In order to determine the desirability and feasibility of introducing methane fermentation technology of a particular type of organic waste, you need to know following things about biogas plants:
Bioreactor's volume
On the territory of our country, mainly due to harsh climatic conditions, unfortunately, we have to admit the fact that BGP with reactor smaller than 50m3 will not work effectively, because our process is anaerobic and thus requires energy. And if, with significant volumes of the reactor, BGP can cover the costs of it's own work regime by converting biogas into electricity or heat, then, in other case, biogas output will be too small to talk about equipment capable of transforming it's energy.
BGP's purpose
The main and only purpose of BGP is processing of organic waste, or rather, their disinfection! Energy production is just a nice bonus that makes the process less costly, and in some cases, when extremely large amounts of processing take place, it can even provide some of the energy for needs of the enterprise. Therefore, the effect should be counted based on cost of recycling and profits of selling fertilizers, or by increasing the yield of your own fields.
Volume of recycling
The maximum suitable volume of daily substrate (waste + water) processing in BGP amounts to about 20% of it's physical volume (ie amount that can be loaded in reactor). Often enough, amounts of substrate that are obtained are double the amount of waste (example - cattle farms). For example, if volume of daily cattle manure output is 1 ton, then amount of substrate will be 2 tonns and then BGP's reactor volume must be atleast 10m3. The lower the humidity of raw materials, the more of it comes out as substrate, because humidity required for efficient anaerobic purification process starts 90-92%.
BGP reactor material
We have gone through test of metal and concrete reactor. We have exploited in laboratory conditions a reactors with it's heat exchanger located within the fermentation tank. Our studies provide serious grounds to doubt the possibility of using these materials in terms of corrosion resistance and resistance to biological and chemical degradation (degradation time of a metal reactor was about 5 years). Thus only stainless steel reactors are left, but in addition to their high cost, they have another significant drawback for use in harsh enviroments - significant ability to transfer heat, which is very importent in -40 degree cold.Fiberglass doesn't just solve those main problems, but also deals with the question of biomass retantion due to porous structure of the wall. Its cost is justified by it's durability, which greatly exceeds it's payback period.
Fermentable organic waste
BGP is capable of fermenting 27 different types of organic waste (the capacity shown in the table below).
Biogas output from organic materials
|
Material category
|
Biogas output (м3) from 1 ton of raw material
|
|
Cow manure
|
39-51
|
|
Cattle manure, mixed with hay
|
70
|
|
Pig manure
|
51-87
|
|
Sheep manure
|
70
|
|
Bird manure
|
46-93
|
|
Adipose tissue
|
1290
|
|
Waste from butchers
|
240-510
|
|
Solid municipal waste
|
180-200
|
|
Feces and Wastewater
|
70
|
|
Distillery dregs
|
45-95
|
|
Biological waste from sugar manufacture
|
115
|
|
Silage
|
210-410
|
|
Potato tops
|
280-490
|
|
Beet pulp
|
29-41
|
|
Beet tops
|
75-200
|
|
Vegetable waste
|
330-500
|
|
Grain
|
390-490
|
|
Grass
|
290-490
|
|
Glycerol
|
390-595
|
|
Spent grain
|
39-59
|
|
Waste produced in the process of harvesting rye
|
165
|
|
Flax and hemp
|
360
|
|
Oat straw
|
310
|
|
Clover
|
430-490
|
|
Milk serum
|
50
|
|
Corn silage
|
250
|
|
Flour, bread
|
539
|
|
Fish waste
|
300
|
The tabulated waste has its own peculiarities durning anaerobic biological treatment:
Cattle manure
One of the most popular types of biogas plants worldwide are biogas plants that can use cow dung as it's raw material. Keeping one cattle will supply around 6,6 - 35 tonns of liquid manure per year. This volume of raw materials can be processed into 257-1785 m3 of biogas. In terms of heat of combustion parameter values correspond to these: 193-1339 m3 of natural gas, 157-1089 kg of gasoline, 185-1285 kg of oil, 380-2642 kg of firewood. One of the key benefits of using cow manure as source of biogas is presence of colonies of bacteria in the digestive tract of cattle, bacteria that produce methane. This means that there is no need for additional application of microorganisms in the substrate, and hence no need for additional investment. Also manure's homogeneous structure makes it possible to use this type of raw materials in devices with continuous cycle. Biogas production is even more effective when bovine urine is added to the fermented biomass.
Pig and sheep manure
Unlike cattle, the animals of these groups are kept in buildings without concrete floors, so the process of biogas production is more complicated. The use of swine sheep manure in devices in continual cycle is not possible, it is only possible with specific loading dosage. Together with raw mass of this type, vegetable waste can fall in, which significantly increases the processing period.
Bird manure
In order to effectively use bird manure to produce biogas it is recommended to equip the bird cages with perches, as this will ensure the collection of litter in large volumes. To produce large volumes of biogas bird manure should be mixed with a cow slurry, that will eliminate excessive release of ammonia from the substrate. The peculiarity of the application of poultry manure in the production of biogas is a need to introduce two-stage technology with the use of a hydrolysis reactor. This is required in order to exercise control over the level of acidity, otherwise the bacteria can be killed in a substrate.
Feces
For efficient recycling of feces it is required to minimize the amount of water per one medical device: a lump sum can not exceed 1 liter.
With recent years, research has established that in the biogas, when feces are used for its production, along with key elements (in particular methane) a plurality of hazardous compounds passes that contribute to environmental pollution .For example, during methane fermentation of such raw material at high temperature regimes at drains bioclearing stations almost all samples of the gas phase had about 90 mg / m3 of arsenic, 80 mg / m33 mercury 500 mg / m3 tellurium 900 mg / m3 tin, 700 mg / m3 lead found in them. These elements are tetrahedral and dimethylated compounds inherent in the processes of autolysis. Identified indicators seriously exceed the MAC specified elements, which demonstrates the need for a more comprehensive approach to the problem of recycling faeces into biogas.
Energy crops
The vast majority of green plants provides an exceptionally high yield of biogas. Many European biogas plants operate on corn silage.This is justified because the corn silage produced from 1 hectare, allows you to develop 7800-9100 m3 of biogas, which corresponds to: 5850-6825 m3 of natural gas, 4758-5551 kg of gasoline, fuel oil kg 5616-6552, 11544-13468 kg of firewood.
A ton of diffirent herbs provide about 290-490 m3 of biogas, with clover having a particularly high yield: 430-490m3. A ton of high-quality raw potato leaves can provide up to 490 m3, a ton of beet tops - from 75 to 200 m3, a ton of waste generated in the process of harvesting rye - 165 m3, a ton of flax and hemp - 360 m3, a ton of oat straw - 310 m3.
It should be noted that in the case of purposefully growing energy crops for biogas production there is the need to invest money in their planting and harvesting. That is how this culture is substantially different from other sources of raw materials for bioreactors. There is no need for fertilizing such crops. In regards to vegetable waste and crop production, their processing into biogas has an extremely high economic efficiency.
"Landfill gas"
From one tonne of dry solid municipal waste can be obtained up to 200 m3 of biogas, more than 50% of which is methane. By activity of methane emissions "landfill sites" are far superior to any other sources. The use of solid waste in biogas production, not only will provide significant economic benefits, but also reduce the flow of polluting compounds into the atmosphere.