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Using biogas

The main method of biogas utilization is converting it to source of thermal, mechanical and electrical energy. However, large biogas plants can be used for production of valuable chemical products for national economy. Biogas can be used in gas appliances for energy production which is then used for heating, lighting, supply of steaming plants, in water boilers, gas stoves, infrared radiators and internal combustion engines.
The simplest method of biogas utilization is using it in burners, as it can be directly supplied form low-pressure gas holders, but it is more preferable to use biogas for mechanical and electrical energy production. This will lead to creation of independent energy base for exploitation needs of the households.

Table 18. Biogas components [21]

Characteristics Biogas components Biogas mixture

(CH4 – 60%,

CO2 – 40%)

CH4 CO2 H2 H2S N2
Inclusion volume fraction, % 55 – 70 20 – 44 1 1 < 3 100
Volume combustion heat, MJ/m3 35,8 - 10,8 22,8 - 21,5
Combustion limit

(content in air), %

5 – 15 - 4 – 30 4 – 45 - 5 – 12
Combustion temperature, 0Ñ +65 +750 - +585 - - +650 +750
Standard density, gram/liter 0,72 1,98 0,9 1,54 - 1,2

GAS BURNERS

Figure 34. Gas stove working on biogas
in Petrovka village.

Photo: Vedenev À.G., PF «Fluid»

The heart of most gas appliances is a biogas burner. In most cases, atmospheric-type
burners operating on premixed air/gas fuel are preferable. Due to complex conditions of flow and reaction kinetics, gas burners defy precise calculation, so that the final design and adjustments must be arrived at experimentally.

Compared to other gases, biogas needs less air for combustion. Therefore, conventional gas appliances need larger gas jets when they are used for biogas combustion. About 5.7 liters of air are required for the complete combustion of one liter of biogas, while for butane 30.9 liters and for propane 23.8 liters are required. [8].

The modification and adaptation of commercial-type burners is an experimental matter. With regard to butane and propane burners, it has to be noted that butane/propane gas has up to three times the caloric value of biogas and almost twice its flame-propagation rate. Conversion to biogas always results in lower performance values.
Practical modification measures include:
• expanding the injector cross section by factor 2-4 in order to increase the flow of gas;
• modifying the combustion-air supply, particularly if a combustion-air controller is provided;

Gas stoves

Before using gas stove burners have to be thoroughly adjusted for obtaining:
• compact blue flame;
• flame has to self-stabilize, i.e. unlighted part of the burner should light up in 2-3 seconds.

Radiation heaters

Figure 35. Water boiler for heating of a house with radiant ceramic heaters in Petrovka village.
Photo: Vedenev À.G., PF «Fluid»

Infrared heaters are used in agriculture for achieving the temperatures required for raising young stock, e.g. piglets and chicken in a limited amount of space. The nursery temperature for piglets begins at 30-35°C for the first week and than gradually drops off to an ambient temperature of 18-23°C in the 4th/5th week.
As a rule, temperature control consists of raising or lowering the heater. Good ventilation is important in the stable / nursery in order to avoid excessive concentrations of CO or CO2. Consequently, the animals must be kept under regular supervision, and the temperature must be checked at regular intervals. Heaters for pig or chicken rearing require some 0,2-0,3 m3l/h as a rule of thumb.

Figure 36. Gas pressure regulator.

Photo: Vedenev À.G., PF «Fluid»

Thermal radiation
Radiant heaters develop their infrared thermal radiation via a ceramic body that is heated to 900-1000°C (red-hot) by the biogas flame. The heating capacity of the radiant heater is defined by multiplying the gas flow by its net calorific value, since 95% of the biogas’ energy content is converted to heat. Small-heater outputs range from 1.5 to 10 kW thermal power [8].

Safety pilot and air filter
Biogas-fueled radiant heaters should always be equipped with a safety pilot, which turns off the gas supply if the temperatures goes low i.e. the biogas does not burn any longer.

Biogas consumption
Household burners consume 0,2 – 0,45 m3 of biogas per hour and industrial burners – from 1 äî 3 m3 of biogas per hour. Biogas volume, necessary for food preparation can be determined from the time spent on daily cooking.

Table 19. Household biogas consumption

Burner Use Biogas consumption, m3
household Preparation of 1 food portion for 1 person 0,15 – 0,3
household Water boiling 0,03 – 0,05
household Heating of premises 0,2 â ñóòêè

MOTORS, WORKING ON BIOGAS

Biogas can be used as a fuel for car engines and its efficiency will depend on the methane content and additional substrates. Both carburetor and diesel engines can operate on biogas, but as biogas is a high-octane fuel it is more effectively utilized in diesel engines. Fuelling engines requires large volumes of biogas and installation of additional devices on internal ignition engines which will enable them to work on benzene as well as on methane.

Gas electrical generators

Experience shows that the use of biogas in electricity generators is economically feasible as with burning of 1 m3 of biogas 1,6 to 2,3 kW of electric energy can be produced. ýëåêòðîýíåðãèè. Efficiency of such utilization of biogas is increased by using heat energy generated from cooling the engine of the electricity generator for heating of the biogas plant digester.

Conditioning of biogas

Figure 37. Biogas electricity generator in Petrovka village.
Photo: Vedenev À.G., PF «Fluid»

Sometimes the biogas must be treated/conditioned before utilization. The predominant forms of treatment aim at removing either water, hydrogen sulfide or carbon dioxide from the raw gas:

Reduction of the moisture content
The biogas is usually fully saturated with water vapor. This involves cooling the gas, e.g. by
routing it through an underground pipe, so that the excess water vapor condenses at the lower temperature. When the gas warms up again, its relative vapor content decreases. The “drying” of biogas is especially useful in connection with the use of dry gas meters, which otherwise would eventually fill up with condensed water.

Reduction of the hydrogen-sulfide content

The hydrogen sulfide in the biogas combines with condensing water and forms corrosive acids. Water-heating appliances, engines and refrigerators are particularly at risk.

Figure 38. Hydrogen sulfide filter and absorber for reduction of carbon dioxide content of biogas in Petrovka village.
Photo: Vedenev À.G., PF «Fluid»
The most simple and effective method of de-sulfurization is dry purification by absorption onto a special filter. As an absorber ferric hydrate (Fe(OH)3), also referred to as bog iron, a porous form of limonite mixed with wood shavings is normally used. With the help of 0,035 m3 of such absorber around 3 kg of sulfur can be extracted. The porous, granular purifying mass can be regenerated by exposure to air.
Minimal material expenses, simplicity of exploitation of this filter and easy absorber regeneration make this method a safeguard against corrosion of gas holders, compressors and internal combustion engines caused by prolonged exposure to hydrogen sulfide contained in biogas. Zink oxide can also be effectively used as an absorbent of hydrogen sulfide and it has an advantage over bog iron: it also absorbs organic sulfide compounds (carbonyl, marcaptan [18].

Reduction of the carbon-dioxide content
The reduction of the carbon-dioxide content is complicated and expensive. In principle, carbon-dioxide can be removed by absorption onto lime milk, but that practice produces “seas” of lime paste and must therefore be ruled out, particularly in connection with large-scale plants. Carbon dioxide itself is a valuable product that can be used in different industrial processes.

Methane utilization
Contemporary chemical research opens wide possibilities for using methane for production of soot (color agent and substrate for rubber industry), acetylene, formaldehyde, methyl and ethyl alcohol, methylene, chloroform, benzol and other valuable chemical products on the base of large-scale biogas plants [18].

Biogas consumption by engines
In Petrovka village of Chui oblast of the KR a biogas plant with digester volume of 150 m3 supplies biogas for household needs of 7 farms of Association “Farmer”, for operation of electricity generator and fuelling of two cars – UAZ and ZIL. The engines of these cars were equipped with special devices for working on biogas and the cars – with balloons for biogas pumping. Average figures of biogas consumption necessary for production of 1 kW of electricity by engines is around 0,6 m3/h.

Table 20. Biogas utilization as a fuel in Petrovka village

Engine Use Balloon quantity Biogas consumption, m3
UAZ-469 Car 3 balloons 42 for 100 km
ZIL MMZ-130 Car 9 balloons 72 for 100 km
GAZ-53 Electricity generator - 20 per hour – 37kW

EFFICIENCY OF BIOGAS

Figure 39. UAZ, working on biogas in Petrovka village.
Photo: Vedenev À.G., PF «Fluid»
The calorific efficiency of using biogas is 55% in stoves, 24% in engines. The most efficient way of using biogas is in a heat-power combination where 88% efficiency can be reached8. The use of biogas in stoves, water boilers, steaming plants and greenhouses is the best way of exploiting biogas energy for farm households in the Kyrgyz Republic.

Excess biogas
In cases when biogas is generated in excessive quantities it is not recommended to let it out in the atmosphere – this leads to unfavorable climate changes, but it is recommended to burn it. For this a torch is installed as a part of gas system that should be located on a safe distance from buildings.