Experimental study for dimethyl ether production from biomass gasification and simulation on dimethyl ether production

Experimental study for dimethyl ether production from biomass gasification and simulation on dimethyl ether production

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b i o m a s s a n d b i o e n e r g y 3 9 ( 2 0 1 2 ) 6 7 e7 2

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Experimental study for dimethyl ether production from biomass gasification and simulation on dimethyl ether production Jie Chang a,*, Yan Fu a, Zhongyang Luo b a b

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China College of Mechanical and Energy Engineering, Zhejiang University, Hangzhou 310027, China

article info

abstract

Article history:

A novel route for DME production from biomass was proposed and tested in a bench scale

Received 23 April 2010

experimental system. A CueZneAl/HZSM-5 (16.7 wt%) was developed for DME synthesis

Received in revised form

from the biomass-derived syngas. At 1113 K, atmospheric pressure, the feed rate of

12 January 2011

0.6 kg biomass/h, ER of 0.28, and CO2/biomass ratio of 0.327, 1.4 Nm3 of raw gas kg1

Accepted 19 January 2011

biomass with LHV of 8.36 MJ m3 was produced. With reforming by the biogas addition of

Available online 17 February 2011

0.54 Nm3 kg1 biomass, the yield of syngas increased to 2.5 Nm3 from 1.4 Nm3 of raw gas,

Keywords:

based on kinetics equations for direct DME synthesis on CueZneAl/HZSM-5 catalyst that

Dimethyl ether

78.5% of CO conversion and 379 g DME per kg biomass could be obtained. Gasification with

and the ratio of H2/CO was adjusted to 1.18. At ideal synthesis conditions, it was simulated

Synthesis

CO2 agent and sequent co-reforming with biogas supplies great potential for high efficient

Biomass

production of DME from biomass. ª 2011 Elsevier Ltd. All rights reserved.

Gasification Simulation

1.

Introduction

Biomass feedstocks, such as agriculture and forestry residues, play an important role in developing alternatives to fossil fuels. Among the renewable energy sources, only biomass offers the possibility to produce liquid, carbon neutral fuels. DME is a colorless gas at an ambient condition and easily liquefied under pressure. Since the physical and chemical characteristics of DME are very similar to those of liquefied petroleum gas (LPG), it is an easy substitute of LPG. The advantages of using DME as a diesel replacement include higher cetane number, reduced NOx emissions and zero smoke emission [1]. Therefore, biomass is considered to be one of the best renewable energy sources to contribute to increasing worldwide fuel needs. DME derived from biomass

as a sustainable energy resource is an extremely promising carbon neutral fuel because CO2 generated by the use of DME can be offset by CO2 fixed by photosynthesis [2,3]. The conventional DME production method is via the dehydration of methanol. In contrast, producing DME directly from syngas in a single step over hybrid catalysts (methanol synthesis catalyst and solid acid catalyst) has many economic and technical advantages over methanol dehydration [4e6]. Thermodynamically, DME production from syngas is more favorable than from methanol, thus the costs of producing DME from syngas should be lower. Syngas can be produced from natural gas, coal, and biomass. Biomass-derived DME production facilities typically consist of the following steps: pre-treatment, gasification, gas cleaning-up, gas reforming (to obtain appropriate H2: CO ratio), and DME synthesis.

* Corresponding author. Tel.: þ86 20 87112448. E-mail address: [email protected] (J. Chang). 0961-9534/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2011.01.044

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Hamelinck et al. [2] calculated the energy efficiency and cost of methanol produced from biomass. Overall HHV (higher heating value) energy efficiency is around 55% for methanol production. Assuming biomass is available at US$ 2 GJ1, a 400 MWth input system can produce methanol at US$ 9e12 GJ1 slightly above the current production from natural gas price. IEA (International Energy Agency) [7] reported that well-to-wheels energy efficiency of biomass gasification and conversion to DME was 56%, and the energy efficiency of petroleum refining to gasoline was 88%. Therefore, biomassderived DME is likely to become competitive fuels tomorrow. Although synthesis of DME from syngas is a conventional commercial process, few researches regarding the synthesis of liquid fuel such as methanol and DME from biomass as a total system have been reported [3]. The present authors built a DME production system from biomass gasification and carried out experiments to study the mechanism and optimize the operating conditions [8e11]. Through the study we found that the gasification of biomass and reforming of produced gas are the key steps in the DME production system, which greatly affect the energy efficiency and cost of the whole process. The production of DME via biomass gasification faces the problem of lower H2/CO ratio and an excess of carbon in the form of CO2 at the produced syngas. Considering the synthesis reaction of DME via syngas in a single step, the CO2 is the by product. According to the features of biomass gasification and synthesis of DME, we developed a novel process for DME production from biomass. The facilities consist of the following steps: biomass gasification with steam/CO2/air, co-reforming of produced syngas with biogas, cleaning-up of syngas, and DME synthesis. After co-reforming with biogas, the per-pass CO conversion and DME yield were calculated by the kinetic models of DME synthesis reactions. The results showed that this process would be a promising route for DME production from biomass.

2.

Experimental

The gasification experiments were performed in an atmospheric pressure, indirectly heated, fluidized-bed gasification system, which is shown schematically in Fig. 1. The experiment system is composed of a fluidized-bed gasifier, a biomass feeder, a steam generator, an air/CO2 compressor, a cyclone, a catalytic reforming reactor, and a DME synthesis reactor. Silica sand (0.2e0.3 mm) was loaded in the gasifier before test. The reforming catalyst NiOeMgO (0.3e0.45 mm) was loaded in the fixed bed reformer. The preparation and reduction method of catalysts was described elsewhere [12]. The DME synthesis catalyst CueZneAl/HZSM-5 was prepared by co-precipitation method [8], HZSM-5 content is 16.7 wt%, Cu/Zn/Al mole ratio is 6/3/1. The biomass was fed into the gasifier through a screw feeder driven by a variablespeed metering motor. The air/CO2 (as the gasification agent) was preheated to 338 K before entering the gasifier for better performance. The steam of 427 K from a steam generator was fed into gasifier. The raw gas from gasifier passed through the cyclone to separate particles, then after mixing with biogas (CH4 68 mol%, CO2 32 mol%, purchased from a gas company) entered into the reforming reactor. The reforming catalyst (NiOeMgO) was reduced under the conditions of 1023 K, atmospheric pressure, for 30 min by the raw syngas from the gasifier. The gas was sampled using gasbag from points A and B separately. The gas sample was analyzed on a gas chromatograph (Model GC-2010, SHIMADZU, Japan), which is fitted with a GS-Carbon plot column (30 m  0.530 mm  3.00 mm), with helium as carrier gas. After the removal of water, oxygen, sulfur and nitrogen compounds, the obtained syngas was pressurized by a compressor into the syngas container for DME synthesis.

Point B Point A

Filter

Dehydration Deoxgenization Syngas compressor

Reformer Cyclone Biomass feeder

Syngas container

Product Gasifier

Biogas

Steam boiler

Synthesis reactor

Flowmeter Air/CO2 compressor

Fig. 1 e Schematic diagram of bench scale experimental system.

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3.

Results and discussion

3.1.

Performance of gasification

The gasification of biomass can produce combustible mixture gas at 1073e1173 K. With suitable treatment the mixture gas can be used for the production of heat, power (by gas engines, gas turbines, fuel cells, etc.), transportation fuels or chemicals (hydrogen, methanol, dimethyl ether, or diesel through chemical conversion). Gasification is a robust proven technology that can be operated either as simple, low technology system based on a fixed bed gasifier, or as a more sophisticated system using fluidized-bed technology [13]. Most of gasification processes are air-blown gasification processes, which produce low calorific value gases with a typical higher heating value (HHV) of 4e7 MJ Nm3. Recently CO2 as a gasification agent attracted attentions for its higher gasification efficiency and higher heating value of produced gas [14e16]. The present authors carried out the gasification tests using CO2/air/steam as agents and using wood sawdust as feedstock, followed by reforming of produced gas. Eucalyptus sawdust (0.3e0.45 mm) obtained from a timer mill in Guangzhou city, China, was employed as biomass feedstock in the experiments, whose properties are listed in Table 1. The effects of different agents, such as air, air/steam, air/CO2, and air/CO2/steam, on the yield and quality of gasification products were thoroughly studied in experiments. The gas samples were obtained in points A and B as shown in Fig. 1. The analytical results of samples from point A were listed in Table 2. From Table 2, it is found that gasification using air only as the agent has the lowest gas yield and lower LHV (lower heating value) of produced gas. Both the addition of steam or CO2 can enhance the gasification of biomass and increase the gas yield and LHV of produced gas. Biomass was gasified in runs 1 through 5 using air as agent with the addition of steam in different steam/biomass ratio (S/B) at ER (equivalence ratio) of 0.28. With the increasing of S/B from 0 to 1.45, the yields of H2, CO, CH4 and C2 increased obviously, for example, the yield of H2 increased from 24.3 to 38.0 g kg1 biomass. However, with the further increase of S/B ratio, the content of these components decreased, as well as the LHV of produced gas. Gasification using CO2 and air obtained the highest LHV of produced gas (8.36 MJ Nm3), has similar gas yield with air/ steam gasification. The addition of CO2 effectively promoted the conversion of biomass, especially the intermediate, char (mainly made of carbon). The following are the reasonable reactions during air/steam/CO2 gasification [17].

Table 1 e Proximate analysis and ultimate analysis of eucalyptus sawdust (dry base). Proximate analysis wt%

16.99

0.29

8.30

DH298 ¼ 246.4 kJ/mol

(1)

C þ O2 / CO2

DH298 ¼ 408.8 kJ/mol

(2)

2CO þ O2 / 2CO2

DH298 ¼ 571.1 kJ/mol

(3)

C þ CO2 / 2CO

DH298 ¼ þ162.4 kJ/mol

(4)

C þ H2O (g) / CO þ H2

DH298 ¼ þ131 kJ/mol

(5)

CO þ H2O (g) / CO2 þ H2

DH298 ¼ 41.2 kJ/mol

(6)

CH4 þ 2H2O (g) / CO2 þ 4H2

DH298 ¼ þ165 kJ/mol

(7)

C þ 2H2 / CH4

DH298 ¼ þ74.85 kJ/mol

(8)

During the gasification process, the addition of CO2 increased the yield of CO (reactions (1), (4) and (5)). The increase of hydrogen content resulted in the increase of CH4 content in produced gas comparing to gasification with air only (reaction (8)). The above reactions can reasonably explain the results in Table 2. Gasification with air/CO2 agent is far better than that with air/steam from the point view of energy conservation. For example, run 6 obtained the gas with LHV of 8.36 MJ Nm3, which is higher than that in run 4 (steam/air gasification). However, the energy consumption per kg biomass in run 4 for heating water from room temperature to gasification reaction temperature (1113 K) was 5.93 MJ, which is about 21 times more than that heating CO2 in run 6. Therefore, gasification with CO2 as agent has great benefit in the utilization of biomass gasification.

3.2.

Performance of gas reforming

Obviously the produced gas from biomass air/steam/CO2 gasification is unfavorable for DME synthesis due to the low H2/CO ratio (less than 1.0), excess CO2 content (15 mol %-20 mol%), and more hydrocarbons (11 mol%-17 mol%). Increasing the H2 content for desired H2/CO ratio and useful syngas (CO þ H2) content is needed [18]. Reforming the raw gas with biogas (CH4 68 mol%, CO2 32 mol%) over NiOeMgO catalyst supply the potential to meet this demand and avoiding removing of CO2. The related reactions are as following:

CH4 þ CO2 / 2CO þ 2H2

(9)

CH4 þ 2H2O / CO2 þ 4H2

(10)

CO þ H2O / CO2 þ H2

(11)

Ultimate analysis wt% HHV

Volatile Fixed Ash Moisture C Carbon (wet base) 82.72

2C þ O2 / 2CO

H

O

N

S

MJ kg1

46.92 5.73 46.73 0.53 0.09 18.29

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Table 2 e Results of air/steam/CO2 gasification of eucalyptus sawdust (Nitrogen free). Run

1

2

3

4

5

6

7

8

9

10

0 0 1.03

0 0.754 1.40

0 1.06 1.45

0 1.45 1.54

0 1.67 1.40

0.327 0 1.40

0.327 0.754 1.62

0.327 1.06 1.59

0.327 1.45 1.37

0.327 1.67 1.25

Gas composition (mol %) in Point A H2 26.5 CO 41.7 20.8 CO2 7.9 CH4 3.1 C2

28.8 33.7 25.4 8.7 3.4

27.9 34.6 25.0 8.8 3.7

27.7 35.5 23.8 9.1 3.9

28.4 34.5 24.6 8.7 3.7

25.0 45.7 12.1 12.2 4.9

27.2 43.9 15.1 9.6 4.1

28.6 39.2 19.2 9.1 3.9

28.8 38.9 20.1 8.4 3.7

29.3 37.8 20.7 8.5 3.7

Gas yields (g kg1 biomass) in Point A 24.3 H2 CO 537.3 421.0 CO2 58.5 CH4 40.1 C2 6.70 LHV (MJ m3)

35.8 591.5 693.7 86.5 60.3 7.49

36.0 625.9 710.4 90.6 67.8 7.75

38.0 684.1 719.6 99.7 76.1 8.13

35.4 603.6 676.5 87.2 65.9 7.67

27.5 705.3 291.9 107.6 76.7 8.36

35.4 803.2 433.9 100.6 75.1 8.20

36.2 697.0 537.9 93.1 69.1 7.67

30.8 583.1 472.4 72.3 55.3 6.91

28.1 509.3 437.8 65.3 50.4 6.53

1

CO2/Biomass (kg kg ) Steam/biomass (kg kg1) Gas yields (Nm3 kg1 biomass)

Temperature ¼ 1113 K, sawdust feeding rate 0.6, air ¼ 0.7, ER 0.28, LHV 0.126 CO þ 0.108 H2þ0.359CH4 þ 0.665CnHm.

CnHm þ nCO2 / 2nCO þ m/2H2

(12)

In the runs 1 and 6, the produced raw gas was reformed by addition of biogas to adjust the H2/CO ratio for the better synthesis of DME. 0.54 Nm3 of biogas per kg biomass was preheated and fed into the reformer after point A to co-reform with raw gas at temperature of 1023 K. The composition and yield of samples from point B (after reforming) were listed in Table 3. With the biogas addition of 0.54 m3 per kg of biomass, the raw gas in run 6 was reformed at temperature of 1023 K to obtain 2.5 Nm3 of syngas kg1 biomass with the following typical composition (in mol%): H2 43.5, CO 36.9, CO2 13.7, CH4 5.0 and C2 0.9. The H2/CO ratio was adjusted to 1.18 from 0.55. After reforming by the addition of biogas, the syngas from run 1 had the similar composition with that from run 6. However, the syngas yield was about 53% lower than the one from run 6. Therefore, the DME production costs from the reformed syngas in run 1 should be remarkably higher than that in run 6. The reformed syngas from run 6 (air/CO2 gasification) was used to simulate the DME production by the following model.

3.3.

The production of liquid fuels, such as methanol and DME, from biomass gasification is one of the highest interests

Table 3 e Results of reforming by addition of biogas (Nitrogen free).

Gas yields (Nm3 kg1 biomass) H2/CO ratio Gas composition, mol% H2 CO CO2 CH4 C2

CO þ 2H2 / CH3OH

(13)

CO2 þ 3H2 / CH3OH þ H2O

(14)

2CO þ 2H2O / 2CO2 þ 2H2

(15)

2CH3OH / CH3OCH3 þ H2O

(16)

Therefore, the sum of these reactions is:

Simulation of CO conversion and DME yield

Run

among the conversion technologies of biomass to energy. Some researchers studied biomass gasification to supply syngas for the synthesis of liquid fuel and developed different catalysts based on reaction mechanisms [19e21]. From the standpoint of the supply of syngas for the synthesis of liquid fuel, the content of CO and H2 in gasification product is important. The following are the related reactions for the synthesis of methanol from hydrogenation of CO or CO2, and dehydration of methanol to produce DME.

1

6

1.62 1.23 43.9 35.5 15.8 4.1 0.7

2.50 1.18 43.5 36.9 13.7 5.0 0.9

3CO þ 3H2 / CH3OCH3 þ CO2

(17)

Based on the above reactions of direct DME synthesis, the kinetics equations for direct DME synthesis on CueZneAl/ HZSM-5 catalyst have been studied by the present authors [22]. The kinetic equations are simulated by Matlab software and listed as following:   k1 fCO fH22 1  fM =K2 fCO fH22 r1 ¼ rcohydrogenation ¼  pffiffiffiffiffiffiffiffiffiffiffiffiffiffi3 1 þ KCO fCO þ KCO2 fCO2 þ KH2 fH2   5:9917  104 k1 ¼ 1:796  105 exp  RT

;

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  k1 fCO2 fH32 1  fM =K1 fCO2 fH32 r2 ¼ rco2 hydrogenation ¼  pffiffiffiffiffiffiffiffiffiffiffiffiffiffi4 1 þ KCO fCO þ KCO2 fCO2 þ KH2 fH2   1:0727  105 k2 ¼ 3:257  107 exp  RT   k3 fM2 1  fD fH2 O =K3 fM2 r3 ¼ rMeOHdedyation ¼ 2  1 þ KM fM   2:3739  104 k3 ¼ 54:765exp  RT

;

;

We had carried out the experiments for DME production from biomass gasification using this bench scale experimental system. The syngas was produced by air/steam gasification of wood sawdust and followed by reforming with addition of biogas. The obtained N2-containing syngas was used directly for DME synthesis. The results showed that 75% of CO conversion and maximized DME yield, 224 g DME kg1 biomass were achieved under a gasification temperature of 1073 K, ER of 0.24, S/B of 0.72, and reforming temperature of 1023 K with the addition of biogas 0.54 Nm3 kg1 biomass [12]. Based on the obtained kinetic model, we used Matlab to simulate the direct DME synthesis reactions over CueZneAl/ HZSM-5 catalyst for syngas from run 6 (air/CO2 gasification combined with addition of biogas). The calculation showed that the optimized reaction conditions are 553 K, 4 MPa, and GHSV (gas hourly space velocity) of 1800 . Under those conditions, 78.5% of CO conversion could be obtained, and the corresponding DME yield was 379 biomass. Comparing with air/steam gasification, which got 224 g DME kg1 biomass, the yield of DME in this novel process increased about 65%. In addition, the energy consumption in air/CO2 gasification process was remarkably lower than that in air/steam gasification process, as discussed in 3.1. This showed great potential of DME production from biomass gasification with CO2 agent and co-reforming with biogas. Although bench scale indirectly heated fluidized-bed gasifier was employed in our experiments, the commercial utilization of this process is feasible. For example, the BCL (Battelle Columbus) atmospheric indirectly fired gasifier has some commercial experience (demo in Burlington, USA) for the gasification of biomass [2]. The indirectly heated BCL is fired by air; there is no risk of nitrogen dilution nor need for oxygen production. It produces a gas with a low CO2 content, but contains heavier hydrocarbons. Therefore, the produced gas from BCT has the potential to be converted into high quality syngas and into DME at subsequent step using the route we proposed in this paper.

4.

Conclusions

A novel route for DME production from biomass was proposed and tested in a bench scale experimental system. Gasification of biomass and reforming of produced gas are the key steps, which greatly affect the energy efficiency and cost of the whole process. Gasification of biomass with CO2 as agent has benefit for increasing syngas yield and saving energy comparing to that with air/steam agents. At 1113 K,

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atmospheric pressure, the feed rate of 0.6 kg biomass/h, ER of 0.28, and CO2/biomass ratio of 0.327, 1.4 Nm3 of raw gas/kg biomass with LHV of 8.36 MJ/m3 was produced. The ratio of H2/ CO is about 0.55. After reformed by the biogas addition of 0.54 Nm3 kg1 biomass, the yield of syngas increased to 2.5 Nm3 from 1.4 Nm3 of raw gas, and the ratio of H2/CO was adjusted to 1.18. At ideal synthesis conditions, it was simulated based on kinetics equations for direct DME synthesis on CueZneAl/ZSM-5 catalyst that 78.5% of CO conversion and 379 g DME kg1 biomass could be obtained. Gasification with CO2 agent and sequent co-reforming with biogas supplies great potential for high efficient production of DME from biomass.

Acknowledgments Financial support received from National Natural Science Foundation of China (Project no. 90610035 and 50811120044) and National Basic Program (Project no. 2010CB732205) is gratefully appreciated.

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