Compact Biogas System

A.D. Karve1, Gandhali Kulkarni, Priyadarshini Karve 1Appropriate Rural technology Institute (ARTI), Pune411 041, India. 2Smt.Kashibai NavaleCollege of Engineering (for Girls), Pune411 041, India.

E-mail:adkarve@vsnl.com

Introduction
Conventional biogassystems used organic wastes such as human or animalfaecaes, distillery effluents, or municipal solid waste.
These feedstock materials cannot be digested by the bacteria in the system.
These systems require 40-50 kg of feedstock and have a retention time of bout 40 days, to produce daily 1000 lit ofbiogas.
Minimum capacity of such systems is 2000 lit.
The gas is a mixture of CH4and CO2, containing about 40-50% CO2 by volume.
They generate daily 80-100 lit of effluent slurry.
Appropriate Rural Technology Institute (ARTI), developed during 2002-03 an innovative, compactbiogasplant, which uses starchy or sugary material as feedstock.
The newbiogasplant has the capacity of just 400 to 500 lit, requires just 2 kg starchy or sugary material to produce about 800 lit ofbiogas, and the reaction time is only 6 to 8 hours.

Special Features of the Compact Biogas System
The potential candidatefeedstocks, namely rain damaged or insect damaged grain, flour spilled on the floor of a flour mill,oilcakefrom non-edible oilseeds, seed of various tree species, non-edible rhizomes (banana,arums,dioscoreas), leftover food, spoiled and misshapen fruits, non-edible and wild fruits, spoilt fruit juice, etc. are readily available in rural areas.
1 kg of sugar or starch yields about 400litresofbiogas, within a period of 6 to hours. This quantity is enough for cooking one meal for 5 to 6 persons.
The effluent slurry generated daily by the plant is just a couple oflitres. It can be used as manure for plants growing around the house.
The compactbiogasplant, mass produced, would cost aboutRs. 3,500 (US$78). The smallest cattle-dung based domesticbiogasplant costs aboutRs. 12,000 (US$267).


Field Testing Objectives:
–To test the quantity and quality of biogas produced from different feedstock materials and their mixtures, under field conditions.
–To generate data tables of gas yield Vs feedstock used for the convenience of the users.

Methodology:
–Use an experimental biogas plant producing 25 lit biogas every 24 hours by consuming about 50 g feedstock.
–Study quantity and quality of biogas produced using a starchy and an oily feedstock separately.
–Study quantity and quality of biogas produced using mixtures of starchy and oily feedstock.
–Quantity of biogasestimated from the increase in the height of the floating drum.
–Quality of biogas deduced from chemical analysis.



It is not advisable to transport the biogas plant while in operation.
After installation, the plant requires about 4-5 days to stabilise.
Once stabilised, the plant routinely produced daily 25 lit biogasusing 50 g of a starchy feedstock.
Chemical analysis indicated that the gas contained <>
Sudden replacement of starchy feedstock by oily feedstock led to stopping of gas production.
Preliminary experiments involving mixtures of starchy and oily feedstock materials in different proportions indicate that biogas yield may be increased by using a proper combination of feedstock materials.

Conclusions
There is a need for generating extensive data tables ofbiogasyield and systemstabilisationperiods for various feedstock materials used individually and in mixtures.
An attractive feature of the compactbiogasplant is the exceptionally high methane content of the gas. This implies that the gas is useful not only as a clean cooking fuel but also as an excellent transportation fuel.
Due to simplicity of operation, easy access to feedstock materials, and user-favouringeconomics, the compact biogasplant has a potential torevolutionisethe household energy scenario in India by offering a more accessible alternative to LPG.
The dream of a ‘blue flame revolution’ –putting a blue flame in each and every rural kitchen in India -has become more realistic and achievable in the near future with this invention.

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Several members asked me to provide more details about the compact

biogas plant being developed by us. I give below the latest status of

this technology.

The biogas plant consists of two cylindrical vessels telescoping into

one another. The larger vessel, called the fermenter, has a total

internal volume of about 500 lit. A drum having diameter of 85 cm and

height of 85 cm would have the desired volume. The smaller vessel, which

telescopes into the larger one, serves as the gas-holder. The diameter

of the gas holder is about 2 cm smaller than that of the fermenter. The

fermenter vessel is provided with appropriate inlet and outlet pipes for

introducing the feedstock into it and for removal of spent slurry from

it. The gas holder is provided with a gas tap, through which the gas is

led to the burner. This system uses starchy or sugary material as

feedstock. 1kg of sugar or starch yields about 400 litres of methane,

within a period of 6 to 8 hours. This quantity is enough for cooking one

meal for 5 to 6 persons. The biogas produced by this system contains

theoretically about equal volumes of carbondioxide and methane, but in

reality, it turned out to have less than 5% carbondioxide. This

phenomenon is explained by the fact that carbon dioxide dissolves in the

water in the fermenter vessel and diffuses out of it through the 1 cm

gap between the fermenter and the gas holder. The gas produced by this

system has thus almost the same calorific value as LPG. It burns without

smoke or soot, producing an almost invisible bluish flame similar to

that of LPG.

Several prototypes, in operation for more than a year, have been

successfully tested using various feedstocks.  The potential candidate

feedstocks, namely rain damaged or insect damaged grain, flour spilled

on the floor of a flour mill, oilcake from non-edible oilseeds, seed of

various tree specie

s, non-edible rhizomes (banana, arums, dioscoreas),

leftover food, spoiled and misshapen fruits, non-edible and wild fruits,

spoilt fruit juice, etc. are readily available in rural areas. This

system is much easier to operate than the dung based biogas plant,

because of the relatively small quantities of feedstock and effluent

slurry to be handled. The effluent slurry generated daily by the plant

is just a couple of litres. It can be used as manure for plants growing

around the house. The 500 litre biogas plant, mass produced from moulded

plastic drums, would cost about Rs. 3,500 (US$ 78).  The smallest

cattle-dung based domestic biogas plant costs about Rs. 12,000 (US$267).

It requires daily 40kg dung, and owing to the retention period of almost

40 days, such plants have a minimum capacity of 2000 litres. They

generate daily 80 to 100 litres of effluent slurry. Daily handling of

such large quantities of feedstock and effluent is conside

red to be

arduous and bothersome by users.

Preliminary studies indicated that the amount of biogas produced and the

retention period varied from feedstock to feedstock and from season to

season.  Also, when the feedstock was changed from one form to another,

the system took a few days to stabilise. Our studies also indicated that

the gas yield could be increased by using combinations of feedstock

materials. We are now looking at additives such as micronutrients,

nitrogen, phosphorous compounds etc., which might bacterial action and

yield more gas at a faster rate. Since the users would depend mainly

upon locally available feedstock, field trials are essential to

determine the retention periods and gas yield for different raw materials.

Many people in India, who read my article in a local neuspaper, copied

our design and have started to use this biogas plant in their

households. A schoolgirl submitted a working model of it in a statewi

de

science project competition and won the first prize in the state. A

company supplying science equipment to educational institute wants to

manufacture models (50 litre capacity) for supply to schools and colleges.

We have supplied 200 litre models to 10 voluntary agencies in different

regions for demonstrating this technology to villagers in their

respective areas. This model is meant for areas where the main diet is

rice. This model yields enough gas to operate a pressure cooker to cook

rice, beans, vegetables or meat for a family of five. In areas, where

the main diet of the people consists of unleavened flat bread, somewhat

like the tortilla, each piece of bread is made individually, and

therefore the stove has to be in operation for a longer time. In such

cases, we recommend the five hundred litre model.

A.D.Karve