Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 170

Traditional gas cleaning and air pollution control technologies for pollutant gases, such as adsorption, absorption, and combustion, were developed to treat high concentration waste gas streams associated with process emissions from stationary point sources. Although these technologies rely on established physico-chemical principles to achieve effective control of gaseous pollutants, in many cases, the control technique yields products which require further treatment before disposal or recycling of treatment materials. In the case of treatment of dilute waste gas streams, however, these traditional methods are relatively less effective, more expensive, and wasteful in terms of energy consumption and identification of alternative control measures is warranted. A suitable alternate air pollution control technology is biofiltration, which utilizes naturally occurring microorganisms supported on a stationary bed (filter) to continuously treat contaminants in a flowing waste gas stream.

Biofiltration by definition is the aerobic degradation of pollutants from (in the current context) a gas stream in the presence of a carrier media. The early development work on biofiltration technology concentrated on organic media, such as, peat, compost, and wood bark. In general terms, organic compounds are degraded to carbon dioxide and water, while inorganic compounds, such as sulfur compounds, are oxidized to form oxygenated derivatives. The formation of these acidic compounds can lead to a lowering of pH of the filtration media which in turn impacts on the performance of the system. Removal of the oxidized compound from the media is an important consideration in the design of biofiltration systems. Biofiltration, like many processes based on bio-treatment in the crude oil refining and gas processing industries, is successfully emerging as a reliable, low-cost option for a broad range of air treatment applications. It is now becoming apparent that biological treatment will play a far more significant role in achieving environmental control on air emissions.

In the biofiltration process, three types of bioreactor designs are usually considered: the biofilter, (ii) the biotrickling filter, and (iii) the bioscrubber. The main differences between these systems concern their design and mode of operation: microorganism conditioning, the nature of the fluid phase (gas or liquid), and the presence or absence of stationary solid phases. Nevertheless, gas stream desulfurization has been carried out by the biotrickling filter and the bioscrubber.

The biofilter is a pollution control technique which involves using a bioreactor that contains living material to capture and biologically degrade pollutants such as hydrogen sulfide. Common uses include processing wastewater, capturing harmful chemicals or silt from surface runoff, and the macrobiotic oxidation of contaminants in gas streams. The technology finds greatest application in treating malodorous compounds and water-soluble volatile organic compounds (VOCs). Compounds treated are typically mixed VOCs and various sulfur compounds, including hydrogen sulfide. Large airflows may be treated, although a large area (footprint) has typically been required. Engineered biofilters, designed and built since the early 1990s, have provided significant footprint reductions over the conventional flat-bed, organic media type.

In the process, the polluted gas flow is purified with biofiltration by conducting the gas flow upward through a filter bed, which consists of biological material, e.g., compost, tree bark, or peat. The filter material is a carrier of a thin water film in which microorganisms live. The pollutants in the gas flow is held back by adsorption and/or absorption on the filter material, and then decomposed by present microorganisms. The filter material serves as a supplier of necessary nutrients. The products of the conversion are carbon dioxide, sulfate, and nitrate. The dry weight of the filter varies typically from 40 to 60 %. To reduce desiccation of the bed, the gas flow must be saturated with water. For this reason, polluted gas flow is moistened before it goes through the biofilter, which is achieved by using a pre-scrubber. The relative humidity of the gas must be 95%. In practice, it is always better to apply a moistener to protect the biofilter against dehydration.

A biotrickling filter is a packed bed bioreactor with immobilized biomass. The gas flows through a fixed bed coor counter-currently to a mobile liquid phase. Synthetic carriers are usually used and these include plastic, ceramic, lava rocks, polyurethane foam, etc. The synthetic carrier does not provide any nutrients, so the liquid mobile phase must contain nutrients for the growth and maintenance of the biomass. Programmed or continuous discharge of recirculation medium helps to remove the oxidation products. The hydrogen sulfide must be transferred from the gas to liquid phase, and the degradation is finally carried out in the biofilm.

According to the microbial cycling of sulfur, the biological oxidation of hydrogen sulfide to sulfate is one of the major reactions involved. There are numerous sulfur-oxidizing microorganisms, but hydrogen sulfide is exclusively oxidized by prokaryotes. In any case, the biotrickling filter is typically inoculated with aerobic active sludge, and during the process, a specific biomass population is developed.

A biotrickling filter can be operated under aerobic or anoxic conditions. The removal of hydrogen sulfide from the gas stream has been mainly studied under aerobic conditions, and the overall reaction is:

In summary, two compounds can be produced: sulfate and elemental sulfur, and the production ratio of sulfate (SO₄^2-) will be dependent on the oxygen-hydrogen sulfide ratio and the trickling liquid velocity. Air is used to supply oxygen and must, therefore, be supplied in excess. 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.