Biomass Power Generation

Biomass, or biofuels, are essentially clean fuels in that they contain no sulfur and the burning of them does not increase the long-term carbon dioxide (CO2) levels in the atmosphere, since they are the product of recent photosynthesis (note that peat is not a biofuel in this sense).

This is by no means an unimportant attribute when seen in the context of the growing awareness across the globe of the pollution and environmental problems caused by current energy production methods, and the demand for renewable energy technologies. Biomass can be used to provide heat, make fuels, and generate electricity. The major sources of biomass include:

Standing forests

Wood-bark and logging residues

Crop residues

Short rotation coppice timber or plants

Wood-bark mill residues

Manures from confined livestock

Agricultural process residues

Seaweed

Freshwater weed

Algae

A few facts and figures might help to put the landbased biomass sources in perspective. The first three of the above list produce in the U.S. approximately the equivalent of 4 million barrels of oil per day in usable form. If all crop residues were collected and utilized to the full, almost 10 percent of the total U.S. energy consumption could be provided for. Although the other land-based sources of biomass are perhaps not on the same scale as this, the combined resource represents a huge untapped reservoir of potential energy.

An interesting point to note is that current practices in forestry and food crop farming are aimed directly at optimizing the production of specific parts of a plant. Since biomass used for energy would make use of the whole plant, some significant advantage might be gained by growing specifically adapted crops designed to maximize the energy yield rate. It is from this origin that the energy farm concept is born.

In addition to land-based biomass, there is potential in aquatic biomass, and there are various methods by which to approach this resource. The first is by direct farming of methane as a byproduct of photosynthesis in marine plants. An example of this would be to farm huge cultivated kelp beds at great depth off the coast in suitable sea areas. It should be noted that of the solar energy incident on the earth’s surface, only 0.1 percent is harnessed through photosynthesis.

Since about 2 _ 1012 tons of vegetable matter grows worldwide each year, it would require only a small increase in the percentage of solar energy used in plant processes to yield a large increase in potential biomass fuel. The conversion of human and animal waste product to useful fuels has long been an interesting prospect. One method for doing just that is to employ microbial processes. It has been demonstrated that a practical regenerative system can be developed in which waste materials are used as the feedstock upon which to grow algae.

Methane can be produced by fermenting the algae and the remaining nutrient-rich waste can be recycled to grow further algae. In a biomass farm, which combines elements of both the above techniques, algae is grown in open ponds of water in the presence of carbon dioxide and recycled inorganic nutrients. Gas lift pumps introduce CO2 to the system and growing algae. After an incubation period, algae are collected in a trough by clumping, sedimentation, or floatation techniques, and then dewatered.

The harvested biomass is deposited in a biophotolytic reactor where, under a carefully controlled environment, the algae cells use sunlight to split water molecules, forming hydrogen and oxygen. The processes for producing energy from biomass can be divided into four areas:

Digestion of vegetable matter

Thermal processing

Combustion of biofuels

Anaerobic digestion of animal waste

The first, digestion of vegetable materials, has a resource size of 3 to 4 million tons of coal equivalent per year (Mtce per year). The economics are critically sensitive both on costs of collection and the digester equipment. The resource is characterized also by the seasonal nature of the raw material. Vegetable matter can also be converted directly into liquid fuels for transportation.

The two most common biofuels are ethanol and biodiesel. Ethanol, an alcohol, is made by fermenting any biomass high in carbohydrates such as corn. It is mostly used as a fuel additive to cut down a vehicle’s carbon monoxide and other smog-causing emissions. Biodiesel, an ester, is made using vegetable oils, animal fats, or algae. It can be used as a diesel additive or as a pure fuel for automobiles.

The second area, possibly more promising than the former, goes under the general heading of thermal processing. This includes the gasification, the direct liquefaction, and the pyrolysis (thermal decomposition in the absence of oxygen) of low moisture content biomass. By these means, about 5 Mtce per year of methanol alone could be produced in the U.K., 10 to 15 percent of current U.K. gasoline annual energy requirements. It is unlikely that the resource will become economically viable in the short term or even medium term.

The last two technologies are significantly better prospects and indeed have been demonstrated as commercially viable even at current fuel prices. Combustion of biofuels is said to represent a significant potential energy resource, and with most schemes having a payback period of three to five years, it presents itself as a most inviting investment. The second of these more attractive technologies is anaerobic digestion of animal wastes.

The size of this resource, although still significant, is not on the same scale as the combustion of biofuels, being on the order of 1 Mtce of economically viable potential. Although much work on this resource has been carried out among farming cooperatives in Denmark, there are a number of uncertainties that significantly affect the rate of development of this resource.

The product of this process is methane, and it is most likely that the exploitation of the resource will be carried out on a local scale, perhaps at farm level. Thus the marketability of surplus methane; that is, that which is in excess of the farmer’s needs, is not certain. Although the technology for digester construction is well established, the actual processes that occur during operation are still poorly understood.

Consequently there are uncertainties as to the design performance and flexibility in adapting to any fuel variations that may occur. Despite these drawbacks and the general unsuitability of high technology to the agricultural environment, research is continuing along a number of promising lines that could lead to increased controllability and the reduction of costs on less economic fuels such as dairy cattle waste.

Although the manufacture of digesters for the more attractive fuels such as pig and poultry waste is well established, it remains to be seen how quickly the technology will be taken up and perform in a working environment.