Biogas technology

Premixing/Prestorage container

Liquid feedstock arrives through pipe systems or is pumped from tank trucks into underground prestorage/premixing concrete containers, from where the material is pumped evenly and at regular intervals into the digester. In this container liquid material can be mixed with solid feedstock. The technological advantage of premixing containers is partly the homogenisation of different types of feedstocks, and partly the microbiological decomposition starts in these tanks.

High efficiency biogas plants would need large and costly premixing tanks, therefore they are usually not installed. An alternative solution, which has become more frequent recently, is the transfer of solid feedstock (eg. energy plants) directly into the digester by a pulley-type forwarding mechanism.

In case of premixing/prestorage containers special attention needs to be payed to the prevention and reduction of unpleasant odours, which can be solved by the use of biofilters, for example.

If liquid feedstock (eg. thin and liquid manure) is not formed near the biogas plant, but has to be transported there, it is advisable to build a prestorage container in order to secure controlled feedstock input.

Solid feedstock input

Depending on the nature of waste feedstock, it might need heating and/or to be sorted and/or to be chopped/shredded first.

The input of solid feedstock takes place at regular intervals, for which there are several solutions:

  • wash in shaft, where solid feedstock is washed into the digester through a shaft together with the effluent after the decomposing process,
  • hydraulic feeding through closed pipe system,
  • special pulley-type mechanism,
  • solid substrate bunker.

If the size of solid feedstock particles varies greatly (heterogeneous substrate size), the material needs to be chopped in order to feed the digesters with homogeneous feedstock of uniform particle size. This way the bacteria have better access to the substrates, which has a positive effect on the gas production. The equipment furnished with feed pulleys mixes the substrates when different types of feedstock are used. Usually an automatic feeding regulation is available so that the operator fills the container only once or twice a day, the rest is controlled by a computer softwarte.

Hygienisation (pasteurization)

When processing certain types of animal waste (slaughterhouse waste, blood, expired food products, kitchen waste, etc.) hygienisation (pasteurization) is necessary. Temperature and time of pasteurization are specified by law. According to EU-regulation 1774/2002/EG, in compliance with decree Nr. 71/2003. (VI.27.) of the FVM (Hungarian Ministry of Agriculture and Rural Development), heat treatment has to be performed at a minimum temperature of 70 C° for at least one hour if the particle size is not bigger than 12 mm.

The biogas plant’s pasteurizing unit is a periodically operating, insulated steel container, which is equipped with a mixing and heating devices and a measuring tools. The energy needed for hygienisation is supplied by the waste heat generated in the biogas plant’s combined heat and power station. The capacity of the pasteurizing unit should be enough to receive at lesaat one feeding volume without difficulties. Exact documentation of the heat treatment is an important requirement.

Another solution for hygienisation is thermal treatment of the liquid fermentor effluent before it leaves the biogas plant, as described above.

Decree Nr. 71/2003. (VI.27.) of the FVM classifies three different types of animal waste. One of the most important traditional feedstocks of biogas production, manure and liquid manure are, pursuant to the decree, second-rate waste. These materials do not need to be pasteurized if applied alone.

Pasteurization is obligatory if the materials listed in the appendix of decree Nr. 23/2003. (XII. 29.) of the KvVM (Hungarian Ministry of Environment and Water) are used in biogas plants. This applies for all plants, where organic waste is used.

Digesters (biogas-reactors)

Liquids are pumped, while solid substrates are fed into the digesters by a specific feeding apparatus at regular intervals. Digesters are generally vertical or horizontal, cylindrical or rectangular, insulated concrete (or steel) flow-through-system containers.

Digesters (or fermentation containers) have to comply with the following requirements:

  • exclusion of air,
  • liquid- and gas-leakage safe design,
  • adjustable heating,
  • prevention of crust- and zone-formation and substrate settlement (by mixing),
  • safe input, output and pass-through of substrates,
  • ensuring necessary retention time.

Breakdown of organic material starts in airtight digesters, where it is mixed slowly, periodically or continuously, at a so-called "mesophilic" temperature range of about 37-38C°. There are so-called "thermophilic" biogas digesters as well, which operate at higher temperature range of about 54-55C°. Compared to mesophilic biogas plants, gas output of thermophilic systems is significantly higher and the breakdown of substrates takes less time. The thermophilic microbiological systems are sensitive to abrupt changes of temperature, therefore these plants require reliable temperature regulation, considerable experience and laboratory service back-up. The feedstock composition can be changed gradually and carefully. Mesophilic systems, on the other hand, can tolerate larger fluctuations in plant operation parameters as these are less sensitive.

Digesters can be heated through metal- or plastic tubes fastened to the container wall (or base). The plastic tubing is cheaper, however, its heat-transfer capacity is poorer than that of the more expensive metal tubing. In modern systems, an external heat exchanger is used to help maintain heat balance. Thermal energy consumption of digesters depends on the temperature of feedstocks fed into the system, the digester’s design and effective insulation.

There are one- and two-step biogas technologies. For the one-step version the different stages of the decomposition process take place in the same vessel. In case of two-step technological solutions, horizontal digesters are frequently used as first step.

Some of the horizontal systems operate in the so-called "plug flow" or "Propfenstrom" mode, which means that the fresh substrate is fed in at one end and the material passes through the digester like a plug through a tube. Consequently, fresh substrates mix only partially with the material already fermented. In "plug-flow" digesters, the different stages of decomposition are more separated from each other, enabling more effective operation. These horizontal digesters usually have a relatively small volumetric-capacity and the average dry substance content of feedstock can be increased up to 18-19%.

The evolved biogas is usually collected in a gas-collecting space above the liquid phase of the digester, from where it it is continuously withdrawn.

In case of two-step technologies, the fermentation mash is pumped from the first digester into vertical cylindrical, insulated concrete container(s) with a flow-through-system, furnished with heating-pipes and operating at mesophilic or thermophilic temperature range, functioning as secondary digester(s), where organic matter decomposition and biogas production continue and become completed.

At the First Hungarian Biogas Ltd.we recommend our clients the most suitable digester designs and sizes on the basis of their available feedstocks and other related aspects.

Biogas storage

Biogas storage facilities stock a gas amount produced within 2 to 12 hours. It is generally not economical to store a larger volume of biogas, as the CHP (combined heat and power) units are running continuously. However, a buffer capacity of at least two hours of biogas production is by all means required, since biogas production may fluctuate.

There are usually three types of biogas storage facilities:

  • foil biogas-storage connected to the headspace of the digester(s),
  • foil biogas-storage in separate, light metal construction towers or buildings,
  • hermetic double-foil dome.

The First Hungarian Biogas Ltd. is always at its clients’ service. We design biogas storage units tailored to the customer’s requirements.

Biogas cleaning, dewatering

The biogas leaving the digester is saturated with water vapour, which condenses if the biogas is transported through a cold pipeline. This can take place if the underground biogas pipeline is at least 50 meters long and descends at least 1%. At the deepest point of the biogas pipe, non-freezing condensed water regulator valves need to be installed. Wet biogas pipelines running above ground require extra cooling, so that water condensation can take place. In this case, the gas has to go through a special cooling apparatus, whereupon its moisture content condenses.

Biological desulfurization of biogas in the gas dome of vertical cylindrical digesters is a frequently used solution, whereby small amount of air is pumped into the dome to create the essential conditions for the aerobic bacteria that oxidize hydrogen sulfide to elementary sulfur. This process can be carried out in separate desulfurization towers as well. Biogas desulfurization can also be done by chemical methods.

In addition to cleaning from water and hydrogen sulfide, CO2 should also be removed from biogas if the gas is to be used as vehicle fuel, or be fed into the natural gas network.

We at the First Hungarian Biogas Ltd. are always at the disposal of investors and operators to help them choose the most appropriate biogas cleaning and dewatering methods, while we can also recommend technological solutions to reach natural gas and vehicle fuel quality.

Combined heat and power (CHP) generation

After its dewatering, biogas is usually burned in continuously running combined heat and power (CHP) generating equipment (cogeneration apparatus). In smaller biogas plants, biogas is sometimes directly used for heating, without electric current generation.

There are three types of cogeneration apparatus used in biogas plants:

Type Petrol engine Diesel engine Diesel engine
  Otto gas engine Otto gas engine Oil injection
       
Árszint low very high high
Hatásfok 20 - 25 % 30 - 35 % 30 - 38%
Élettartam low medium medium
Zajszint medium high high
Koromképződés no no yes
Szervízigény high low high
Gyújtóolaj felhasználás no no about 10 %
Tartalék üzemanyag PG gas PB gas Diesel
  (petrol)   (plant oil)
Teljesítmény tartomány (kW) 5 - 30 > 150 30 - 200

This comparative table clearly shows that considering low emission rates and higher efficiency levels, it is best to apply special biogas engines. As a result of increasing oil prices, the operating costs of diesel fuel injection engines are growing as well. The endurance of gas engines varies greatly. If maintained appropriately, they can run up to 60,000 operating hours without decrease of output. Without the proper service, however, the engine’s electric output decreases dramatically and so does the operator’s income. The engines’ systematic and professional preventive maintenance is the operator’s vital interest.

The engines’ endurance can be increased by keeping the hydrogen sulfide content of biogas at a minimum level. Manufacturers generally specify a value of max. 150 ppm, but it is advisable to keep an H2S concentration level of down to 50 ppm in the biogas.

Relying on its market experience and contacts, the First Hungarian Biogas Ltd. can give investors professional advice on how to choose the best CHP station for the planned biogas plant.

Storage of fermentation effluent

The biogas fermentation end product can be used as fertilizer (biohumus). When designing the size of the storage container, official regulatinons regarding the prescribed compulsory storage time have to be taken into consideration. In newer and bigger biogas plants, these containers have a fixed roof or a foil cover in order to decrease nitrogen loss and to capture the biogas that might be produced during storage.

Liquid material is pumped from the digester into the storage container. There are several methods for the phase separation of the effluent, whereby a solid and a liquid phase are produced. The latter can be purified like waste water, so that it can eventually be released into natural waters, or used in technological processes.

On the basis of its professional experience, the First Hungarian Biogas Ltd. can give both investors and operators advice on how to choose the best possible solutions for the treatment and use of fermenation effluent.

Torch

According to safety regulations, biogas plants need to be equipped with a torch, which ensures safe biogas burning in case of a possible breakdown. The gas torch is part of the safety equipment, and is used to burn the biogas surplus in an evironmentaly friendly way, in case the normal CHP apparatus breaks down.

Process control, monitoring

The operation of biogas plants needs constant monitoring. The following parameters have to be checked regularly:

  • quantity of feedstocks
  • dry substance content (in case of change)
  • temperature (in digesters, heat exchangers)
  • quantity of produced biogas
  • biogas composition (methane, carbon dioxide, hydrogen sulfide, possibly oxigen)
  • the pH of fermentation medium
  • condition of the CHP station, exhaust-gas composition
  • own electric current consumption

Main operating parameters of the biogas plant have to be measured and recorded with the help of specially-selected measuring instruments. It is essential to keep record of feedstock input to receive feedback about the plant’s reaction to different feedstock combinations. Microbiological changes can be prognosticated on the basis of the digester’s pH values and alterations in volatile fatty acid composition. Since the pH changes more quickly than the gas composition itself, all the necessary precautions can be taken in time to prevent possible disturbances in gas production. It is also highly important to measure the gas composition and its quantity in order to ensure sufficient operation. It is not compulsory to have an on-line gas analyser in the biogas plant, measuring at preselected intervals, but collecting such data on regular basis through specialized service company facilitates plant management remarkably and makes the operation more secure and balanced. Through continuous record-keeping and operational data analysis, correlations between the quality and quantity of feedstock input and the quantity and composition of produced biogas can more easily be detected.

The most important elements of process control are:

  • regular input of substrates
  • digester’s operating temperature regulation
  • switch on/off regulation of mixing apparatus
  • gas cleaning regulation
  • CHP station switch on/off.

Thanks to automatic process control in modern biogas plants, operators routinely only have to take care of feedstock input and monitor the process.

In order to run the biogas plant efficiently, it is essential to maintain the optimal conditions for microorganisms. Through samples taken from feedstocks and digesters, as well as regular analysis of operational data, the First Hungarian Biogas Ltd. helps its clients through constant biotechnological monitoring and counselling to reach the highest biogas output possible.