|Structure of the experimental gasifer. Click to enlarge.|
Researchers in China are reporting the development and successful field-testing of an improved method for an underground coal gasification (UCG) process that could lead to more economical operation. The researchers describe their work in a paper in the 18 July issue of the journal Energy & Fuels.
The new process, designed for thin, high-angle coal seams, consists of blind-hole electric ignition of an underground gasifier, a forward and backward firepower seepage method, and gasification of a small shaftlike gasifier.
In general, underground coal gasification (UCG) involves injecting steam and air or oxygen (O2) into a coal seam from a surface well. The injected gases react with coal to form a syngas that is brought to the surface in a production well to be cleaned and used as a fuel or chemical feedstock.
As the coal burns, the roof of the seam collapses, resulting in lateral growth of the gasifier in the seam. When the quality of the product gas declines, the operator re-ignites the seam at a new location. When the coal in a particular seam is exhausted, the operator drills new injection and production wells, and repeats the process.
There are two approaches to UCG: shaft and shaftless gasification. The former requires the underground engineering of the gasifier, while the second uses “pushing-through” techniques to build the gasification galleries in the coal seam.
China tried shaftless UCG in the 1960s and the 1980s, but with ineffective results. The Former Soviet Union experimented with a variety of techniques, including electric, hydraulic fracturing, and oxygen-enriched techniques to build gasification galleries. This, according to researchers Lanhe Yang, Shuqin Liu, Li Yu, and Jie Liang, resulted in the consumption of large amounts of energy, as well as high cost and low profits.
These techniques also made it notoriously difficult to control the course of the pushing-through process as it proceeds from one end of the gasifier to the other. Considering the advantages and disadvantages of the methods mentioned above, this paper develops a kind of pushing-through technique with forward and backward firepower seepage and successfully tests this method in the thin high-angle coal seams of Mazhuang Mine, Xuzhou Mining Industry Group, Jiangsu Province, China.
The researchers used a two-phase gasification technique, first blasting air through the inlet hole at a rate of 270-310 m3/h followed by steam.
Pushing through the coal seam to create the gasifier can use either forward combustion, in which the forefront of the flame of the burning coal expands in the same direction as that of the air current, or backward combustion, in which the forefront of the flame expands in the opposite direction to that of the air current. Forward combustion consumes all the combustible substances, but backward combustion passes through the combustible substances and only consumes part of them.
The important difference between the two processes is that while backward combustion tends to lead to a narrow gallery with a fixed diameter, forward combustion expands with a relatively wider forefront. Thus, forward combustion can enlarge the fire source, while backward combustion can form a relatively regular gallery. As a result, the forward and backward interchanging combustion penetration can form a regular gallery with a comparatively wider diameter. In the field test, this interchange pushing-through method was used.
Under the experimental conditions, the two-phase gasification produces underground water gas with a medium-high heating value, in which H2 is over 48% and CO and CH4 are over 13% and 7%, respectively, with an average heating value of 12.00 MJ/m3.
“Experimental Study of Shaftless Underground Gasification in Thin High-Angle Coal Seams”; Lanhe Yang, Shuqin Liu, Li Yu, and Jie Liang; Energy Fuels, ASAP Article 10.1021/ef700231p S0887-0624(70)00231-1