Page 9 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN002C-60 May 17, 2001 20:23

Biomass Utilization, Limits of 165

harvesting costs of the latter are generally greater than biogas. This efficiency varies from 18 to 95%. Dairy cows
for dung. The processing facility can be relatively simple produce 85 kg daily of manure for each 1000 kg of live
and be constructed for about $700. A large facility ca- weight. The total solids in this manure average 10.6 kg,
pable of processing the dung from 320 cows might cost and of these, 8.6 kg are VS. Theoretically, a 100% efficient
about $150,000. The basic principles for both systems are digester could produce 625 l of biogas for every kilogram
similar. of VS in the system. The digester utilized for the data pre-
Manure from a dairy farm or small cattle operation is sented in Table V was 28.3% efficient. It produces 177 l of
loaded or pumped into a sealed, corrosion-resistant diges- biogas per kilogram of VS added or 1520 l of biogas per
tion tank where it is held from 14 to 28 days at temper- 1000 kg live weight of cattle daily. Note, if the total heat
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atures from 30 to 38 C. In some digestion systems, the value of the manure was used in calculating efficiency,
manure in the tank is constantly stirred to speed the diges- then the percentage efficiency would be only 5%.
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tion process and assure even heating. During this period, Biogas has an energy content of about 5720 kcal/m ,
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the mesophilic bacteria break down volatile solids (VS) in compared to 8380 kcal/m for pure methane gas, because
the manure and convert them into methane gas (65%) and carbon dioxide is present in the biogas. Energy costs and
carbon dioxide (35%). Small amounts of hydrogen sul- energy outputs for processing 100 t of manure (wet), with
fide may also be produced. This gas is drawn off through a 7.1 million kilocalories energy input, results in a total of
pipes and either burned directly, similar to natural gas, or 10.2 million kilocalories produced for a net energy yield
scrubbed to clean away the hydrogen sulfide and used to of 3.1 million kilocalories (Table V). Much of the energy
generate electricity. The energy output/input is listed in input or cost comes from the production of electricity to
Table V. run the pumps and stirring system used to reduce the re-
The amount of biogas produced in this system is deter- tention time in the digester. The volume of the digester
mined by the temperature of the system, the VS content is determined by the amount of manure produced by the
of the feedstock, and the efficiency of converting it into animals during the retention time. In this example, with a
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retention time of 14 days, it would be slightly over 75 m .
It is assumed that the electricity is generated from the
biogas and that the electrical conversion efficiency of the
TABLE V Energy Inputs Using Anaerobic Digestion for Bio-
gas Production from 100 t wet (13 t dry) using Cattle Manure entire operation is 33%. The energy needed to heat the di-
(Pimentel et al., 1988) a,b gester is cogenerated by the electric generator via the use
of the generator’s cooling system as the heat source. The
Quantity kcal (1,000)
net energy produced by the digester can either be used to
Inputs generate electricity for the farm or be used as heat source
Labor hours 20 hr — for other on-farm activities.
Electricity 2,234 kWh 5,822 Although material costs are lowered if there is no gen-
Cement foundation (30-year life) 0.9 kg 2 erator or stirring mechanism on the digester, the size of
Steel (gas collector and other 35 kg 725 the digester must be increased because of the increased re-
equipment with 30-year life)
tention time needed to complete the process. Also, some
Pumps and motors 0.5 kg 1
of the biogas will have to be used to heat the digester, per-
Truck/tractor for transport 10 kg 200 haps an additional 610,000 kcal for every 100 wet tons of
(10-year life)
manure digested. The critical heat requirements are calcu-
Fuel for transport (10-km radius) 34 l 340
lated by including the heat losses to the surroundings, the
Total inputs 7,090
heat associated with the feed and effluents, and the heat
Total biogas output 10,200
released by the biological reaction. In the tropics, the over-
a Theretentiontimeinthedigesteris20days.Theunithasthecapacity all efficiency of the biogas systems is enhanced because
to process 1,825 t (wet) per year. Note: the yield in biogas from 100 t is there is no need to heat the system to keep the temperature
estimated at 10.2 million kilocalories. Thus, the net yield is 3.1 million in the 30–38 C range.
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kilocalories. The energy for heating the digester is cogenerated from the
cooling system of the electric generator. Dairy cattle are not the only source of manure for bio-
b It is assumed that anaerobic digestion of the manure takes place at gas systems. They are used as a model since dairy animals
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35 C with a solids retention time of 20 days. The temperature of the are more likely to be located in a centralized system, mak-
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fresh manure is 18 C, and the average ambient temperature is 13 C. ing the collecting and adding the manure to a digestion
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The manure is assumed to have the following characteristics: production
system less time consuming and energy intensive than for
per cow per day, 23.6 kg total; solids, 3.36 kg; and biological oxygen
range-fed steers, or even for draft animals. Efficiencies
demand (BOD), 0.68 kg. The digester is assumed to transform 83% of the
of conversion vary not only from system to system, but
biodegradable material into gas. The biogas produced is 65% methane,
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and its heat of combustion is 5720 kcal/m at standard conditions. also the sources of manure. Swine and beef cattle manure