BIOGAS FROM CROP WASTES

Nearly all organic substrates have potential of significant energy generation via the process of anaerobic fermentation. There are several factors which must be taken into consideration to operate the digester based on alternate feed materials effectively. The factors which effects the biogas production in cattle dung holds good for this material also. So to make use of alternate material for biogas generation it is essential to control the environmental and operational factors. A wide variety of plant wastes as well as crop residues in the farm, terrestrial and aquatic species have been studied for their potential for biogas generation. The characteristics of the plant wastes and cattle dung are quite different therefore, anaerobic digestion of plant wastes need additional requirements for maintaining environmental and operational parameters. Some of basic requirements for crop wastes for biogas production are summarized as below.
 The C/N ratio of wastes varies widely from waste to waste. Maturity and type of species greatly effect the C/N ratio. It is also reported that fresh crop wastes has low C/N ratio, while after some time it increases. These is a necessity to bring C/N ratio to the optimum level of 30:1.
 Pre-processing of crop wastes is also essential in order to increase it's density and feed required quantity in the digestion chamber, this also accelerates the anaerobic reaction. This process includes chopping, cutting, mixing with other feed, steaming (if material is hard such as wood) for bringing the required C/N ratio and the concentration of solids to 7-9 % and reduce retention time.
 The density of crop wastes is less. Therefore, if it is used as such, it may form scum on the top of slurry in the digesters, thus, inhibiting methane production process.
 Pre-digested crop wastes have low hydraulic retention time, and it settles at the bottom, which require a perfect stirring mechanism, either mechanically or through gas recirculation. The system should be more reliable and effective for anaerobic digestion of the crop wastes.
The paddy straw, obtained as spoiled and waste material during collection, storage and use of paddy can be converted into biogas and valuable manure. It has been observed that wheat straw can yield biogas at a rate of 36 l/kg of dry matters, where as from the paddy straw the gas production is 260 l/kg of dry matter fed. Plant materials generate considerably higher biogas yield per kilogram of total solids than several animal wastes. Vegetable matter from young plants generates more gas than from old plants, and dry vegetable matter generates more gas than green vegetable matter. The studies carried out for anaerobic fermentation of crops and organic wastes at loading concentrations of 3-10 % TS.,biogas yield decreased with increasing loading concentrations. Above 5 % total solids, digestion of crops with high soluble carbohydrate content needs continual addition of alkali to maintain a pH of nearly 7. The composition of biogas varied with the material from 50 % methane from newsprint to 68 % from cattle manure. The balance was carbon dioxide in all cases with less than 0.001 % hydrogen sulphide. The potential of Gliricidia leaves is established as feed for biogas production and the use of digested effluent as a rich fertiliser which is superior to fresh glilricidia leaves. Mirabilis leaves produced nearly 400 l of gas/kg of dry matter which is double the amount produced by cattle dung. Methane content in biogas obtained from Mirabilis leaves was 69 % as against 62 % normally obtained from cattle dung.
The stalks of maize and sweet sorghum collected before and after the juice extraction, dried and finely powdered to pass a 40-60 mesh sieve, mixed with cow dung in the ratio of 1:1 on dry weight basis produced highest amount of biogas.This is obtained from a mixture of cow dung and fresh stalks of sweet sorghum followed by maize.
Rabbit droppings slurry at 9 % TS produced biogas at 0.24 m3/Kg as compared to 0.12 m3 gas/Kg of cow dung at the same concentration. In 6 m3 digester, 5 to 5.5 m3 of biogas was produced continuously after reaching steady state. The methane content of biogas from rabbit droppings was 68 to 70 % compared to 50 to 60 % from cow dung.

A successful plant system details for a poultry litter based gas plant

High rate anaerobic digestion technology

When compared to cattle dung poultry litter has a high nitrogen anf phosphorous and hence offers an immence potential for biomethanation in aneorobic digesters with energy recovery in the form of gas besides organic manure. For the gas production conventional digesters are used for biomethanation of organic wastes including cattle dung. For this purpose many types of digesters have been developed. The high rate reactor is one of them. The knowledge base of high rate anaerobic digesters is rather limited in most of the developing countries except for few sewage treatment plants, where commercial in-house or open sale utilization of biogas was practiced. The American Society of Civil Engineers classify the digesters as low rate, high rate, anaerobic contact and phase separation
variations. A low rate digester operates with a very low volatile solids (VS) loading rate of 0.6 to 1.6 kg / m3 / day, a feed solids concentration of about 2- 5%, and high hydraulic residence time of 30 to 60 days. Being open intermittent feed, unless environmental conditions are controlled, these systems are unstable. High Rate Digestion operates at higher feed solids concentration, in general has supplemental heating for either the mesophilic (30 - 38° C) or thermophilic (50- 60° C) ranges and uniform feeding rates. As it is operated at high solids concentration, it would result in reduced tank volumes. The anaerobic contact process involves thickening of digested sludge and recycling to the inlet with a similarity to
contact stabilization in activated sludge. This is mainly meant for high strength soluble wastes only as it has hydrolysis and acid methane formation phase separated. Though a sound process theoretically, this has practical difficulties in keeping the phase separation in the two separate tanks and often requires process readjustments. In general, a vast majority of successful digesters are of the high rate mesophilic type, but the problems arises from either equipment plugging. Line problem incidental to each type accounted for almost 65- 85% of the problems encountered with such systems. Current technologies of digestion The information made available from various digester manufacturers reveals that various technology providers claim differing energy recovery potentials for the same feed and operating temperatures. Theoretically, given a unit weight of feed, and all other conditions remaining unchanged, the conversion to energy and new cells is a fixed percentage and cannot be influenced unduly by any single factor. However, the one variable, which may have some
influence, is mixing, which would either result in a complex mix or stratified regime or subsequently result in higher or lower gas yields.

Underground masonry structure type poultry litter waste biomethanation plant

A typical poultry litter waste biomethanation plant of high rate reactor system which can successfully produce gas is described below.

i) Reactor: A drum type. partially underground masonry structure can be constructed having approximately 5.5 m diameter and 10 m depth. The bottom of the digester is truncated to 1 m dia. The drum diameter is nearly 7 m and height 1.5 m with the dome rise of 0.5 m. Recirculation arrangement can be made to draw the slurry from top layer as well as middle layer and distribute from the bottom. The reactor can also be installed with immobilization arrangements for microbes. The floating drum can be allowed to travel with in a limited height by suitable locking arrangements. The drum can have a water seal at its bottom and fitted with a gas pipe of 75 mm diameter at the top.

ii) Water remover: A cylindrical drum fitted with overflow arrangement can be used as water remover from the moisture laden gas. This can be fitted at the immediate outlet of the gas line. An overflowing arrangement can be made to continuously drain out water during operation. A removable lid can be provided for easy cleaning and refitting.
iii) Water shower: The water can be arranged to be sprayed to remove hydrogen sulphide and CO2 to some extent if present in the gas using a cylindrical drum with counter current flow of gas and water shower.

iv) Activated carbon bed: To remove carbon dioxide from the biogas produced from the gas plant so as to make it rich in methane for operating the engine, an arrangement fitted with activated carbon bed to three fourth of its height in a cylindrical container made of iron can be made.

v) Filter bed for moisture filtering and gas storage: In order to remove gas-laden moisture these two to three beds can be provided as the last components of gas cleaning chain before admitting the gas to compressor. A single or two-stage compressor can be fitted in the line to suck and compress the gas for admitting to a cylindrical storage drum. The outlet of the compressor may be fitted with a non-return valve. The storage cylinder should be capable of withstanding at least 100-psi pressure, also fitted with a non-return valve. The gas stored in the cylinder can used as fuel for running the generator sets to produce electric power.

Operation of the plant: If the poultry shed is a layered type arrangement the poultry litter is not mixed with other bed materials. The feedstock is collected by a tractor and transported to the biomethanation site. The feedstock is unloaded into a shredder where it can be reduced in size and conveyed in to a tank where water is mixed with the shredded feedstock in the appropriate ratio and delivered to the reactor. The gas produced from the reactor is stored in a cylindrical container and used to operate generator sets. Part of the power produced is used to run the shredder, compressor, recirculation pump and other accessories.

Partial oxidation of biogas to hydrogen rich gas

The conversion of gaseous hydrocarbons can be achieved in many ways. Partial oxidation with air is one of the options. In this process methane in the biogas with oxygen in air to form hydrogen in a bed of catalyst.
i. Principle
This commercial process route is basically the result of sequential combustion reactions in which the gas is burnt with deficit oxygen (nearly 30 % of stoichiometric requirement). Depending on the feed -stock the product gas may require purification of sulphur compounds and CO2 - removal. The advantage of dispensing with an external heat source favours the partial oxidation step, since the oxidation of CO supplies the necessary heat.
ii. Process flow
The biogas containing 55-60 % methane , 40 % CO2 and traces of H2 S is first dehydrated and then purified from H2 S. The gas is then sent to the partial oxidizers to form hydrogen. Depending on the composition of the outlet gas from the oxidizer methanation of residual carbon oxides can be incorporated. Finally a Co2 scrubber may be added depending on the type of fuel cell to be used in the power plant.
iii. Process
The process occuring in the partial oxidizer is

CH4 + 0.5 O2 = Co + 2H2
The process requires oxygen, which may be separated and supplied from air and is favoured by moderately high pressure. It is understood that due to residence time limitations, the process approaches equilibrium leaving some residual methane and carbon in the product gas. Hence CO2 needs to be scrubbed and recycled in the plant (Balthasar).
iv.Need for pure oxygen
If air is used instead of oxygen the separation of hydrogen from nitrogen is difficult. Hence oxygen at a purity of at lease 95 % and has to be used in large scale systems. Pressure of notrogen could be accepted in small scale fuel cell applications. After all the cathode gas (air) contains 80% nitrogen. The process has thus to be modified for operation on air in this particuler study.
v.Factors controlling the process
As the partial oxidation proceeds through a flame reaction, it is necessary to moderate the flame temperature, preferably by means of steam. The raw gas composition is controlled by the oxygen to methane ratio and by the steam addition. In order to reduce the oxygen consumption for the oxidation step the biogas and steam has to be preheated and have to be metered precisely to the reactor.
The operating conditions vary with the non-catalytic reactors.

Pressure: 60 to 90 bar, Reaction temperature: 1200 to 1370 C.
Under catalytic partial oxidation a commercial process, topsoe SBA has specified the following conditions.
Pressure: up to 30 bar or more. Temperature: 90C.
Catalyst: NiFeed-stock gas and super heated steam are mixed and preheated to 60c and mixed with oxygen .
vi.Reactor
The reactor may be a refractory lined stainless steel vessel with one or more burners. In order to initiate the reaction part of the gas has to be burnt inside the reactor. Johnson Matthey Ltd. report in their patent a reactor made of SS tube with entry for gas at the middle of the reactor and exit for product gas from the end. As gas entry is made near the middle of catalyst bed, they have observed a higher temperature of 450 c at the tip and a uniform temperature of 280 c surrounding the hot zone. This is claimed to be superior to at once through reactor. The reactor may then have multiple (4 or 5 entries from the sides).
A theoretical model developed by Opris et. al describes the temperature and product distribution profiles along the length of the reactor. This fits well with the commercial data. The reactor requires a minimum length for complete partial oxiation of methane leading to a continuous supply of hydrogen at a fixed concentration.
vii.Catalysts
The catalyst used by Johnson Matthey let. in their Hot Spot TM reactor contained 0.01 to 5 wt % platinum and from 1 to 15 wt % chromium oxide supported on a refractory solid such as silica. The support may be monolith honeycomb or particles with a maximum size of 1.5 mm.
viii.Catalyst deactivation
The successful economic and technical utilization of the process depends on the avoidance of free carbon deposition which decreases the catalyst surface area resulting in lower reaction rates. It is suggested that carbon deposition shall be avoided by operational techniques rather than by inhibition.
ix.Product gas
The hot product gas is expected to have the following composition on a dry basis.Hydrogen and CO, 93 % by volumeCarbon dioxide, 5 % by volumeNitrogen and argon, 1.5 % by volume Methane, 0.6 % by Volume. The reactor effluent needs rapid cooling to freeze the gaseous equilibria established at the high temperature reactor by a direct water quench or by heat exchange. The product gas then has to be washed free of carbon-sulphur compounds, carbon dioxide and inert gases.

Partial oxidation of biogas

The conversion of gaseous hydrocarbons can be achieved in many ways. Partial oxidation with air is one of the options. In this process methane in the biogas with oxygen in air to form hydrogen in a bed of catalyst.
This commercial process route is basically the result of sequential combustion reactions in which the gas is burnt with deficit oxygen (nearly 30 % of stoichiometric requirement). Depending on the feed -stock the product gas may require purification of sulphur compounds and CO2 removal. The advantage of dispensing with an external heat source favours the partial oxidation step, since the oxidation of CO supplies the necessary heat.
The biogas containing 55-60 % methane , 40 % CO2 and traces of H2 S is first dehydrated and then purified from H2 S. The gas is then sent to the partial oxidizers to form hydrogen. Depending on the composition of the outlet gas from the oxidizer methanation of residual carbon oxides can be incorporated. Finally a Co2 scrubber may be added depending on the type of fuel cell to be used in the power plant.
The process occuring in the partial oxidizer is
CH4 + 0.5 O2 = Co + 2H2
The process requires oxygen, which may be separated and supplied from air and is favoured by moderately high pressure. It is understood that due to residence time limitations, the process approaches equilibrium leaving some residual methane and carbon in the product gas. Hence CO2 needs to be scrubbed and recycled in the plant.