Elsevier

Renewable Energy

Volume 151, May 2020, Pages 21-29
Renewable Energy

Evaluation of effects of freezing pretreatment on the grindability, energy consumption and chemical composition of wheat straw

https://doi.org/10.1016/j.renene.2019.10.139Get rights and content

Highlights

  • Freezing pretreatment enhances the grindability compared with torrefaction.

  • Particle morphology obtained by image analysis represents grindability.

  • Sample preparation is improved to reduce particle agglomeration effect.

  • Freezing grinding consumes less energy than torrefaction grinding.

  • Freezing pretreatment maintains the chemical composition of wheat straw.

Abstract

In biomass utilisation, size reduction caused by grinding is a critical procedure because it greatly changes the particle size and shape of the biomass sample, which greatly influences its energy conversion efficiency. Raw biomass is difficult to grind due to its fibrous nature and therefore needs to be pretreated before grinding. Traditional torrefaction pretreatment consumes more energy and changes the chemical composition of biomass. In this study, we select wheat straw with abundant yield in northern China as the biomass sample for testing and introduce a new freezing pretreatment method into the grinding procedure. This method incorporates the freezing of wheat straw with liquid nitrogen. We evaluate the performance of freeze grinding in terms of grindability, energy consumption, chemical composition and energy conversion efficiency, and compare it with that of torrefaction grinding to investigate the effects of pretreatment methods on wheat straw. Particle size and shape distribution are used to characterise grindability. Moreover, we improve sample preparation, image acquisition and segmentation methods to reduce the impact of particle agglomeration. Experimental results demonstrate that freezing as pretreatment improves the grindability and energy conversion efficiency, whilst reducing energy consumption and maintaining chemical composition of wheat straw.

Introduction

The increasing adverse impacts of fossil fuel consumption on the environment, such as greenhouse effect and air pollution have made the search for alternative renewable energy sources that can complement or partially replace fossil fuels as the main energy source imperative [1]. Biomass residues are promising resources for substituting fossil fuels due to their renewable nature, abundant availability and carbon dioxide neutrality. In addition, biomass can be converted into gaseous, liquid and solid fuels and other chemical raw materials or products [[2], [3], [4], [5]].

Pulverised biomass offers important advantages during energy conversion processes. For instance, high surface area changes the number of contact points for chemical reactions, thereby greatly affecting the efficiency of biomass applications [6]. Size reduction is known as the grinding process, which changes the particle morphology, increases bulk density and generates new surface area. It is typically integrated into biomass conversion processes [[7], [8], [9], [10]]. The morphology of ground biomass particle, including particle size and shape, is an important physical property that affects the fluidisation and combustion parameters of particles [[11], [12], [13]]. Lu et al. [14] showed that volatile yields decrease with the increase in particle size of biomass particles. They also found that near-spherical particles exhibit lower volatility and higher tar yields than aspherical ones. Sudhakar et al. [15] reported that the size and shape of wood particles are the two key deciding factors of devolatilisation time in a fluidised bed combustor.

Biomass material exhibits anisotropy on the structural strength due to the high content of structural lignin and cellulose. The material is also hard to grind into small homogeneous particles, which leads to low energy conversion efficiency [16]. Pretreatment prior to grinding is a necessary step to improve the grindability of biomass [17,18]. Torrefaction is widely considered a practical pretreatment method for improving the properties of biomass [19]. Gil et al. [20] demonstrated that the grindability of torrefied biomass is significantly improved compared with raw biomass. Their experimental results also show that high temperature increases the proportion of small-sized particles after grinding. However, the high temperature caused by torrefaction alters the chemical composition of the biomass and influences some applications of biomass in the chemical industry, such as gasification and ironmaking [21]. Moreover, torrefaction grinding consumes a large amount of electrical energy; more than one-third of the electrical energy of the entire energy consumption is required for size reduction [22]. Compared with torrefaction grinding, freeze grinding is used in food processing because it can effectively ensure the chemical composition of the raw material [23]. At extremely low temperature, the raw biomass material becomes brittle and easily broken, which indicates that it can be easily ground into fine particles [24].

The Hardgrove grindability index (HGI) represents the difficulty in grinding a solid sample into powder and is commonly used grindability test for coal [25]. However, the HGI is unreliable indicator of grindability performance for some biomass samples with tough structure [26]. The particle size and shape distribution (PSSD) of ground powder provide a satisfactory way to evaluate the grinding performance in various fields that handle granular biomass materials. Comparisons of the PSSD parameters of input feed and output product can quantify the effectiveness of the grinding process [27]. A series of methods and equipment is used to characterise PSSD. For example, mechanical sieving with a series of sieves is the standard method adopted by the American Society of Agricultural and Biological Engineers to analyse the particle size distribution (PSD) of ground biomass [28]. However, sieving analysis has several issues, such as the process involves a costly mechanical shaker and limits the sieve selection to only a finite set. Moreover, the measured size of biomass particles using sieve analysis is usually less than the actual size of the particles and lacks particle shape information. Other existing methods of PSSD measurement use the principles of light scattering, acoustic spectroscopy and laser diffraction. Such methods often assume the particles to be spherical, which is not always the predominant case for ground biomass materials [29]. Image analysis technology is considered an alternative for mechanical sieving analysis because of its high accuracy and observability [[30], [31], [32], [33], [34], [35], [36]]. However, limited studies have identified agglomerated fine-grained biomass particles.

Considering the high energy consumption of torrefaction grinding and its characteristic that changes the chemical composition of biomass, the freezing pretreatment method is introduced into the grinding procedure. This method incorporates the freezing of biomass material with liquid nitrogen. The present study aims to evaluate the performance of freezing pretreatment by comparing the effects of freeze and traditional torrefaction on grindability, chemical composition and energy consumption. In consideration of the great variation in particle size range, the initial ground sample is divided into five size fractions by mechanical sieving. Ultrasonic dispersed sample preparation method and high-magnification scanning electron microscopy (SEM) are used to avoid particle agglomeration and acquire high-quality images of fine biomass powders, respectively. The grindability after pretreatment is evaluated by PSSD as obtained by image segmentation based on contour information. In addition, the changes in chemical composition after pretreatment with different grinding methods are determined by fourier transform infrared spectroscopy (FT-IR). Finally, we quantify the energy consumption of grinding process by monitoring the electrical energy consumption, and evaluate the effect of pretreatment on the energy conversion efficiency by an experiment on chemical oxidative degradation of wheat straw.

Section snippets

Material and pretreatment

Wheat, which is a traditional crop with a high yield, is harvested from the suburb of Xuzhou in China. And wheat straw is used as the test biomass material in this study. Before grinding, the wheat straw with an initial moisture content 27% wet basis as measured by an infrared moisture metre is cut into approximately 15 mm long cylinders as shown in Fig. 1.

To compare the improvement in grindability by pretreatment, torrefaction and freeze processes are exploited for the pretreatment of the

Grindability evaluation based on PSD analysis

Two granular products produced by freeze and torrefaction grinding are shown in Fig. 6. The particles generated from freeze grinding are finer than those from torrefaction grinding, which initially show the effectiveness of the freezing pretreatment on the improvement in biomass grindability. Considering the heterogeneity of the particle size, we divide the ground particles into five groups by mechanical sieving method. In general, the PSD characteristics determined through sieving analysis are

Conclusions

Wheat, which is an annual herbaceous crop, is a widely used biomass raw material. In its growth process, the division of stem cells is mainly follows the growth direction. This condition causes distinct structural anisotropy that is not conductive to the grinding process. In this study, a freezing pretreatment method that freeze the wheat straw material with liquid nitrogen prior to grinding is introduced into the size reduction procedure. At an extremely low temperature, the samples are

Acknowledgements

This study was supported by the National Natural Science Foundation of China (Nos. 51604271, 61771473, 61379143), Fundamental Research Funds for the Central Universities (2015XKMS100), Six Talent Peaks High-level Talents in Jiangsu Province (XYDXX-063), Natural Science Foundation of Jiangsu Province (BK20181354), and Qing Lan Project of Jiangsu Province.

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