measurements for the base case experiments (excluding 2 and 8 which are repetitions of 1 and 9, respectively, and test 7, for which tar and water measurements are not available). The information displayed in Table 4 includes the inputs given to the model (ER, T, tar, LHC, and C6H6), the predicted values of gas species (CO, H2, CO2, H2O, CH4, and N2), and the performance indicators (carbon conversion, cold gasification efficiency, lower heating value of the gas). The last row of the table displays the value of the heat exchanged (HE) calculated by the model, expressed as the percent of the HHV of the input fuel. It is observed that the model predicts fairly well the gas composition as well as the performance indicators, with the exception of the value of water vapor in the gas, whose error is more significant than those for other species. The error in the values predicted for test 1 (800 °C and ER = 0.24) is also significantly greater than for other tests. These two disagreements are explained, respectively, by the uncertainty in the measurement of water vapor and the possibility of some K


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Energy & Fuels

ER (0.24), so the only carbon losses in the experiments were caused by entrainment of char particles from the bed. These losses were significantly reduced by decreasing the gas velocity in the freeboard from 0.46 to 0.30 m/s and increasing the fuel particle size from 0.5−4 to 2.5−4 mm (i.e., eliminating the finest fraction of fuel particles). A previously developed model was employed to analyze the experimental results and scale-up the process. The following conclusions were obtained: (1) Most of the experiments were conducted under allothermal conditions (with significant amounts of heat lost or added to the system). Only three tests were identified to be close to autothermal conditions (with heat losses close to 2% of the HHV of the fuel fed) and, therefore, can be approximately used to represent the operation of industrial FB gasifiers. (2) Extended simulations to predict the operation of industrial FB gasifiers, indicated that the highest cold gas efficiencies are achieved around 950 °C, with values of up to 0.70, producing a lower heating value of about 6 (excluding tars and benzene) and 6.7 MJ/(Nm3) (including tars and benzene).

It is seen that the CC and CGE increase with temperature, while the low heating value decreases. The CC and CGE increase significantly from 800 to 850 °C due to an increase in char reactivity but level off at higher temperatures. The optimum value (without further consideration of other factors such as ash-related issues, not analyzed here) is 950 °C. The experimental values of CC, CGE, and LHV (considering only the light gas) for the tests identified as operating close to autothermal regime (1, 4, and 9−10) are also displayed (dots in the figure), supporting the trend given by the model. The temperature at which each test was carried out can be read directly on the horizontal axis, but it should be noted that the upper horizontal axis shows the ER for autothermal conditions and not the ER at which the experiments were conducted. For example, test number 4 was carried out at 850 °C and ER of 0.24, while the ER for autothermal operation (with heat losses of 2%) at that temperature shown in Figure 9 is close to 0.26. The disagreement between model and experiments is explained by the different values of the HE (none of tests had an exact HE of 2% as seen in Table 3). As an example, the CGE and gas LHV are higher for test 9 compared to the model results because test 9 was conducted with a lower ER (0.24) than that representing autothermal operation at 900 °C (0.28). The relatively high differences found between the predicted LHV of gas without tar and benzene with the experimental values (the circles in Figure 9b) show how important it is to compare “equivalent” tests (those with equal heat losses). The differences between the various LHV displayed in Figure 9b point out the relative contribution of benzene and tars to the LHV of the gas. The LHV that takes into account all fuel species (total) and the LHV that considers only the light gas both decrease with temperature. However, the LHV of the gas accounting for the benzene but not tar is seen to increase with temperature. This means that the increase in benzene and other fuel species resulting from the tar decomposition compensates the higher combustion of fuel species.

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AUTHOR INFORMATION

Corresponding Author

*Corresponding author.Tel.: +34 954487223 E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support of the Foundation Advanced Technological Centre for Renewable Energy (CTAER) in the project hybridization solar-biomass PI-0918/ 2012 and the Junta de Andaluciá in Project P12-TEP-1633 MO (FLETGAS2). P.H. acknowledges the post-doctoral grant (Contrato de Acceso al Sistema Español de Ciencia, Tecnologiá e Innovación, VPPI-US) of Universidad de Sevilla. The help in the experimental work by Julio Guillén and Inmaculada del Valle is also acknowledged.

4. CONCLUSIONS The gasification of olive tree pruning, an agricultural residue, has been studied in a laboratory fluidized bed (FB). The influence of temperature (800−900 °C), ER (0.12−0.35), fuel particle size (from 0.5−4 mm to 2.5−4 mm), biomass throughput (485−725 kg/(h m2)) and O2 enrichment of the air (21 and 40% O2 in air) was investigated. The fluidization velocities varied between 0.30 and 0.90 m/s and gas residence times between 0.82 and 1.76 s. The process was characterized by measuring the composition of the gas at the exit of the gasifier and taking samples from the cyclone and bed to estimate the char conversion in the process. The experimental results showed the following: (1) Using air, operation at low temperature (800 °C) and ER (0.24) resulted in char conversion and cold gasification efficiency below 0.58 and 0.82, respectively. Increasing the temperature up to 900 °C gave cold gasification efficiency above 0.70 and char conversion above 0.96 for all ER tested (0.24−0.35). (2) Using enriched air with an oxygen content of 40% indicated that cold gas efficiency higher than 0.80 and a gas LHV higher than 9.3 MJ/(Nm3) (excluding tars and benzene) can be achieved. (3) The tests were conducted without extraction of solids from the bed and accumulation of char in the bed was only detected to some extent at low temperature (800 °C) and low

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NOMENCLATURE AND ABBREVIATIONS CC = carbon conversion CGE = cold gas efficiency ER = equivalence ratio FB = fluidized bed FBG = fluidized bed gasification or fluidized bed gasifier GC-MS = gas chromatography-mass spectrometry HE = heat exchanged HGE = hot gas efficiency HHV = higher heating value LHC = light hydrocarbons LHV = lower heating value OTP = olive tree pruning T = temperature TC = tar class xi = mass of species i, g/(g of dry fuel) yi = yield of species i, g/(g of dry fuel) REFERENCES

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