Biogas is a gaseous fuel, like Methane Gas (CH4), produced by the fermentation or breakdown of organic matter.
Biogas is not typically produced at the time or in the quantity needed to satisfy its conversion. Therefore, storage systems are used 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 a 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 (0.9-psi) 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.
TMC Fluid Systems supplies turn-key Biogas Blower Systems.
The 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 in 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 it may also be injected into gas pipelines and provide illumination and steam production. Finally, through catalytic chemical oxidation, methane can also 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.
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.