ENERGY RECOVERY FROM BIOGAS GENERATED FROM FISH PROCESSING
PLANTS EFFLUENTS
Author(s):


Sindra Lutchmee Summoogum, Research Centre for Energy, University of Newcastle,
Australia
Vikram Seebaluck, University of Mauritius
Abstract
This article reveals the potentials and opportunities of methane recovery from biogas
generated from effluent wastewater in fish processing plants for use as an energy
product. It focuses on a commercial scale tuna processing factory whereby the methane
gas recovered could be potentially used to satisfy part of the internal energy
requirement of the plant, thereby displacing significant amount of heavy fuel oil. Indeed,
methane gas is a combustible that can be converted to other energy forms and can be
used as a fuel for steam production, electricity generation, in spark-ignition engines or
for cooking purposes. In this study, three potential applications were considered namely
heat and steam production, electricity generation, and compression and liquefaction of
the gas for alternative utilizations such as a fuel for vehicle.
Biogas is primarily composed of methane and carbon dioxide but its composition
depends on the type of material fermented, the design and efficiency of the anaerobic
reactor. Typical values of methane (CH4) and carbon dioxide (CO2) lie in the range of
55-65 % and 35-45 % respectively. Hydrogen sulphide (H2S) is usually reported in ppm
and varies significantly depending mostly on the substrate. The tuna processing plant,
where the study was conducted, had a maximum flow rate of 74.6 m3/hr of biogas
derived from its wastewater having a maximum chemical oxygen demand of 4080
kg/day. The biogas generated was analyzed and found to have an average
concentration of 63.5 % methane, 16.7 % carbon dioxide, 1.9 % oxygen and 17.9 % of
other gases which contained 367 ppm carbon monoxide and 2500 ppm hydrogen
sulphide.
The first scenario, methane recovery for heat and steam production for the internal use
of the plant, necessitated the existing two boilers of 10 tonnes steam/hr in the plant to
be revamped to accommodate gas burning in the boiler house. This required the rotary
burner to be retrofitted to a 669 kW dual burner with an increase in capacity of the
forced draft fan from 23 W to 12 kW. The usual arrangement is to have a fuel oil supply
available on site, and to fire it in the boiler when gas is not available. Due to the lower
calorific value of biogas (28.89 MJ/kg, equivalent to 26 MJ/ m3) compared with heavy
fuel oil (40.7 MJ/kg), 680 kg/hr of biogas was required to displace the 563.6 kg/hr of
HFO for the production of 10 tonnes/hr of steam. However, the maximum flow rate of
biogas that could be produced was 67.1 kg/hr (74.6 m3/hr) such that the percentage of
HFO that could be displaced by biogas was found to be 9.9 %. The capital investment
needed for this set-up was around 22 million MUR which could be paid off in 5.05 years.
Moreover, a net present value of around 29 million MUR and an internal rate of return
(31.4 %) greater than the discount rate (10 %) were obtained. Based on these findings,
it was suggested that the fish processing plant could implement this set-up in the short
term given that this option was technically and economically feasible.
In the other proposed two scenarios for energy recovery from biogas, hydrogen
sulphide gas needed to be removed to prevent formation of sulphuric acid, given that it
has a high tendency for causing equipment corrosion. A 15m 3 biological filter was
proposed to be used which could achieve a 99 % H2S reduction, thus decreasing its
concentration from 2500 ppm to a value as low as 10 ppm. The resulting biogas could
then be combusted with an air to fuel ratio of 100:1 in an open gas turbine. The turbine
and thermal efficiency were found to be 85 % and 31.4 % respectively, with an
electricity generation potential of 0.38 MWh of electricity. For this set-up, a total capital
investment of 75 million MUR was required with an estimated unit production cost of
around Rs 3/kWh. In Austria, a similar estimated unit production cost of around Rs
4/kWh was obtained (€0.103/kWh). A higher investment was required compared to the
first scenario and this was due to the additional cost of the desulphurization unit and the
gas turbine. However, electricity generation from biogas was found to be the second
best option for energy recovery from biogas. The breakeven cost of unit production of
electricity was found to be around Rs 2/kWh which could be achieved if appropriate
measures are adopted to optimise the biogas plant.
For the liquefaction process, further purification was essential and carbon dioxide (CO 2)
was proposed to be eliminated by water scrubbing technique. Subsequently, the sweet
biogas could be compressed and liquefied from 824.3 m3/hr to 3.3 m3/hr at a pressure
of 250 bars and a temperature of -161°C. Based on a cost benefit analysis, a capital
investment of 181 million MUR and an exorbitant unit product cost of Rs 22,615/ m 3,
compared to a cost price of Rs 890/ m 3 to Rs 1373/m3 in the United States. Hence, this
option was not found to be appropriate at current time.
In light of this study, it was found that biogas derived from fish processing plant effluents
could be used for energy recovery either as steam or electricity. A number of other
substrates exist where biogas could be derived such as oats, wheat, and barley. In
Mauritius, potential sources where biogas could also be tapped are for instance the
existing breweries where the by-product obtained from the anaerobic digestion of the
wastewater, rich in organic matter, could be recovered for steam or electricity
production. Application of such techniques and strategies for energy recovery could
help in reducing the country’s reliance on imported fossil fuels and at the same time
improve the production cost and competitiveness of product manufactured.
Keywords: Fish processing plant, biogas, methane, energy.