Q$2.$ Biomass Energy

Various forms of biodegradable waste from industry, agriculture, and forestry can also be

converted to biogas through anaerobic digestion by microorganisms. An agri$-$food business

produces $1\mathrm{.}0$ tonne per hour of waste at $15C.$ The owner is considering generating biogas

from the waste in an anaerobic digestor and combusting this biogas in a small reciprocating

internal combustion engine $($RICE$)$ to generate electricity for the business. The RICE

manufacturer stipulates that impurities in the biogas $($such as hydrogen sulphide and

ammonia$)$ are removed before combustion. The 'cleaned' biogas and $10\%$ excess air enter

the RICE at $25C$ and $1$ atmosphere while combustion products exit the RICE at $427C.$ The

rate of heat loss from the RICE is estimated to be equal to the shaft power developed. The

electrical generator coupled to the RICE has an electrical efficiency of $90\%.$ A sample of the

oven$-$dry waste has an elemental composition $($by weight as a percentage of dry solids$)$ of

$48\mathrm{.}3\%,7\mathrm{.}6\%,35\mathrm{.}8\%,$ and $3\mathrm{.}3\%$ of carbon, hydrogen, oxygen and nitrogen, respectively. The

remaining $5\%$ of dry solids is inert $($or ash$).$ The moisture content of the waste is $70\%$ while

$95\%$ of the dry solids are volatile. A well$-$designed anaerobic digester aims at a minimum of

$70\%$ conversion of volatile solids to biogas.

Your task is to design and analyze the proposed waste$-$to$-$energy $($WtE$)$ system. Prepare a

"mass balance diagram" for the entire WtE system. Determine the biogas composition, the

theoretical biochemical methane potential of the waste, the required air$-$fuel ratio for the

combustion of biogas in the RICE, the net electric power generated and the net electric LHV

$($lower heating value$)$ heat rate of the RICE.

$(50$ marks$)$