4.3.1 Bio-gas

Bio-gas is a combustible with high calorific power (4,500-6,500 kcal/N m3 depending on the chemical composition of the gas) that is obtained by anaerobic digestion (see par. 2.1, Chapter 4) of an organic substance. The main components of bio-gas are methane (CH4) and carbon dioxide (CO2); other substances with a lesser percentage are carbon monoxide (CO), nitrogen (N2), hydrogen (H2) and hydrogen sulphide (H2S).

Figure 24: Bio-gas spinneret.

Table 15: Medium gas composition [2]. Compounds % (on dry gas) Methane (CH4) 50-80 Carbon dioxide (CO2) 35-45 Hydrogen sulphide (H2S) 0.02-0.2 Water vapour Saturation Hydrogen, ammonia Traces Oxygen, nitrogen Traces

Before being used for energy production, the bio-gas must be subjected to appropriate treatments (see Table 17) which, by raising the percentage of methane to the detriment of other gases, improves the calorific power.

Table 16: Energy equivalence between 1 m3 of bio-gas and the main combustibles.

Gasoline 0.8 l

Methane 0.7 m3

Ethylic alcohol 1.3 l

Wood coal 1.4 kg

Wood 2.7 kg

In fact, it is the concentration of methane in the mixture that determines the final calorific power of the gas: the higher the concentration of methane, the higher is the LCP; the presence of carbon dioxide, nitrogen and water has a contrary effect. The treatment to which the bio-gas is subjected should reduce the percentage of corrosive agents, such as hydrogen sulphide, which can damage the utilization plants. The choice of the treatment, or treatments, to which the bio-gas is subjected depends on its initial characteristics and on the final calculated utilization [1, 2, 31].

Table 17: Bio-gas treatments (*only in the presence of excessive H2S). Consequence Treatment Usage Water Condense that provokes Condense separators In boiler malfunctioning Refrigerator condensation Cogeneration « Corrosive potential action systems natural gas H2S Engine corrosion Iron oxides filters In boiler* Electrical elements Active coal filters Biofilters NaOH washing Iron salts solution washing Cogeneration* « natural gas CO2 Removal of CO2 is necessary to improve the level of methane in the bio-gas (« natural gas) Absorbing in water with subsequent stripping and emission in atmosphere Half-permeable Membranes that selectively retain CH4 « Natural gas

Table 18: Biomasses and anaerobic digestion organic wastes and their bio-gas yield (m3 per volatile solid ton) [32].

m3 bio-gas/t SV

Animal dejections (piggish, bovine, birds and rabbit) 200-500

Cultural residuals (straw, beet collar, etc.) 350-400

Agroindustrial organic wastes (serum, vegetable wastes, 400-800

yeast, muds, distillery effluents, beerhouse, cellar, etc.)

Abattoir wastes (fats, stomach and intestinal content, 550-1,000

flotation sludge, etc.)

Depuration muds 250-350

Urban wastes organic fraction 400-600

Energy cultures (maize, sugar sorghum, etc.) 550-750

Currently, the main uses of bio-gas are relative to the thermal and/or electrical energy production. In detail, it is possible to produce [2, 32]:

• heat, as hot water, vapour or air, with a medium energy efficiency of 80-85%;

• electricity, generally in engines with vapour or gas turbines, for plants with a high capacity whose medium yield is 30-35%;

• combined production of heat and electricity (cogeneration) in endothermic engines with total medium yields of 80-85% (medium thermal yield: 50%, medium electric yield: 35%), which is currently the most used solution.

Other emerging applications are [2, 32]:

• fuel production for vehicles;

• cold production (three-generation), for example, with absorbing machines;

• the use in industrial ovens as a primary or auxiliary combustible.

In Europe, since 1990 we have witnessed a continuous growth in bio-gas pro­duction from 2,304 tons in 1999 to 3,219 tons in 2003. The leading country in this sector continues to be England, with more than 15,000 technicians in the sector in 2003 and a production of raw bio-gas which is equal to 1,151 tons; then there is Germany which has declared 2,000 installations for bio-gas production that is equal to 685 tons [33].

Currently, Europe can count on [32]:

• 1,600 operative digestion tanks for the stabilization of depuration sludge;

• 400 bio-gas plants for the effluent industrial waters with high organic load;

• 450 plants that work on the recovery of bio-gas from the urban rubbish dumps;

• more than 2,500 plants that work on effluents from intensive animal breeding, particularly in Germany (>2,000), Italy, Austria, Denmark and Sweden;

• 130 plants for anaerobic digestion; each of them processes more than 2,500 tons in a year of urban rejections and or industrial organic residuals with organic content.

In Italy, on the contrary, there are [32]:

• 120 digestion tanks for the stabilization of sludge and purification of effluent urban waters;

• few experiences (seven plants) of anaerobic digestion of urban rejections with organic content;

• several bio-gas plants in the agro-industry;

• more than 100 bio-gas plants for effluents from intensive animal breeding.