1, The nature and use of methane
Biogas is organic matter (such as straw, weeds, human and
animal feces, garbage and sewage sludge and industrial organic wastewater,
etc.) in anaerobic environment and certain conditions, after various kinds and
great number, the function of different kinds of anaerobic microbe catabolism,
the final form is given priority to with CH4 and CO2 gas mixture, is a
biochemical process.The main component of biogas is methane (CH4).Usually
accounts for the total volume of 50%~70%;The second is carbon dioxide, which
accounts for 25%~40% of the total volume.Other gases, such as hydrogen sulfide,
nitrogen, oxygen, hydrogen and carbon monoxide, make up about 5% of the total
volume.Methane, carbon monoxide, hydrogen and hydrogen sulfide in biogas are
flammable gases, oxygen is combustion-supporting gas, carbon dioxide and
nitrogen are inert gases.Unburned biogas is a colorless, smelly, toxic
(hydrogen sulfide), lighter than air, easy to diffuse, difficult to tolerate
water flammable gas mixture.Biogas becomes a non-toxic, odorless, smokeless gas
after full combustion.
As a kind of biomass energy, biogas has been
used in China for nearly a hundred years.It is a valuable renewable source of
energy that can be used directly as fuel or converted to produce syngas.After
purification and purification, biogas is biomethane. As a biogas, it has four
characteristics: clean, efficient, safe and renewable. The efficient
preparation and comprehensive utilization of biogas is a very representative
two-way cleaning process, and as a new type of energy, it will occupy a place
in the energy pattern.
As a
by-product of anaerobic reaction, biogas has been ignored, random emissions,
causing safety risks.With the progress of technology, as well as the national
encouragement in the level of energy conservation and emission reduction
policies, more and more biogas after purification is connected to the grid,
used as vehicle gas (CNG), through boilers, power generation (cogeneration) and
so on are used up.
Based on
China's national conditions, biogas power generation is difficult to access the
Internet, generally only for self-use;The technology for fuel cells or
wastewater purification is not mature;As a chemical raw material, due to the
general biogas production is small, does not have a certain scale effect.With
the national energy shortage and the high oil price, the global gas price,
including China's soaring, CNG price has reached 4~5 yuan /m3, therefore, the
use of biogas to produce alternative natural gas or vehicle gas, will have
better economic and social benefits.
2. Biogas purification technology
From the analysis of the
utilization direction of biogas to make CNG, other components except CH4 are
harmful components or impurities, which need to be removed separately.The main
impurity is CO2. Due to its high content, the more it exists, the lower the
calorific value of the gas will be. According to the technical requirements of
the first and second class gas in China's natural gas standard GB 17820-1999,
it should be removed to less than 3.0%.Sulphur content is too high will corrode
equipment and pipelines, reduce their service life, so different standards have
different requirements for sulfur content;Because the biogas is produced in an
anaerobic environment, the O2 content will not exceed the standard, but it
should be noted that in the CNG standard, the O2 content after purification
shall not be higher than 0.5%;In order to prevent ice from clogging pipes, CNG
and LNG production also requires deep water removal;Other trace N2, H2, etc.
can not be removed if it does not affect the calorific value required by
natural gas standard.
3. Methane desulfurization
The
small amount of sulfur contained in biogas will cause environmental pollution
and corrosion of equipment, pipelines, instruments, etc., so whether it is
directly used for combustion or biogas power generation, the necessary
desulfurization treatment of biogas must be conducted.
Sulfur
in biogas mainly exists in the form of hydrogen sulfide, and there are many
removal methods. At present, the commonly used industrial desulfurization
methods include wet desulfurization and dry desulfurization.
3.1 Wet desulfurization
Wet desulfurization can be divided into
physical absorption method, chemical absorption method, oxidation method and
biological method.Physical and chemical methods have the problem of retreating
hydrogen sulfide and have been gradually phased out.Oxidation method takes
alkaline solution as absorbent and adds oxygen carrier as catalyst to absorb
H2S and oxidize it to elemental sulfur, and the desulfurization efficiency can
reach more than 99.5%.The process flow is as follows: first, the desulfurizer
is dissolved in water to prepare the desulfurization liquid, the liquid enters
the equipment and mixes with the biogas, and the hydrogen sulfide (H2S) in the
biogas reacts with the desulfurization liquid to generate elemental sulfur.
The basic principle of biological absorption
method is that sulfur containing pollutants in oxidation state first generate sulfide
or H2S through biological reduction, and then generate elemental sulfur through
biological oxidation process and then remove
it.In most bioreactors, the microbial species are dominated by bacteria, with
fungi as the minor species, and yeasts are rare.Commonly used bacteria are
thiobacillus ferrooxidans, thiobacillus denitrifying and thiobacillus.The most
successful example is Thiobacillus ferrooxidans, which grows at an optimal pH
of 2.0 to 2.2.
Wet
desulfurization of biogas usually refers to wet oxidation method, which can be
divided into tannin extract method, PDS method, COS method and 888 method
according to the different catalysts selected. It is suitable for the condition
of large gas volume and high sulfur content, and maintains desulfurization
efficiency with pH value, and the operation cost is relatively low.The
desulfurization accuracy is low, generally suitable for crude desulfurization
occasions, the device is relatively complex, there are many equipment need
regular maintenance.In large-scale desulfurization projects, the combination of
dry and wet desulfurization process is generally used to purify andpretreatment biogas.
3.2 Dry desulfurization
The basic principle of dry desulfurization equipment
is a method of oxidizing corrosive gas hydrogen sulfide into elemental sulfur
or sulfide, also known as dry oxidation method.The dry desulfurization
equipment is composed of: a desulfurizer is placed in the desulfurizer, gas
passes through the desulfurizer bed at a low flow rate from one end, hydrogen
sulfide (H2S) is oxidized into sulfur or sulfide and remains in the bed, and
the purified gas is discharged from the other end of the container.
Dry desulphurization has the advantages of
simple structure, convenient operation, small number of equipment (usually 2
desulphurizers, one for use and one for standby, running alternatively), small
footprint, high desulphurization accuracy and low labor intensity.No personnel
need to be on duty during the work.It increases linearly with the increase of
sulfur content in the raw gas. When the upper limit of desulfurizer is reached,
the desulfurizer only needs to be regenerated and can be recycled for use. The
main characteristics are high desulfurization efficiency, simple and convenient
operation, and generally used in precision desulfurization occasions.
4．the conclusion
Biogas is a kind of recyclable green energy, but its
content in addition to the main methane also contain other gases, especially
methane ingredients contain strong causticity and highly poisonous hydrogen
sulfide gas, in order to be able to make biogas to health, safety, stability,
no secondary pollutants produced by green, using effect,Before use, it must be
desulfurized and purified. Our company (Jingcheng Weiye Environmental
Protection) has professional technical support and a number of successful
projects in biogas purification and biogas recovery and utilization.Many
achievements of technology is mainly used in livestock breeding, slaughtering
industry and agricultural organic waste industry organic waste treatment
industry, food, eat hutch waste treatment, alcohol industry, papermaking
industry organic waste such as sewage treatment industry, equipment in the
running process achieved good economic benefits, ecological benefits and social
benefits.
About biogas purification or biogas recovery and utilization business
welcome to inquire or mail message!
