**3. Biomass conversion processes**

The development of conversion technologies for the utilization of biomass resources for energy is growing at a fast pace. Most developing countries find it hard to catch up because

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typical mixture consists of 65% methane and 35% CO2 with traces of other gases. The methane producing bacteria (called methanogenic bacteria) generally require a pH range for growth of 6.4 to 7.2. The acid producing bacteria can withstand low pH. In doing their work, the acid producing bacteria lower the pH and accumulate acids and salts of organic acids. If the methane-forming organisms do not rapidly convert these products, the conditions become adverse to methane formers. This is why the first type of reactors developed for conversion of biomass wastes into methane have long retention times seeking equilibrium

Municipal wastes and livestock manures are the most suitable materials for anaerobic digestion. In the US, numerous landfill facilities now recover methane and use it for power generation. Aquatic biomass such as water hyacinth or micro-algae can be digested and may become valuable sources of energy in the future. Anaerobic digestion of organic wastes may constitute an effective device for pollution control with simultaneous energy generation and nutrient conservation. A major advantage of anaerobic digestion is that it utilizes biomass with high water contents of as high as 99%. Another advantage is the availability of conversion systems in smaller units. Also the residue has fertilizer value and can be used in crop production. The primary disadvantage of anaerobic digestion of diluted wastes is the large quantity of sludge that must be disposed of after the digestion process including the wastewater and the cost of biogas storage. In cold climates, a significant fraction of the gas produced may be used to maintain the reactor operating temperature. Otherwise,

microorganisms that thrive on lower or moderate temperatures should be used.

Fig. 2. Steps in anaerobic digestion process with energy flow represented as % chemical

Three main types of biogas facilities have been successfully developed in Asia for widespread biogas production in households and industrial use. These are the "Chinese Digester" of fixed dome type, the "Indian Gobar Gas Plant" of floating gas holder type and the rectangular commercial size biogas digesters developed in Taiwan. These are what we may call the first generation biogas reactors. Shown in Figure 3 is the common Chinese digester design. These

between acid and methane formers.

(Source: American Chemical Society)

**3.1.1.1 The first generation biogas reactors** 

oxygen demand (COD).

the level of technology is beyond their manpower as well as their manufacturing and technological capability. Added to this is the unavailability of local materials and parts for the fabrication of these conversion units. Figure 1 shows the different methods for converting biomass into convenient fuel. Biomass conversion into heat energy is still the most efficient process but not all of energy requirement is in the form of heat. Biomass resources need to be converted into chemical, electrical or mechanical energy in order to have widespread use. These take the form of solid fuel like charcoal, liquid fuel like ethanol or gaseous fuel like methane. These fuels can be used in a wide range of energy conversion devices to satisfy the diverse energy needs. In general, conversion technologies for biomass utilization may either be based on bio-chemical or thermo-chemical conversion processes. Each process will be described separately.

Fig. 1. Methods of using biomass for energy.

### **3.1 Bio-chemical conversion processes**

The two most important biochemical conversion processes are the anaerobic digestion and fermentation processes.
