James G. Speight - Encyclopedia of Renewable Energy

Effective gas generation in bioreactors under saturated conditions of rapid differential settlement without a low permeable cover is impossible. The result of this practice, which significantly increases early gas generation without effective gas capture, is a significant near-term increase of potent greenhouse gas emissions into the atmosphere.

The use of clay caps and advanced liner systems and bentonite is useful for the prevention of leachate and landfill gas release, but a consequence of this is that moisture, which is required for the biodegradative processes, is often excluded. The result of this is that much of the waste can remain intact entombed inside the landfill, potentially for longer than the lifetime of the barriers. One method to reduce this effect, by enhancing and accelerating waste stabilization, is to operate the landfill as a bioreactor. The bioreactor landfill attempts to control, monitor, and optimize the waste stabilization process rather than contain the wastes as prescribed by most regulations. This is carried out in an aerobic environment as opposed to the normal situation in landfill sites, where conditions within the wastes are commonly anaerobic.

The future of the bioreactor may well be found in its use with an anaerobic system i.e., a hybrid arrangement. For example, air could be added to the anaerobic processes after the degradation of waste has occurred, thus removing excess moisture from the landfill and fully composting the waste. The cycling of both conditions (aerobic and anaerobic) also offers the possibility of treating a greater range of chemicals such as the nitrification and denitrification of ammonia.

See also: Anaerobic Digestion, Landfill Gas.

A biorefinery is the means by which biomass can be converted to other products using a biochemical platform or a thermochemical platform which can be simply represented ( Table B-29). Thus:

Table B-29Simplified representation of a biorefinery.

In the current context, the other products are biofuels which have the potential to replace certain crude oil-derived fuels. Thus, biorefining offers a key method to accessing the integrated production of chemicals, materials, and fuels. The biorefinery concept is analogous to that of an oil refinery except that biomass (instead of crude oil) is the feedstock to the refinery.

Biomass refers to (i) energy crops grown specifically to be used as fuel, such as fast-growing trees or switch grass, (ii) agricultural residues and by-products, such as straw, sugarcane fiber, and rice hulls, and (iii) residues from forestry, construction, and other wood-processing industries. These products can range from biomaterials to fuels such as ethanol or important feedstocks for the production of chemicals and other materials.

Biomass is a renewable energy source, unlike the fossil fuel resources (natural gas, crude oil, and coal) and, like the fossil fuels, biomass is a form of stored solar energy but produced in lesser time than the fossil fuels. Thus, a biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The concept is based on the modern crude oil refinery concept, which produces multiple fuels and products from crude oil. Industrial biorefineries have been identified as the most promising route to the creation of a new domestic biobased industry.

Plants are effective chemical mini-factories or refineries insofar as they produce chemicals by specific pathways. The chemicals they produce are usually essential manufacture (called metabolites) which include sugars and amino acids that are essential for the growth of the plant, as well as more complex compounds.

In a manner similar to the crude oil refinery, a biorefinery would integrate a variety of conversion processes to produce multiple product streams such as motor fuels and other chemicals from biomass ( Table B-30). In short, a biorefinery would combine the essential technologies to transform biological raw materials into a range of industrially useful intermediates. However, the type of biorefinery would have to be differentiated by the character of the feedstock.

Table B-30Simplified representation of a biorefinery.

For example, the crop biorefinery would use raw materials such as cereals or maize and the lignocellulose biorefinery would use raw material with high cellulose content, such as straw, wood, and paper waste.

In addition, a variety of methods and techniques can be employed to obtain different product portfolios of bulk chemicals, fuels, and materials. Biotechnology-based conversion processes can be used to ferment the biomass carbohydrate content into sugars that can then be further processed. As one example, the fermentation path to lactic acid shows promise as a route to bio-degradable plastics. An alternative is to employ thermochemical conversion processes which use pyrolysis or gasification of biomass to produce a hydrogen-rich synthesis gas which can be used in a wide range of chemical processes. Thus, a biorefinery is a facility that integrates biomass conversion processes and equipment to produce fuels, power, and chemicals from biomass. The biorefinery concept is analogous to the crude oil refinery, which produce multiple fuels and products from crude oil.

A biorefinery can have different options for the production of biofuels from wood and other biomass materials. There is the (i) bioconversion, (ii) thermal conversion, and (iii) thermochemical conversion. Each of these options has merits, but selection is dependent on the feedstock and the desired product slate.

By producing multiple products, a biorefinery can take advantage of the differences in biomass components and intermediates and maximize the value derived from the biomass feedstock. A biorefinery might, for example, produce one or several low-volume, but high-value, chemical products and a low-value, but high-volume, liquid transportation fuel, while generating electricity and processing heat for its own use and perhaps enough for the sale of electricity. The high-value products enhance profitability, the high-volume fuel helps meet national energy needs, and the power production reduces costs and avoids greenhouse-gas emissions.