A review on biomass torrefaction process and product properties for energy applications

Torrefaction of biomass can be described as a mild form of pyrolysis at temperatures typically ranging between 200 and 300°C in an inert and reduced environment. Common biomass reactions during torrefaction include devolatilization, depolymerization, and carbonization of hemicellulose, lignin, and cellulose. The torrefaction process produces a brown to black uniform solid product, as well as condensable (water, organics, and lipids) and noncondensable gases (CO2, CO, and CH4). Typically during torrefaction, 70% of the mass is retained as a solid product, containing 90% of the initial energy content, while 30% of the lost mass is converted into condensable and noncondensable products. The system’s energy efficiency can be improved by reintroducing the material lost during torrefaction as a source of heat. Torrefaction of biomass improves its physical properties like grindability; particle shape, size, and distribution; pelletability; and proximate and ultimate composition like moisture, carbon and hydrogen content, and calorific value. Compared to raw biomass, the carbon content and calorific value of torrefied biomass increases by 15–25% wt, while the moisture content decreases to <3% (w.b.). Torrefaction decreases the grinding energy by about 70%, and the ground torrefied biomass has improved sphericity, particle surface area, and particle size distribution. Torrefied biomass pelletization at temperatures of 225°C decreases the specific energy consumption and increases the capacity of the mill by a factor of 2. The loss of the OH functional group during torrefaction makes the material hydrophobic (i.e., loses the ability to attract water molecules) and more stable against chemical oxidation and microbial degradation. These improved properties make torrefied biomass particularly suitable for cofiring in power plants and as an upgraded feedstock for gasification. 384 INdustrIal BIotechNology october 2011 research K.13.PRV75 Hess 384-401.indd 384 10/18/11 10:40 AM High moisture in raw biomass is one of the primary challenges, as it reduces the efficiency of the process and increases fuel production costs.3 High moisture content in biomass leads to natural decomposition, resulting in loss of quality and storage issues such as off-gas emissions. Another consequence of high moisture content is the uncertainty it causes in biomass’s physical, chemical, and microbiological properties. Irregular biomass shapes constitute another issue, especially during feeding in a cofiring or gasification system. In addition, biomass has more oxygen than carbon and hydrogen, making it less suitable for thermochemical conversion processes. Considered collectively, these properties make raw biomass unacceptable for energy applications. To overcome these challenges and make biomass suitable for energy applications, the material must be preprocessed. One of the commonly used preprocessing operations is grinding, which helps to achieve a consistent particle size; however, the moisture content of the biomass limits the performance of many grinders.3 Furthermore, grinding can be very costly when smaller particle sizes are desired and, in some cases, impractical for biomass with high moisture content. High moisture content can also result in inconsistent particle sizes (especially when the particles are less than 2 mm), which may not react consistently, thereby reducing the efficiency and increasing the costs of the conversion process. Also, raw biomass is thermally unstable due to high moisture, which results in low CVs and inconsistent particle-size distribution issues when used in thermochemical processes such as gasification. This can lead to inconsistent products and the formation of condensable tars, which results in problems like gas-line blockage.4 A viable option is to pretreat the biomass before the end-use application. Pretreatment helps alter biomass’s physical properties and chemical composition and makes it more suitable for conversion.5 The pretreatment can be a chemical, thermal, or mechanical process, like ammonia fiber explosion, torrefaction, and steam explosion, respectively. These pretreatment processes help alter the amorphous and crystalline regions of the biomass and bring significant changes in structural and chemical compositions. Figure 1 shows how the pretreatment of biomass makes the biomass easier to convert.5 Torrefaction, which is a thermal pretreatment process, is a viable technology that significantly alters the physical and chemical composition of the biomass. Torrefaction is defined as slowly heating biomass in an inert environment and temperature range of 200–300°C. This process improves the physical, chemical, and biochemical composition of the biomass, making it perform better for cofiring and gasification purposes. Many researchers have studied the effect of torrefaction process time and temperature on the physical and chemical composition.6–15 However, a detailed literature review is lacking on the torrefaction process in terms of biomass reactions (such as depolymerization, devolatilization, and carbonization) and product properties. The primary focus of this research is to conduct a detailed literature review on biomass torrefaction, which includes (a) biomass reactions, including chemical and structural changes, (b) torrefaction product yields in terms of condensable, noncondensable, and solid product, and (c) the solid torrefied product’s physical, chemical, and storage properties for energy applications. Biomass components The plant cell wall is the tough, usually flexible but sometimes fairly rigid layer that provides structural support and protection from mechanical and thermal stresses. The major components of the primary cell wall are cellulose (carbohydrates), hemicelluloses, and pectin. The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix. The outer part of the primary cell wall is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.16 Cells and tissues of the plant body play an important role in the growth of the plant. The structural complexity of the plant body results from variations in the form and function of the cells and also from differences in the manner of combination of cells into tissue and a tissue system. The three different types of plant tissues are (1) meristematic tissue, (2) dermal tissue, and (3) vascular tissue.17 Secondary cell walls contain a wide range of additional compounds that modify their mechanical properties and permeability. The polymers that make up the secondary cell wall include (1) cellulose, (2) xylan, a type of hemicellulose, (3) lignin, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose, and pectin components and which drives out water and strengthens the wall, and (4) structural proteins (approximately 1–5%), which are found in most plant cell walls.16 Figure 2 shows the plant cell wall and lignocellulosic biomass composition.5 Table 1 shows the typical lignocellulosic content of some plant and woody biomass.12,18 Torrefaction process overview Torrefaction is a thermal pretreatment technology. It is also defined as isothermal pyrolysis of biomass occurring in temperature ranges of 200–300°C and performed at atmospheric pressure in the absence of research © mary ann liebert, inc. • Vol. 7 no. 5 • october 2011 INdustrIal BIotechNology 385 Figure 1. Pretreatment effect on lignocellulosic biomass 5 Cellulose