Biomass can be used to generate different forms of energy (Fig. 1.3). It can be either burnt directly to generate heat, or the flue gases generated during the burning of biomass can be used to provide process heat. The heat generated from biomass can be used to generate steam which can again be used either directly to provide process heat or it can be converted into electricity via steam turbines. As such, biomass is very low in terms of energy density. It can be upgraded into high energy density fuels such as charcoal, liquid fuels (mainly transportation fuels), and gaseous fuels such as hydrogen, producer gas, or biogas. These biofuels form the major, most important product of the bioconversion processes.

Biofuels are classified into four categories depending on the nature of biomass used to produce it. Table 1.1 gives a concise classification of biofuels with rep­resentative examples for each category.

First-generation biofuels are already commercially produced, and an estab­lished technology is available for their production. However, the major problem with first-generation biofuels is that their production largely depends on raw material feedstock that could otherwise be used for food and feed purposes. This food versus fuel controversy gave rise to the development of second-generation biofuels which are produced from non-feed crops, forest residues, agricultural, industrial, and domestic waste. Second-generation biofuels are produced mainly by thermochemical and biochemical methods. The thermochemical methods are more amenable to commercialization as these are based on technologies estab­lished over a number of years. The biochemical methods have not yet been commercialized but these methods have a greater potential for cost reduction. Research efforts toward their optimization are currently ongoing and may soon result in commercialized, low cost alternatives to first-generation biofuels.

Although second-generation biofuels are able to circumvent the food versus fuel controversy, they still need arable land for the generation of feedstock required for their production. Thus land which would otherwise have been used for growing of food crops would still be required. This gave rise to third-

generation biofuels such as biofuels produced from seaweeds and algae. This algal biomass is capable of flourishing in marshy land, sea water, and land which is totally unproductive with respect to cultivation of agricultural crops. Concerted efforts are underway to bring out successful technologies which produce biofuels from algae.

Fourth-generation biofuels are still at a conceptual stage and many more years may be required for these types of biofuels to become a reality. These biofuels are produced by technologies which are able to successfully convert biomass into fuel in such a manner that the CO2 consumed in their generation is much more than that produced as a result of their burning or use. Hence, these biofuels would be instrumental in reducing atmospheric GHGs, thus mitigating the problem of global warming to a significant extent. The technologies for the production of fuels other than first-generation biofuels are yet to prove them­selves as commercially viable alternatives to fossil fuels and are under various stages of development. The following section gives an overview of the different biomass conversion technologies developed till date. These are broadly classified as shown in Fig. 1.4.

An important aspect about the use of biomass as an alternative to fossil fuel for generation of energy is that biomass has a high volatility compared to fossil fuels due to the high levels of volatile constituents present in biomass. This reduces the ignition temperature of biomass compared to that of fossil fuel such as coal. However, biomass contains much less carbon and more oxygen. The presence of oxygen reduces the heat content of the molecules and gives them high polarity.

PROCESSES FOR
CONVERSION
OF BIOMASS INTO
ENERGY

Fig. 1.4 Processes for biomass conversion into energy

Reproduced with permission from [1] a wt% of dry fuel b wt% of dry ash

Hence, the energy efficiency of biomass is lower than that of coal and the higher polarity of the biofuel which is obtained from biomass causes blending with fossil fuel difficult. Table 1.2 gives a comparison between the physicochemical and fuel properties of biomass and coal.

It can be seen from Table 1.2 that the properties of biomass and fossil fuel vary significantly. Although biomass has a lower heating value, the emission problems especially, emission of CO2, NOx, SOx for biomass are much less than those for coal due to the lower carbon, sulfur, and nitrogen contents of biomass.