Fluidized Bed Two-Stage Gasification Process for Clean Fuel Gas

Aug 19, 2016 - A new two-stage gasification process, decoupling complex biomass gasification from biomass pyrolysis and char gasification, has been ...
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Fluidized Bed Two-Stage Gasification Process for Clean Fuel Gas Production from Herb Residue: Fundamentals and Demonstration Xi Zeng,† Yuping Dong,‡ Fang Wang,†,§ Pengju Xu,⊥ Ruyi Shao,†,∥ Pengwei Dong,† Guangwen Xu,*,† and Lei Dong*,⊥ †

State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China ‡ School of Mechanical Engineering, Shandong University, 250061 Jinan, PR China § School of Chemical & Environmental Engineering, China University of Mining & Technology, Beijing 100083, PR China ⊥ Shandong Baichuan Tongchuang Energy Company Ltd., 250101 Jinan, PR China ∥ College of Chemical Engineering, Inner Mongolia University of Technology, Hohhot 010051, PR China ABSTRACT: A new two-stage gasification process, decoupling complex biomass gasification from biomass pyrolysis and char gasification, has been proposed for the production of clean industrial fuel gas. In this work, Chinese herb residues will be used as raw material, and the fundamental studies and demonstration of this process were conducted on an externally heated laboratory two-stage gasification setup and an industrial demonstration plant, respectively. The fundamental studies found that the appropriate operation of the upstream fluidized bed (FB) pyrolyzer occurred at 700 °C and the suitable conditions for effective tar removal in the downstream gasifier were as follows: 850 °C, an equivalent ratio of air (ER) at 0.04, and a retention time of tarcontaining fuel gas above 0.9 s. On the basis of these fundamental data, an autothermal demonstration plant treating 600 kg of herb residue per hour was built and successfully commissioned for its continuous running to verify the technology feasibility. The running data showed that the tar content in the gasified gas was as low as 400 mg/Nm3 at temperatures of 700 °C for the FB pyrolyzer and 850 °C for the transport fluidized bed gasifier. The produced fuel gas had a heating value of ∼5.0 MJ/Nm3. All of these displayed well the technical characteristics and demonstrated the process feasibility for this newly developed gasification technology. circulating fluidized bed gasifier (CFB) at a feeding rate of 300 kg/h. Under the optimized operating conditions for gasifying herb residue, the LHV of the generated fuel gas and the tar content were in the range of 4−5 MJ/Nm3 and 14.4−15.0 g/ Nm3, respectively. Hofbauer et al.16 developed a representative dual fluidized bed (DFB) gasification process composed of a FB gasifier and a transport fluidized bed (TFB) combustor, and an industrial demonstration plant was built at Güssing and has run successfully since 2002. The corresponding heating value of the generated fuel gas and the tar content in it were ∼12−14 MJ/ Nm3 and ∼5−15 g/Nm3, respectively. As a summary of the typical FB gasification processes for biomass, including a BFB gasifier, a CFB gasifier, and a DFB gasifier,17,18 it can be found that the larger amount of tar in the generated gasified gas is still a great challenge,19 which will condense below its dew point, resulting in the clogging of the fuel lines, filter, and downstream equipment, the poisoning of the catalyst, and the production of phenol-containing wastewater. Generally, tar removal technologies can be mainly divided into in-bed and out-bed eliminating technology by the methods of physical treatment, thermal cracking, partial oxidation, catalytic reforming, etc.20 Among the numerous catalysts, char is considered as a promising alternative in large-scale

1. INTRODUCTION Today, China is the world’s largest producer and consumer of herb medicine, having more than 1500 companies and a production of herb residue of ∼15 million t per year.1,2 After active ingredients have been extracted, herb residue has a high moisture content (>70%), nutrient components, and drug residue, making it easy for them to decay and thus pollute water resources and release irritant gas.3 However, because of their easy collection, C and H richness, and good reaction activity, herb residue is also considered as a promising renewable energy source.4 Therefore, from the viewpoint of industry utilization, determining how to reuse and recycle herb residue cleanly and highly effectively is very necessary and essential. At present, gasification is considered as an attractive technology for efficiently converting carbon-containing solid waste into syngas and fuel gas.5−7 According to the differences in reactor structure, the existing gasification process can be mainly divided into a fixed bed gasifier,8−11 a fluidized bed gasifier, and an entrained-flow bed gasifier. However, for producing industry-use fuel gas, a fluidized bed gasifier is always more promising and suitable with obvious merits of high throughput, wide distribution for raw material size (10 g/Nm3, respectively. Guo et al.15 designed and developed a © XXXX American Chemical Society

Received: April 1, 2016 Revised: July 22, 2016

A

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Properties of Herb Residue Used in Our Experimentsa proximate analysis (wt %)

a

ultimate analysis (wt %)

Mad

Aad

Vad

FCad

Cdaf

Hdaf

Sdaf

O*daf

Ndaf

LHV (MJ/kg)

14.12

4.32

66.98

14.58

51.64

5.60

0.16

41.22

1.38

16.37

Legend: ad, air-dried basis; daf, dry and ash-free basis; asterisk, by difference. lower heating value was ∼16.37 MJ/kg, showing the essential properties required for its use as a fuel. The particle size of herb residue employed in this study was in the range of 0.5−1.0 mm. Prior to its use in experiment, the feedstock was dried in air at 105 °C for 2 h. Figure 1 shows the laboratory two-stage gasification apparatus used for the fundamental studies, mainly including a screw feeder, two

applications because of the developed porous textural structure, high reliability, low cost, natural production from a gasifier, and easy recovery after deactivation.21,22 On this basis, the gasification process combined with tar removal technology by hot char bed becomes more and more attractive.23,24 For example, Kim et al.25 developed a two-stage gasification process to examine the tar removal ability, which mainly consisted of a fluidized bed gasifier and an updraft fixed bed reactor filled with activation carbon. Zeng et al.26 conducted experiments on a pilot autothermal two-stage gasifier composed of a fluidized bed pyrolyzer and a downdraft fixed bed gasifier. Henriksen et al.27 designed and built a typical two-stage gasifier for wood chips, mainly including a screw convey pyrolyzer and a downdraft fixed bed gasifier. Although all of these processes have proved the availability of a hot char bed on tar removal, some limitations remain, such as a larger operating pressure drop in a fixed bed reactor, scale-up difficulty, and poor feedstock flexibility. On this basis, a new fluidized bed two-stage (FBTS) gasification technology has been proposed in this work to produce clean fuel gas from carbon-containing solid fuel. It is mainly composed of a FB pyrolyzer (the first stage) and a TFB gasifier (the second stage). In the first stage, biomass is autothermally pyrolyzed or partially oxidized. Then, all the pyrolysis products will overflow to the second stage, in which it mainly performs the reactions of char gasification and fuel gas improvement. Depending on the catalytic reforming of char in gasifier, the tar in fuel gas will be decomposed and converted into incondensable gas compositions with small molecules. Via combination of the merits of the two reactors adopted, the newly proposed FBTS gasification process is very suitable for the treatment of granular feedstock (particle size of 3 11.66

w0 − wi w0 − wash

0.5 τ0.5

(1)

(2)

where w0, wi, and wash are the initial quality, the actual quality of the char sample, and the ash quality in it, respectively, and τ0.5 is the reaction time at x of 50%. The greater the value of Rs, the better the reactivity of char sample. In the industrial demonstration plant, the gas products from the gasifier were sucked and flowed through a condensing apparatus to collect tar and fuel gas. The gaseous product was conducted by purification treatment by removing the moisture and impurities before analysis via a combustible gas analyzer from Wuhan Cubic Co., Ltd. The tar-containing liquid was separated by removing acetone in a vacuum rotary evaporator below 30 °C.

Figure 2 shows a flowchart of the industrial demonstration plant, with a feeding rate of 600 kg/h for feedstock and a total height of 17

3. RESULTS AND DISCUSSION 3.1. Pyrolysis Characteristics of Herb Residue. The pyrolysis of herb residue in the upstream FB reactor is closely related to the reaction of char isothermal gasification and tar removal in the reactor in the second stage. Figure 3 shows the distribution of (a) pyrolysis product and the yield of all the gas components (b) under pyrolysis temperatures from 400 to 800

Figure 2. Process diagram of the demonstration plant: (1) herb residue, (2) conveyor, (3) hopper, (4) screw feeder, (5) pyrolyzer, (6) gasifier, (7) primary cyclone, (8) loop seal, (9) secondary cyclone, (10) primary heat exchanger, (11) seconday heat exchanger, (12) air compressor, (13) boiler, (14) draft fan, amd (15) chimney. m. This autothermal testing apparatus mainly included a screw feeder, a FB pyrolyzer, a TFB gasifier, two cyclones, a loop seal, two heat exchangers between air and fuel gas, two air compressors and gassupplying systems (for two reactors), a fuel gas burner, and a sampling port for measuring the gas of gasification. The upstream and downstream stages were connected by a high-temperature-resistant pipe. The effective heights for the pyrolyzer and gasifier were 1.2 and 13 m, respectively. The temperature and pressure in this system were monitored by thermocouples and pressure sensors, respectively, and the data were recorded at an interval of 10 s by computer. In operation, both of the FB and TFB reactors were first preheated via lighting hardwood char to elevate the reaction temperatures to the desired values, and the rates of flow of the gasification agent into the reactors were adjusted gradually to form a good fluidized state of the solid heating carrier. Then, the screw feeder was started, and the feeding rate was gradually regulated to reach the desired value. At the same time, all the pyrolysis products from the first stage were transported into the second stage to conduct char gasification and tar catalytic reforming. During this period, the temperatures in the bottom of FB and TFB reactors were controlled at ∼750 and ∼850 °C, respectively. After the separation of the solid particles by two cyclones and recycling heat by two heat exchangers, the produced fuel gas was

Figure 3. Product distribution of herb residue pyrolysis at different temperatures. C

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels °C in a N2 atmosphere with a feeding rate of 10.0 g/min. From them, one can see that with the increase in reaction temperature, the char yield quickly decreased from 53.9 to 40.2 wt %, while the yield of pyrolysis gas increased strongly from 13.6 to 46.5 wt %. Interestingly, the yield of tar first increased with a maximal value at 500 °C and then decreased gradually, which was strongly related to the occurrence of secondary cracking reactions of tar at high temperatures. As for the gas components, at 400 °C, the gas mainly consisted of CO and CO2. With the increase in pyrolysis temperature, the yield of H2 and CO increased quickly while that of CH4 and CnHm displayed a tendency to first increase and then decrease, with maximal values at 600 and 700 °C, respectively. Moreover, the content of CO2 increased quickly until 700 °C, and then with an increase in degree, the rate became slow above 700 °C. Figure 4 illustrates the change in gasification reactivity of chars produced at different pyrolysis temperatures. One can see

Figure 5. Tar content and gas composition at different temperatures in the second stage of the laboratory testing apparatus without oxygen and char.

CO2, and CH4 increased gradually, and that of CO had a tendency to first increase below 800 °C and then become steady above 800 °C. In addition, the yield of CnHm reached its maximum at 800 °C. During the period of thermal cracking, the major sources of H2 and CO are the decomposition reaction of alkane compounds, the cyclodehydrogenation reaction of oxygen-containing heterocyclic compounds, and the aromatization reaction of other aromatic compounds.32,33 Thus, such enhanced reactions at higher temperatures led to the increase in the level of production of CO, H2, and CH4. The partial oxidation of tar was examined by adding a given amount of O2 to the fixed bed reactor operated at 850 °C. As illustrated in Figure 6a, with an increasing ER from 0 to 0.06, the yield of tar decreased very sharply, showing that, at the experimental temperature, even a small mass of O2 was quite available for tar elimination. The contents of CO and CO2 in the produced gas increased with the rise of ER because of the effect of partial oxidation on tar components. The levels of H2 and CH4 produced first increased and then declined, reaching their maxima at an ER of 0.04. Without a doubt, too much O2 would decrease the tar content but also burn the effective components in the fuel gas, such as H2 and CH4. Thus, there should be an optimal ER. The catalytic effect of char on tar removal was investigated by varying the height of the char bed in the range of 3−11 cm in the second stage but holding the reaction temperature at 850 °C and the ER at 0.04. Under these conditions, the retention time of the tar-containing pyrolysis gas from the first stage passing by the char bed in the second stage changed from 0.3 to 0.9 s. Figure 7a indicates that char had a significant effect on tar destruction, making the yield of tar decrease from 0.49 to 0.08 wt % under the experimental conditions. Increasing the height of the char bed will prolong the retention time of tar

Figure 4. Gasification reactivity in CO2 of chars made at different pyrolyzing temperatures in the laboratory testing apparatus.

that the reaction reactivity of char with CO2 increased with an increase in its preparation temperature and reached the maximum at a pyrolysis temperature of 700 °C. Many studies reported that the reaction reactivity of the char sample with a gasification agent was strongly related to the carbon microcrystal structure and porous structure of char.29 Elevating the pyrolysis temperature in a N2 atmosphere would promote the release of volatiles and thus lead to formation of more micro- or mesopores. Above 700 °C, the microcrystalline structure of carbon in the char became more ordered, lowering the char gasification reactivity strongly.30 Therefore, in light of the adaptation to the FBST gasification technology displayed in Figure 2, the suitable operating parameters for the FB reactor in the first stage should be at ∼700 °C. 3.2. Tar Removal over Char. The performance of the tar reformer has a significant effect on the quality of fuel gas. Generally, a high reaction temperature, partial oxidation, and catalytic cracking were the main methods for tar elimination in a gasifier.31 The actual performance of tar removal was examined in the fixed bed reformer by varying the temperature, oxygen content in the cracking atmosphere, and retention time of pyrolysis gas through the reactor. Upstream pyrolysis was performed under the optimal conditions mentioned in section 3.1. Figure 5 displays the influence of thermal cracking on tar removal and fuel gas upgrading. With an increase in the reaction temperature of the tar reformer from 750 to 900 °C, the tar yield decreased from 18.0 to 11.0 wt %, showing the strong cracking ability at high temperatures. The yields of H2, D

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX

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oxidation, and catalytic removal by the active ingredient in the char sample.34,35 In comparison with the quality of the gas from the second stage without char, adopting the char bed layer in the second stage had an obviously promoting effect on the yield of H2 and CO (especially H2) but decreased the content of CH4. These variations in terms of gas yield were strongly related to the tar thermal and catalytic conversion. 3.3. Verification in the Demonstration Plant. The industrial demonstration test of the newly proposed FBTG gasificaiton was conducted using air as the gasification reagent. The total steady-state operation time for the plant shown in Figure 2 was up to 1000 h, and each test usually lasted ∼170 h. Figure 8a shows the partially operating temperatures in the FB

Figure 6. Tar content and gas composition under different excessive air ratios in the second stage of the laboratory testing apparatus at 850 °C.

Figure 8. Typical time series temperature (a) and pressure drop (b) in the pyrolyzer and gasifier of the demonstration plant.

pyrolyzer and TFB gasifier for a typical operation with an actual feeding rate of 510 kg/h for herb residue. According to the fundamentals mentioned above, during steady-state operation, the operating temperatures of the reactors for the first and second stages were maintained at 700 and 850 °C, respectively. Under these conditions, the air flow rates for the pyrolyzer and gasifier were ∼385 and ∼295 Nm3/h, respectively. From Figure 8a, one can see that within the tested period, the pilot plant ran continuously and steadily. Figure 8b displays the pressure drops at the bottom of the FB and TFB reactors during stable operation for the case mentioned above. During the initial stage, the pressure drops of the pyrolyzer and gasifier were relatively stable, with values of 1.5 and 2.5 kPa, respectively. This further indicated the good particle fluidization in reactors during their operation. The main fluctuation in the pyrolyzer was strongly related to the batch operation of the loop seal, which returned char and ash from cyclone once or twice for half an hour, while the main fluctuation in the gasifier was attributed to the discharge of ash from the gasifier for maintaining a stable bed height of char and ash in the gasifier. Figure 9 shows the time series of fraction variations for each gas product and the corresponding higher heating value (HHV) for the combustible gas components from the exit of the second stage, which further demonstrated the relatively stable operation in the pilot plant. The average values of the main

Figure 7. Tar content and gas composition at different residence times in the second stage of the laboratory testing apparatus at 850 °C and an ER of 0.04.

components in the second stage, providing more opportunities for undergoing the reactions of thermal decomposition, partial E

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX

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technical features. Rather improved performance would be also expected for this two-stage gasification process running with rather high capacities and under further optimized conditions.

■

AUTHOR INFORMATION

Corresponding Authors

*Phone and fax: 86-10-8254886. E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The study was financed by the Natural Science Foundation of China (21306209), National High-tech Research and Development Projects (2015AA050505-02), and the Major Program of National Natural Science Foundation of China (U1302273).

Figure 9. Typical time series composition and heating value of the produced fuel gas in the demonstration plant.

■

gas species, H2, CO, CH4, and CO2, were 7.80, 18.05, 4.25, and 10.13 vol %, respectively, and the corresponding HHV was ∼5.0 MJ/Nm3. After the tar in the cooling system had been collected, the tar content in the gasified gas can be calculated to be ∼400 mg/Nm3. In fact, Guo et al.15 had conducted herb residue gasification on a BFB with a feeding rate of 300 kg/h using air as the gasifying agent. The resulting H2, CO, CH4, and CO2 gas contents were ∼6.9, ∼10.2, ∼4.0, and ∼17.8 vol %, respectively, and the corresponding gas HHV and tar content were ∼3.9 MJ/Nm3 and ∼15 g/Nm3, respectively. Comparing with the results from literature listed in Table 3, one can see that fluidized bed two-stage gasification can effectively crack tar and thus promote the formation of effective gas components, such as H2, CO, and CH4, and improve the gas heating value. This fully agreed with the fundamentals.

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4. CONCLUSIONS A new FBST gasification technology composed of a FB pyrolyzer and a TFB gasifier has been researched and developed to treat herb residue for the production of clean industrial fuel gas. This article mainly focused on the results from the related laboratory research and the demonstration test. Using the externally heated laboratory two-stage apparatus, experiments clarified that the appropriate operation for the first stage occurred at 700 °C and the suitable conditions for the effective tar removal over char bed of the downstream gasifier should be as follows: 850 °C, an equivalent ratio of air at 0.04, and a retention time of tar-containing fuel gas of >0.9 s. On the basis of fundamentals, a self-heating demonstration plant with a feeding rate of ∼600 kg of biomass per hour was designed, built, and operated to treat herb residue in an air atmosphere. The typical operating temperatures of the plant were 700−750 °C for pyrolysis and 800−900 °C for gasification. At the stable stage, the tar content in the produced fuel gas was as low as 400 mg/Nm3 and the produced fuel gas had a heating value of ∼5.0 MJ/Nm3. Both of these verified the process feasibility and

Table 3. Comparison of Gasification Properties from the Literature and the Newly Proposed Process gasification technology

author

feedstock

treating capacity (t/h)

heating value (MJ/Nm3)

tar content (g/Nm3)

bubbling FB CFB double FB FB two-stage

Bhaird et al.14 Guo et al.15 Hofbauer et al.36 Zeng et al.

wheat straw herb residue wood chips herb residue

0.075 0.3 1.5−2.0 0.6

3.6 4−5 12−14 >5

>10 14.4−15.0 5−15 11.66

F

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX

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G

DOI: 10.1021/acs.energyfuels.6b00765 Energy Fuels XXXX, XXX, XXX−XXX