Biomethane

The global market of biofuels is led by bioethanol and biodiesel. Bioethanol is industrially produced from sugarcane, wheat, corn, and sugar beet. Biodiesel is made from vegetable oils and, in limited cases, from fats and waste cooking oils. In comparison, global biogas production was 27% of the global biofuel market or about 0.25% of the global energy market in 2011. Any kind of biomass has potential to be a substrate for biogas production as long as it contains carbohydrates, proteins, fats, cellulose, and hemicelluloses. Biogas is a gas formed in an anaerobic process which is the decomposition of organic matter without oxygen. Anaerobic digestion consists of four stages, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis to break down biodegradable materials in the absence of oxygen by a consortium of microorganisms. Waste from agriculture or general household waste, manure, plants, or food waste can produce biogas. It is regarded as a renewable energy source as organic matter can be grown indefinitely. As it is produced from waste, it has advantages over other renewables that use organic non-waste as their raw material, such as certain ethanol plants, firewood, and charcoal. Biogas contains methane, which is useful, but also several gases that are not so useful such as carbon dioxide and hydrogen sulfide. Biomethane is a gas that results from a process that improves the quality of biogas by reducing the levels of carbon dioxide, hydrogen sulfide, moisture, and other gases. If these gases could be removed entirely, the biomethane that remains is pure methane. The name biomethane refers to the method of production, rather than the gas content. This chapter explains the difference between biogas and biomethane and explores the production of biomethane in selected countries.

This chapter deals with the processing of raw biogas, directly from the anaerobic digester. This biogas generally contains undesirable components which are corrosive to metals and toxic to health and the environment. Before the biogas is sent to an upgrading unit, it requires some pretreatment to remove and minimize contaminants which might cause damage. Depending on the anaerobic digester, the contaminants might vary but the two main ones are hydrogen sulfide and moisture. In agricultural digesters, the H₂S level can range from 1000 to 4000 ppm. Even in small local digesters, used only for generating on-site power in an old generator, the hydrogen sulfide should be kept below 200 ppm or else it seriously affects the life of the engine. For larger scale upgrading plants, especially membrane and PSA technologies, a more systematic approach should be taken to biogas pretreatment. All possible contaminants are briefly introduced and discussed but only the engineering design for H₂S removal and moisture reduction is explained in detail in this chapter. Chapter 3 deals with specific upgrading technologies.

Many end use applications require a high quality of biogas, which means the gas must contain a higher percentage of methane than found in raw biogas. In such situations, it is common to improve the quality of the biogas by reducing the contaminants and unwanted gases. Contaminants, in this case, are defined as any substances that are not methane. For example, if used in natural gas vehicles, the raw biogas heating value should increase from \(23\ \text {MJ}/{\text {m}}^{3}\) to \(37{-}42\ \text {MJ}/{\text {m}}^{3}\). Upgrading involves two key steps, biogas cleaning, which is a pretreatment process readying the gas for the second process. This second process, calling upgrading, purifies the low methane, high carbon dioxide biogas into high methane, low carbon dioxide—biomethane.

This chapter deals with transportation applications of biomethane. If biomethane is injected to a natural gas grid, subsequently it can be used in transportation through natural gas filling stations. Alternatively, in certain situations where there is no natural gas grid available, a stand-alone filling station can be constructed. This would use biomethane produced from the upgrading stations discussed in the previous chapter. The similarities and differences between the performance of biomethane and natural gas in transportation applications will be discussed.

This chapter is concerned with substituting biomethane in situations where LPG fossil fuel is normally used. In many parts of the world, domestic stoves use liquid petroleum gas (LPG), supplied in portable tanks. LPG is a gas that has been liquefied in storage and consists of 60% propane and 40% butane. It is also used in transportation and agricultural applications. In industries, such as ceramic making, many firing kilns also use LPG. If biomethane is to be used as a LPG replacement then methods of (a) biomethane storage and (b) stove/furnace modification to allow biomethane combustion are required. This chapter will outline a solution to both of these hurdles. Upgrading plants to produce biomethane was discussed in Chap. 3. In this chapter, a storage and delivery solution will be discussed and a methodology was developed and implemented for converting stoves and industrial furnaces for biomethane use. Biomethane can be produced, delivered, and combusted safely and efficiently in LPG-powered applications.

Biomethane, that satisfies national grid standards, can be injected into national natural gas pipelines. This is the easiest procedure for handling excess biomethane. The biomethane gets mixed with the natural gas and is used wherever the pipeline delivers, for example, power plants, industries, or homes. The quality of the biomethane necessary for injection depends on the country. It is set on a country-by-country basis. In cases where a natural gas pipeline is not locally available, a small-scale biomethane gas grid can be constructed to deliver gas to the local neighborhood and industries. This chapter deals with the design, construction, and operation of a local biomethane gas grid.

Given global greenhouse gas reduction pressures and incentives for renewables there are strong forces supporting the future growth of the global biomethane industry. This chapter deals with future technologies and uses of biomethane. Developments in improving anaerobic digesters, such as acid-phase digesters, are examined. Future sources of biogas from second-generation biofuels, including from algae and lignocellulosic biomass, are discussed. Following that discussion, technologies that could be used to improve the upgrading of biogas are included. These include energy-efficient methods such as in situ methane and enzyme enrichment. Future methods for storing biomethane, such as adsorbed natural gas and liquefied biomethane, are discussed. The book finishes with a recommended pathway for future biomethane development.