BIOTECHNOLOGY - V. H. Herasymenko - 2006

Part II. Special Biotechnologies

Chapter 22. BIOTECHNOLOGIES FOR WASTE MANAGEMENT AND BIOCOVERSION IN THE AGRO-INDUSTRIAL COMPLEX

22.3. NON-TRADITIONAL METHODS. BIOTECHNOLOGY FOR BIOGAS PRODUCTION VIA ANAEROBIC WASTE DIGESTION

22.3.4. Fractions formed during the biomethanogenesis process

22.3.4.1. Biogas, its composition and utilization

Biogas is a gas mixture consisting of methane (50-85%), carbon dioxide (15-50%), a small amount of hydrogen sulfide (up to

2%), as well as trace amounts of hydrogen, ammonia, and nitrogen oxides. Methane is the primary component of biogas. The presence of carbon dioxide limits the calorific value of biogas as a fuel, which, depending on the CH₄/CO₂ ratio, reaches 20.9-33.4 MJ/m³.

The ENERGY VALUE OF 1 m³ of biogas containing 50%

methane reaches 17.8 MJ, and when the methane content increases to 70%, its energy potential rises to 25 MJ. The energy value of traditional energy carriers such as natural gas and liquid fuel, calculated per 1 m³ and 1 kg, is 34 and 42 MJ, respectively.

The main impurities in biogas that reduce its quality are carbon dioxide and hydrogen sulfide. When biogas is used as a fuel, these impurities (especially hydrogen sulfide) cause significant corrosion of technological equipment. To purify biogas from contaminating impurities, Methods such as dry, "wet", and alkaline scrubbing, liquid absorption, and pressure Separation are used.

To increase the economic efficiency of large-scale industrial biogas plants, Water gas scrubbers are used, which increase the specific weight of methane in biogas up to 95%. This helps to almost double the calorific value of the biogas.

The efficiency of biogas plant utilization is determined by the methods of converting the produced biogas as an energy carrier. The following directions for biogas utilization can be identified:

- combustion in boiler units for water heating and its utilization in technological processes or by other consumers;

- conditioning of biogas in accordance with regulatory and technical documentation requirements and feeding it into the gas distribution networks of local natural gas consumers (blending with natural gas);

- purification, drying, compression, and refueling of gas-cylinder vehicles, tractors, and other agricultural machinery with biogas;

- electricity generation.

Biogas possesses all the advantages inherent to natural gas. It is easily transported through pipelines, Burns without smoke, soot, or residue (ash, slag). Gas-powered appliances are simple, safe, quickly put into operation, easily regulated, and adapted to automated modes.

Biogas can be used directly in gas-burning devices for heating and lighting, for supplying energy to feed preparation units, for the sterilization and dehydration of the solid fraction of manure after separation, for internal combustion engines, etc.

The simplest application is burning biogas instead of natural gas in gas burners, to which it can be supplied from a storage tank (gas holder) under low pressure. Most modern heat-engineering equipment on livestock farms operating on natural or liquefied gas can be converted to biogas without significant upgrades or a noticeable decrease in operating efficiency. Industries in Germany, the USA, and other countries have begun supplying unit heaters with biogas burners, as well as heat generators that use biogas as fuel.

However, it is more profitable to use biogas to generate mechanical and electrical energy. This makes it possible to create an independent energy base capable of meeting the needs of a livestock farm and even a residential sector. From 1 m³ of biogas, 1.6-2.3 kWh of electricity can be obtained.

The Italian company Fiat has developed a universal energy unit called "Totem", which operates on any type of fuel, including biogas. The power unit consists of a kit that includes a diesel engine, an electric generator, a heat exchanger that captures exhaust gas heat from the internal combustion engine and channels it for thermal needs. The total coefficient of performance (COP), taking into account the heat generated during electricity production, reaches 80-85%, while 33% of the chemical energy of biomethane is directly converted into electricity.

Biogas is also used as fuel for automotive engines, and its efficiency in this case depends on the methane content and the presence of impurities. Both carburetor and diesel engines can run on methane. However, since methane is a high-octane fuel (its octane rating is 110-115, and the lower flash point is 645 ^oC), it is more efficient to use it in diesel engines.

Industries manufacture devices for internal combustion engines that can run on both gasoline and methane. The Use of methane in motor transport has several advantages: relatively low cost and significantly lower environmental pollution compared to gasoline and diesel fuel.

Methane Fermentation plants are particularly promising for rural energy supply due to The complexity of centralized natural gas distribution and the gradual depletion of traditional energy resources (oil, gas, coal, etc.). In rural areas, biogas can be used as a substitute for natural gas for cooking, hot water supply, space heating of residential and administrative buildings, greenhouse heating, grain and fruit drying, and refueling agricultural machinery. According to experts' calculations, 1 m³ of biogas is sufficient to cook a meal for 6-7 people or illuminate a medium-sized room for 6-8 hours.

1 m³ of biogas is equivalent in energy content to: 0.65 m³ of natural gas, 0.7 l of crude oil, 0.65 l of diesel fuel, 0.64 l of gasoline, 0.6 l of kerosene, 3.5 kg of firewood, and 1.5 kg of coal.

In some cases, comprehensive biogas utilization is implemented: a portion of it is directed to meet the farm's energy needs—including powering the methane fermentation process itself (such as Reactor heating)—while the remainder is commercially sold to the local or national gas supply grid. An example of such application is the gas supply to Chicago (USA), managed by the Calorific Recovery Anaerobic Process company, whose Processing facility is located near Guymon, Oklahoma. The facility processes 500 tons of manure daily, yielding 45.3 thousand m³ of biogas. This amount is sufficient to heat 3.5 thousand residential homes in Chicago. The gas is delivered via a specially constructed pipeline. After preliminary removal of carbon dioxide, hydrogen sulfide, and other impurities, the incoming biogas consists of 99.8% methane.

According to some authors, producing biogas from livestock waste and utilizing it for agricultural needs can reduce the consumption of other Energy Sources by 3–4%.

Additionally, the carbon dioxide contained in biogas can be used as a feed preservative, to enhance Photosynthesis efficiency in greenhouses, and in the cultivation of aquatic organisms (such as Chlorella and Spirulina), among other Applications.