Page 10 - 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

166 Biomass Utilization, Limits of

appears to yield more gas per kilogram of VS than dairy TABLE VI Energy Inputs for Anaerobic Digester in the Trop-
cattle manure. Poultry manure is also used, but sand and ics for Biogas Production using 8 t (1 t dry) of Cow Manure
(Pimentel et al., 1988) a
other forms of heavy grit in this dung cause pump main-
tenance problems and require more frequent cleaning of Quantity (kg) kcal
the digester.
Inputs
Manure processed in the digester retains its fertilizer
Cement foundation (30-year life) 0.07 140
value and has the advantage of less odor. Therefore, it can
Steel (30-year life) 0.33 7,000
be spread on fields and may be easier to pump if the ini-
Total inputs 7,140
tial pumping system used a cutter pump to break up stray
Total biogas output 820,000
bits of straw or long undigested fibers. Biogas systems
Net return per 1 t dry manure 812,840
have the advantage of being able to adjust in size accord-
ing to the scale of the operation. The pollution problem a The retention time is 20 days without a means of storing the biogas.
associated with manure in a centralized dairy production The gas is used as delivered. The digestion takes place at 35 C. The
◦
◦
system is the same whether or not it goes through a biogas temperature of the fresh manure is assumed to be 21 C, and the average
ambient temperature is 21 C. The efficiency of the digester is 25%.
◦
generator.
The biogas produced is 65% methane and its heat of combustion is
In developing countries, such as India, the situation is 5720 kcal/m .
3
different. There, a substantial percentage of the manure as
dried dung is burned directly as fuel. Although burning
3
utilizes a significantly higher percentage of the total en- an estimated 2277 m of biogas per year at a conversion
ergy in the manure, it results in a complete loss of nitrogen efficiency of 25% (Table VI). The energy value of this
and loss of substantial amounts of the other valuable nutri- gas totals 13.0 million kcal. Assuming $8.38 per 1 million
ents. Whether or not biogas is a useful energy alternative kcal, the economic value of this much energy is $109 per
in India and other similar countries is highly problem- year. Then if no charge is made for labor and dung and
atic in spite of the higher overall energy efficiency of the the capital cost is assumed to be only $14 per year, the net
conversion system. return is $95 per year. These costs are not equally appli-
If it is not desirable to produce electricity from the bio- cable to Kenya where the energy replacement of biogas
gas, the energy data listed in Table V will change consider- in terms of woodfuel saved is appropriate. Using an aver-
ably. For instance, less energy will be lost in the conversion age of 4000 kcal/kg of woodfuel, this amount of biogas
to electricity if all the energy is used directly for heating. would replace3tofwoodand since biogas is generally
However, compressing biogas for use in tractors involves more efficient than wood when used for cooking, the total
the input of significant amounts of additional energy for amount of wood replaced might be double.
“scrubbing” the biogas to remove hydrogen sulfide and Although the labor requirement for the described bio-
water. gas generator is only 5–10 min/day, the labor input for col-
lecting and transporting biomass for the generator may be
significant. If the source for the 400 kg of manure required
A. Biogas for Smallholders
for the digester was, on average, 3 km from the digester,
The economics of biogas production in a rural area of a de- it would take 2 laborers working an 8-hr day to collect
veloping nation, like Kenya or India, illustrates that costs manure, feed it into the digester, and return the manure to
and benefits are complex and results mixed. The capital cropland where it could be utilized as fertilizer. On a per
costs of constructing a simple biogas digester with a ca- hour basis, the laborers would have to work for 3c / per hour
pacity to process 8 t (wet) of manure per 20-day retention for the biogas digester to have costs equal to the amount
time, or 400 kg/day, are estimated to be between $2000 of gas produced. In some situations, especially in densely
and $2500 (Table VI). Such a unit would have usable life populated parts of a country, the amount of transport re-
of 30 years, so the capital costs are only $80 per year. quired will be too costly.
If rural workers construct the biogas generator them- Although the profitability of small-scale biogas produc-
selves, material costs might range from $300 to $700. At tion may be low even without the charge of labor, biogas
$400 for materials, without any charge for labor, the in- digesters have significant advantages in rural areas. The
vestment would be only $14 per year with the costs spread biomass can be processed and fuel energy obtained with-
out over the life of the digester. out losing the valuable nutrients (N, P, and K) present
A digester this size in India, where cows weigh an aver- in the manure. Nitrogen and phosphorus are major limit-
age of between 225 to 330 kg each, would require access to ing nutrients in tropical agriculture and these are returned
manure from about 20 cows. This system would produce to the cropland. The only loss that the processed manure