Elsevier

Bioresource Technology

Volume 99, Issue 17, November 2008, Pages 8252-8258
Bioresource Technology

Total recovery of resources and energy from rice straw using microwave-induced pyrolysis

https://doi.org/10.1016/j.biortech.2008.03.026Get rights and content

Abstract

This article presents the application of microwave-induced pyrolysis to total recovery of resources and energy from rice straw. The microwave power and particle size of feedstock were both key parameters affecting the performance of microwave-induced pyrolysis. Under 400–500 W microwave power, the reduction of fixed carbon in the biomass was significant. From the experimental results of specific surface area, zeta potential, and Cu2+ adsorption, the applications of solid residues in the water and wastewater treatment could be expected. The major compositions in gaseous product were H2, CO2, CO, CH4 of 55, 17, 13, 10 vol.%, respectively. The high H2 content might imply that microwave-induced pyrolysis of biomass waste has the potential to produce the H2-rich fuel gas. Alkanes, polars, and low-ringed polycyclic aromatic hydrocarbons were three primary kinds of compounds in the liquid product.

Introduction

Biomass waste was not properly disposed in the past, but now there is a growing trend to account it as a source of resources and energy. Biomass is a mixture of hemicellulose, cellulose, lignin and minor amounts of other organics which each pyrolyze or degrade at different rates and by different mechanisms and pathways (Bridgwater et al., 1999). Wood, crops, and agricultural and forestry residues are some of the main renewable energy resources available, besides, the biodegradable components of municipal solid waste (MSW) and commercial and industrial wastes are also significant bioenergy resources (Bridgwater, 2006). In the past, many of the agricultural and forestry residues and MSW are directly incinerated, but the CO2 emission problem needs to be further concerned. For the purpose of more resources and energy recovery and less CO2 emission, there are many alternative technologies that include thermal, biological, and other treatments. Thermal treatments except incineration mainly contain carbonization, pyrolysis, and gasification. The older literature generally equates pyrolysis to carbonization, in which the principal product is a solid char. Today, the term pyrolysis often describes processes in which oils are preferred products (Mohan et al., 2006). Generally speaking, pyrolysis is the thermal process in the absence of oxygen to produce solid (char), liquid (tar), and gas (Demirbas, 2005), whose proportion is governed by feedstock properties and operating parameters (Mohan et al., 2006).

Microwaves are a kind of electromagnetic wave, whose frequencies lie in between 300 MHz and 300 GHz by general definition. Not all materials can absorb microwaves. The materials can be classified into three types according to their interactions with microwave, i.e., conductors (reflective), insulators (transparent), and dielectrics (absorptive). Thus microwave heating is also referred to as dielectric heating (Jones et al., 2002). In conventional heating manner, heat is transferred into the material through convection, conduction, and radiation of heat from the surfaces of the material. On the contrary, microwave energy is delivered directly into materials through molecular interaction with the electromagnetic field. In heat transfer, energy is transferred due to thermal gradients by conventional heating, but microwave heating is the transfer of electromagnetic energy to thermal energy and is energy conversion, rather than heat transfer. This difference in the way energy is delivered can result in many potential advantages to using microwaves for processing of materials (Thostenson and Chou, 1999).

There are so many application accomplished by microwave technology, including drying, heating, synthesis, digestion, extraction, etc. This may be due to the rapid, uniform, and selective heating of microwave radiation, and there is no direct contact between the microwave source and the heated material. The microwave heating has been applied to pyrolysis of many kinds of feedstocks, including the oil-palm stone (Guo and Lua, 2000), oil shale (El Harfi et al., 2000), paper (Miura et al., 2001), plastics (Ludlow-Palafox and Chase, 2001), rock phosphate (Bilali et al., 2005), sewage sludge (Menendez et al., 2002, Dominguez et al., 2003), wood (Miura et al., 2004), and coffee hulls (Dominguez et al., 2007). Compared with the conventional pyrolysis conducted by electric furnace, the microwave pyrolysis produces more content of H2 and CO (Menendez et al., 2004), which is the so-called syngas. Besides, microwave pyrolysis generates less polycyclic aromatic hydrocarbons (PAHs), so it provides less hazardous compounds (Dominguez et al., 2003). However, the key effecting factors of these results and the possible application of the pyrolytic products are still ambiguous and need to be further explored.

In Taiwan, the annual average of rice production was ca. 1.6 million tonnes in 2001–2005 (Agriculture and Food Agency, COA, Executive Yuan, 2006). For every tonne of grain harvested, ca. 1.35 tonnes of rice straw remain in the field (Kadam et al., 2000). Therefore there is ca. 2.2 million tonnes of rice straw generated every year. Furthermore, to refer to the database from the Food and Agriculture Organization of the United Nations, the worldwide rice production is ca. 600 million tonnes every year (Food and Agriculture Organization, United Nations, 2007), so ca. 810 million tonnes of rice straw can be generated. This is really a large quantity of waste, or the source of resources and energy. Therefore, this study was aimed (1) to determine the key parameters affecting the microwave-induced pyrolysis, (2) to assess the characteristics and applicability of products, and (3) to evaluate the feasibility of total recovery of rice straw via microwave-induced pyrolysis.

Section snippets

Materials

The rice was planted in Pingtung, the southern Taiwan. After harvest, the residual rice straw was weathered for 10 days, till the constant moisture was reached. After shredding, the rice straw was sieved by 20/40 mesh (0.850/0.425 mm opening), to collect the designate sieved part as sample. The general characteristics and constituents of rice straw were analyzed and listed in Table 1. The proximate and elemental analyses were referred to ASTM Standard Test Method D 5142 and D 5291, respectively.

Temperature profiles

Under different microwave power, the temperature profiles of pyrolytic reaction are shown in Fig. 1. All the samples were ca. 3 g, and the particle size was between 20/40 mesh (0.425–0.850 mm). During the time of 30 min of microwave radiation, the reaction temperatures slowly increased, and the maximal reaction temperatures were only 105–158 °C, under the microwave power of 50–150 W. These conditions only offered the desiccation and slight pyrolysis of samples. For the microwave power of 200 W and

Conclusions

The microwave power and particle size of feedstock were both key parameters affecting the performance of microwave-induced pyrolysis. For a certain outcome, the less microwave power will be satisfactory if the particles get smaller. After the pyrolysis of rice straw, three-phase products were generated and collected separately. The adsorption-related analyses of solid residues had shown the potential for the application in the removal of metallic contaminants. About half of rice straw sample

Acknowledgements

The authors are grateful to the National Science Council, Taiwan for the financial support (Contract No. NSC 96-2218-E-002-004).

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