In situ gasification of steeply dipping coal beds (UCG-SDB) has significant advantages over the more conventional horizontal UCG. In fact, the UCG-SDB process appears to be both technically and operationally competitive with surface gasifiers. The results of the Rawlins UCG-SDB field test program suggest that the process can compete with program suggest that the process can compete with more conventional sources of synthesis gas on an economic basis. The SDB process mechanism has several advantages over the horizontal process and performs in a fashion similar to surface performs in a fashion similar to surface packed-bed reactors. The oxygen requirements for the packed-bed reactors. The oxygen requirements for the process are quite low and the degree of process process are quite low and the degree of process control observed at Rawlins is very attractive.
In the last decade, there has been considerable activity in the development of technology related to underground coal gasification in the United States. Much of the work was funded by the Department of Energy and its predecessor, ERDA. There have also been field tests performed by industrial concerns, individually and collectively. All of the federally funded and much of the privately funded work is documented in the open privately funded work is documented in the open literature. The bulk of the activity has been directed towards gasification of horizontal seams of low-rank coal and lignites. In 1977, Gulf Research and Development Company was awarded a contract by DOE to demonstrate underground coal gasification in a steeply dipping coal seam in the United States. The site selected for the field test was in the Knobs area 8 miles west of Rawlins, Wyoming. The second test in the two-test program was completed in the fall of 1981. Both program was completed in the fall of 1981. Both tests achieved their defined objectives. The results of the program suggest that the UCG process is significantly different in steeply dipping process is significantly different in steeply dipping seams as compared to horizontal seams.
UCG process chemistry has been studied extensively utilizing both physical and mathematical simulations (1). The primary reactions include:
Oxidation: C + O2 = CO2 + 94.05 kcal/mole 2C + O2 = 2CO + 26.42 kcal/mole
Gasification: C + H2O = CO + H2 - 31.4 kcal/mole C + CO2 = 2CO - 41.2 kcal/mole
Pyrolysis: coal + heat = char + tar + Pyrolysis: coal + heat = char + tar + water + gases
Secondary reactions involving gasification products include: products include: Water gas shift: CO + H2O = CO2 + H2 + 9.8 kcal/mole
Methanation: C + 2H2 = CH4 + 17.9 kcal/mole CO + 3H2 = CH4 + H2O + 59.0 kcal/mole
During the UCG process both kinetic and thermodynamic effects are significant. Residence times and temperature gradients in the reactor will determine the degree to which these effects will be significant for a given test site.
The horizontal UCC process using reverse burn linking is shown in Figure 1. The process consists of drilling two wells into a horizontal coal seam, establishing a link zone between the wells, igniting the seam, and gasifying the coal by injection of oxygen or air into one well and removing the gasification products from the second well. As the process progresses, the reaction zone grows to the roof and laterally as well as along the link zone between the wells. The oxidation zone is continually growing, while the reduction and pyrolysis zones remain constant or diminish in size.
UCG in steeply dipping beds (UCG-SDB) is depicted in Figure 2. Again, the reaction zone grows to the roof and laterally as well as along the link zone between the wells. However, the primary mechanism of growth is the weakening of primary mechanism of growth is the weakening of the coal above the injection well and its falling into the bottom of the reactor forming a firepit.
