1.3.1.Biomass conversion technologies

Except the cases the direct combustion is suitable, raw biomass requires conversion into solid, liquid or gaseous fuels, which can be used for the production of heat and electricity and as a fuel for transportation. This conversion is carried out by mechanical, thermo-chemical or biological processes (Fig. 1). The mechanical processes are used for biomass pre-treatment as: waste sorting and compacting, wood residues compacting in bales, pellets and briquettes, chopping of straw and stalks and oilseeds pressing.

Figure 1. Main conversion processes of biomass.

A comparison of energy carriers produced from biomass can be made based on their ability to produce heat, electricity and transportation fuels. A useful way of comparing biomass and fossil fuels is based on their ratios O/C and H/C, known as Van Krevelen diagram (Fig. 2). The energy content of fuels is higher when the ratios are lower.

Figure 2. Chemical composition of solid fuels [2].

1.2.2.1. Biomass combustion

During the combustion process, a biomass particle passes through several phases, more or less distinct. First, the particle is dried until the temperature reaches 100C and then pyrolysis and/or gasification occurs followed by gas phase oxidation and combustion of char (Fig. 3). The time used for each reaction depends on the fuel size and properties, temperature and conditions of combustion. Combustion of a small particle shows a distinct separation with time between volatile and char combustion phases. At large particle combustion the phases overlap to a certain extent. In the continuously operating combustion systems the consecutive reactions occur simultaneously at different places in the furnace. A distinct separation of different combustion steps allows combustion optimisation by furnace design. The relative importance of each phase depends on the type of biomass, technology and process. Drying is realised by heat absorbing from the environment. The maximum water content of biomass to support combustion should not exceed 60% (mass). Humidity is a very important biomass property which influences the combustion plant design and the progress of combustion process.

Figure 3. Combustion phases of a biomass particle [2].

The main combustion parameter is the excess air ratio (λ) defined as the ratio between the available and the stoichiometric amount of combustion air.

Considering that CH_1.44O_0.66 is the average composition of typical solid biofuel, the combustion reaction can be described with the following equation [3]:

(1)

The conversion efficiency of biomass into electricity is about (20-25)%. Biomass can be burned directly or burned together with coal (co-combustion). Modern boilers are designed to use the co-combustion to reduce CO₂ emissions. In the design of the combustion system the characteristics of the fuel that is to be used, environmental legislation, cost and performance of the available equipment are taken into account.

The solid biofuel combustion is influenced by its physical characteristics and chemical composition.

Properties of solid biofuels

Biomass differs from coal in respect of organic and inorganic materials content, heating value and physical properties. Biomass has generally less carbon, aluminum and iron and more oxygen, silica, and potassium, lower calorific value, higher water content, lower density and lower friability. Biomass combustion process differs from coal combustion due to the biomass composition, especially higher volatile content. pulverized biomass combustion is dominated by devolatilisation and gas phase combustion as opposed to coal combustion, which is dominated by the formation of char and gas-solid oxidizing. The calorific value of biomass is much lower than that of the coal because of the high content of moisture and oxygen. Solid biofuels have higher volatile matter content (70-80%) than coal (10-15%).

Combustion properties of biomass can be classified into physical, chemical, thermal and mineral. The physical properties have large variation and those like density, porosity and inner surface are connected to the wood specie, and the bulk density and granulation are related to preparation methods of solid biofuel. Chemical properties are: elemental analysis, proximate analysis, higher heating value, calorific value of volatiles and calorific value of char. The values of thermal properties such as specific heat, thermal conductivity and emissivity vary with the moisture content, temperature and degree of thermal decomposition. The products of thermal decomposition of wood-based fuels are: moisture, volatile, char and ash. Some properties vary with species and growing conditions of biomass. Other properties depend on the combustion conditions. Biomass characteristics that influence combustion are: granulation, ash and moisture content and content of the structural constituents (cellulose, hemi-cellulose and lignin). table V shows the properties of solid biofuels and their main effects.