Process simulation of rice straw torrefaction

Fu-Siang Syu and Pei-Te Chiueh*

Graduate Institute of Environmental Engineering National Taiwan University

Taipei 106, Taiwan

Key Words: Rice straw, torrefaction, combustibility, energy model, autothermal operation

ABSTRACT

INTRODUCTION

Crop residues are considered to be important feedstocks for partial fossil fuel substitution in energy production. Therefore, utilizing crop residues such as rice straw which is abundant in Taiwan has been proposed for electricity generation that enhances energy security and sustainability. How-ever, the high moisture content and low heating value of rice straw affect its biomass utilization efficiency. Furthermore, poor grindability of rice straw decreases rates of co-milling with coal at coal-fired power plants. Torrefaction is a thermal pretreatment technology which improves the prop-erties of biomass in order to deal with the above problems. Simulation of the process which produces biocoal from rice straw by torrefaction was conducted in this paper. It was found that the volatile gas is combustible even though the gas consists of 49 wt% non-combustible content. Results also show that the process which is carried out at 250 °C for 30 min can be operated autothermally when the input (wet rice straw) moisture content is less than 12 wt%. The process product yield increases with decreasing wet rice straw moisture content. Moreover, the process thermal energy efficiency was estimated to be 0.85 when the process was operated above the point of autothermal operation. According to the results, rice straw could be sun dried off in the field to reduce moisture content before transport to the torrefaction plant in order to improve process performance. The obtained information should be useful for the design of logistic studies of biomass, or future applications of rice straw torrefaction for energy production. .

*Corresponding authorEmail: ptchueh@ntu.edu.tw

1. Torrefaction The utilization of crop residues as raw materials in a bioenergy supply chain is considered to be an alter-native to energy crops. Crop residue such as rice straw is produced in abundance in Taiwan, and can provide a sustainable biomass resource used to make biocoal for producing bioelectricity enhancing energy security in Taiwan. However, biomass has high moisture content and low energy density [1,2]. These properties have negative impacts during thermochemical conversion. Moreover, rice straw has lignocellulosic structure which is more tenacious than coal. This characteristic makes it less grindable when it is to be used for cofiring with coal in pulverized coal power plants [3]. The pretreatment of biomass can resolve the draw-backs of raw biomass mentioned above. One of the pretreatment methods is torrefaction [4]. Torrefaction

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is a thermochemical treatment method which is con-ducted under standard atmospheric pressure conditions in the absence of oxygen. Operating temperature is within the range of 200 to 300 °C. A lower heating

-1rate (< 50 °C min ) and a longer residence time (typi-cally 1 h) are typical operation conditions for this proc-ess [5]. Numerous reaction products are formed during the process. The products are divided into solid char (torrefied biomass), aqueous compounds, and gases [6]. For wood briquette, torrefied biomass's heating value may increase by 15%, and the moisture content decreases by 73% [7]. Furthermore, torrefied biomass showed better grinding properties [8]. Bridgeman et al. [9] also conclude that torrefied biomass can be success-fully pulverized. The fuel characteristics obviously improved following torrefaction processes. Biomass loses relatively more oxygen and hydrogen compared to carbon. Consequently, the lower heating value (LHV) of torrefied biomass is within the range of 18

-1to 23 MJ kg (dry basis) depending on the torrefaction

177Sustain. Environ. Res., 22(3), 177-183 (2012)

conditions. In contrast, the LHV of coal is typically in -1the range of 25 to 30 MJ kg (dry basis) [5]. The

characteristic and yield of torrefied biomass strongly depend on the torrefaction conditions (temperature and residence time) and the raw biomass initial properties [5,10].

2. Rice Straw and its Torrefaction

Rice is the main grain crop in Taiwan. The annual average production of paddy rice was about 1.6 Mt from 2001-2005 [11]. Rice straws are the non-edible plant parts left in the field after harvesting paddy rice. According to Shie et al. [11], the production of rice straw is about 2.2 Mt annually. The considerable amount of rice straw requires appropriate disposal to avoid local pollution problems resulting from open burning. Literature on the torrefaction of woody bio-mass is abundant. Studies on torrefaction of herba-ceous biomass are much less than for woody biomass. The discussion focused on rice straw is limited. Sadaka and Negi [12] demonstrated the changes of torrefaction temperature (260 °C) and residence time (15-60 min) on the physical and thermochemical characteristics of rice straw [12]. The results showed that more than 80% of the mass was retained as a torrefied rice straw over a 30 min residence time. In addition, Deng et al. [13] also investigated rice straw torrefaction. The torrefaction of rice straw and rape stalk from Anhui Province in eastern China was performed in a vertical reactor at 200-300 °C for 30 min. Composition of the different product groups (solid, liquid and gases) was also obtained. Furthermore, the grindability of the torrefied rice straw was evaluated by milling it in a ball mill [13]. The results showed that the mass yield of solid product was only equal to 40%. Most of the mass were transformed into gaseous products con-sisting of CO, CO and some CH . The authors con-2 4

cluded that the liquid product included mostly water, acetic acid and other oxygenates. Moreover, they fur-ther pointed out that an increase of torrefaction tem-perature led to a decrease in torrefied biomass yield and the type of raw biomass influenced the conversion rate.

3. Simulation of Torrefaction Process

Developing a model of a process is considered to be an effective method for analyzing technical and economic feasibility. The torrefaction model is used to serve as a predictive tool for a local torrefaction plant. Dudgeon [14] developed a model in Aspen Plus© for simulating the unit operations associated with the torrefaction process. The model determined the opti-mal operating conditions for torrefaction and drying. The conditions for autothermal operation were also evaluated. Cost and energy models for torrefaction were developed by Maski et al. [15]. The study

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quantified torrefaction energy and cost components, and analyzed sensitivity of torrefaction energy and cost according to key model parameters. Bergman et al. [5] conducted torrefaction process simulation con-structed from experimental data to evaluate process characteristics. This paper conducted process simula-tion assuming production of rice straw biocoal through torrefaction based on the experimental results in the literature. The purpose of the simulation was to evalu-ate process yield (torrefied biomass) and energy effi-ciency with respect to raw biomass moisture. Another purpose was to determine the conditions for autothermal operation. .

MATERIALS AND METHODS

1. Process Simulation and Evaluation Parameter Simulation results are used for providing useful information for design and operation of torrefaction facilities. The critical decisional factors are explored in this demonstration of the feasibility of producing biocoal from rice straw by torrefaction. Specifically, these key aspects deal with the process structure, proc-ess performance characteristics, autothermal operation, and volatile gas combustibility. Combustibility is im-portant as regards how the gas can be utilized so that its heat energy is recovered through combustion and waste streams prevented. The major parameters for evaluating torrefaction process performance character-istics are defined below.

1.1. Solid and energy yield of torrefaction unit

The main parameters in the evaluation of torrefac-tion unit are the solid and energy yield. They indicate the transition of mass and chemical energy from rice straw to biocoal. According to the literature [5], the solid yield (y ) is defined on a mass basis as shown M

below.

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.

(1)

where m is the mass of biomass input after drying in

(dry rice straw) by torrefaction unit and m is the out

mass of product output (biocoal). And the definition of energy yield (y ) is as follows. E .

(2)

The y and y of the torrefaction unit are expressed M E

on the reactive portion of the material (dry-ash-free, daf, basis); while the y is based on the LHV which E

corresponds to the case where none of the water is assumed to condense. As a result, the y is expressed E

on the basis of the energy that can effectively be re-trieved from the torrefied biomass output after its combustion. .

178 Syu and Chiueh, Sustain. Environ. Res., 22(3), 177-183 (2012)

1.2. Process product yield

The process product yield (C ) shows how much P

biocoal the process produces per unit mass of rice straw input. It is represented by,

.

.

(3)

(4)

where m is the mass of feedstock input (wet rice feed

straw) to the process and m is the mass of product prod

output from the process (biocoal). m is expressed on prod

a daf basis.

1.3. Process energy efficiency (ç ) P

Referring to the literature [5], the ç is defined as P

below.

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The ç is defined as the ratio of available energy P

content of product (biocoal) over the sum of energy input through the feedstock (rice straw) and utilities (E ). Utilities include steam, fossil fuels, or electricity util

which are used for operating the process. The ç is P

directly connected to the feedstock demands, and lower energy efficiency reduces the sustainability profile of biocoal. According to the analysis of Bergman et al. [5], the electricity consumption of torrefaction process could be less than 5% of net efficiency. Therefore, the ç is termed thermal energy efficiency when electricity P

consumption is excluded from the utilities in this study.

2. Performance Related Assumptions

2.1. Applied torrefaction condition

Bergman et al. suggested that ideal operating con-

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Table 1. Experimental results quoted in this study

Parameters

Characteristics of rice straw-1 Higher heating value (MJ kg , dry)

Moisture (wt%)

Proximate analysis (dry basis, wt%)

Ash

Volatiles

Fixed carbon

Elemental analysis (Dry ash free basis, wt%)

C

H

N

S

O

Solid yield (%)*

Increase of higher heating value (%)*

Values

15.3

8.3

13.4

72.2

14.4

45.4

6.3

1.0

0.2

47.1

79.2

5.3

Note

Reference [17]

260 °C, 30 min. Reference [12]

250 °C, 30 min. Reference [13]

*Recalculated by this study.

ditions are within the range of 250 to 280 °C with a residence time of < 30 min, considering economics and product quality [5]. Arias et al. [3] and Svoboda et al. [16] recommended torrefaction temperatures for woody biomass of 240, 270-280 °C and holding time about 0.5 h. Deng et al. [13] suggested that the torre-faction temperature of 250 °C and residence time of 30 min are suitable for torrefaction of rice straw because the energy loss of agricultural residues is higher than that of woody biomass during torrefaction. Therefore, this study selected a torrefaction temperature of 250 °C and residence time of 0.5 h.

2.2. Torrefaction unit performance characteristics

This paper conducted the process simulation based on the experimental results in the literature shown in Table 1. Moreover, the constituents of volatile gas referred to the results of wheat straw because of a shortage of data for rice straw [6]. Figure 1 provides composition of volatile gas which consisted of the liquid and gas products of the torrefaction unit. The volatile gas consists of about 49 wt% of non-combus-tible components which include water and CO . 2

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*The constituents were recalculated by this study. Reference [6].

Fig. 1. The constituents of volatile gas.

179Syu and Chiueh, Sustain. Environ. Res., 22(3), 177-183 (2012)

wt%

CO2 CO Water Acetic acid

Formic acid

Methanol Lactic acid

Furfural Hydroxy acetone

3. Energy and Mass Flow Model Concept

3.1. Process description

A typical torrefaction process mainly comprises of a dryer, torrefaction reactor, combustor, and heat ex-changer. Figure 2 presents the process diagram of torrefaction used in this study. First, a dryer (drying unit) is applied to dry wet rice straw in order to have the input achieve constant moisture content to the tor-refaction unit. Then, the torrefaction reactor (torrefac-tion unit) heats the rice straw to generate volatile gas and biocoal. The combustor recovers the heat energy of volatile gas through combustion. The hot flue gas is used to provide heat to the torrefaction unit through a heat exchanger. After passing through the heat ex-changer, the same flue gas enters the dryer to serve as a gaseous heat carrier. Finally, the biocoal is cooled down to room temperature before leaving the process.

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The point of autothermal operation represents the condition where the energy content of the volatile gas is equal to the sum of the required energy for drying and the torrefaction unit and any heat losses encoun-tered elsewhere in the process. According to the rela-tion between the energy content of the volatile gas and the total energy required in the process, two cases are possible. (1) The process is operated below the point of autothermal operation when the volatile gas does not contain sufficient energy such that an additional fuel is required. (2) The process is operated above the point of autothermal operation when the volatile gas contains more energy than the process required, meaning that the process is self-supporting. However, if the volatile gas contains too much energy, the process energy efficiency and biocoal yield will decrease.

3.2. Model concept

The energy and mass flow models in this study are partly based on the research of Maski et al. [15]. Inthis study, wet rice straw is considered to be a mixture of mass of dry straw and water. For example, 1 kg wet rice straw with 10% moisture content consists of 0.9

Fig. 2. The process diagram of torrefaction.

kg of dry rice straw and 0.1 kg of water. It is assumed that the moisture content is totally removed in the drying unit when the wet rice straw is heated to 100 °C. Hence, the energy required for drying 1 kg of wet

-1rice straw (E (MJ kg )) is represented by R,D .

(5)

(6)

(8)

(7)

where M (wt%) is the moisture content of wet rice wet

straw, DB (wt%) is the percentage of dry rice straw wet

in wet rice straw, and C is the specific heat of water p,w-1 -1(0.0042 MJ kg K ). Besides, T is the initial tempera-i

ture of raw rice straw (298 K), L is the latent heat of v,w-1water at boiling (2.27 MJ kg ), C is the specific heat p,b

-1 -1of straw (0.0015 MJ kg K ) [18], and e is the effi-f,D

ciency of drying unit, assumed to be 0.85 (note: M + wet

DB = 100 wt%). wet

Torrefaction is a slightly endothermic process re--1quiring about 0.6-1 MJ kg [3,16]. The energy which

torrefaction reaction absorbs is assumed to be 0.8 MJ -1kg . Hence, the energy required for torrefying 1 kg of

-1dry rice straw in a torrefaction unit, E (MJ kg ) is R,T

represented by,

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.

where the T (K) is the torrefaction temperature and e T f,T

is the efficiency of the torrefaction unit, assumed to be 0.85. The total energy required for the process can be considered as the sum of energy required to dry the moisture and torrefy the dried straw. Therefore, the total energy required by the process for 1 kg of wet

-1rice straw input, E (MJ kg ) can be represented by, R,total .

The heating value of the volatile gas which is gen--1erated from the torrefaction unit, LHV (MJ kg ) is volatile

represented by,

-1where LHV (MJ kg , daf) is the LHV of rice straw. Accordingly, the available energy which is derived from the combustion of volatile gas and heat exchange

-1for 1 kg of wet rice straw input, E (MJ kg ) isA,TG

represented by, .

where Ash is the ash content of wet rice straw, e is wet f,c

the efficiency of combustor (0.85), and H is the heat L

losses of heat exchanger (0.005). .

(9)

Flue gas Air Electricity

Combustor

Wet Biomass

Drying

Volatile gas

Torrefaction

Heat Exchange

Cooling Torrefied Biomass

Waste Heat

180 Syu and Chiueh, Sustain. Environ. Res., 22(3), 177-183 (2012)

(10)

(11)

where m is the mass of air and m is the mass of air TGAS

volatile gas. Ö is the ratio of actual air-fuel ratio ((A/F) ) to stoichiometry for a given mixture of fuel wt

and air. .

Ö of 1.0 is at stoichiometry, rich mixtures are less than 1.0, and lean mixtures are greater than 1.0. Aspen Plus , a process modeling tool for concep-tual design, optimization, and performance monitoring of chemical processes (http://www.aspentech.com/products/aspen-plus.aspx), is used to simulate the combustion of volatile gas. The adiabatic flame tem-perature is obtained in the simulation results of flue gas. Figure 3 shows the Aspen Plus model that con-sists of two reactors.

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®Fig. 3. Aspen Plus model of combustion of volatile gas.

RESULTS AND DISCUSSION

1. Combustibility of the Volatile Gas

According to the constituents of volatile gas pre-sented before, the higher ignition temperature is 609

Fig. 4. The adiabatic flame temperature with respect to Ö.

181Syu and Chiueh, Sustain. Environ. Res., 22(3), 177-183 (2012)

2500

2000

1500

1000

500

00 0.2 0.4 0.6 0.8 1 1.2 1.4

the moisture content of wet rice straw. When moisture content of wet rice straw is 10% the process operated above the point of autothermal operation, as shown in Fig. 6. The process C is 0.74 (0.62, daf). Because p

additional fuel is not required in autothermal operation, the process ç has a maximum of 0.85, which equals P

the E of the torrefaction unit. Y

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The above findings are valuable for the future application of straw torrefaction for energy production, because it is crucial to know the thermal efficiency, the constitutions of flue gas and their behaviour during thermochemical conversion. Simulating the feasibility of autothermal of torrefaction for rice straw helps to increase authority confidence in conducting full-scale testing and potentially adopting this technique. More specifically, the high cost of logistics of biomass-to-energy system has interfered with the development of renewable energy. If practical pretreatment process, such as torrefaction for increasing energy density is worked out, then the agriculture residues may be suit-able candidates for torrefaction application, contrib-uting to the local development.

Fig. 6. The energy and mass flow of an autothemal process.

CONCLUSIONS

This paper conducted process simulation for pro-duction of biocoal from rice straw by torrefaction based on the experimental results in the literature. According to the results, we draw the following con-clusions: (1) Volatile gas can be applied as a fuel for drying and torrefaction and does not lead to waste streams, despite it consists of about 49 wt% of non-combustible content. (2) The process which is carried out at 250 °C for 30 min can be operated autothermally when the input (wet rice straw) moisture content is less than 12 wt%. (3) The process has a maximum of çP

0.85 when the process is operated above the point of autothermal operation. And (4) Torrefaction facilities which performed torrefaction process with autothermal operation do not require additional fuel to operate the process. Therefore, it can be located in areas rich in rice straw to reduce transportation fuel costs and fossil fuel emissions. .

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Discussions of this paper may appear in the discus-sion section of a future issue. All discussions shouldbe submitted to the Editor-in-Chief within six monthsof publication. .

Manuscript Received: Revision Received:

and Accepted:

November 20, 2011January 11, 2012February 8, 2012

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