This is the most common design used in biomass combustion systems. The bio­mass fuel is fed at the upper part of the furnace where pre-drying of the biomass takes place. The dried biomass slowly tumbles down over the sloping grate onto a reciprocating grate in the lower portion of the furnace, where combustion takes place. The grate is either water cooled or air cooled which obviates the require­ment of an insulating ash layer in order to protect it from abrasion. Thus, this type of a combustion design is suitable for biomass with a lower ash content.

Fluidized-bed combustion

In this type of system, finely comminuted biomass particles are fed onto a bed of coarse sand particles present at the bottom of the furnace. Fluidizing air is passed through this bed in an upward direction through uniformly distributed perforations on which this bed rests. The velocity of this air is critically controlled such that it is just sufficient to fluidize the fuel particles in the air above the bed. The bed appears like a bubbling liquid at this air velocity. The coarse sand particles assist the mixing of the fuel with the air and also increase the heat transfer to the fuel for initial drying and subsequent ignition. Figure 1.8 shows a schematic diagram of a typical fluidized — bed combustion system designed for a boiler. The critical air velocity or the minimum fluidization velocity at which fluidization occurs is a function of the biomass particle size, density and pressure drop across the bed. An increase in air velocity beyond the minimum fluidization velocity causes the bed to become turbulent, and subsequently to circulating. This results in increased recycling rates of the material in suspension. Commercial designs are either bubbling fluidized-bed or (BFB) or, circulating fluidized-bed (CFB). The entire system may operate at atmospheric pressure or may be pressurized. Air or oxygen may be used for fluidization.

BFB system uses air velocities of 1-3 m/s. The primary air supply is through nozzles beneath the bed, whereas the secondary air flow enters the furnace above the bed. The ratio of the primary to secondary air supply controls the bed tem­perature. The bed temperature can also be controlled by recirculating some of the flue gases that are formed as a result of combustion of the biomass.

In the CFB systems, a higher air velocity of 4-9 m/s is employed. This causes the bed material to circulate within the furnace. As in BFB, here also, there are

primary and secondary air supplies. Due to the higher air velocities used, the smaller biomass particles tend to get entrained along with the flue gases generated as a result of combustion. Cyclone separators are provided to collect the biomass and sand particles which are then returned to the feed bed. CFB designs are more expensive than the BFB ones. However, CFBs operate at lower operating tem­peratures than the BFBs, which reduces the NOx emissions significantly.

Fluidized-bed combustion systems are much more versatile compared to fixed — bed design systems. A wide range of biomass with varying compositions such as higher moisture contents and varying ash properties can be handled without encountering slagging problems. Varying loads, ranging from full capacity to as low as 35% of full capacity can be handled. At any given time, compared to the fixed-bed design, only a small quantity of fuel is present in the combustion chamber, hence, giving good conversion efficiencies. However, the fluidized-bed designs are costly compared to the fixed-bed designs and are suitable for large — scale operations only.

Pyrolysis

Pyrolysis is the thermochemical decomposition of organic material at high temper­atures in the absence of oxygen, producing gas and liquid products and leaving behind a carbon-rich residue. It is invariably the first step in combustion and gasi­fication of biomass. If sufficient oxygen is provided subsequent to initial pyrolysis, it can proceed to combustion or gasification. The liquid products obtained from pyrolysis include water and oils, whereas the gaseous products include carbon monoxide, carbon dioxide, and methane. A solid residue that is left behind is a carbonaceous solid, i. e., charcoal. The solid residue can be used as such for heating. The gas produced can be processed through a gas burner and under a restricted air supply can be used as a heat source for the pyrolyzer, or it can be used in gas turbines or gas boilers for production of electricity. The liquid product, bio-oil can have multiple uses: it can be used as such for heating, or for power generation, or it can be upgraded to transportation fuel, or can be used for conversion into suitable chemicals. Figure 1.9 shows the different energy products/forms that can be obtained from pyrolysis. Figure 1.10 shows a general schematic diagram of a pyrolysis process.

The fact that one of the products of pyrolysis is a liquid product (viz. bio-oil) makes this process very important because liquid fuels are easy to transport and hence, it is possible to have the conversion plant remote from the point of use, which is not possible in case of the combustion process. Pyrolysis is not an exothermic process like combustion. It is an endothermic process where heat is required to be supplied for the process. Different types of pyrolysis processes, resulting in different types of products, are possible depending on the temperature and the rate of heating employed. The nature of the biomass also largely affects the

Fig. 1.10 Schematic diagram of a pyrolysis process (Adapted from [11]) yield of pyrolysis, as the rate of heating depends on the nature of biomass. Typically, lignocellulosic materials such as wood, stalks, straw, etc. are poor heat conductors. Hence these materials require pretreatment such as size reduction before they can be used for pyrolysis, so that an acceptable yield can be obtained. Lower processing temperatures and longer vapor residence times are favorable for production of charcoal (solid product); higher processing temperatures and longer vapor residence times favor production of gas, whereas under moderate temper­atures and short vapor residence times, a liquid product is obtained. The product dependence on the processing conditions and vapor residence times can be explained on the basis of the composition of the biomass and the chemical nature of pyrolysis [10]. Biomass mainly comprises polymers in which large chains of carbon atoms are linked with each other, or to oxygen atoms, or sometimes to other elements like nitrogen or sulfur, to form macromolecules. The most com­monly occurring macromolecules in biomass are hemicelluloses and cellulose.

Unprocessed biomass consists of a small number of such large polymers or macromolecules. Cellulose is a linear chain polymer, whereas hemicellulose is a branched chain polymer with side chains or branches present at random locations along the chain. As heat is supplied, the chemical bonds linking the monomer units in the large polymer begin to break off. In cellulose, the bonds are broken ran­domly along the chain whereas in hemicelluloses, first the side chain or branches break off followed by breaking of the straight chains. As more heat is supplied, a large number of smaller molecules are generated i. e. the degree of polymerization (Dp) reduces. When Dp reduces to <10, the polymer is no longer a polymer but an oligomer. These oligomers (especially those having Dp less than around 8), are volatile. These are generated at typical pyrolysis temperatures between 400 and 800°C. These oligomers, comprising anhydro sugars, evaporate from the solid mass as volatiles. These are required to be removed from the solid biomass. If they are not removed, under continued influence of high temperature, they undergo thermal fragmentation to produce highly reactive, small intermediates. These fragments, if removed and quenched immediately, can be used as such as chem­icals or as fuels. If, again, these are not removed from the solid biomass, they undergo chemical reaction with the remaining solid material to form new polymers or, accelerate breakdown of original chains. These reactions are exothermic and thus accelerate the overall pyrolysis reaction. Thus, depending on the processing conditions and vapor residence times, different types of pyrolysis processes have been developed, which result in a different product mix. These are summarized in Table 1.4.

A brief description of the different variants of the pyrolysis process is given below.