- Biogas production
– 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.
- Heat
requirement – 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.
- Dilution
- 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.
- Chemicals
-
chemicals 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.
Animal By-Products Regulations
(Northern Ireland) 2003
Table
1:
Biogas production and biogas energy values for cattle and pig slurries as derived from the literature.
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
|
32
|
|
Plant electricity consumption (kWh/t fresh feedstock)
|
3.1
|
3.1
|
