Subdue the "devil" and create miracles: let ultra-low concentration gas "turn evil back to good"

As a byproduct of coal mining, coal mine gas is produced throughout the entire coal mining process, posing a threat to coal mine safety production, polluting the air and causing a greenhouse effect. For hundreds of years, humans have tried various methods to subdue coal mine gas. We have developed appropriate development and utilization methods for high-concentration gas with a methane concentration greater than 30%, and low-concentration gas with a methane concentration greater than 7% and less than 30%; however, the development and utilization of ultra-low-concentration gas has long been a difficult problem for the coal mining industry, and it is also a difficult problem that industry insiders have been struggling to solve.

Methane gas processing (copyright image from the library, reprinting may cause copyright disputes)

01 What is ultra-low concentration gas?

According to the "Design Code for Coal Mine Gas Power Generation Project" (GB51134-2015), ultra-low concentration gas refers to coal mine gas with a methane concentration of less than 7% . This part of gas cannot be used to generate electricity directly. Its total amount is huge, accounting for more than 80% of China's coal mine gas.

The ultra-low concentration gas extracted through buried pipes in goafs generally has a methane concentration greater than 1% , and accounts for a small proportion overall; the other type is wind-exhaust gas , also known as lack of air gas, which refers to the wind-exhaust gas (also known as lack of air gas) produced in the ventilation of coal mine tunnels, and its methane concentration is less than 0.75%.

According to a report in China Energy News in November 2023 , the methane discharged into the atmosphere by coal mine ventilation in China each year is equivalent to the annual gas transmission volume of the West-East Gas Pipeline , and the greenhouse effect produced is equivalent to 200 million tons of carbon dioxide equivalent . Therefore, it is of great significance to make full use of the lack of wind gas.

West-East Gas Pipeline (copyright image from the gallery; reprinting may lead to copyright disputes)

02 The Difficulty of Subduing Demons

For ultra-low concentration gas with a methane concentration greater than 1% and less than 7%, the main utilization method is lean combustion , which specifically includes technologies such as porous media combustion, pulsating combustion and catalytic combustion .

The exhaust gas with methane concentration lower than 0.75% can be used as fuel and auxiliary fuel . Fuel is the main utilization method, firstly enrich the methane concentration of ultra-low concentration gas to more than 1% , and then utilize the heat energy through thermal countercurrent thermal storage oxidation and catalytic lean-burn gas turbine technology.

At present, although the thermal countercurrent oxidation technology and catalytic lean-burn gas turbine technology have been industrialized, there are still two major problems in realizing the fuel utilization of wind exhaust gas:

Enrichment problem : How to increase the methane concentration in ultra-low concentration gas and achieve ultra-low concentration methane enrichment.

Safety issues : In addition to methane, ultra-low concentration gas also contains a certain concentration of oxygen and nitrogen. When the methane concentration in ultra-low concentration gas is enriched to 5%~15%, it is easy to cause an explosion when it encounters an open flame. Therefore, in order to ensure that no safety accidents occur during the gas enrichment process, the use of ultra-low concentration gas must also consider the problem of air deoxygenation.

Coal mine gas explosion (copyright image from the gallery, reprinting may cause copyright disputes)

03 The Demon-Subduing Pestle—Common Techniques for Turning Waste into Treasures

At present, the main technologies for enriching methane from ultra-low concentration gas with a methane concentration below 0.75% include low-temperature liquefaction, adsorption, absorption, membrane separation, and hydrate method .

At present, pressure swing adsorption separation technology and low-temperature liquefaction separation technology are widely used in industry. Among them, the former has a more mature separation technology, a higher methane recovery rate, and more advantages in energy consumption and cost. It is currently the technology with the greatest potential and application value.

Pressure swing adsorption separation uses the different methane adsorption amounts of adsorbents at different temperatures to achieve gas separation. The key to its success is the selection of adsorbents . At present, the most widely used adsorbents include carbon-based adsorbents, zeolite molecular sieves, metal organic frameworks, etc. Among them, carbon-based adsorbents are divided into activated carbon, carbon molecular sieves, graphene and carbon nanotubes . Among them, activated carbon has become the most commonly used adsorbent in pressure swing adsorption separation processes due to its advantages such as wide range of raw materials, simple preparation process, developed pore structure, large specific surface area, and good mechanical and regeneration properties.

04 Outlook

Overall, the current ultra-low concentration gas utilization technology has achieved some breakthroughs, but it is still in the laboratory research stage and cannot effectively guide production practice. The reasons are:

First, the high adsorption pressure and high vacuum pressure swing adsorption process increases the risk of explosion and operating costs;

Second, the economy, adaptability and safety of the enrichment process still need to be improved.

In recent years, with the development of membrane separation technology , new membrane separation technology has been increasingly used in ultra-low concentration gas deoxygenation technology.

In the future, ultra-low concentration gas utilization:

On the one hand, we should continue to work on the research and development of adsorption materials with high selectivity, high capacity and high tolerance;

On the other hand, we also need to continue working hard to optimize the overall process and seek breakthroughs.

At dusk, the empty pond is full of music, and the poisonous dragon is subdued by meditation (Wang Wei, Tang Dynasty). The "poisonous dragon" of ultra-low concentration gas will eventually be conquered by us in the future!

References:

[1] Gao Pengfei. Current status and prospects of the utilization technology of air-deficient gas-saturated concentrate[J]. Mining Safety and Environmental Protection, 2017(003):044.

[2] Zhu Yan. Review and Prospect of Low-Concentration Gas Utilization Technology[J]. Energy Technology and Management, 2022(004):047.

[3] Sang Shuxun, Yuan Liang, Liu Shiqi, et al. Prospects of carbon neutral geological technology and its application in low-carbonization of coal[J]. Journal of China Coal Society, 2022, 47(4): 1430-1451.

[4] Li Shuohao, Gong Yuqing, Fu Shenguang, et al. Research progress in purification of methane from coal mine gas by pressure swing adsorption[J]. Chinese Science: Chemistry, 2023, 53(6):992-1007.

[5] Qu Peiran. New methane emission control policy forces the acceleration of low-concentration gas utilization[N]. China Energy News. 2023-11-20.

Author: Liu Hanbin, Senior Engineer, Shanxi Coal Geology Bureau

Reviewer: Li Yong, Professor, School of Earth Sciences and Surveying Engineering, China University of Mining and Technology (Beijing)

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.