Physicochemical Properties of Lignocellulosic Biomass for Engineering Applications
The most highly available renewable resource in the globe for biofuel generation is lig- nocellulosic biomass. Dependence on biofuel extracted from lignocellulosic biomass can lead to less reliance on fossil fuels and can help significantly in climate change mitigation. The analytical characterization methods and physicochemical properties will give suitable biomass conversion techniques. The main objective of this chapter is to present a complete study of the physical and chemical properties of biomass, which include grind ability, flowability, particle size, density, thermal properties, moisture sorption, calorific value, and elemental composition. It also focuses on offering systematic information to users by describing techniques and physicochemical properties of biomass for converting it and for the biofuel’s application (Fatma et al. 2018)
Density
The density of the biomass or any substance is defined as mass per unit volume. The density of biomass is classified as particle density, and bulk concentration is mentioned below.
8.4.1.1 Particle Density
Excluding the pore space volume for a group of biomass particles, the particle density is defined as the ratio of the mass of all particles to the volume occupied by the particles.
where pp is the density of the particle, mp is the mass of a particle, and Vp is the true volume of the particle. The pressure difference can be used to determine the true volume with a known pressurized gas quantity flowing into a reference volume cell contains biomass matter (Xue and Fox 2015).
Where Vc is the sample cell volume, VR is the reference volume, P, is the pressure after pressurizing the reference volume, and P2 is the pressure after including Vc. The biomass particle density will be an important factor considered in a computational fluid dynamics simulation (Himmel et al. 2007).
8.4.1.2 Bulk Density
The biomass particle mass upon the total volume of biomass particle is known as bulk density, which includes the volume of pore space in and around the biomass matter. This is a physicochemical property which is used in planning the biomass system of logistics, transport, and handling, which depends on the size and shape of biomass matter, the moisture content, the density of particles, and the characteristics of the surface. According to the ASTM standard E873-82, the bulk density measurement of biomass is performed (ASTM E873-82 2006). A regular box (300 mm width x 300 mm depth x 600 mm height) is used to receive the poured biomass sample, and the remaining materials are removed by scraping across the top of the container with a straight edge. The bulk density can be found from the net mass of the sample (Rabieret al. 2006).
Extreme variations in bulk densities such as from 15-201 kg nr3 for cereal grain straws to 280-480 kg nr3 for hardwood biomass chips are shown by different biomass samples, which are low compared to of coal’s bulk density, of about 901 kg rrr3 (Jose and Bhaskar 2015). The bulk density is proportional to the fill tightness degree; the loose fill had a bulk density of 50 to 265 kg rrr3 and packed fill had 69 to 326 kg nr3 for switchgrass (after tapping). Porosity, defined as the bulk samples’ pore spaces, is shown by the following formula;
where e0 is the bulk biomass sponginess, pb is the bulk density, and pp is the density of the particle (Lam et al. 2007).
Flowability
The property of flowability plays a major role in handling, storage, and transportation of biomass. The feeding subsystem of a biomass conversion system is usually bridging the biomass particles (Ueki et al. 2011). Flowability is a means to measure the flow of biomass from one place to another. The parameters to describe biomass flowability are the cohesion coefficient, the angle of repose, the flow index, and the compressibility index. The steepest angle where a large quantity of biomass particles remains constant without slumping, known as angle of repose, ranges from 0° to 90° (Rouse 2014). The methods used for measuring the angle of repose are fixed funnel, titling box, and revolving cylinder. This method shown here is the method of static funnel represented in the ASTM standard C144 (Rosillo-Calle 2012). A cone is formed from a funnel, which is used to pour biomass matter slowly into it. The poring should be stopped when the biomass matter has accumulated and reaches a defined elevation or base of determined width. The formula for finding the angle of repose is as follows:
where the angle of repose is ф, and H and R are the piling cone’s height and radius, respectively (Mani 2015).
The various types of flowability of biomass are low, medium, cohesive, high, and very cohesive flowing. Mani has clearly shown that the flowability results in different angles of repose. When the angle of repose is less than 55°, the flow index is FI<1 and the flowability grade is very cohesive. When it is between 45° and 55°, then FI lies between 1 and 2 and the flowability grade is cohesive. When it is between 38° and 45°, then FI lies between 2 and 4 and the flowability grade is low flowing. When it is between 30° and 38°, then FI lies between 4 and 10 and the flowability grade is medium flowing. When it is less than 30°, then FI is higher than 10 and the flowability grade is high flowing (Szalay, Kelemen, and Pintye-Hodi 2015). The bulk density of biomass increases under consolidated pressure and the compressibility index of biomass is determined. At any given consolidation pressure, this can be determined by the following equation:
where Cb, ры, and pbf are the compressibility, initial bulk density before consolidation, and final bulk density, respectively (Jenike 1964). The flow behavior of biomass material can be quantified using a shear tester. This method is done as per the ASTM standard D6128-16. The unconstrained yield stress (oc) and the main associating stress (cr,) are noted. The slope of the linear fit of versus o, plot is used to get the flow function for biomass particles. Flow index is the transpose of the flow function (Conshohocken 2001).
Particle Size
The fluidization and mixing, the surface area for mass and heat transfer, and the flow behavior of biomass particles are all affected by the size and shape of biomass feedstock particles (Vidal et al. 2011). Hence, the efficiency of various conversions and the energy input requirements can be seen for feedstock particles of varying shapes and sizes (van Loo and Koppejan 2012). Several difficulties arise due to irregularity in the shape of lignocellulosic biomass such as elemental quantities of width, length, and thickness. The handling, storage, and processing facilities require accurate knowledge of biomass particle size and shape. The most important parameter for the particles is the aspect ratio (AR), which affects the transfer of heat in the feedstock thermal conversion (Gera et al. 2002). It is shown as AR = bl where AR, b, and / are aspect ratio, width (i.e., the smallest space between of 2 parallel lines at a tangent to the outline projected by the particle), and length (i.e., the biggest space between two parallel lines at right angles to the width tangents), respectively. The two main particle size description methods are analysis of sieving and imaging particle analysis (Shastri et al. 2014).
Moisture Sorption
Another important factor for biomass harvest, handling, transport, and storage is the moisture sorption. In order to preserve the biomass feedstock quality, suitable ventilation and storing operations are necessary (Lin et al. 2016). The characterization of the water sorption behavior of biomass is done with equilibrium moisture content (EMC), which is described as the moisture content of a substance in equilibrium in a specified atmosphere with regard to temperature and comparative humidity. It depends on the specific surface area and the composition microstructure of the biomass (Kaur 2009). The equilibrium moisture content is determined using various static and dynamic methods. From the saturation of different concentrations of salt solution, several different relative humidities are obtained with respect to saturated and dried-out air (Mann et al. 2015).
Thermal Properties
The thermochemical conversion characteristics of biomass are greatly influenced by the thermal properties of biomass. The important properties of thermal are specific heat and thermal conductivity, w'hich are explained here.
8.4.5.1 Thermal Conductivity
The heat conduction in and around the fiber is changed with respect to the biomass material when it is heated during a thermochemical conversion process, which affects their behavior. Masonet al. used a custom-built test apparatus which is a part of a method that gives their thermal conductivities (Mason et al. 2016). It was observed that wood materials have high thermal conductivity in the range of 0.080 to 0.419 W nr1 K-1 and residues from agriculture have a very low thermal conductivity of around 0.05 W nr1 K_l. Being an anisotropic material, the thermal conductivity of biomass is dependent on the porosity, density, moisture, heating direction, and temperature (Dahlquist 2013). The correlations interrelating the thermal conductivities, density and moisture content were given as follows:
where Kcf„ Мл, and .v„ are the thermal conductivity (W nr1 K_l), specific gravity, and moisture percentage of biomass, respectively (Pandey 2008).
8.4.5.2 Specific Heat
One more important thermal property of biomass is the specific heat, which is used to indicate the capacity of the heat of the material, which in turn is required for calculations regarding thermodynamics of the material depending on the moisture content in the biomass and its temperature. Ranging from 0°C to 106°C, the expression for the specific heat of dried wood is given by the following:
where 0 and Cp() are temperature and specific heat at the temperature 0 (in°C and J g_l K_l, respectively). The relationship between the specific heat of residual samples from dried agricultural products withparticle sizes of less than 200 pm and temperature is given below:
where T and CpT are the temperature (314 К < T <355 K) and specific heat at the temperature T(in К and J kg-1 K_l, respectively). The following expression shows the impact on specific heat by moisture content.
where MARB and Cw are the moisture content as received and water-specific heat, respectively.
