(257f) The Experiment Study On Low Concentration Coal Bed Methane Enrichment by Proportion Pressure Swing Adsorption(PPSA)

Coal bed methane is abundant in China, with a total amount of about 36,000 billion m³ within 2,000 meters beneath the earth's surface. As a by-product, the amount of coal bed methane produced in coal mines in China is approximately 6 billion m³ per year at present, but only 0.3 to 0.4 billion m³ can be utilized as industrial material, whilst the major part is purged directly.

When the concentration of methane in coal bed methane drops below 30%, it gets close to the methane explosion range, which is 5 %-16 % at normal temperature and pressure. For the purpose of safety, it is required in the Chinese National Standard of ‘Safety Regulations in Coal Mine’ that coal bed methane with a concentration of methane lower than 30% should not be utilized in industrial processes. According to a statistical review, low concentration (<30%) coal bed methane accounts for ca. 2/3 of  the methane emissions in China. As a result, the low concentration of coal bed methane is one of the major reasons which hinders the large-scale utilization of coal bed methane in China.

On the other hand, methane is a typical type of greenhouse gas. The impact factors of methane for the green house effect and for the atmosphere ozone laye damage are 21 times and 7 times, respectively, as high that of carbon dioxide. Therefore, increasing the concentration of the low concentration coal bed methane becomes an important issue of utilization of coal bed methane.

PSA is a process of substantial interest in coal bed methane concentrating due to its low energy requirements and cost advantages. A majority of the studies have described the separation of coalmine methane by PSA. But these studies taken the high-concentration coal bed methane or the binary system of CH₄/N₂ as the research object. Few papers can be found on the concentrating of low concentration oxygen-bearing coal bed methane. Low concentration oxygen-bearing coal bed methane contains lots of oxygen, the conventional PSA methods with single adsorbent would usually make the methane concentration rather close to its explosion limit and lead to explosion risks in the adsorption process. This was the most difficulty in coal bed methane concentrating. In this paper, it took the oxygen-bearing coal bed methane with the methane concentration of 20% as the research object. And an experimental study was carried outfor the upgrade of the concentration of coal bed methane by PSA, aiming at a safer and more effective enrichment method for low concentration coal bed methane.

II    A New Safety Enrichment Method for Low Concentration Coal Bed Methane

In a PSA process, concentration distribution in the adsorber, vent gas and desorption gas were changed with different adsorbents used. If active carbon was used as an adsorbent alone, much methane would be adsorbed in PSA process, resulting in a decrease of methane concentration and an increase of oxygen concentration. Therefore, the mixture gas concentration would go into the explosion range and may explode in adsorber. If carbonaceous molecular sieve (CMS) was used as the adsorbent alone, there was no explosion risk in adsorption process due to the reduced oxygen concentration by CMS. But it would still have explosion risk in desorption process, because the concentration of desorption oxygen mixed with feed gas left in adsorber would be over the explosive limit.

Basing on the analysis presented above, the traditional approach of using single adsorbent could not ensure the security of low concentration coal bed methane enrichment in PSA process. So a new method should be brought forward for the process. The new safety method was put forward, referred to as Proportion Pressure Swing Adsorption (PPSA), for enrichment of low concentration coal bed methane, The new method uses the mixture of active carbon and CMS as the adsorbents. So methane and oxygen in low concentration coal bed methane could be adsorbed simultaneously, where methane was mostly adsorbed on active carbon and oxygen was mostly adsorbed by CMS. By this way, the concentration of CH₄ and O₂ could be controlled well and not over explosive limit both in adsorption and desorption processes.

The new safely enrichment method for low concentration coal bed methane (PPSA), could make the methane concentration away from the explosive limit in whole adsorption process and implement the safe enrichment of low concentration coal bed methane.

III   Experimental Results

A two adsorbers of PPSA experimental unit was used for low concentration coal bed methane separation. The separation process of coal bed methane gas was carried out at room temperature with adsorption pressure no more than 320 kPa abs., and desorption pressure about 25 kPa abs. The effects of adsorbent’s quality ratio, adsorption pressure, desorption pressure, adsorption time and methane concentration in feed gas were investigated.

1)The low concentration coal mine methane could be separated securely by PPSA with the help of mixxed adsorbents. In the mixxed adsorbents system of active carbon and CMS, the coal bed methane of 20% could safely enriched to higher than 30%, when the CMS/AC was 3.4. The concentration of O₂ in vent gas decreased when the ration of CMS5/AC1 increased. The concentration of O₂ in vent gas with the CMS5/AC1 ration of 3.4 was ca. 1% less than that with CMS5/AC1 of 2.6 on the same adsorption time. And the concentration of O₂ in vent gas could be reduced to 12.8%, when CMS5/AC1 was 3.4. It was because that the increase of CMS mass resulted in the increase of adsorption capacity for oxygen in adsorber. So the concentration of O₂ in vent gas decreased as the ration of CMS5/AC1 increased.

The concentration of O₂ in vent gas could be less than 12%, if the ratio of CMS5/AC1 was further increased, or adsorption time was further extend. According to Coward triangle, when the oxygen concentration was less than 12%, it could not explode in vent gas and the PPSA process safety was guaranteed.

2)Under this experiment condition, the higher the adsorption pressure was, the better the separation effect was. When the highest adsorption pressure was 320 kPa, the methane and oxygen concentration of vent gas could decrease to below 3% and 12% respectivlely.

And the lower the desorption pressure was, the better the separation effect was. The methane concentration showed an increase of 23%, when desorption pressure increased from 21 kPa to 67 kPa at adsorption pressure of 190 kPa.

3)The experimental results showed that the methane concentration in desorption gas was increased with the adsorption time expanding, but the recovery of methane was reduced. So when the adsorption time expanded in order to increase the methane concentration, the adsorber could be lengthened to maintain the recovery of methane. Further more, at same adsorption time, there was an optimum ratio between product flow rate and feed flow rate, which could make the concentration of methane maximum.

4)The methane concentration in desorption gas and vent gas both increased with the methane concentration of feed gas increasing. And the change of methane concentration in feed gas had great influence on components of desorption gas and vent gas. For example, when the methane concentration in feed gas increased from 16.1% to 20%, he methane concentration in desorption gas would increased from 20.9% to 30.4%. However, the change of methane concentration in feed gas had little influence on oxygen concentration in desorption gas and vent gas. According on the Safety Analysis, it was found that the methane concentration could be enriched upon 30% safely by PPSA, and it would not have explosion risk in adsorption process, when the methane concentration in feed gas was more than 20%. Therefore the new proportion pressure swing adsorption (PPSA) method could implement the safe enrichment for low concentration coal bed methane.