Experimental Study on Herb Residue Gasification ... - ACS Publications

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Experimental Study on Herb Residue Gasification in an Air-Blown Circulating Fluidized Bed Gasifier Feiqiang Guo,† Yuping Dong,*,‡,§ Tonghui Zhang,‡ Lei Dong,§ Chuwen Guo,† and Zhonghao Rao*,† †

School of Electric Power Engineering, China University of Mining and Technology, Xuzhou 221116, China School of Mechanical Engineering, Shandong University, Jinan 250061, China § Shandong Baichuan Tongchuang Energy Company Ltd., Jinan 250101, China ‡

ABSTRACT: The gasification characteristics of herb residue for producing syngas in a pilot-scale circulating fluidized bed were investigated experimentally in this paper. The results indicated that the gas composition and tar yield were affected by the parameters including the equivalence ratio (ER), biomass feeding rate (FR), and steam-to-biomass mass ratio (S/B). The concentrations of combustible gases (H2, CO, CH4, and CnHm) showed a decreasing trend with increasing ER, while the lower heating value (LHV) exceeded 4.0 MJ/N·m3 in all cases when the ER was lower than 0.4. An increase in the ER can improve the reaction temperature, facilitate carbon conversion, and decrease the tar yield. The LHV increased gradually from 4.2 to 5.7 MJ/ N·m3 with an increase in the FR from 210 to 352 kg/h. The cold gas efficiency and carbon conversion efficiency reached their maximum values of approximately 68% and 93% at a feeding rate of 352 kg/h, whereas both of them decreased with a further increase in the feeding rate. The supplement of steam into the gasifier improved the LHV of the product gas, which reached a maximum value of about 6.0 MJ/N·m3 when S/B was 0.23−0.43 and the cold gas efficiency and carbon conversion efficiency were within 62−73% and 87−94%, respectively.

1. INTRODUCTION At present, there are about 1500 Chinese medicine enterprises, and approximately 12 million tons of herb residue is produced annually. In China, herb residue, which is rich in cellulose with a high original water content of more than 70 wt %, is seen as a representative of a kind of concentrated biomass resource, whereas it also represents a kind of potential pollution because it is highly susceptible to rot.1 Furthermore, the biomass fuels represent a renewable energy resource with CO2 neutral impact. In order to fully take advantage of the CO 2 neutralization function of these kinds of fuels, gasification of herb residue was recently tested by the authors to produce fuel gas (H2, CO, CO2, CH4, and CnHm) to replace fossil natural gas.2 As a thermochemical conversion technology, gasification is an important method to convert biomass into combustible gaseous fuels by partial oxidation at high temperature. Different gasifiers are employed in this process, mainly including fixed bed and fluidized bed.3 Compared with the fixed-bed gasifier, the fluidized-bed gasifier, particularly a circulating fluidized bed (CFB), is normally employed for large-scale biomass conversion, and many researchers have paid special attention to characterize the biomass gasification process through experimental studies.4 Cordiner et al.5 designed a prototype bubbling fluidized-bed gasifier in the 85 kW power range fitting with small-scale sustainable distributed generation systems with special focus on energy recovery from paper production process sludge. They found that the energy content in the sludge may be recovered along with a significant reduction of the residue volume and mass, and the concept may then be used to increase the overall sustainability of paper production. Link et al.6 studied the gasification characteristics of untreated and pretreated olive residues in a fluidized bed. It © XXXX American Chemical Society

was found that the addition of woody fuels and reed at elevated proportions resulted in variation of the LHV and tar content of the product gas, indicating that the proportions of different fuels in the mixture play a role in the composition of the producer gas. Gasification tests using a catalytic fluidized-bed gasifier were carried out by Ruoppolo et al.7 to obtain a dihydrogen-rich stream by feeding different pellets made of wood, biomass/plastic, and olive husks to the gasifier. The use of biomass/plastic pellets in a catalyst bed yielded good results in the hydrogen concentration (up to 32% by volume), even with an increase in tar production. Xu et al.8 tried to produce a middle caloric gas by dualfluidized-bed gasification using distilled spirits as the feedstock, and it was verified that this technology worked well for materials with good granularity. They identified also the superior reactor combination for such a kind of dualfluidized-bed gasification system and made efforts to optimize the fluidized-bed gasifier in terms of raising the gasification efficiency and reducing the tar formation in tar gasification, for example, via using a two-stage fluidized-bed gasification reactor.9 These provided a good basis for developing biomass waste gasification technologies based on a fluidized bed. The effect of steam gasification conditions on the product properties was investigated by Weerachanchai et al.10 in a bubbling fluidized-bed reactor. They found that steam gasification gave a higher amount of gas product and a higher H2/ CO ratio and different bed materials affect the gas composition and tar yield. Couto et al.11 studied a pilot thermal gasification Received: May 26, 2014 Revised: July 31, 2014 Accepted: August 8, 2014

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dx.doi.org/10.1021/ie5021238 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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furnace (XL-2006). The ash of herb was measured by inductively coupled plasma atomic emission spectrometry, showing that the ash of herb residues is rich in Ca, Mg, K, Fe, and Mg, which have been verified to have an influence on the gasification characteristics in our previous report.2 The size of the tested fuel is below 2.0 mm, and its bulk density is about 480 kg/m3. The cumulative particle-size distribution of herb residue is shown in Figure 1. As is obvious from the figure, the majority of herb residue particles after pretreatment have sizes approximately between 0.5 and 1.5 mm.

plant with a processing capacity of 70 kg/h and operating at around 800 °C using coffee husks as the fuel material, and the gasification tests were performed continuously for several days in order to optimize the heat value and composition of produced syngas. In this study, the gasification characteristics of herb residue in a CFB were investigated. In order to improve the conversion efficiency of the raw material, two high-temperature cyclones were designed to capture coarse particles in the gas, and the particles are recycled to the riser through air-driven loop seals. A water-jacketed heat exchanger and an air preheater were designed to make full use of the sensible heat of the gas and produce steam and hot air as the gasification agent. An experimental study was completed to examine the effects of the operating parameters [equivalence ratio (ER), biomass feeding rate (FR), and steam-to-biomass mass ratio (S/B)] on the gas composition, gasification efficiency, and tar yield. The present paper intends to provide some process fundamentals about the conversion of a concentrated biomass resource by gasification in the fluidized beds.

2. EXPERIMENTAL SECTION 2.1. Feed Material and Pretreatment. Herb residue used for the experiments is obtained from Henan Wanxi Pharmaceutical Co., Ltd. (China). The water content of herb residue was dropped to about 12.5% by mechanical dewatering and drying before the gasification process.2 The chemical composition of the fuel was analyzed and is summarized in Table 1, showing that the fuel is rich in volatiles and oxygen. The following analytical methods were used for chemical analysis: The moisture and heating values were measured by means of a DHG-9240A drying oven and a HY-A9 calorimeter system, respectively. Ultimate and proximate analyses were done by an elemental analyzer (Vario ELCHNO) and a muffle

Figure 1. Particle-size distribution of herb residue.

2.2. Experimental Setup and Procedures. An atmospheric-pressure air-blown CFB gasifier was designed for largescale gasification research of herb residue. Some technical details of this facility are available in a few papers that have been published elsewhere.2 In this study, some improvements in the structure have been done to improve the conversion efficiency of the raw material. The pilot configuration of the facilities used in the experiments is presented in Figure 2. The riser has an inner diameter of 0.3 m and a total height of 9.0 m. The inert bed material used was sand, which has a mean particle size of 300−800 μm with a bulk density of 1549 kg/m3. Air was supplied as the oxidant and fluidizing agent after passage through a start-up burner near the bottom of the riser. The gasifier was preheated to 400−550 °C by the start-up burner before coal or biomass fuel was fed to the riser to further raise the temperature to the desired level. Diesel oil was used as the fuel in the preheating process, and sand was sent into the gasifier intermittently. When a total weight of about 250 kg of sand was fed into the gasifier and preheated to above 600 °C, the system was then switched to the gasification mode, with herb residue as the only feed material. The gasifier employs two independent feeding systems: one for the main fuel (herb residue) and the other for sand and the auxiliary fuel (coal) used during the start-up process. The biomass feeding system consists of a sealed hopper, a rotary valve with a variable-speed direct-current motor and controller, and a screw to introduce biomass into the riser. The fuel flow rate can be regulated by means of changing the rotational speed. Feed particles undergo moisture evaporation, pyrolysis, and char gasification primarily in the riser. The feed rate of the

Table 1. Chemical Composition of Herb Residue fuel moisture (wt %, a.r.) proximate analysis (wt %, dry) volatiles fixed carbon ash ultimate analysis (wt %, dry) carbon hydrogen oxygen nitrogen sulfur others LHV (MJ/kg) elemental ash analysis (wt %) Al2O3 MgO Fe2O3 K2O CaO P2O5 SiO2 SO3 Cr2O3 Na2O

herb residue 12.5 82.99 14.18 2.82 42.40 6.20 47.39 1.86 0.15 2.00 14.9 6.64 6.47 8.31 6.24 25.02 16.32 23.60 3.13 1.47 1.08 B



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Figure 2. Schematic diagram of the fluidized-bed apparatus: 1, gas valve; 2, mass flow controller; 3, start-up burner; 4, motor; 5, screw feeder; 6, hopper; 7, temperature and pressure recorder; 8, riser; 9, sand and auxiliary fuel feed system; 10, primary cyclone for particle circulation; 11, secondary cyclone for particle circulation; 12, cyclone for ash removal; 13, ash drum; 14, water-jacketed heat exchanger; 15, air preheater; 16, water washing tower; 17, venturi scrubber; 18, draft fan.

by a sampling bag for measurement. Thereafter, MgSO4 was added into the tar-contained solution with 10−15 g per 100 mL to absorb water, and then the solution is filtered. The acetone solution was distilled in a water bath at the normal pressure and 40 °C for about 2.5 h, and the residues were defined as tar. As for gas compositions, the volume fractions of H2, CH4, CO, CO2, CnHm (which includes C2H4, C2H6, and C3H8), O2, and N2 were measured by a gas chromatograph (Micro-GC 3000A, Agilent). To ensure the reliability of the test results, each experiment continued for around 3 h after the gasification system became stable; each test was repeated for three to five times, and the variability of measuring data was within 5%. Process data of the gasification, such as local temperatures and pressures, were logged into a computer. This system consists of four pressure transducers and temperature thermocouples, and from bottom to top, the temperature and pressure reading dates are T/P1, T/P2, T/P3 and T/P4. The measurement point of T/P1 is at 1 m, T/P2 at 4 m, T/P3 at 6 m, T/P4 at 8 m above the bottom. All of the detectors are introduced into the riser only to about 15 mm to avoid interference with the flow characteristics inside the riser. 2.4. Investigating Variable Definitions. The following variables are applied to characterize the gasification conditions. The equivalence ratio, ER, is defined as the ratio of the actual air supply to the stoichiometric air required for complete combustion on a dry-ash-free (daf) basis.

fuel particles was estimated to be 210−410 kg/h, corresponding to a gas production of around 300−800 m3/h. Two high-temperature cyclones were designed to capture coarse particles in the gas immediately downstream of the riser. The solid captured in these two cyclones is recycled to the riser through air-driven loop seals. Hot gas after ash removal at a high temperature of 600−800 °C is cooled by a water-jacketed heat exchanger and an air preheater, which produce steam and hot air as the gasification agent, before entering the purification system. The tar and fine ash purification system mainly includes a cyclone, a water washing tower, and a venturi scrubber. The remaining ash is first removed by the cyclone, and tar is then removed by the washing tower and venturi scrubber. The content of ash and tar in the product gas after purification is lower than 10 mg/N·m3. During the experiment, a sample flow of the product gas is continuously extracted from the main stream downstream of the gas outlet of the cyclone. A total of 19 different tests have been carried out to investigate the effect of different operational parameters (ER, FR, and S/B) on the product gas composition and tar formation, as shown in Table 2. 2.3. Product Gas Sampling and Measurement Procedures. In this study, the tar concentrations at the exits of the gasifier were measured by the following sampling system. It consists of a sampling port, tar impinge bottles, an ice water pool, a cartridge filter, a gas meter, and a vacuum pump. Acetone, which acts as a tar absorbent, was added into the tar impinge bottles before each test, and samples of condensates including tar, water, and a few soot particles were collected for 20−30 min. The product gas after tar absorption was collected

ER =

[Φm,air /Φm,fuel(daf)]actual [Φm,air /Φm,fuel(daf)]stoich

(1)

where C


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[Φm,air /Φm,fuel(daf)]stoich

2.6 310 368 0.31 0.55

19


Cdaf H S O ⎞ 1.293 ⎛ ⎜1.866 + 5.55 daf + 0.7 daf − 0.7 daf ⎟ ⎝ 0.21 100 100 100 100 ⎠

18 17

2.5 310 368 0.31 0.35

16

2.7 310 368 0.31 0.28

2.8 310 368 0.31 0.43

=

(2)

where Φm,i(daf) is the mass flow rate of i on a daf basis [kg/h] and Idaf is the mass fraction of I on a daf basis. The dry gas lower heating value (LHV) at the standard state of 101.3 kPa and 273 K can be estimated from the gas composition by eq 3:

15 11

2.7 210 249 0.31 0

10

2.5 310 487 0.41 0

9 8

[LHV]gas Gv [LHV]fuel

× 100%

(5)

Q aφN

2

φN ′

[N·m 3/h] (6)

3.5 352 418 0.31 0

where Qa is the volume of air fed in 1 h (N·m /h), φN2 represents the volume fraction of N2 in air at the standard state (78.12 vol %), and φN2′ is the volume fraction of N2 in the product gas at the standard stage (vol %). The carbon conversion efficiency (ηc) is defined as the ratio of carbon that is converted from the added fuel into gaseous carbon components to the carbon in the added fuel.12 Because the tar was removed from the gas at ordinary temperatures, the carbon in tar was not taken into account in the experiments. Therefore, the carbon conversion efficiency at the standard state can be calculated as

4.5 255 381 0.39 0

3.5 310 416 0.35 0

3

4.0 210 330 0.41 0

3

(4)

2

run time (h) FR(kg/h) air flow (N·m3/h) ER S/B

4

Gv =

2

Φm,fuel(a.r.)

where [LHV] gas is the lower heating value of the product gas (MJ/m3), [LHV] fuel is the lower heating value of the biomass fuel (MJ/kg), and Gv is the specific gas yield at the standard state (N·m3/kg). A nitrogen tracer method was developed to determine the gas yield from the percentage of nitrogen in the gases produced during gasification. From the concentration of nitrogen in the product gas and the total amount of nitrogen entering the reactor along with air in the gasification process, the total dry volumes of gas produced can be estimated using the following formula:

2.6 310 368 0.31 0

6

3.5 310 321 0.27 0

5

3.5 310 285 0.24 0

7

2.7 310 451 0.38 0

η=

1

Φm,steam

The cold gas efficiency of the gasifier in this study is calculated as

2.5 310 344 0.29 0

test no.

(3)

where CO, H2, CH4, and CnHm are percentages of the volume fraction of carbon monoxide, hydrogen, methane, and hydrocarbons (C2H4, C2H6, and C3H8) in the product gas. The steam-to-biomass, S/B, mass ratio is calculated as the ratio of steam supplied to biomass supplied on an as-received (a.r.) basis.

S/B =

Table 2. Experimental Conditions

1 (107.98H 2 + 126.36CO + 358.18CH4 1000 + 629.09CnH m) [MJ/N·m 3]

2.8 310 368 0.31 0.16

13

3.2 410 487 0.31 0

12

2.7 255 303 0.31 0

14

2.8 310 368 0.31 0.23

LHV =

ηc =

12(CO2 + CO + CH4 + nCnH m) × Gv × 100% 22.4Cc (7)

where Cc is the mass fraction of carbon in the fuel (wt %). D



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3. RESULTS AND DISCUSSION During the tests, the temperature and pressure of each measurement point were read every 15 min. The changing trends of the temperature, product gas composition, and pressure as measured versus time during the first four tests (about 15.5 h) are presented in Figures 3 and 4. It can be seen

temperature of measurement point 3 is taken as the characteristic operating temperature unless specified otherwise. The concentrations of CO, H2, and CO2 obtained in different tests were significantly different, indicating that the ER and FR have a direct effect on the temperature and gas composition. Thus, the ER and FR were chosen as two main variables in the study. No significant fluctuation was observed in the measured values of the pressure drop during the four tests. The pressure of measurement point 1, which represents the pressure in the gasifier after resistance from an air distribution plate, was fairly stable at about 8 kPa. There is a significant drop between the measurement point 1 and the others, while the pressure drop between the other three points is rather small, which can reflect that the fluidization is in good condition.15 3.1. Effect of the ER. The ER represents the oxygen feed in the gasifier, which is a crucial factor that affects the performance of the gasification process. Experiments were performed by changing only the ER, with the FR kept constant and no steam feed. The fast fluidization flow regime was maintained at the operating temperature, with the superficial velocity in the range of 3.8−7.6 m/s corresponding to an air flow rate of 285−487 N·m3/h. The experimental results of gas composition are shown in Figure 5. The main chemical reactions in the riser are

Figure 3. Temperature and gas composition of the gasifier for the first four tests.

Figure 5. Gas composition and LHV as a function of the ER. Figure 4. Measured pressure files of the gasifier for the first four tests.

considered to be reactions (8)−(18),16 and the effect of the ER on the gas composition is mainly attributed to the oxidation reactions via reactions (9)−(12), which release heat and create high temperature for the whole gasification process. It is considered that an increase of the ER led to further combustion of the product gas and dilution of the gas by nitrogen in air, resulting in a decrease in the concentrations of H2 and CO. It was found that H2 and CO decreased from 9.81 to 4.64 vol % and from 17.53 to 9.06 vol %, respectively, with an increase in the ER from 0.24 to 0.41. Conversely, CO2 showed an increasing trend from 14.23 to 19.82 vol % as a result of an increase in the ER from 0.24 to 0.41.

that the gasifier temperature remained fairly stable and similar at different parameters. The temperatures at the upper two measurement points (T3 and T4) are higher than the lower two points (T1 and T2) for about 20−40 °C. The coarser particles settled at the bottom and cooled there, while intense solids recycle minimized the temperature gradient.13 Compared to test nos. 1 and 2, test no. 3 gained higher temperature in the gasifier through lower ER. This can be explained by the fact that the FRs of test nos. 1 and 2 are less than that of test no. 3, and this means that the combustion heat and hot product gas yield are less than those of test no. 3, while the heat loss to the wall or sensible heat loss of the product gas to the fresh feedstock is relatively similar to that of test no. 3.14 The temperature of test no. 4 decreased with an increase in the feeding rate and a decrease in the ER. Because the temperature in the gasifier is rather stable and uniform, the mean

pyrolysis: Cx HyOz (biomass) → charcoal + tar + volatile gases (8) E


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C(s) + O2 = CO2 + 393 kJ/mol

(9)

CO + 0.5O2 = CO + 283 kJ/mol

partial oxidation:

(10)

hydrogen oxidation: H 2 + 0.5O2 = H 2O(g) + 286 kJ/mol

(11)

methane oxidation: CH4 + 1.5O2 = CO + 2H 2O + 357 kJ/mol water−gas:

(12)

C + H 2O ↔ H 2 + CO − 131.3 kJ/mol (13)

water−gas shift: CO + H 2O ↔ CO2 + H 2 + 41.2 kJ/mol

Boudouard:

C + CO2 ↔ 2CO − 172.1 kJ/mol

(14) Figure 6. Cold gas efficiency and carbon conversion efficiency as a function of the ER.

(15)

steam reforming: CH4 + H 2O ↔ CO + 3H 2 + 206.2 kJ/mol

tar reforming:

(16)

⎛ 1 ⎞ CpHq + pH2 O → pCO + ⎜p + q⎟H 2 ⎝ 2 ⎠ (17)

CpHq +

⎛p q⎞ q ⎜ + ⎟O2 → pCO + H 2O ⎝2 4⎠ 2

(18)

A higher value of the ER results in more CH4 burning with O2 and inhibiting the formation of CH4 at higher temperature. Therefore, the volume fraction of CH4 decreased with increasing ER. In addition, CnHm, which represents light hydrocarbons (C2 and C3) produced in the gasification process with a total volume fraction of around 2%, decreased slightly with the varying ER. Although the concentrations of CH4 and CnHm were changed slightly, they have much higher LHV values than those of CO and H2. Thus, a slight change in their concentrations can have a significant influence on the value of LHV. Because the volume fraction of combustible gases (H2, CO, CH4, and CnHm) decreased as a result of increasing ER, the LHV of the product gas decreased as well. Especially with ER > 0.38, more N2 was supplied to the gasifier, which diluted the combustible gases, resulting in rapidly decreasing LHV. From Figure 5, it can be seen that in all cases LHV exceeded 4.0 MJ/ N·m3 when the ER was lower than 0.4, which means the syngas produced is suitable for use as a fuel for syngas engines.14 Variation of the cold gas efficiency and carbon conversion efficiency is used to investigate the effect of the ER on the energy and mass conversion. The results are shown in Figure 6. With an increase in the ER, the cold gas efficiency increased first and reached its maximum of 63.28% at ER = 0.27 and remained nearly constant with a further increase of the ER to 0.35. As the ER further increased, although the temperature in the riser still increased, as shown in Figure 7, the cold gas efficiency showed a sharp decrease, which meant that more combustible gas was burned during the process. The carbon conversion efficiency represents the carbon conversion from biomass fuel to product gas, which is the main element of conversion. With an increase in the ER from 0.24 to 0.35, the carbon conversion efficiency increased from 75.24% to 90.22%.

Figure 7. Temperature and tar yield as a function of the ER.

As the ER further increased, the carbon conversion efficiency did not show any significant change. Furthermore, higher temperature elicited a passive effect on the formation of tar and a positive effect on the tar decomposition via reaction (18). As a result, the tar yield in the product gas decreased with the ER, as shown in Figure 7. It was observed that the mean temperature in the riser increased from about 755 to 914 °C with an increase in the ER from 0.24 to 0.41, and the tar yield in the gas decreased rapidly from 38.2 to 10.7 g/m3. Similar trends were described in a report by Wei et al.:17 the gas yield increased with the reactor temperature due to enhanced endothermic steam reforming, tar cracking reactions, and char gasification at elevated temperatures. Kinoshita et al.18 also reported that the product gas yield and gas efficiency increased as the temperature increased because higher temperatures facilitated tar conversion. The above analysis suggests that an increase in the ER can improve the reaction temperature, facilitate carbon conversion, and decrease the tar yield. However, excessive increase in the ER also decreases the LHV, and the cold gas efficiency decreased sharply with ER > 0.35. Here, from the viewpoint of obtaining higher LHV and cold gas efficiency and lower tar yield, 0.31 is considered to be the appropriate ER. Hence, in the F



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following tests, ER = 0.31 was used to maintain a satisfactory gasification performance. 3.2. Effect of the FR. For a specified gasifier, the effective gasification space is certain (about 0.64 m3 of the riser in this study). Thus, accelerating the FR is beneficial for increasing the solid suspension in the gasifier, which has a positive effect on the heat and mass transfer between solids. Therefore, a study is carried out to investigate its effect on the product gas and tar yield. Five experiments (test nos. 4, 8, and 11−13) were performed at different FRs, with the ER kept at 0.31. Figure 8 showed the effect of the FR on the gas composition and LHV. It was found that the volume concentration of H2 in

Figure 9. Cold gas efficiency and carbon conversion efficiency as a function of the FR.

conversion efficiency increased from 77.37% to 93.03% as the FR increased to approximately 352 kg/h and then decreased to 92.24% as the FR continuously increased to 410 kg/h. This may be explained by the fact that the decreasing residence time causes more energy and mass loss due to the decreasing time for the gasification reactions as discussed above. This finding suggests that increasing FR is favorable toward promoting the mass and energy conversion when its value is lower than 352 kg/h. Usually, in a certain range, a higher value of the FR accelerates the rate of reactions in the gasifier, especially stronger oxidization reactions promoting an increase of the temperature, while excessive FR leads to more water evaporation of the fuel particles in the riser, resulting in a decrease in the temperature. As shown in Figure 10, the

Figure 8. Gas composition and LHV as a function of the FR.

the product gas increased initially from 5.82 to 8.09 vol % by increasing the FR from 210 to 310 kg/h and decreased with a further increase in the FR. The CO concentration increased from 11.81 to 17.38 vol %, whereas the CO2 concentration decreased from 19.73 to 17.48 vol % with variation of the FR from 210 to 410 kg/h. It was estimated that the effective contact between particles increased with increasing FR and the mass and heat transfer was accelerated also, especially in reactions involving carbon [reactions (13) and (15)]. Increasing FR led to increases of the concentrations of CH4 and CnHm from 2.61 to 3.82 vol % and from 1.94 to 2.44 vol %, respectively. Thus, the LHV increased gradually from 4.2 to 5.7 MJ/N·m3 with an increase in the FR from 210 to 352 kg/h due to increases of the concentrations of CO, CH4, and CnHm, as shown in Figure 8. However, no significant change was observed as the FR further increased. According to the study of Andreux et al.,19 the mean residence time decreases with increasing particle feeding rate. The mean residence time of the fuel particles in the gasifier was estimated according to Miccio et al.,20 showing that its value decreased from 19.3 to 14.8 s with an increase in the FR from 210 to 410 kg/h. Thus, the decreasing residence time at high FR may be unbeneficial for biomass gasification cracking and reforming reactions, which may reduce the concentrations of H2 and CO. The results of the cold gas efficiency and carbon conversion efficiency varying with the FR are presented in Figure 9. Higher FR favored more biomass conversion into gas initially, and the cold gas efficiency increased to approximately 68% at FR = 352 kg/h. The variation trend of the carbon concentration efficiency was similar to that of the cold gas efficiency. The carbon

Figure 10. Temperature and tar yield as a function of the FR.

temperature in the riser increased first and reached a maximum value of 847 °C at FR = 310 kg/h and then decreased to 772 °C as the FR continuously increased to 410 kg/h. The temperature in the riser plays an important part in tar generation and secondary cracking reactions. In addition, the higher gas yield as a result of higher FR resulted in a shorter gas residence time in the riser, which was unfavorable to tar G



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cracking and reforming reactions. As a result, the tar yield variation was contrary to the temperature and reached a minimum value of 24 g/Nm3 at FR = 310 kg/h. 3.3. Effect of the S/B. During the experiments, the temperature of the gas in the pipeline ahead of the washing tower was about 700−800 °C, and this sensible heat can used to produce steam by a heat exchanger. An electrical boiler was used to produce additional steam as a complement if necessary. The temperature of the steam was about 150 °C. With changes in the steam flow rate supplied into the riser, the effect of the S/ B on the gasification performance was investigated. Here, the FR was kept at 310 kg/h, the ER was kept at 0.31, and only the S/B changed from 0 to 0.55 (test nos. 14−19). The steam flow rate was measured with an inline vortex steam flowmeter. The results of the effects of the steam ratio on the gas composition and LHV are shown in Figure 11. When steam Figure 12. Cold gas efficiency and carbon conversion efficiency as a function of the S/B.

conversion efficiency reached their maximum values of approximately 72.92% and 93.86% at S/B = 0.23 and 0.35, respectively. As the S/B further increased, the cold gas efficiency and carbon conversion efficiency showed significant decreases. For all of the runs in the present study, the overall cold gas efficiency and carbon conversion efficiency were within 62−73% and 87−94%, respectively. Analysis of tar and the fraction of unconverted solid carbon had not been included in the study, and these components might account for the rest. Variation of the tar yield with an increase in the S/B is shown in Figure 13. The increase of the water concentration in the

Figure 11. Gas composition and LHV as a function of the S/B.

was supplied, the LHV of the product gas increased to around 5.8−6.0 MJ/N·m3 when the S/B was 0.23−0.43. However, when the steam ratio was higher than 0.43, the LHV decreased again. This can be explained by variation of the gas composition. Increasing the steam ratio from 0 to 0.55 led to an increase of the CO2 concentration (from 18.21 to 23.63 vol %) and a decrease of the CO concentration (from 15.62 to 13.01 vol %). The concentration of H2 increased gradually and reached maximum values of 16.05 vol % at S/B = 0.35, whereas it decreased a little to 14.78 vol % as the S/B further increased to 0.55. The CH4 concentration also increased first and then decreased in a small range, whereas no significant changes were detected in the formation of CnHm with a concentration of around 2 vol %. The changes of the CO and H2 concentrations were mainly due to reaction (13) of char with steam and the water−gas shift reaction (14). When the S/B was higher than 0.43, the decrease in the LHV was probably due to the fact that excess steam led to a decrease in the riser temperature, as shown in Figure 13. Therefore, reaction (14) might become more dominant, which led to a CO2 concentration increase in the product gas, resulting in a decrease in the combustible gas concentration.21 According to variation of the gas composition, the calculated cold gas efficiency and carbon conversion efficiency are given in Figure 12. As the S/B increased from 0 to 0.35, it was observed that the higher steam supplement favored more energy and carbon conversion. The cold gas efficiency and carbon

Figure 13. Temperature and tar yield as a function of the S/B.

gasifier increased the tar secondary reaction via reaction (17), leading to a little decrease of the tar yield in the gas, whereas a further increase in the steam resulted in a significant increase in the tar yield due to decreasing temperature. According to the experimental results of Li et al.,13 the tar concentration primarily depends on the operating temperature. Steam injection lowers the operating temperature, and this could lead to a higher tar yield in a gasification system without an external heat source. Meng et al.15 also reported that a higher steam ratio promoted the tar decomposition when the temperature was kept at 770−880 °C. This suggests that the H



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temperature is a major factor for the reduction of tar compared with the steam ratio, and the temperature primarily depends on the air ratio, which determines the operating temperature.

4. CONCLUSIONS Air-blown gasification was investigated for the production of syngas in a pilot-scale (CFB) gasifier using herb residue as the fuel material. Two high-temperature cyclones were designed to capture coarse particles in the gas to improve the mass and energy conversion of the system. Pilot-plant tests of biomass gasification indicate that the product gas composition and heating value depend heavily on the ER, FR, and S/B. An increase in the ER led to a decrease of the volume concentration of combustible gases (H2, CO, CH4, and CnHm), resulting in a decrease in the LHV, whereas the LHV exceeded 4.0 MJ/N·m3 in all cases when the ER was lower than 0.4. An increase in the ER can improve the reaction temperature, facilitate carbon conversion, and decrease the tar yield. However, the cold gas efficiency decreased sharply with ER > 0.35. The concentrations of CO, CH4, and CnHm increased with increasing FR, leading to an increase in the LHV of the product gas. With an increase of the FR from 210 to 352 kg/h, the LHV increased gradually from 4.2 to 5.7 MJ/N·m3, whereas no significant change was observed as the FR further increased. Higher FR favored more biomass conversion into gas initially and the cold gas efficiency and carbon conversion efficiency reached their maximum values of approximately 68% and 93% at FR = 352 kg/h, whereas both of them decreased with a further increase in the FR. Furthermore, the tar yield in the gas increased when the FR was higher than 310 kg/h due to decreasing temperature and shorter gas residence time. When steam was supplied, the LHV of the product gas increased and reached a maximum value of about 6.0 MJ/N·m3 when the S/B was 0.23−0.43. When the steam ratio was higher than 0.43, the LHV decreased again. The higher concentration of H2O in the riser favored H2 generation, and the maximum value of the H2 concentration reached 16.05 vol % at S/B = 0.35. The overall cold gas efficiency and carbon conversion efficiency were within 62−73% and 87−94%, respectively, for all of the tests in the present study. The increase of the water concentration in the gasifier increased the tar secondary reaction, leading to a decrease of the tar yield in the gas, whereas a further increase in the steam resulted in an increase in the tar yield due to decreasing temperature.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86 516 83592000. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Fundamental Research Funds for the Central Universities (China University of Mining and Technology; Grant 2014QNB04).

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REFERENCES

(1) Dong, L.; Xu, G. W.; Suda, T.; Murakami, T. Potential approaches to improve gasification of high water content biomass I



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