Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 194

Anaerobic digestion is the decomposition of biological wastes by microorganisms, usually under wet conditions, in the absence of air (oxygen), to produce a gas comprising mostly methane and carbon dioxide. A digester system (the anaerobic digester) is a device that promotes the decomposition of manure or digestion of the organics in manure to simple organics and gaseous biogas products.

During anaerobic digestion of an organic material such as biomass, a varied mixture of complex compounds is converted to a very narrow range of simple compounds, mainly methane and carbon dioxide. The anaerobic bacteria are responsible for the biochemical transformation of the biodegradable organic fraction (BOF). The AD of organic material basically consists of hydrolysis, acidogenesis, acetogenesis, and methanogenesis. These transformations are involved in the breakdown of complex polymers, such as cellulose, fats, and proteins to long and short chain fatty acids, and finally to methane, carbon dioxide, and water.

Any organic substance can become subject to anaerobic digestion so long as there are warm, wet, and airless conditions. For example, marsh gas is a product of the anaerobic digestion of vegetation at the bottom of ponds; this gas rises to the surface and bubbles, and the gas is also combustible. With the aid of human intervention, there are two products of this process, biogas and landfill gas. The chemical processes behind the production of these gases are complex.

The anaerobic digestion process focuses on hastening the natural process of biomass conversion to a gaseous fuel (biogas). Research has been conducted to ascertain optimal conditions for anaerobic digestion. These include (i) the feedstock, (ii) the nutrients, (iii) the temperature, (iv) the moisture content of the feedstock, (v) the pH of the system, and (vi) the atmospheric conditions. Most of the biomass waste feedstocks (municipal solid waste, agricultural waste, farm waste, crop waste, and forestry waste) studied have produced a biogas rich in methane. This medium to high Btu gas can, in some instances, be upgraded to a substitute natural gas (SNG). Depending on the feedstock, sulfur may also be produced.

Anaerobic digestion is a multi-stage biological waste treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane, and water. In this way, the waste sludge is stabilized and the obnoxious odor is removed. The process can, however, be described adequately and simply as occurring in two stages, involving two different types of bacteria. The process occurs in the absence of air; the decomposition in this case is caused not by heat but by bacterial action. In the first stage, the organic material present in the feed sludge is converted into organic acids (also called volatile fatty acids) by acid-forming bacteria. In the second stage, these organic acids serve as the substrate (food) for the strictly anaerobic methane-producing bacteria, which converts the acids into methane and carbon dioxide. The end result of the process is a well-established sludge in which 40 to 60% of the volatile solids are destroyed. Finally, a combustible gas is produced consisting of 60 to 75% methane and the remainder largely being carbon dioxide.

The anaerobic digestion process is a multi-stage biological waste treatment process whereby bacteria, in the absence of oxygen, decompose organic matter to carbon dioxide, methane and water. In this way, the waste sludge is stabilized and the obnoxious odor is removed. The process can, however be described adequately and simply as occurring in two stages, involving two different types of bacteria. In the first stage, the organic material present in the feed sludge is converted into organic acids (also called volatile fatty acids) by acid-forming bacteria. In the second stage, these organic acids serve as the substrate (food) for the strictly anaerobic methane-producing bacteria, which converts the acids into methane and carbon dioxide. The end result of the process is a well-established sludge in which 40 to 60% v/v of the solids are consumed by the process. Finally, a combustible gas consisting of approximately 60 to 75% v/v methane is produced with the remainder being predominantly carbon dioxide.

Anaerobic digestion is a complex process which requires strict anaerobic conditions [oxidation reduction potential (ORP) < -200 mV] to proceed and depends on the coordinated activity of a complex microbial association to transform organic material into mostly carbon dioxide and methane. Despite the successive steps, hydrolysis is generally considered as rate limiting. The hydrolysis step degrades both insoluble organic material and high molecular weight compounds (lipids, polysaccharides, proteins, and nucleic acids) into soluble organic substances (amino acids and fatty acids). The components formed during hydrolysis are further split during acidogenesis, the second step.

Volatile fatty acids (VFAs) are produced by acidogenic bacteria along with ammonia (NH₃), carbon dioxide (CO₂), hydrogen sulfide (H₂S), and other by-products. The 3^rd stage in anaerobic digestion is acetogenesis, where the higher organic acids and alcohols produced by acidogenesis are further digested by acetogens to produce mainly acetic acid and as well as carbon dioxide and hydrogen. The final stage of methanogenesis produces methane by two groups of methanogenic bacteria: (i) the first group splits acetate into methane and carbon dioxide and (ii) the second group uses hydrogen as an electron donor and carbon dioxide as the acceptor to produce methane. Thus:

The digestion process is continuous – fresh feedstock must be added continuously or at pre-determined frequent intervals. The gas formed during digestion is removed continuously. In high-rate digestion, stabilized sludge is displaced from the digester during feeding. In low-rate digestion, supernatant sludge is typically removed as the feed sludge is added; stabilized sludge is removed at less frequent intervals.

In a typical anaerobic digestion process, biomass is simply allowed to degrade in an anaerobic environment. A number of factors affect the entire process. They include (i) the temperature of the substrate, (ii) the loading rate, (iii) the pH, (iv) the residence time, (v) the concentration of nutrients, and (vi) the presence of any toxic substances.

In terms of temperature, the optimal temperature (where digestion and gasification proceeds at the highest rate) is 35°C (95°F). Below 15°C (59°F), the rate is so slow that little gas is produced. Temperature is also dependent on the bacterial populations: mesophilic or thermophilic. Mesophilic bacteria prefer temperatures ranging from 30 to 40°C. Thermophilic bacteria prefer temperatures ranging from 50 to 60°C (140°F).

The loading rate is the amount of fermentable matter that is fed into the digester per cubic meter of digester capacity. Any change in loading rate affects the balance inside the digester, so it should be kept constant. For a given capacity, if the loading rate is increased, the fermentation period is correspondingly increased. The common range of solid concentration is 7 to 9%, and digression from this range can cause fermentation to be retarded. In terms of the alkalinity/acidity (pH), the optimal gas formation occurs at pH of 7 to 8. If the pH becomes too acidic, gas production could stop altogether.

The retention time is the amount of time fermentable material resides inside the digester. It has been observed that maximum gas production takes place within the first four weeks, and then gradually tapers off. Detention time can be significantly reduced if the temperature is raised or the contents of the digester are agitated, or the supply of nutrients in the digester is augmented. Usually, some type of nutrient source is needed to help stimulate gas production. Bacteria use nitrogen, phosphorus, and potassium for their nutrients. Once these are available, fermentation proceeds very quickly. However, caution is advised that, although rare, toxic substances such as copper can inhibit gas production if found in large quantities.

It is essential that the organic acids formed in the first stage of the waste treatment process are converted to methane at the same rate at which they are formed. If not, they accumulate and ultimately lower the pH, leading to inhibition of the second stage of the digestion process and digester failure. The temperature must be maintained within certain ranges - heating increases the activity of the anaerobic bacteria reducing the required digestion time. A pH of 7.0 to 7.5 is recommended to encourage the methane-producing stage. A correctly operating digester will have sufficient buffer capacity (alkalinity) introduced from the breakdown of organic matter. Also, the gas leaving the digester is almost saturated with water vapor which condenses as the gas stream cools there causing problems – the problem is more severe when digesters are heated. To mitigate the problem, it is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices. For this reason, water traps should be located as close to the digester as possible and all piping should be sloped a minimum of 1% toward the water trap, which should be situated at a low point in the gas line.

Several types of digesters have been developed including the floating drum, the fixed dome, the bag, the plastic tube, the plug flow, and the up-flow anaerobic sludge blanket digesters. A digester is an airtight vessel or enclosure in which bacteria decomposes biomass in water to produce biogas. Both batch and continuous digesters are commercially available. While batch systems may be suitable for some applications, they are labor intensive, and in developed countries are probably not appropriate to the steady production of gas on any significant scale.

Mixing may be used in both batch and continuous digesters to enhance contact between the bacteria and substrate. It has, however, been argued that while mixing may increase conversion rates, it is not energy efficient. In two-stage systems, only the first stage is mixed. Multistage systems provide optimum conditions for the successive digestive steps, and further development may prove them to be economical for some biomass materials.

In a process of manure and straw mixture digestion, for the first 72 hours, the yield of methane was minimal (essentially 0%) and the yield of canon dioxide is virtually quantitative (approximately 100%) – in this period, digestion occurred as aerobic fermentation to carbon dioxide. The yields of methane and carbon dioxide gases were approximately equal after 250 hours, and after 500 hours, the digestion reached the stationary phase. The methane content of the biogas was in the range of 73 to 79% for the runs, the remainder being principally carbon dioxide. During a 720-hour digestion period, approximately 80 to 85% v/v of the biogas was produced in the first 360 to 430 hours which is an indication that the digester retention time can be designed to a shorter period (360 to 430 hours) instead of the longer period (720 hours).

The final stage in the process is the disposal of the any waste. Essentially all the water entering the system will be present in the effluent. Direct recycle of the water after separation from the sludge cannot be practiced, as this will result in a buildup of toxic substances and eventual failure of the system. It is possible that some fraction of the water may be safely recycled, although data on this aspect have not been seen. Treatment of this large volume of water to remove the toxic constituents is costly and is likely to be uneconomical other than in large-scale operations.

Typically, it can be assumed that the water, which is considered to have some nutritional value, can be used for irrigation of growing crops. However, serious consideration must be given to the effect of trace metals and other toxic materials present in the water and the effects of these constituents on the crops. In addition, subject to the presence of non-toxic constituents (or the lack of toxic constituents) in the sludge, it also may be used for land application and will generally have higher nitrogen content than the original material. On an absolute (feedstock-to-sludge) basis, there may be less nitrogen retention due to losses of the nitrogen as ammonia (NH₃), and there may be additional losses on land application since the nitrogen in the sludge is in a more volatile form than the nitrogen in the original feedstock. Nevertheless, the sludge resulting from the digestion of animal manure is generally considered (subject to a critical analysis of the constituents of the sludge) to have improved fertilizer value over the original feedstock. During digestion, the volatile fatty acid concentration is lower and the pH higher. Nearly all digester plants have ancillary processes to treat and manage all of the by-products. Before storage and use, the gas stream is dried and sweetened by removal of sulfur compounds. The sludge liquor mixture has to be separated by one of a variety of ways, the most common of which is filtration. Excess water is also sometimes treated in sequencing batch reactors (SBR) for discharge into irrigation systems.

Essentially all organic material can be digested except for the stable woody materials since the anaerobic microorganisms are unable to degrade lignin. The biogas which is formed has a high calorific value (heat content) and is considered as a renewable energy source. The main disadvantages of this process are (i) the possible presence of volatile siloxanes in the biogas that can cause serious damage in the energy users’ engine and boiler due to the formation of microcrystalline silica; and (ii) the increased concentration of heavy metals and various industrial organic chemicals in the residual sludge due to the significant reduction of the organic fraction during digestion, leaving the mineral and non-degradable fraction untouched.

Anaerobic digestion produces a clean and environmentally friendly fuel, although it contains carbon dioxide, water vapor, hydrogen, hydrogen sulfide, and siloxane derivatives (i.e., compounds having a molecular structure based on a chain of alternate silicon and oxygen atoms, -O-Si-O-Si-, especially – as in silicone derivatives – with organic groups attached to the silicon atoms).

As a note of caution, a mixture of biogas (predominantly methane) and air can be explosive. Methane in concentrations of between 5% and 15% v/v in air is explosive, and air should not be allowed to enter the digester or gasholder. All piping and equipment must be sealed properly to prevent gas from escaping to the outside. In addition, all electrical installations must be of the explosion-proof type, as the smallest spark could ignite escaped gases.

Biomass feedstocks generally produce a biogas rich in methane. This medium-to-high Btu (heat content) gas can, in some instances, be upgraded to a substitute natural gas (SNG). However, depending on the feedstock, non-negligible amounts of sulfur are also produced (Table B-13).

Table B-13 Comparison of different biogas feedstocks.

Feedstock	Sulfur content*	Product sulfur*
Liquid and solid manure	300-500	0.5
Organic waste	100-300	0.3
Wood chips	300-1000	0.3
Sewage sludge	300-500	0.6
*mg/m3

See also: Anaerobic Digestion – Gas Production.