FTIR study of pyrolysis products evolving from typical agricultural residues

doi:10.1016/j.jaap.2010.03.004

Journal of Analytical and Applied Pyrolysis

Volume 88, Issue 2, July 2010, Pages 117-123

https://doi.org/10.1016/j.jaap.2010.03.004 Get rights and content

Abstract

Pyrolysis of agricultural residues (maize stalk, rice straw and cotton straw) was studied using a thermogravimetric (TG) analyzer and a laboratory scale fixed bed coupled with Fourier transform infrared (FTIR) analyzer. Pyrolysis characteristics of three materials were discussed. The characteristic parameters were determined for the main devolatilization step. Maize stalk showed the highest thermal reactivity, followed by cotton straw and rice straw. Their pyrolysis processes underwent three consecutive stages, corresponding to the evaporation of water, the formation of primary volatiles and the subsequent release of small molecular gases. In order to further study the pyrolysis mechanisms of agricultural wastes, the release of the main volatile and gaseous products were on-line detected by FTIR spectroscopy. The results showed that the major pyrolysis gases for the three materials were similar, including CO₂, CO, methane, ethane, ethylene and some organics such as methanol, formaldehyde, formic acid and acetone. HCN was the major nitrogen containing product. At higher temperatures several small molecular gases, such as CO₂, CO and methane, could still be monitored.

Introduction

With the excessive use of fossil fuels and the concerns over environmental protection, the utilization of biomass resources has attracted increasing worldwide interest. Biomass including agricultural residues is one of the main renewable energy resources available especially in an agricultural country such as China. With the rapid increase of energy demand in recent years, China has become the largest coal-consuming country in the world, causing serious issues in energy sustainability and environment. So the development and utilization of biomass energy have been paid more and more attention to in China. At present China energy policies are strongly encouraging the use of biomass for energy purposes mainly owing to three aspects: economic and social development, elimination of wastes and reduction of CO₂ emissions. Thus, the research and utilization of biomass resources are further promoted. China produces a large amount of agricultural wastes annually, including 600 million ton for straws, 389 million ton for agricultural processing wastes [1]. However, most of the agricultural residues are disposed of by burning in open fields, causing environment and public health problems. At present, the ratio of biomass being utilized is approximately 3% [1]. Therefore, it is very important for the stability and sustainable development of society to find a way that can effectively make use of agricultural residues and realize the conversion of energy.

A good solution for technical application of biomass energy is conversion of biomass by pyrolysis, combustion and gasification. Pyrolysis is considered as a promising alternative technology that converts biomass to clean energy and valuable chemicals. Moreover, pyrolysis is also a vital process in biomass combustion and gasification [2]. The pyrolysis process is extremely complex and generally goes through a series of complex reactions [3], [4], [5]. Thus, it is very essential to study the fundamentals and mechanisms of agricultural residues pyrolysis.

So far, numerous investigations on biomass pyrolysis have focused on the effects of various parameters (temperature, heating rate, pressure, particle size, etc.) on the distribution and characterization of solid and liquid products [6], [7], [8], [9], [10], [11], [12], [13]. However, few detailed studies on the real time analysis of gas release during the pyrolysis of agriculture residues are found in the available literature. Information on the release properties of gas products is essential to understand the fundamentals and mechanism involved in pyrolysis of biomass. Bassilakis et al. [14] studied the pyrolysis products evolution characteristics of wheat straw, three types of tobacco (Burley, Oriental, and Bright), and three biomass model compounds (xylan, chlorogenic acid, and d-glucose) using thermogravimetric analysis coupled with Fourier transform infrared analysis (TG/FTIR). They found that the majority of the volatile products had multimodal evolution patterns. Later they used a pyrolysis model based on parallel, independent, first-order reactions with Gaussian distributions of activation energies to predict product yields [15]. Yan et al. [16] studied the evolution of various gaseous products during the pyrolysis of palm oil wastes based on TG/FTIR analysis. They pointed out that the different reaction pathways were involved during pyrolysis. Worasuwannarak et al. [17] investigated the gas formation behaviors during the pyrolysis of rice straw, rice husk, and corncob with the thermogravimetry analysis/mass spectrometry (TG/MS) technique. They found that the differences in the gas formation rates were due to their differences in the composition of hemicellulose, cellulose, and lignin. The lack of data, combining with the large variety and complexity of agricultural residues, leads to difficulties in understanding the evolution characteristics and formation mechanism of gas products during the pyrolysis of agricultural residues. Besides, the structural properties of biomass are found to influence greatly the pyrolysis characteristics [18]. However, the releasing characteristics of gas products and their relationships with the chemical structures of biomass have not been discussed in-depth in the previous studies. A better understanding of the release properties and formation mechanism of gas products during pyrolysis is essential to achieve high yields of the targeted products.

In the present paper, the pyrolysis of agricultural residues was studied using a thermogravimetric (TG) analyzer and a laboratory scale fixed bed coupled with Fourier transform infrared analyzer (FTIR). From these measurements, the releasing behaviors and formation mechanisms of various gas products were investigated in detail. This study is essential for developing comprehensive biomass pyrolysis models and can provide valuable inputs for predictive modeling of agricultural waste pyrolysis. Further studies are needed to obtain the kinetic parameters and pyrolysis models that can predict product yields and evolution patterns of selected volatiles products for CFD applications.

Section snippets

Materials

Maize stalk, rice straw and cotton straw were used in this study as the representatives of Chinese typical agricultural residues. The samples were crushed and sieved to the fraction of particle sizes <0.295 mm for subsequent studies. The small particle size used in this study ensured that temperature gradients within the samples were minimized. Proximate and ultimate analysis were carried out in a TGA-2000 (Navas Instruments, Spain) and a Euro-CA 3000 (HEKAtech, Italy) elemental analyzer. The

Chemical structure of biomass materials

The FTIR spectra of maize stalk, rice straw and cotton straw (Fig. 2) are similar. Band assignments indicate that the three agricultural residues contain a number of atomic groupings and structures. Band intensities reveal that the most abundant chemical bonds are O–H, C–H, olefinic C double bond C and C–O. CO and aromatic CC bonds are also detected in three materials. The FTIR spectrum of cotton straw is taken as an example to account for the band assignments.

The band at ≈3410 cm⁻¹ is ascribable to ν(O–H)

Conclusions

Maize stalk, rice straw and cotton straw are the major agricultural wastes in China. In this study, the pyrolysis behaviors and gas products evolution patterns for the three materials were investigated in detail. The following conclusions could be drawn.

(1)
Thermal decomposition of the three materials mainly occurred between 200 and 500 °C. Their pyrolysis processes underwent three consecutive stages, corresponding to the evaporation of water, the formation of primary volatiles and the subsequent

Acknowledgements

This work is supported by the National Natural Science Foundation of China (NSFC) (No. 50776037, No. 50721005), Program for New Century Excellent Talents in University (No. NCEF-07-0335) and the Major State Basic Research Development Program of China (No. 2004CB217704). These supports are gratefully acknowledged. The authors would also like to acknowledge the extended help from the Analytical and Testing Center of Huazhong University of Science and Technology.

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FTIR study of pyrolysis products evolving from typical agricultural residues

Abstract

Introduction

Section snippets

Materials

Chemical structure of biomass materials

Conclusions

Acknowledgements

J. Anal. Appl. Pyrol.

Bioresour. Technol.

J. Anal. Appl. Pyrol.

Biomass Bioenergy

J. Anal. Appl. Pyrol.

Fuel Process. Technol.

J. Anal. Appl. Pyrol.

J. Ind. Eng. Chem.

Bioresour. Technol.

Bioresour. Technol.

J. Anal. Appl. Pyrol.

Fuel

J. Anal. Appl. Pyrol.

Combust. Flame