Biogas contains 55-80% methane, the higher the proportion the higher the calorific value. The quantity of biogas produced per unit volume of feedstock and per unit volume of digester determines digester performance and economic viability. In volume terms, on-farm anaerobic digestion (AD) in Northern Ireland has the greatest potential for cattle and pig farms. Values in the literature for biogas production from cattle and pig manures are variable. Data presented in Table 1 below are estimated and calculated from values cited in the literature. It is not economic to store large quantities of biogas and as a consequence, it is usual to utilise biogas as it is produced.
The heat required to maintain digester temperature depends upon climate, digester type, insulation and digester design. This requirement can be more than 50% of the energy available from the biogas produced. Mesophilic digesters require less heat input than thermophilic digesters. Since water does not produce biogas, heating water is an unproductive energy demand. Farm slurries are usually low in dry matter (2-10%); therefore on-farm AD is normally mesophilic.
Hydraulic Retention Time
The longer the hydraulic retention time (HRT) of the feedstock in the digester the greater the biogas yield per unit of feedstock. For a given feedstock loading rate, a longer HRT will correspond with a bigger digester and hence greater capital cost. In practice there is an optimum HRT that is a compromise between gas production and digester volume.
For pumping and mixing, feedstocks below 10% DM are required, with 8% considered as optimum for mesophilic digestion. Due to dilution from yard water and washings, many slurries on farms are often only 3-4% DM. Generally the higher the dry matter the higher the gas production per unit of slurry and the smaller the digester needs to be for a given number of animals.
Cchemicals that kill bacteria will inhibit anaerobic digestion. It is recommended to keep them out of the feedstock, e.g. disinfectants.
EU Animal By-Products Regulation
The EU Animal By-Products Regulation and the Animal By-Products Regulations (Northern Ireland) 2003 divide organic material into three different categories. These regulations must be complied with, particularly AD plants that treat combinations of categories.
Table of Biogas production and biogas energy values
|Parameter||Cattle slurry||Pig slurry|
|Dry matter (proportion)||0.08||0.05|
|Ratio Volatile Solids to Total Solids (VS:TS proportion)||0.80||0.75|
|Biogas production (m3 biogas / kg total VS)||0.25||0.30|
|Biogas approximate energy value (MJ / m3)||22.0||22.0|
|Hydraulic Retention Time (days)||24.0||15.0|
|Gas boiler efficiency (% gross biogas energy to hot water)||85.0||85.0|
|Overall combined heat and power (CHP) efficiency (% gross biogas energy)||85.0||85.0|
|Conversion efficiency of engine to electricity (% biogas gross energy to electricity)||32.0||32.0|
|Heat recovery from combined heat and power (CHP) (% gross biogas energy)||53.0||53.0|
|Heat to digester (kWh/t fresh feedstock)||31.0||32.0|
|Plant electricity consumption (kWh/t fresh feedstock)||3.1||3.1|