The purification of biogasPhysical and chemical properties of hydrogen sulohideThe origins of hydroqen sulphide in biogas plantsThe effect of H2S on the biogas plant and the gas-utilization equipmentEnginesThe odour of biogasDetermination of the H2S content of biogasMethods for removing H2S from biogasRegenerationHydrogen sulphide (H
2S) is particularly harmful when biogas is used in internal combustion engines. Its chemical reactions and those of its combustion product - sulphur dioxide - lead to corrosion and wear on engines. The only practical way of removing the hydrogen sulphide on a small scale is by dry desulphurization, using ferrous substances. Locally available, iron-containing soil is suitable for use as the purifying agent in Developing Countries. This chapter contains a detailed description of criteria for designing the purification chamber. It also presents the basic steps for manufacturing the purifying agent or absorbent.
Physical and chemical properties of hydrogen sulohideHydrogen sulphide is a colourless, very poisonous gas. It is inflammable, and forms explosive mixtures with air (oxygen) H
2S has a characteristic shell of "rotten eggs", apparent only in a small concentration range (0.05 - 500 ppm). It is soluble in water, forming a weak acid. A combustion product of H
2S is SO
2, which makes the exhaust gases very corrosive (sulphuric acid) and contaminates the environment (acid rain). H
2S is very poisonous (comparable to hydrogen cyanide): with a lower toxic limit of 10 ppm. 1.2 - 2.8 mg H
2S per litre of air (0.117%) kills instantly, 0.6 mg H
2S per litre of air (0.05%) kills within 30 minutes to one hour. H
2S changes the red blood pigment; the blood turns brown to olive in colour. The transport of oxygen is hindered. The person suffocates "internally". The symptoms are irritation of the mucous membranes (including the eyes), nausea, vomiting, difficulty in breathing, cyanosis (discoloration of the skin), delirium and cramps, then respiratory paralysis and cardiac arrest. At higher concentrations immediate respiratory paralysis and cardiac arrest are the only symptoms. Even if a person survives poisoning, long term damage to the central nervous system and to the heart may remain.
The origins of hydroqen sulphide in biogas plantsHydrogen sulphide is formed in the biogas plant by the transformation of sulphur-containing protein, which can be from plants and fodder residues. However, when animal and human faces are used, bacteria excreted in the intestine" are the main source of protein. Inorganic sulphur, particularly sulphates, can also be biochemically converted to H
2S in the fermentation chamber. While plant material introduces little H
2S into biogas, poultry droppings introduce, on average, up to 0.5 volume percent of H
2S, cattle and pig manure about 0.3 volume/percent. Protein-rich waste (e.g. molasses, etc.) can produce large amounts of hydrogen sulphide (up to 3 vol. %). Inorganic sulphates (from salty, stall rinse water or diluting water) also produce considerable H
2S.
The effect of H2S on the biogas plant and the gas-utilization equipmentDissolved H
2S is contained in the fermentation slurry, and when dissolved in high concentrations can be toxic to the bacteria in the slurry. It can inhibit the production of biogas and cause its composition to alter. This can be remedied by putting less sulphur-rich material in the plant or diluting with water. In less serious cases, stir vigorously to drive H
2S out of the slurry. The presence of H
2S gas in biogas makes it corrosive to metal parts: iron and galvanized parts are subject to surface attack, although not to major corrosion. The effect on non-ferrous metals in components, such as pressure regulators, gas meters, valves and mountings, is much more serious.
The combustion product, SO
2 combines with water vapour and badly corrodes the exhaust side of burners, gas lamps and engines. Burning biogas in stoves and boilers can also result in damage to the chimney.
EnginesThe acid which is formed corrodes engine parts in the combustion chamber, exhaust system and in various bearings. This is enhanced by frequent starts, short running times and the relatively low temperatures when starting up and after cutting off the engine. The water cooling system also provides the means (water needed to form sulphuric acid) for corrosion. Running engines with gas containing H
2S can reduce the service time to the first general overhaul by about 10 - 15%. The sulphur content of biogas used in gas engines shortens the time between oil changes and overhauls. SO
2 from combustion and water vapour both dissolve in the lubricating oil. The oil becomes acidic, and its properties change, losing its ability to lubricate and sometimes corroding metal components. Under continuous operating conditions, the interval between oil changes is reduced to 200 - 250 hours. If biogas is burned for cooking and lighting in poorly ventilated rooms, the occupants will be burdened by SO
2 in the air. Indicators are coughing, irritation of the mucous membranes, watering of the eyes and the corrosion of metal surfaces.
The odour of biogasAdequate desulphurization of biogas causes it to lose its characteristic, warning smell. This increases the danger of unnoticed leaks from pipes or equipment. SO
2 formed during combustion pollutes the environment by creating "acid rain". Even small concentrations of SO
2 in the atmosphere damage plants. Its concentration in the soil slowly causes land which is lacking in lime to become acidic. These effects should be negligible when biogas is used in rural areas in Developing Countries, since only small amounts of biogas are produced.
As noted, the desulphuring of biogas is necessary for its use in gas engines. Under some circumstances it is expedient to desulphur for boders. Desulphurization is also required when the biogas is produced from sulphur-rich materials. If people are not adversely affected, desulphuring is not required when biogas is burned openly.
Determination of the H2S content of biogasThe H
2S content of the purified gas can be measured to check the effectiveness of the desulphuring process. In the laboratory, the H
2S content of gases is usually measured iodometrically, using cadmium acetate. However, the necessary techniques are too involved for application in the field.
A simple way to determine the presence of H
2S in biogas is a test with lead acetate paper: when a piece of paper soaked with lead acetate solution is held in the gas stream for a short time, the presence of H
2S colours the strip black. The difficulty with this method is its high sensitivity, since a small amount of H
2S is not an indication of greatly reduced efficiency of the desulphurization. Simple desulphuring plants may still posses an adequate purifying performance.
Another method for detecting H
2S is with an alcoholic solution of iodine, such as often available in first aid kits. A small amount of biogas is carefully introduced into the iodine solution. If H
2S is present the reddish brown solution will decolour, causing a milky turbidity.
The test-tube method is a very exact and simple method of determining the H
2S concentration in biogas. Suitable tubes are available for measuring the concentration in both raw and purified gas. However, the gas detector apparatus and the individual test tubes are relatively expensive. Also, the test tubes can only be preserved for a limited time. This method is only expedient in the regional biogas extension service or similar advisory services. The apparatus can then be used to provide empirical field values for individual plants. The intervals for recharging the purifying agent can then be laid down.
As yet there is no simple, cheap, test method available. For this reason a close control of the desulphuring plant is strongly recommended.
Methods for removing H2S from biogasOf the many processes traditionally and presently employed that have been used for large-scale desulphurization of gases, only the so-called "dry" process is suitable on a smaller scale for biogas plants. The desulphuring of biogas is based on a chemical reaction of H
2S with a suitable substance, such as quicklime, slaked lime in solid form, or slaked lime in liquid form. The process using quick or slaked lime has not been applied on a large scale for a long time, because the large amount of odorous residue that is produced cannot be satisfactorily disposed of. Large concentrations of CO
2 which are present in biogas make the satisfactory removal of H
2S difficult: the CO
2 also reacts with the quick and slaked lime and uses it up quickly. The Ca(HCO
3)
2 formed reacts with Ca(SH)
2 which is formed by the reaction of H
2S with Ca(OH)
2 thus resulting in the recurrence of H
2S. A large scale biogas plant in Germany, with the co-generation of heat and power, has recently been constructed, using a lime purifier, but the results of long term tests are not yet available. In as far as enough lump quicklime is available in the countries concerned, this process could be considered for desulphurization. The apparatus for utilizing quicklime corresponds in construction and function to that used for the desulphurization with iron- containing substances.
Ferrous materials in the form of natural soils or certain iron ores are often employed to remove H
2S. The ferrous material is placed in a closed, gas-tight container (of steel, brickwork or concrete). The gas to be purified flows through the ferrous absorbing agent from the bottom and leaves the container at the top, freed from H
2S.
The absorbing material must contain iron in the form of oxides, hydrated oxides or hydroxides. This process terminates, of course, after some time. The greater part of the iron remains as a sulphide.
RegenerationHowever, by treating the sulphided absorbent with atmospheric oxygen, the iron can be returned to the active oxide form required for the purification of the gas. The used absorbent can, therefore, be "regenerated". This regeneration cannot be repeated indefinitely. After a certain time the absorbent becomes coated with elementary sulphur and its pores become clogged.
Purifying absorbents in gasworks (coke plants) acquire a sulphur content of up to 25% of their original weight,
There are three different, dry desulphuring processes available:
Without regeneration The purification chamber consists of a box or drum. The absorbent is placed inside it on several, intermediate trays (sieve floors) to ensure that the depth of the absorbent is not more than 20-30 cm. The biogas is fed in at the bottom of the box, flows through the absorbent and leaves the purification chamber at the top, freed from H
2S. When the absorbent becomes loaded with iron sulphides, the gas leaving the chamber contains more and more H
2S. The chamber is then opened at the top, the trays with the spent absorbent are removed, and then fresh absorbent is placed on the trays. After the air in the purification chamber has again been displaced with biogas the gas connection to the user is re-opened.
With regeneration The spent, sulphide-containing absorbent can also be regenerated by exposing it to oxygen. This can either be done by taking the used absorbent out of the chamber and exposing it to the air, or inside the purification chamber by simply sucking ambient air through it.
Since regeneration inside the chamber requires precautions against the formation of unwanted and dangerous air-gas mixtures and would require powerful fans, regeneration outside the chamber is usually preferred. The absorbent that is to be regenerated is spread out on the ground in as thin a layer as possible. From time to time it is turned over with a shovel. After a few days it is ready for use again. This regeneration process can be repeated up to ten times, after which the absorbent is finally spent.
Simultaneous regeneration and loading Simultaneous regeneration and loading of the absorbent is a special case. Here, a small amount of air is added to the biogas, so that sulphide formation and regeneration occur at the same time and place. The absorbent acts effectively as a catalyst. Expensive gas-measuring and mixing equipment is required for this process, however, so that it is not suitable for small biogas plants.
Alongside the traditional, commercially available absorbents, certain substitutes can be used. Various tropical and subtropical soils contain sufficient iron in a suitable form, but must be prepared to obtain the proper purifying characteristics. The material must be loose, porous, moist and granular. The raw soil has to be ground and mixed with a filler and water to obtain a homogeneous texture. Using two or more purification chambers, connected in series, ensures a continual production of purified gas, and allows a good capacity utilization. The spent absorbent can be disposed of safely by burying it. Various factors must be considered in calculating the dimensions of the purification chambers. A certain maximum flow speed should not be exceeded. The gas volume to be purified, per unit time, determines the cross section of the purification chamber. The chamber volume, and hence, the amount of absorbent, determine the operating time for the purification process before regeneration or exchange of the absorbent. A calculation procedure simplifies working out the dimensions of the desulphuring unit.
Biogas production and utilization in ChinaGas production of all household digesters in China totals about 2,000 million m
3 per year. In southern China, the total gas yield of family size digesters averages 300 m
3 per year (over 8 months). In the north, production is 200 m
3 biogas per year, or less, depending on ambient temperatures. Biogas production in RMP plants is usually 10% higher, due to the heat absorption effect.
Biogas in China is used by about 25 million people for cooking and lighting for 8 - 10 months a year. Many rural households are equipped with both biogas stoves and improved cooking stoves. With the latter type, the peasants burn straw and wood, as usual, during the winter months, for cooking and heating. Improved and cheap biogas stoves and lamps have been developed and are distributed to every biogas owner. The cost of one biogas lamp varies between 6 - 12 Yuan. Lamps and burners are adapted to low pressures of about 2 cm, at which RPM digesters operate. Commercial and industrial burners are also being investigated in China. Furthermore, the use of biogas is manifold: There are about 400 biogas power stations, with a total capacity of 5,800 HP, 800 biogas electrical stations with a total capacity of 7,800 kw, providing electricity to over 17,000 households. China has sound experience in running diesel and gasoline engines with biogas.
The net energy obtained by anaerobic digesting plants refers to the difference between total energy (biogas) generated during the process of anaerobic digestion and the energy consumed during the process in maintaining anaerobic digestion. This value is a key to achieving profitability.
The net energy output of mesophyllic anaerobic digesting installations serves as an important criterion for measuring their economic effects. In his case study on the 3-year operation of the biogas plants at Gold Star Dairy Farm and Nan-ge-zhuang Fish Farm in Beijing, Tang (1989) analyzed the factors that affect the output of total energy (biogas output), such as types of manure, technological process, and means of stirring. He studied the factors that consume energy in maintaining a mesophyllic operation, such as heating the slurry, heat dissipated from digesters and their pipes, and the energy consumption of various apparatus, in particular the pumps. Through this analysis of various energy consuming factors, their proportion in energy consumption was obtained.
Use of biogas in IndiaBiogas is commonly used for cooking and lighting: there are a number of enterprises in each State that produce stoves and lamps. At some Community and Institutional Biogas Plants, biogas operates engines or agricultural equipment. Only three enterprises in India manufacture or adapt diesel engines with optional operation on biogas.
Activities in using biogas in India have gained momentum since the National Project on Biogas Development was launched in 1981 and the Department of Non-conventional Energy Sources in 1982. Today, it is generally accepted among richer farmers that a biogas plant is desirable. The earlier period was taken up with problems, such as convincing bankers to give loans and setting up the organizational structure, subsidy system, etc.
The introduction of biogas technology in the rural areas of India requires technological improvements and financial help for successful operation. The technological improvements should be:
(a) To nullify the effect of low temperature on gas production;
(b) To devise simple, economical and labour-saving equipment for dung collection;
(c) Effective techniques for drying and transporting the effluent.
During the initial stages, the Government may provide the funds to meet the operational losses, so that the technology may be absorbed by the rural masses. Intensive efforts are made to upgrade technology, to produce more gas without excessive sophistication.
Experience of full scale plant of biogas generation at Hambran, PunjabTo save commercial energy, prevent deforestation, reduce soil erosion and preserve organic biomass for recycling, and to produce more food grain, a comprehensive program for the use of non-conventional energy was introduced by the Government of India. Special emphasis was laid on the biogas program. In the first phase, stress was laid on the family size biogas plants, but later emphasis was shifted to the installation of community/institutional biogas plants to provide cheap and smoke- free cooking gas to the rural populace who can not install their own plants, due to poverty, lack of dung and land for installation of the digester. The DNES, Government of India, provided a 10% subsidy for the installation of community biogas plants. There were 20 community biogas plants in operation and several more under construction in the State of Punjab in 1988. In India there were over 250 community/ institutional biogas plants in operation. Production of biogas during the winter, costly dung collection arrangements, inadequate slurry handling systems and lack of outside financial help are some of the constraints in the promotion of the program.
Vyas et al. (1989) examined a community biogas complex that had been installed at the village of Hambran, Punjab, to study aspects of the introduction of biogas technology into rural households. The village had 293 households, with a human and animal population of 1711 and 1400 respectively. An area of 936.4 ha was under cultivation, out of total geographical area of 1136.6 ha, with mechanical and electrical power inputs of 0.58 and 0.06 kw/ha respectively, plus animal power. The main occupation of the households was agriculture, followed by agricultural labour. The average family had 5 members. The households used cow dung cakes, fuel wood and agricultural waste as cooking fuel. The specific objective of the study was to collect information about the technical, social and economic feasibility of the community biogas plant.
The plant consisted of 4 floating drum digesters, with an average gas production capacity of 505 m /day, together with the necessary infrastructure, such as dung collection platform, mixing chamber, silt excluder, slurry inlet pipe line, slurry outlet, slurry recycling system, a pump for slurry storage, slurry handling machine, latrine block, generator room, wind mill and drying beds. The digesters had special constructional features: doubly reinforced raft foundation, reinforced brick walls, elimination of steel frame by a central pillar as guide pipe, no diametric dividing wall, two concentric well digesters with an outer to inner diameter ratio of 1:8, diameter depth ratio of 1:35, 3 outlets per digester, and provision for the installation of stirring and heating arrangements. The biogas was supplied to individual households by a pipe line. The plant was put into operation in 1986. People's participation in the community work was the main prerequisite for the success of entire program and needed careful planning at all stages. An association of users was formed and registered. A managing committee of 8 members was elected for the day to day management of the plant, aided by professors from the University.
Effect of temperature variation on gas productionThe variation in the mean ambient temperature from 32.9-12.7°C, during the year 1986-87, affected gas production. The coefficient of correlation between gas production per unit of input and ambient temperature was 0.92.
The reduction in gas production forced the management to reduce the hours of gas supply. Dung supply from the consumers was also closely correlated to the ambient temperature (r=0.89). With the fall in the mean ambient temperature from 32.9°C in July to 12.7°C in January, the gas supply time decreased from 6.5 to 3.00 hours and the dung supply correspondingly decreased sharply from 39 to 18 quintals per day (1625 - 750 kg/day). Similarly, with the rise in ambient temperature from January onwards, both the gas supply time and dung supply touched the previous level. The reduction in gas supply time affected the income of the plant adversely, because gas charges per connection had to be reduced in proportion to the duration of gas supply, while operational costs remained the same.
Source :http://www.fao.org/docrep/T0541E/T0541E0b.htm
