Biogas Conversion Options
The choice in the final means for utilization of biogas impacts the design and equipment requirements for biogas processing, storage and the economics of the biogas conversion system. The biogas may be applied in direct combustion systems (boilers, turbines, or fuel cells) for producing space heating, water heating, drying, absorption cooling, and steam production. The gas used directly in gas turbines and fuel cells may produce electricity. An alternative choice in biogas conversion is the use in stationary or mobile internal combustion engines which may results in shaft horsepower, cogeneration of electricity, and/or vehicular transportation. A final opportunity exists for sale of the biogas through injection into a natural gas pipeline.
Treatment of Biogas
The hydrogen sulfide contained in biogas caused odors, corrosiveness, and sulfur emissions when the gas is burned. High levels of sulfide in biogas may require removal to protect equipment if the gas is to be used in internal combustion engines, turbines, or fuel cells. The concentration of hydrogen sulfide in the gas is a function of the digester feed substrate and inorganic sulfate content. Wastes which are high in proteins containing sulfur based amino acids (methionine and cysteine) can significantly influence biogas hydrogen sulfide levels. For instance, layer poultry waste containing feathers made of keratin may produce biogas sulfide levels up to 20,000 ppm. Also, sulfate present in the waste, either from an industrial source (eg. pulping of wood) or from seawater (marine aquiculture) will be reduced by sulfate reducing bacteria in the digester and end up contributing to sulfide levels in the gas.
The treatment of biogas may include removal of components including hydrogen sulfide, water, mercaptans, carbon dioxide, trace organics, and particulates. Due to the corrosive nature of hydrogen sulfide, removal processes for this component are well developed and include both dry and wet removal processes. In a wet process the biogas is passed up-flow through a stripping tower where the aqueous solutions are sprayed counter-currently. The tower is generally separated by distribution trays which maximize contact between the biogas and the solution.
For small-scale biogas producers, an alternative to the wet absorption systems described above is dry adsorption or chemisorption. Several dry processes are available, using particles of either activated carbon, molecular sieve, iron sponge or other iron-based, granular compounds to remove sulfide from the gas phase to the solid phase. These are sometimes referred to as dry oxidation processes because elemental sulfur or oxides of sulfur are produced (and can be recovered) during oxidative regeneration of the catalyst.
In addition to those aqueous absorbents described for H2S removal in the previous section, there are many chemical solutions commercially available which can be used to remove CO2 and H2S concurrently. In general, these processes employ either solvation solutions where the objective is to dissolve CO2 and H2S in the liquid, or solutions which react chemically to alter the ionic character of these gases and therefore drive them into solution. Solutions of the former category include the solvents and the latter include the alkanolamines and alkaline salts.
There are membrane materials which are specially formulated to selectively separate CO2 from CH4. The permeability of the membrane is a direct function of the chemical solubility of the target compound in the membrane. To separate two compounds such as CO2 and CH4, one gas must have a high solubility in the membrane while the other is insoluble. Accordingly, rejection (separation) efficiencies are typically quite high when the systems are operated as designed.
Storage of Biogas
Biogas is not typically produced at the time or in the quantity needed to satisfy the conversion system load that it serves. When this occurs, storage systems are employed to smooth out variations in gas production, gas quality and gas consumption. The storage component also acts as a reservoir, allowing downstream equipment to operate at a constant pressure.
A wide variety of materials have been used in making biogas storage vessels. Medium-and high-pressure storage vessels are usually constructed of mild steel while low-pressure storage vessels can be made of steel, concrete and plastics. Each material possesses advantages and disadvantages that the system designer must consider. The newest reinforced plastics feature polyester fabric which appears to be suitable for flexible digester covers. The delivery pressure required for the final biogas conversion system affects the choice for biogas storage.
Compression of Biogas
The operating gas pressure for most anaerobic digesters rarely exceeds 24 “ WC and can be used without some form of compression, only in the simplest direct combustion devices such as flares and simple boilers. In addition the pressure drop along delivery piping and in clean-up processes can entail the need for some type of blower or compressor to overcome these losses. The use of biogas in mobile engines requires compression to high pressures to achieve minimal storage volume.
Biogas can be used readily in all applications designed for natural gas such as direct combustion including absorption heating and cooling, cooking, space and water heating, drying, and gas turbines. It may also be used in fueling internal combustion engines and fuel cells for production of mechanical work and/or electricity. If cleaned up to adequate standards is may be injected into gas pipelines and provide illumination and steam production. Finally, through a catalytic chemical oxidation methane can be used in the production of methanol production.
Biogas conversion in direct combustion provides the simplest method of direct utilization on-site. Most combustion systems designed for either propane or natural gas may be easily modified for biogas. Care must be taken to consider the heat input rate, the fluid handling capability, the flame stability and the furnace atmosphere when such modifications are made. Due to the lower heating value of biogas equipment may operate at a lower rating and the size of gas inlet piping may need to be increased.
If cogeneration is employed in the biogas conversion system heat normally wasted may be recovered and used for hot water production. In the gas of gas turbines, the waste heat may be used to make steam and drive an additional steam turbine with the final waste heat going to hot water production and this is termed a combined cycle cogeneration system. Combining hot water recovery with electricity generation, biogas can provide an overall conversion efficiency of 65-85%. Modern gas turbine plants are small, extremely efficient, and visually unobtrusive.
An additional direct combustion conversion process which should be considered is the use of steam to run adsorption refrigeration systems. Such systems can be employed to provide heating and cooling and can utilize waste heat from a topping cycle. In typical adsorption systems, a fluid is contacted with a salt brine and the heat of solution is rejected. Input heat then boils the fluid from the brine, it is condensed and then used as a refrigerant fluid in a standard expansions valve arrangement. Multi-staged adsorption systems can be combined to improve the coefficient of performance of the overall system.
Internal Combustion Systems
For smaller biogas installations shaft horsepower and electrical generation is most effectively met by the use of a stationary internal combustion engine. Adequate removal of hydrogen sulfide to below 10 ppm is important to reduce engine maintain requirement. Often more frequent changing of engine oil and testing for oil sulfur content can increase engine component life. Some applications have used a dual-fuel carburetor so that propane or natural gas can be employed to start-up and shut down the engine system effectively removing trace sulfide from the internal parts.
When waste heat from engine cooling and exhaust gases is recovered and used the efficiency of the engine cogeneration system improves. Waste heat may be used for digester heating, space heating, hot water and or refrigeration.
Biogas, if compressed for use as an alternative transportation fuel in light and heavy duty vehicles, can use the same existing technique for fueling already being used for compressed natural gas vehicles. In many countries, biogas is viewed as an environmentally attractive alternative to diesel and gasoline for operating buses and other local transit vehicles. The sound level generated by methane-powdered engines is generally lower than that generated by diesel engines and the exhaust fume emissions are considered lower than the emission from diesel engines, and the emission of nitrogen oxides is very low. Application of biogas in mobile engines requires compression to high pressure gas (>3000 psig) and may be best applied in fleet vehicles. A refueling station may be required to lower fueling time and provide adequate fuel storage.