A co-digestion biogas unit is designed with an annual solid processing capacity of 5,000 ton/yr.

A co-digestion biogas unit is designed with an annual solid processing capacity of 5,000 ton/yr. Assume the feedstock consists of low water content vegetable wastes (45%), untreated household biowaste (5%), raw untreated rumen content (25%) and fruit mash (25%). Elemental analysis confirms that the feedstock processed by this biogas unit has the following characteristic: Analysis results:

 An average chemical composition of   C13H25O7N3S

 The water content of the feedstock is 500 kg H2O/ton feedstock.

  It contains 32% dry matter (DM) and 80% volatile dry matter (VDM)with an density of 0.75 ton/m3 

The biogas production is at optimum when the digester is operated at 55 C.

 i. Use the biogas production table provided in your lecture, calculate upper and lower yields of biogas (in m3/day) from this plant.

 ii. Provide three reasons and explain why the actual biogas yield is always lower than the theoretical value.

 iii. Determine the concentration (in %) of CH4 and CO2 in the final biogas produced. 

iv. Determine the energy content of the biogas.

v. What is the parasitic thermal and electrical energy requirements for this operation? (the temperature of the surrounding varies throughout the year but you may assume an average temperature of 20 C in your calculation)

vi. If the HRT is fixed at 30 days, what should be the size (in m3) of the digester?

table:

Vegetable waste

DM (%) and oDM per DM (%) 5-20

Biogas yield (m3/kg) 0.4

Household biowaste

DM (%) and oDM per DM (%) 40-75

 Biogas yield (m3/kg) 0.3-1.0

Rumen content (untreated)

DM (%) and oDM per DM (%) 12-16

Biogas yield (m3/kg) 0.3-0.6

Mush from fruit

DM (%) and oDM per DM (%) 2-3

Biogas yield (m3/kg) 0.3-0.7