Why to make LNG from coke oven gas?

Why is LNG?

LNG (liquefied natural gas) is the abbreviation of liquefied natural gas. Natural gas is a kind of combustible gas produced naturally in the gas field, and its main component is composed of methane. LNG is cooled to – 162 ℃ by natural gas in gas state under normal pressure to make it condense into liquid. After natural gas liquefaction, storage and transportation space can be greatly saved, and it has the characteristics of high calorific value and high performance.

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LNG is a clean and efficient energy. LNG trade is becoming a new hot spot in the global energy market because imported LNG helps energy consuming countries to diversify energy supply and ensure energy security, while exported LNG helps natural gas producing countries to effectively develop natural gas resources, increase foreign exchange income and promote national economic development. Natural gas is becoming more and more popular as a clean energy. Many countries have listed LNG as the preferred fuel, and the proportion of natural gas in energy supply has increased rapidly. LNG is growing at a high speed of about 12% every year, becoming one of the fastest growing energy industries in the world. In recent years, the global LNG production and trade are becoming more and more active. LNG has become a scarce clean resource, and is becoming a new hotspot in the world’s oil and gas industry. In order to ensure the diversification of energy supply and improve the structure of energy consumption, some energy consuming countries pay more and more attention to the introduction of LNG. Japan, South Korea, the United States and Europe are building LNG terminals on a large scale. Large international oil companies have also turned their new profit growth point to LNG business, LNG will become the next global hot energy commodity after oil.
China’s natural gas utilization is extremely unbalanced, and the proportion of natural gas in China’s energy is very small. From the perspective of China’s natural gas development situation, natural gas resources are limited, natural gas production is far less than demand, and the gap between supply and demand is growing. Although it has not yet formed a scale, the characteristics of LNG determine its rapid development. It can be predicted that LNG will become the main force of China’s natural gas market in the next 10-20 years. In 2007, China imported 2.91 million tons of LNG, more than three times of that in 2006. From January to November 2008, China imported 3141475 tons of LNG, an increase of 18.14% over the same period of 2007. At the same time of China’s sustained and rapid economic development, the energy power to protect the economy is extremely scarce. China’s energy structure is dominated by coal, while oil and natural gas account for only a small proportion, far below the world average. With the increasing energy demand of the country, the introduction of LNG will play an important role in optimizing China’s energy structure, effectively solving the dual problems of energy supply security and ecological environment protection, and realizing the sustainable development of economy and society.
China pays more and more attention to the development of LNG industry. The coastal LNG projects that China is planning and implementing are: Guangdong, Fujian, Zhejiang, Shanghai, Jiangsu, Shandong, Liaoning, Ningxia, Hebei Tangshan, etc. these projects will eventually form a coastal LNG terminal and transmission network. According to China’s LNG use plan, it will be 240 billion cubic meters in 2020. In terms of natural gas import, the national development and Reform Commission expects that by 2020, China will import 35 billion cubic meters, equivalent to 25 million tons / year, seven times the total amount of receiving stations in Guangdong Province.

LNG basic knowledge and Application

Natural gas is a colorless and odorless combustible gas produced in oil and gas fields.

The main component of CH4 is methane (CH4), accounting for 80-99%, followed by ethane, propane, total butane, total pentane, carbon dioxide, carbon monoxide, hydrogen sulfide, total sulfur and water. Under the standard state, the boiling point is – 161.52 ℃, the density of natural gas is generally 640-750g / m3, and the relative density relative to air is 0.55-0.62.

Working principle of natural gas as fuel CH4 + 2o2-2h2o + CO2 + 38mj / m3 3. Main uses of natural gas

Natural gas is an important industrial raw material and fuel. 

Its main uses are as follows:

  • 1) Main fuel for power plant, thermal energy, heating, air conditioning and other industrial enterprises;
  • 2) Fuel for catering and civil use;
  • 3) The main raw materials of chemical industry can synthesize or refine various industrial raw materials and semi-finished products;
  • 4) As a clean fuel for motor vehicles that meet the requirements of environmental protection and an ideal substitute for gasoline and diesel, this is a new technology developed rapidly at home and abroad in recent years, which can be used in automobiles, ships, trains and even airplanes.

LNG basic parameters

The main components of LNG are methane (over 90%), ethane, nitrogen (0.5-1%) and a small amount of C3-C5 alkanes. LNG is another form of energy transformed from natural gas.

  • 1) The main component of LNG is methane, chemical name is CH4, and a small amount of ethane C2H6, propane C3H8, nitrogen N2 and other components.
  • 2) The critical temperature is – 82.3 ℃.
  • 3) The boiling point is – 161.25 ℃, and the ignition point is 650 ℃.
  • 4) The liquid density is 0.420-0.46t/m3, and the gas density is 0.68-0.75kg/m3.
  • 5) The calorific value of gas is 38mj / m3, and that of liquid is 50mj / kg.
  • 6) Explosion range: upper limit is 15%, lower limit is 5%.
  • 7) Octane number ASTM: 130.
  • 8) Colorless, tasteless, non-toxic and non corrosive.
  • 9) The volume is about 1 / 625 of the same volume of natural gas.

Six advantages of LNG

  • 1) The volume of LNG is 625 times smaller than that of natural gas of the same mass, so it is convenient to transport LNG to places without natural gas by car and ship.
  • 2) LNG has high storage efficiency and small land occupation. In the province of investment, 10m3 LNG storage capacity can provide 10000 households with one day’s domestic gas.
  • 3) Compared with gasoline, LNG has higher octane number, better antiknock performance and longer engine life. Low fuel cost. Good environmental protection performance. It can reduce HC by 72%, NOx by 39%, CO by 90%, Sox and Pb by zero.
  • 4) The latent heat of LNG vaporization is high, and the cooling capacity in the liquefaction process can be recycled.
  • 5) As the density of LNG after vaporization is very low, only about half of the air, a little leakage will fly away immediately, which will not cause explosion.
  • 6) LNG is a good clean fuel because of its pure composition, complete combustion and the generation of carbon dioxide and water after combustion, which is conducive to environmental protection and urban pollution reduction.

Main use of LNG

  • 1) As a clean fuel vaporized for urban residents, it is safe, convenient, fast and has little pollution.
  • 2) It is used as alternative vehicle fuel. When LNG is used as the fuel of automobile engine, the engine only needs to be changed properly. The operation is not only safe and reliable, but also has low noise and pollution. Especially in today’s increasingly strict emission regulations, the exhaust of automobile using LNG as the fuel is obviously improved. According to reports: compared with compressed natural gas (CNG), the fuel cost of LNG vehicle is 20% lower and the weight is 2/3 lighter for medium-sized and medium-sized heavy-duty vehicles under the same travel distance and operation time. At the same time, the cost of fuel supply system is at least 2/3 lower. It can be proved that liquefying natural gas and storing it in liquid state is the most economical and effective way to promote its application in transportation fuel. Among all clean fuels, natural gas is regarded as the most suitable alternative fuel for automobiles by many countries and experts in the world because of its mature application technology, safety, reliability and economic feasibility. Compared with gasoline, the emission of HC, NOx, Co, Sox and Pb are reduced by 72%, 39%, 90% and zero respectively. 40% noise reduction. Therefore, promoting the use of natural gas fuel will play a positive role in reducing air pollution and improving the environment.
  • 3) As a cold source, it can be used in the production of fast cooling food, the low-temperature grinding of plastics and rubber, as well as desalination of seawater and cable cooling.
  • 4) As industrial gas fuel, it is used in glass shell factory, process glass factory and other industries.

LNG transportation mode

LNG is mainly transported by ship, train and truck. Within the economic transportation radius of 500-800 km, it is an ideal way to transport LNG by truck. The tank body of tank car adopts double wall vacuum powder insulation, equipped with operation valve safety system and infusion hose, etc. The manufacturing technology of domestic cryogenic liquid tank car is relatively mature, and the tank car is safe to use. LNG products are stored in cryogenic liquid tanks with double wall vacuum powder insulation. The daily evaporation rate of LNG can be controlled within 0.46%, and the storage period is 4-7 days.

What is coke oven gas?

The raw gas produced in coking process of coking coal is called clean gas after recovery and purification. The part of clean gas is returned to coking plant for self use, and the rest is called surplus gas, usually called coke oven gas. In 2008, the national coke production was 280 million tons, of which the output of independent coking plant accounted for 65%, and the output of mechanical coke reached over 95%. It is estimated that the annual remaining gas supply of independent coking plant is 28 billion Nm3, but most of the remaining gas has not been effectively utilized, so the comprehensive utilization of coke oven gas is imminent.
How to use the remaining gas reasonably and effectively is a very serious problem faced by coking plants of non steel enterprises. If such a large number of coke oven gas is discharged into the air, it will cause serious air pollution and waste valuable chemical resources. Coke oven gas, also known as coke oven gas, English name is coke oven gas (COG). Because there are many combustible components, it belongs to high calorific value gas. After high temperature carbonization of coal in the coking furnace, it also produces a kind of combustible gas while producing coke and tar products. Therefore, coke oven gas is a by-product of coking industry. Coke oven gas is a mixture, and its yield and composition are different due to different coking coal quality and coking process conditions. Generally, each ton of dry coal can produce coke oven gas of 300 ~ 350nm3 (standard state). Its main components are hydrogen (55% ~ 60%) and methane (23% ~ 27%), in addition, it contains a small amount of carbon monoxide (5% ~ 8%), unsaturated hydrocarbons above C2 (2% ~ 4%), carbon dioxide (1.5% ~ 3%), oxygen (0.3% ~ 0.8%), nitrogen (3% ~ 7%). Among them, hydrogen, methane, carbon monoxide and unsaturated hydrocarbons above C2 are combustible components, while carbon dioxide, nitrogen and oxygen are non combustible components.

Why should China develop coke oven gas to LNG?

In recent years, China has implemented the access system for coking industry, and the comprehensive utilization of coke oven gas has become the key to the survival and development of coking enterprises. At the same time, natural gas is becoming more and more popular as a clean energy. Many countries have listed liquefied natural gas as the preferred fuel. The proportion of natural gas in energy supply is increasing rapidly, especially liquefied natural gas, which is growing at a rate of about 12% every year, becoming one of the fastest growing energy sources in the world.
The growth rate of China’s natural gas production (including unconventional natural gas) is far from meeting the demand of natural gas consumption. In 2017, China’s natural gas import volume has reached 92 billion cubic meters, with an external dependency of nearly 39%. In this case, as long as the price can be accepted by the market, coke oven gas to natural gas will develop steadily. At the same time, the dependence of natural gas on foreign countries can be reduced by at least 1-2 percentage points.
For coking enterprises, coke oven gas to natural gas is undoubtedly a breakthrough under the pressure of environmental protection and survival. In the past, coke oven gas has been burned out in coking enterprises, which not only wastes energy, but also causes serious air pollution. Recycling it to produce natural gas is not only an environmental protection measure, but also has considerable economic benefits. At the same time, it also provides clean energy for the society, which is a good way to achieve one stroke. In terms of economic benefits, the price of coke oven gas is only about 0.5 yuan / m3. After purification, methanation and liquefaction, the cost is about 2 to 2.5 yuan / m3. In central and eastern China, where the price of natural valve station is higher, the cost of natural gas pipeline is much higher than this cost price. This means that in these areas, coke oven gas to natural gas has a strong market competitive advantage. It is reported that the investment yield of natural gas from coke oven gas is about 28% higher than that of methanol from coke oven gas. More Than This. The methane content of natural gas produced by coke oven gas can reach more than 99%, which is obviously better than that of natural gas produced by general pipeline.
In fact, China does have the advantage of vigorously developing coke oven gas to natural gas.

First, China is the world’s largest producer of coke and coke oven gas.

China’s coke producing areas are widely distributed. Except Tibet and Hainan, other provinces have coke production, with an annual output of more than 1.5 million tons. Shanxi, Hebei, Shaanxi, Shandong and Inner Mongolia all have an annual output of more than 30 million tons. There are more than 600 coke production enterprises in China, with an annual production capacity of more than 1 million tons, more than 200, covering 28 provinces, cities and autonomous regions. This is not the same as shale gas (mainly in Sichuan and Chongqing), coal-bed gas (mainly in Shanxi) and coal to gas (mainly in Xinjiang and Inner Mongolia). In 2016, China’s coke production was about 480 million tons, accounting for 67.44% of the total global coke production. In 2017, the coke output after capacity removal was 431.426 million tons, still ranking the first in the world. For each ton of coke produced, about 430 cubic meters of coke oven gas can be produced, half of which can be used for back burning and the other half can be used to produce natural gas.

Second, China’s coke oven gas to natural gas technology has made a breakthrough.

At present, there are two main technologies for producing natural gas from coke oven gas in China, namely, the methanation process represented by “coke oven gas methanation to produce natural gas” of Southwest Chemical Research and Design Institute Co., Ltd. and the separation process represented by “coke oven gas combined purification and separation to produce natural gas” of Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Among them, the methanation process can be further divided into two types: no carbon supplement methanation process and carbon supplement methanation process. As the volume fraction of methane in coke oven gas is only 23% – 27%, there will be surplus hydrogen in the process of methanation without carbon supplement, and the raw materials can not be fully utilized reasonably. Through carbon supplement of feed gas, the process makes full use of various effective components in coke oven gas, so that all the feed gas can be methanized, which can increase the production of natural gas by about 30%, which is conducive to the subsequent gas separation and low-temperature liquefaction, and is an ideal way to convert coke oven gas into natural gas.

Third, China’s coke oven gas to natural gas has begun to take shape.

China began to use coke oven gas to make natural gas in 2012. The first project undertaking enterprise is Inner Mongolia hengkun Chemical Co., Ltd. of Xinwen Mining Group, with a daily output of 360000 cubic meters. By 2016, China had nearly 30 coke oven gas to natural gas plants with a daily capacity of 12.6 million cubic meters. In the same year, the total number of natural gas plants in China was 172, with a daily capacity of 96.96 million cubic meters. In other words, the capacity of coke oven gas to natural gas has accounted for 13% of the total capacity of natural gas. In addition to conventional natural gas, coke oven gas has become the second largest source of natural gas in China.

Technology of LNG production from coke oven gas

The coke oven gas from the coking plant after pre purification still contains some tar, benzene, naphthalene, sulfide (H2S, thiophene, thioether, mercaptan, cos and CS2), etc. the presence of these impurities will have a serious impact on the normal operation of the methanation catalyst, which will not only reduce the activity of the catalyst, shorten the service life, affect the product quality, but also cause equipment corrosion And pipe plugging. After purification, the coke oven gas contains a lot of H2 and a small amount of CO, CO2, which can be converted into CH4 by methanation reaction to improve the product output. After methanation, the semi product gas still contains more H2, which can be separated at the same time of liquefaction to obtain product LNG.

Process flow of LNG from coke oven gas

Coke oven gas contains a certain amount of impurities, which need to be purified to meet the requirements of the process. The process production unit includes four sections: compression, desulfurization, synthesis and liquefaction. The total process flow of the whole plant is as follows:

Coke oven gas pretreatment

The coke oven gas from the coking plant contains a variety of impurities, especially benzene and Tsai, which are relatively high, about 3000 mg / nm; and 300 mg / nm, which will cause harm to the downstream purification and separation process and need to be removed.
Benzene, Tsai and tar were removed by adsorption. In other words, the sorbent is used to adsorb heavy components such as benzene, Tsai and tar at a lower pressure and temperature, and then desorb and regenerate at a high temperature and a low pressure to form the adsorption and regeneration cycle of the sorbent, so as to achieve the purpose of continuous gas separation. In this way, the subsequent catalyst can be protected, and the equipment such as pipes and coolers can be prevented from being blocked by Cai crystallization after boosting.

Hydrogen purification

At present, there are two widely used hydrogen separation technologies in industry, pressure swing adsorption and membrane separation.
Since PSA technology has the advantages of low investment, low operating cost, high purity, simple operation, flexibility, small environmental pollution and wide adaptability of raw material and gas source, it has been widely used in petrochemical industry, metallurgy, light industry, environmental protection and other fields since 1970s. PSA separation process has the advantages of simple operation, high degree of automation and no need for special materials. The most widely used adsorption separation technology is the separation and purification of industrial gas. The adsorption capacity of hydrogen on adsorbent is far lower than other common components such as CH2, N2, CO and CO2, so PSA technology is widely used in the field of hydrogen purification and recovery. In order to make the product hydrogen have high purity, PSA technology is used to purify hydrogen.

Methanation reaction

Methanation is a strong exothermic chemical reaction in which CO and CO2 react with hydrogen to form methane under the action of catalyst.
Methanation belongs to catalytic hydrogenation. Equation of catalytic hydrogenation:
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Generally, there are two kinds of methanation reactions in industrial production:
One is used in ammonia synthesis and hydrogen production plant. Under the action of catalyst, a small amount of carbon oxide (generally CO + CO2 < 0.7%) in the synthesis gas reacts with hydrogen to generate water and inert methane, so as to remove the influence of carbon oxide on the catalyst in the subsequent process.
The catalysts and processes used for methanation are mainly used to remove a small amount of carbon oxides (CO and CO2) remained in synthesis gas. Since the invention of nickel based catalysts for Methanation in 1902, most of the catalysts and processes used for Methanation in fertilizer production have been studied around such catalysts.
The other is Methanation in the synthetic natural gas process. The carbon oxide (CO + CO2) concentration in the feed gas is relatively high.
The research of syngas (methanation) using coal to syngas (high CO content) as raw material began in the 1940s. After the oil crisis in the 1970s, people began to pay attention to the research of syngas production using coal as raw material, so that the research of syngas entered a period of rapid development.
A technology for producing natural gas by methanation, which uses catalyst, can convert coal or biomass into natural gas.

Desulfurization of coke oven gas

There are many sulfides in coke oven gas, including H2S, cos, CS2, RSH, RSSR and c4h4s. These sulfides have strong toxicity to methanation catalyst. Therefore, the sulfides should be removed to 20200217031956 51575 - Why to make LNG from coke oven gas? before coke oven gas enters methanation reactor. Generally, in coal chemical projects, the process of wet desulfurization dry fine desulfurization is adopted for gases containing organic sulfur and complex inorganic sulfur. The main purpose of wet desulfurization is to reduce the cost of dry fine desulfurization (operation cost and desulfurizer cost).
Since the effective components CO and CO2 in coke oven gas will be subject to methanation reaction in the subsequent section, any technology that desulfurization process will cause CO2 and co consumption or removal is not applicable. Therefore, wet oxidation desulfurization process is selected for wet desulfurization. The advantages of this method are: only H2S and part of organic sulfur can be removed without CO2, hydrogen sulfide can be oxidized into elemental sulfur in the liquid phase and separated, and desulfurizer can be recycled and recycled, with low operation cost.

Due to the complex sulfur composition in coke oven gas, in order to ensure the desulfurization of raw coke oven gas, the key hydrogenation + ferromanganese desulfurizer + ZnO fine desulfurization process is adopted for dry desulfurization. Firstly, the iron key hydrogenation catalyst with low activity and gentle reaction is used for hydrogenation to avoid the catalyst bed temperature rising too fast due to the intense reaction. After the gas is hydrogenated, the cheap iron manganese desulfurizer with low sulfur capacity is used to remove the converted hydrogen sulfide; then the zinc oxide fine desulfurizer with high activity, high organic sulfur conversion rate, high sulfur capacity and high price is used to check, and at the same time, the partial Hydrogenation of unsaturated hydrocarbon to saturated hydrocarbon.

Refrigeration and liquefaction

The mixed refrigerant cycle is selected as the refrigeration process of LNG production. This technology was developed in the late 1960s on the basis of the cascade refrigeration liquefaction cycle. Its process is shown in the figure.

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Chart: mixed refrigerant liquefaction cycle

It uses a multi-component mixture as a refrigerant instead of a single component multi refrigerant in a cascade refrigeration liquefaction cycle, thus simplifying the process. It is characterized by simple equipment and low energy consumption, but the refrigerant needs to be specially prepared, which is suitable for large-scale natural gas liquefaction and gas component stability.

LNG storage

Large LNG stations generally use atmospheric pressure storage tanks to store LNG entering the storage tanks, which are low-pressure and high-density products. The storage and transportation efficiency of LNG products is high, and the refrigeration energy consumption is slightly large. However, the liquid is in the supercooled state after pressurized loading, and the gasification volume is small during the transportation process. The storage tank is much larger than the parent-child tank in the same appearance volume and floor area.
Considering the production capacity, sales and transportation, the storage tank in the plant area shall be able to meet the storage capacity of about 7-10 days at least. When filling the tank, a certain space shall be reserved as the medium for thermal expansion, and the tank shall not be filled.

Source: China Natural Gas Pipeline Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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