Category Advanced renewable energy systems

Storage of liquefied biomethane

Biomethane can also be liquefied, creating a product known as liquefied biomethane (LBM). Two of the main advantages of LBM are that it can be transported relatively easily and it can be dispensed to either LNG vehicles or CNG vehicles (the latter is made possible through a liquid-to-compressed natural gas [LCNG] refuelling station equipment which creates CNG from LNG feedstock). However, if LBM is to be used off-farm, it must transported by tanker trucks, which normally have a 10,000-gallon capacity. For obvious economic reasons, the LBM must be stored on-farm until 10,000 gallons have accumulated.

Figure 17.6 shows the generalised process of storing LBM prior to use or transport. The low-pressure storage tank is a buffer for LBM after it exits the biomethane liquefaction equipment...

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Biomethane storage

Biomethane is less corrosive than biogas and also is potentially more valuable as a fuel. For these reasons, it may be both possible and desirable to store biomethane for on – or off-farm uses.

(a) High-pressure storage of compressed biomethane

Biomethane can be stored as CBM to save space. Gas scrubbing is even more important at high pressures because impurities such as H2S and water are very likely to condense and cause corrosion. The gas is stored in steel cylinders such as those typically used for storage of other commercial gases. Storage facilities must be adequately fitted with safety devices such as rupture disks and pressure relief valves...

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Medium-pressure storage of cleaned biogas

Biogas can also be stored at medium pressure between 2 and 200 psi, although this is rarely, if ever done, in the USA. To prevent corrosion of the tank components and to ensure safe operation, the biogas must first be cleaned by removing H2S. Next, the cleaned biogas must be slightly compressed prior to storage in tanks. Typical propane gas tanks are rated to 250 psi. Compressing biogas to this pressure range uses about 5 kWh per 1000 ft3.

Assuming the biogas is 60 per cent methane and a heat rate of 13,600 Btu/ kWh, the energy needed for compression is approximately 10 per cent of the energy content of the stored biogas.

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Biogas storage

Both biogas and biomethane can be stored for on-farm uses. In practice, however, most biogas is used as it is produced. Thus, the need for biogas storage is usually of a temporary nature, at times when production exceeds consumption or during maintenance of digester equipment. Important considerations for on – farm storage of biogas include: (i) the needed volume (typically, only small amounts of biogas need to be stored at any one time), (ii) possible corrosion from H2S or water vapour that may be present, even if the gas has been partially cleaned, and (iii) cost (since biogas is a relatively low-value fuel).

(a) Low-pressure storage of biogas

Floating gas holders on the digester form a low-pressure storage option for biogas systems...

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Storage systems and costs

There are two basic reasons for storing biogas or biomethane: storage for later on-site usage and storage before and/or after transportation to off-site distribution points or systems. The least expensive and easiest to use storage systems for on-farm applications are low-pressure systems; these systems are commonly used for on-site, intermediate storage of biogas. The energy, safety, and scrubbing requirements of medium – and high-pressure storage systems make them costly and high-maintenance options for on-farm use. Such extra
costs can be best justified for biomethane, which has a higher heat content and is therefore a more valuable fuel than biogas.

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Storage and transportation of biogas and biomethane

Dairy manure biogas is generally used in combined heat and power applications (CHP) that combust the biogas to generate electricity and heat for on-farm use. The electricity is typically produced directly from the biogas as it is created, although the biogas may be stored for later use when applications require variable power or when production is greater than consumption.

Biogas that has been upgraded to biomethane by removing the H2S, moisture, and CO2 can be used as a vehicular fuel. Since production of such fuel typically exceeds immediate on-site demand, the biomethane must be stored for future use, usually either as compressed biomethane (CBM) or liquefied biomethane (LBM). Because most farms will produce more biomethane than
they can use on-site, the excess biomethane must be transpo...

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Research and development

We may summarise some of the research and development tasks that need to be undertaken as follows.

In basic research

1. Studies on the choice, culture, and management of the micro­organisms involved in the generation of methane.

2. Studies on bacterial behaviour and growth in the simulated environment of a digester (fermentation components: rate, yield of gas, composition of gas as a function of variables— pH, temperature, agitation—with relation to substrates—manure, algae, water hyacinths).

In applied research

1...

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Bottlenecks, considerations and research and development

Bioconversion of organic domestic and farm residues has become attractive as its technology has been successfully tested through experience on both small – and large-scale projects. Feeding upon renewable resources and non­polluting in process technology, biogas generation serves a triple function: waste removal, management of the environment, and energy production. Nevertheless, there are still several problems that impede the efficient working of biogas generating systems (Table 17.3).

Table 17.3 Considerations relating to bottlenecks in biogas generation.

Подпись: RemarksAspect Bottlenecks

Подпись: Planning Availability and ease of transportation of raw materials and processed residual products Подпись:Use of algae and hydroponic plants offsets high transportation costs of materials not readily at hand. Easily dried residual products facilitate transportation.

Bottlenecks, considerations and research and development

Nature of subsoil, water table, and availabilit...

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