Pak Sui Lam, Zahra Tooyserkani, Ladan Jafari Naimi and Shahab Sokhansanj

Abstract Pretreatment is a first crucial step to modify the structure of wood via physical, chemical, and biological treatment for cost effective and sustainable fu­els and chemicals production. Different pretreatments would be selected to upgrade the characteristics of wood with respect to different applications and process effi­ciencies. High-temperature pretreatment (e. g., torrefaction) at the temperature range greater than 250 °C led to higher degradation rate of sugars and extractives, which is not preferable for fuel and chemicals production from ligno-cellulosic biomass. Instead, high-temperature pretreat-ment was used to upgrade the solid fuel for thermo-chemical conversion (e. g., combustion and gasification). It can remove the moisture and volatiles with a low-heating value of the native biomass, which favors for the ease of fuel combustion compared to the raw wood. In addition, it can in­crease the hydrophobicity of the biomass which improves their handling and storage performance. In this chapter, the production chain of the wood pellet production with incorporating recent novel pretreatment technologies (torrefaction, steam ex­plosion, and hydrothermal carbonization) were discussed. The resulted pellets are a uniform feedstock for producing chemicals, heat, and energy via biochem-ical and thermochemical conversion, respectively.

Keywords Pretreatment ■ Wood pellet ■ Drying ■ Grinding ■ Biomass preprocessing ■ Torrefaction ■ Steam explosion ■ Hydrothermal carbonization ■ Pellet quality

P. S. Lam (H) ■ Z. Tooyserkani ■ L. J. Naimi ■ S. Sokhansanj Biomass and Bioenergy Research Group, Clean Energy Research Center, Department of Chemical and Biological Engineering,

University of British Columbia,

2360 East Mall, Vancouver, B. C., V6T 1Z3, Canada e-mail: wilsonlam82@yahoo. com

Z. Fang (ed.), Pretreatment Techniques for Biofuels and Biorefineries,

Green Energy and Technology,

DOI 10.1007/978-3-642-32735-3_5, © Springer-Verlag Berlin Heidelberg 2013

5.1 Introduction

Renewable energy has been targeted as a strategic important area for many countries for both environmental and economic reasons [1]. The establishment of a clean energy supply can provide greater energy independence and security, has notable environmental benefits due to reduced CO2 emissions, as well as promoting positive economic growth for the local area. Among various types of renewable energy, bioenergy is attractive as biomass is considered to be carbon neutral that absorbs CO2 from the atmosphere during production [2]. Besides, bioenergy systems can create the highest job creation effect, particularly in the rural areas with high unemployment rate, and resulting in the stimulation of economic growth [2, 3]. Since biomass is dispatchable, it is economically preferable to deploy when required. The biomass feedstock supply logistic cost contributes around 30-50 % of the total bioenergy production cost [4]. An optimized pre-processing of the biomass into densified pellets is essential to achieve a cost-effective production process for bioenergy.

Wood pellets are a type of solid fuel made from sawdust with uniform shape and dimensions. Pellets are made by densifying the ground particles of woody biomass. Wood residues usually come as sawdust from saw mills. Their bulk densities are around 40-60 kg/m3 (wet basis) depending on species and moisture content (MC)

[4] . Drying is required to ensure that the size-reduced feedstock is good for pel­letization (densification) to produce durable wood pellets. The bulk densities of the biomass pellets are around 550-700 kg/m3 depending on the size of the pellets [4]. The volume reduction reduces the space required for storage and transportation. For storage, biomass densification helps to reduce the space required to store the materi­als. Biomass pellets improve the heating efficiency and have lower emissions during combustion than using the low bulk density and fluffy biomass. The typical example is the co-firing plant using wood pellets and coal as feedstock where the difference between these two materials’ densities cause difficulties in feeding due to the uneven, fluffy, and low bulk density of the biomass feedstock [4].

The typical production process of biofuel pellets is collecting the residues from saw mill and following by drying using a rotary drum dryer, further size reduction to the granular form by hammer mill, and finally pelletizing into fuel pellets using a pellet mill (Fig. 5.1). The fuel pellets are then cooled, screened, and transported to an export port by trains. They are usually transported on conveyor belts and dropped from the height of 10-15 m above the storage silos for temporary storage. They are stored under a well-monitored environment to prevent self-heating and off-gas accumulation. Pellets are then loaded into the ocean vessel. The details of each unit operation will be discussed in the following sections. The pellet quality needs to be maintained during transportation in order to meet the import specification of the European standard [5].

Biomass preprocessing is aimed at enhancing the energy density of a bulk biomass. Further optimization of the process can be achieved by enhancing the production yield and reducing the energy required for the preprocessing process. Two major technical problems during the preprocessing process need to be addressed. Poor mechanical strength of biomass pellets contributes to disintegration of pellets into fines during

transportation. This usually happens for the pellets transporting on the conveyor and loading from the top of the silo to form piles and pellets break into fines due to impact. The fines cause blockage of the conveyor or hopper during processing and also lead to an occupational health problem to the workers inhaling the fines [6]. Moreover, the fines also lead to dust explosion which causes severe fire damage to the expensive handling facilities. This is related to the lack of natural binding between the fibers of the pellets, and most biomass species including straws and stover are difficult to densify without any expensive binders [7-10]. Only wood pellet can be formed with good durability due to their binderless characteristics.

Pellets easily adsorb moisture and disintegrate into fines under high-humidity conditions. The high surface area of the small fines favors the susceptibility of the attack by the micro-organisms during storage [11]. Anaerobic conditions lead to local heat generation and generation of toxic off-gassing that may include terpenes [12]. Local heat generated may ignite the volatiles in the pellets to cause fire, and the off-gas accumulations inside the storage silos are toxic to the workers. High MC reduces pellets combustion efficiency at the power plant.

In the following, we will focus on discussing the development and optimization of the entire biomass pellet production chain by different pretreatment techniques. This not only aims to produce fuel pellets for ease of chemical conversion and energy production, but also to reduce the preprocessing cost and improve the safe handling of pellets during transport and storage.