3.2.2 Digestion

Anaerobic digestion is a biological process, which produces biogas. It is performed on the waste (at T = 30-35°CoratT = 50-55°C) by two types ofbacteria and it involves two steps: (i) breakdown of complex organics in the waste by acid-forming bacteria: into simpler compounds, including volatile acids (e. g. such as acetic and propionic); and (ii) the conversion of these acids by methane — producing bacteria into CO2 and CH4 called “biogas”. Typically both steps are performed in a single tank (GTI reports) and biogas contains mainly CH4 (~60%), CO2 (~35%) a mixture of H2, N2, NH3, CO and H2S (~5%). The heat value of biogas is about 22350kJ/m3 for a mixture of CH4:CO2:inerts = 60:35:5. The investigations on digester-based energy conversion systems involving high moisture and/or high ash (HA) animal biomass (typically collected with soil, Fig. 3b) have mostly dealt with capturing biogas from biological systems such as anaerobic digesters. The percentage of CH4 may be reasonably predicted using atom conservation equations for the reaction between digestible solids and H2O (Annamalai and Puri, 2007):

CH1.98O0.83N0.086S0.0084 (s) + 0.09 H2O (£) ^ 0.54 CH4 (g) + 0.46 CO2 (g) + N0.086S0.0084 (s)

(3.1)

Even though very little water is consumed (0.09 kmoles of H2O per unique carbon atom in fuel (or empirical kmole of fuel), the bacteria can survive only in dilute slurry of water and digestible or volatile solids (VS). Element conservation yields 54% CH4 and 46% CO2. One SI tonne of liquid manure with 5% dry matter (DM) produces about 20 m3 biogas (Gregersen, 2009). There were only 40 operational systems in this country as of June 2004 (USEPA, 2004). Further discussions of biological energy conversion of manure-based biomass solids can be found in Meyer (2003), Matthews et al. (2003) and Schmidt et al. (2000). The digester efficiency is defined as the ratio of volatile solids (VS) converted into gas to VS fed in. For onion waste the digester efficiency is about 54% (Romano, etal., 2004; Gunaseelan, 1997, 2004).

Up to 162 digestion systems were operating in the USA as of 2010 generating 450 million kWh (402 million as electricity and others as supplemental fuel, mixed with natural gas; 2.8 million barrels or equivalent to 25,000 homes per year i. e. average power consumption per home 20.5 kW) with 15 new digesters every year (Agstar Bulletin 2011; also see GTI reports). Digesters pro­duce a renewable fuel in the form of CH4 which also has a higher global warming potential when compared to CO2 (20 times the global warming potential (GWP) compared to CO2). Thus approx­imately 246,000 tonnes of CO2 avoided; by capturing CH4 and CO2 in digester (as opposed to releasing CH4 and CO2 during atmospheric natural digestion), 1.1 million tonnes of CO2 equiva­lent is destroyed. The majority of the plug flow reactors operate at mesophilic temperatures of about 35-40°C; others include covered lagoons; and about 25% of them co-digest with other organic wastes (food waste, agricultural wastes, cheese whey, etc.). Typical yields are as listed in Table 3.1.