Encyclopedia of Renewable Energy. James G. Speight
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There is also interest in partial upgrading to a product that is compatible with refinery streams in order to take advantage of the economy of scale and experience in a conventional refinery. Integration into refineries by upgrading through cracking or hydrotreating is a viable option, but in such cases where the bio-oil is blended with a crude oil product, there may be incompatibility issues that arise.
Upgrading bio-oil to a conventional transport fuel such as diesel, gasoline, kerosene, methane, and liquefied petroleum gas (LPG) requires full deoxygenation and conventional refining, which can be accomplished either by integrated catalytic pyrolysis or by decoupled liquid phase hydrodeoxygenation. There is also growing interest in partial upgrading to a product that is compatible with refinery streams in order to take advantage of the economy of scale and experience in a conventional refinery, and the main methods are: (i) hydrodeoxygenation and (ii) catalytic cracking, in situ or ex situ gasification to synthesis gas followed by synthesis to hydrocarbon derivatives or alcohol derivatives.
Hydrothermal liquefaction under a hot-water environment has been proposed as an alternative process to provide better energy efficiency and unique characteristics of bio-oil and other related products compared to pyrolysis-based processes. The optimization of the operating parameters, including temperature, pressure, time, and catalyst, is crucial for improving the performance of these processes. In addition to bio-oil production technologies, several upgrading technologies based on catalytic approaches (e.g., hydrotreatment and esterification) have also been developed to further improve bio-oil quality for a variety of applications.
See also: Bio-oil, Refining.
Bio-oxidation
Bio-oxidation (biological oxidation) is one of the most used methods of desulfurization, based on injection of a small amount of air (2 to 8% v/v) into the raw gas stream. This way, the hydrogen sulfide is biologically oxidized either to solid free sulfur or to liquid sulfurous acid (H2SO3):
In practice, the precipitated sulfur is collected and added to the storage tanks where it is mixed with digestate, in order to improve the fertilizer properties of digestate. Biological desulfurization is frequently carried out inside the digester, and, for this kind of desulfurization, oxygen and Sulfobacter oxydans bacteria must be present, to convert hydrogen sulfide into elementary sulfur, in the presence of oxygen. Typically, Sulfobacter oxydans is present inside the digester (does not have to be added) as the anaerobic digester substrate contains the necessary nutrients for their metabolism. In the process, the air is injected directly in the headspace of the digester and the reactions occur in the reactor headspace, on the floating layer (if existing) and on reactor walls. Due to the acidic nature of the products, there is the risk of corrosion. The process is dependent of the existence of a stable floating layer inside the digester, and the process often takes place in a separate reactor.
Chemical desulfurization of gas streams can take place outside of digester, using a base (usually sodium hydroxide). Another chemical method to reduce the content of hydrogen sulfide is to add commercial ferrous solution (Fe2+) to the feedstock. Ferrous compounds bind sulfur in an insoluble compound in the liquid phase, thereby preventing the production of gaseous hydrogen sulfide.
See also: Biofiltration, Bioscrubbing, Gas Cleaning – Biological Methods Gas Processing, Gas Treating.
Biophotolysis
The photosynthetic production of gas (e.g., hydrogen) hydrogen employs microorganisms such as cyanobacteria, which have been genetically modified to produce pure hydrogen rather than the metabolically relevant substances. The conversion efficiency from sunlight to hydrogen is small, usually under 0.1%, indicating the need for large collection areas.
The current thinking favors ocean locations of the bio-reactors. They have to float on the surface (due to rapidly decreasing solar radiation as function of depth), and they have to be closed entities with a transparent surface (e.g., glass), in order that the hydrogen produced is retained and in order for sunlight to reach the bacteria. Because hydrogen buildup hinders further production, there further has to be a continuous removal of the hydrogen produced, by pipelines to, for example, a shore location, where gas treatment and purification can take place. These requirements make it little likely that equipment cost can be kept so low that the low efficiency can be tolerated.
A further problem is that if the bacteria are modified to produce maximum hydrogen, their own growth and reproduction are quenched. Presumably, there has to be made a compromise between the requirements of the organism and the amount of hydrogen produced for export, so that replacement of organisms (produced at some central biofactory) does not have to be made at frequent intervals. The implication of this is probably an overall efficiency lower than 0.05%.
In a life-cycle assessment of bio-hydrogen produced by photosynthesis, the impacts from equipment manufacture are likely substantial. To this, one should add the risks involved in production of large amounts of genetically modified organisms. In conventional agriculture, it is claimed that such negative impacts can be limited, because of the slow spreading of genetically modified organisms to new locations (by wind or by vectors such as insects, birds, or other animals).
In the case of ocean bio-hydrogen farming, the unavoidable breaking of some of the glass- or transparent plastic-covered panels will allow the genetically modified organisms to spread over the ocean involved and ultimately the entire biosphere. A quantitative discussion of such risks is difficult, but the negative cost prospects of the biohydrogen scheme probably rule out any practical use anyway.
Biopower
Biopower (biomass power, renewable electric energy) is the use of biomass feedstock to produce electric power or heat through direct combustion of the feedstock, through biopower system technologies (such as: direct-firing, co-firing, gasification, pyrolysis, and anaerobic digestion), gasification and then combustion of the resultant gas, or through other thermal conversion processes. Power is generated with engines, turbines, fuel cells, or other equipment.
Most biopower plants use direct-fired systems. They burn bioenergy feedstocks directly to produce steam. This steam drives a turbine, which turns a generator that converts the power into electricity. In some biomass industries, the spent steam from the power plant is also used for manufacturing processes or to heat buildings. Such combined heat and power systems greatly increase overall energy efficiency. Paper mills, the largest current producers of biomass power, generate electricity or process heat as part of the process for recovering pulping chemicals.
Co-firing refers to mixing biomass with fossil fuels in conventional power plants. Coal-fired power plants can use co-firing systems to significantly reduce emissions, especially sulfur dioxide emissions. Gasification systems use high temperatures and an oxygen-starved environment to convert biomass into synthesis gas, a mixture of hydrogen and carbon monoxide. The synthesis gas (syngas) can be converted into other fuels or products, burned in a conventional boiler, or