However, control of the air supply is an important issue for safety and operational reasons – the lower and upper explosive limits for methane in air are 5% v/v and 15% v/v, respectively.
On the other hand, the gas stream can be diluted (mainly by the nitrogen content in air) and undesired residual oxygen can be found in the outlet stream. Therefore, a high oxygen supply is a drawback because the calorific power is also decreased and, moreover, a lack of oxygen increases clogging problems caused by the formation of elemental sulfur. The gas stream dilution depends on the efficiency in the mass transfer and the concentration of hydrogen sulfide. An alternative to the direct supply of air mixed with the gas stream involves the use of venture-based devices to increase the oxygen mass transfer. However, the installation of an aerated liquid recirculation system could produce hydrogen sulfide stripping of the dissolved sulfide.
The anoxic biotrickling filter is based on dissimilatory nitrate reduction. Dissimilatory nitrate reduction is carried out by certain bacteria that can use nitrate and/or nitrite as electron acceptors instead of oxygen:
Complete denitrification vs. partial hydrogen sulfide oxidation
Complete denitrification vs. complete hydrogen sulfide oxidation
Partial denitrification vs. complete hydrogen sulfide oxidation
Partial denitrification vs. complete hydrogen sulfide oxidation
See also: Bio-oxidation, Bioscrubbing, Gas Cleaning – Biological Methods, Gas Processing, Gas Treating.
Biofuels
The term biofuel is a generic name for gaseous, liquid, or solid fuels that are not derived from fossil fuels or contain a proportion of non-fossil fuel. For the purposes of this text, the term alternate fuels is used to represent those fuels that are produced from plant sources as well as from other sources such as the organic constituents’ (predominantly biological in nature) municipal and industrial waste. Thus, biofuels are bio-materials that are produced made from renewable biological sources which are now contemplated as grown specifically for the purpose of providing useful heat upon combustion. Biofuels are produced from sources such as: corn, soybeans, flaxseed, rapeseed, sugarcane, palm oil, sugar beet raw sewage, food scraps, animal parts, and rice.
Biofuels are fuels derived from plant materials – entering the market, driven by factors such as oil price spikes and the need for increased energy security. Examples of solid biofuels include wood, sawdust, grass cuttings, domestic refuse, charcoal, agricultural waste, non-food energy crops, and dried manure. Biofuels are also known as non-conventional fuels or alternative fuels. Alternative fuels can be classified as any fuel that is not derived from conventional sources like natural gas, crude oil, and coal.
Other than biofuels, some more examples of alternative fuels are solar, wind and tidal power, hydrogen, air engine power, and non-conventional oil. Since mankind discovered fire, wood has been the first biofuel used for heating and cooking which has also been used to produce electricity, and liquid biofuel have been used in the automotive industry since its inception. Unlike fossil fuels, which are necessarily derived from long deceased and metamorphosed biological organisms, biofuels (otherwise known as agrofuels) are obtained from only recently deceased or from living biological organisms, or in other words, derived from biomass or bio-waste.
When raw biomass is already in a suitable form (such as firewood), it can be combusted directly in a stove or furnace to provide heat or (industrially) to raise steam. When raw biomass is in an inconvenient form (such as sawdust, wood chips, grass, urban waste wood, and agricultural residues), the typical process is to densify the biomass. This process includes grinding the raw biomass to an appropriate particulate size, which depending on the densification type can be from 0.5 to 1.5 in., which is then concentrated into a fuel product. The current types of processes are pellet, cube, or puck. The pellet process is most common in Europe and is typically a pure wood product. The other types of densification are larger in size compared to a pellet and are compatible with a broad range of input feed-stocks. The resulting densified fuel is easier transport and feed into thermal generation systems such as boilers.
Bioprocessing routes have a number of compelling advantages over conventional petrochemicals production (Table B-9). However, it is only in the last decade that rapid progress in biotechnology has facilitated the commercialization of a number of plant-based chemical processes.
Table B-9 Example of sources and use of biomass for energy products.
| Resources | Collection | Conversion | Products |
|---|---|---|---|
| Agricultural crops | Harvesting | Biochemical | Biodiesel |
| Energy crops | Collection | Thermochemical | Heat |
| Forestry crops | Physical processes | Electrical power | |
| Herbaceous biomass | Chemical Processes | Other fuels | |
| Oil-bearing plants | |||
| Wastes | |||
| Woody biomass |
It is widely recognized that further significant production of plant-based chemicals will only be economically viable in highly integrated and efficient production complexes producing a diverse range of chemical products. Also contrasting with fossil fuels, biofuels do not contribute to the stock of total carbon dioxide in the atmosphere, since the sources of many biofuels are plants which normally remove carbon dioxide from the atmosphere, and then give off the same amount when burned and are, therefore, considered to be carbon dioxide neutral, although some observers would doubt this balance as being the result of mathematical manipulation without taking into consideration other important and influential factors.
In any given location, the type of biofuel used will depend on, among other minor factors, the available natural resources and local energy needed for processing requirements. Some of the main reasons for the shifting of interest from fossil fuels to biofuels are the rising prices of oil and increasing emissions of greenhouse gases, low barriers to entry of biofuels, and increasing government support. There is also a requirement for the utilization of biofuels which is the reduction or total elimination of noxious emissions in an economical manner.
Biofuels can be classified into (i) first generation biofuels, (ii) second