UCG trials in Russia began in 1937 and by 1939 the Soviets had successfully begun operating a UCG plant in the Ukraine. This was shut down during World War but right after the war the UCG program was restarted, culminating in the operation of fourteen industrial scale UCG plants by the end of the 1960s. The main projects were:
Gorlovka, Ukraine - early work in a steeply dipping seam tested the advantages of using an oxygen enriched oxidant compared with air;
Lisichansk, Ukraine Donbass coalfield. Bituminous coal with 6-16% ash, and thin (0.4-1.5 m) seams down to 400m, some dipping steeply. The early trials were in the shallow part of the seam;
Yuzhno-Abinsk, the Kuzbass coalfield. Bituminous coal with 4-10% ash, and up to twenty-three seams 2-9 m thick, dipping steeply;
Podmoskova, Moscow basin. Lignite, with 27-60% ash and 20-30% water. It lay horizontally in seams 2-4 m thick and 40-60 m deep;
Angren, Uzbekistan. The coal is lignite, with 11% ash and 30% water. Seams dip at 5-15â£ in a seam that is 4-24 m thick, and at depths of 110-250 m. The plant was restarted in 1961 and 100MW of electrical power is still generated from syngas out of the 600MW that the plant produces.
Many of these projects were conducted in thin, low quality seams which contributed to the low gas quality produced. The data on the test and projects is limited but it is known that experiments into air and oxygen injection rates were made as well as heating the input air by passing it through previously gasified panels. The Lisichansk project undertook important design work in gasifying steeply dipping seams.
A number of UCG trials have been undertaken in Europe, below are the most significant.
. 1912 Co. Durham UK First Test
. 1948 Bois-la Dame, Belgium
. 1950/60 Early European Trials
. 1955 Newman Spinney, UK National Coal Board
. 1980 European Studies and First Trial
. Mid to late 1990 El Tremedal, Spain European Trial
. DTI UCG Initiative (1999-2005)
. Firth of Forth Feasibility Study, Scotland
In the UK trials involved several innovations, including. In-seam linkages through drilling from a coal face using several boreholes per panel
Tests using a single in seam borehole with an air pipe produced better results with gas heat values at just below 4MJ/m3. A small commercial project of about 5MW was planned, using in-seam holes about 6m apart and 100m deep as individual reactors, but this was not successful and the project was eventually abandoned in 1959.
In France, two main trials took place between 1979 and 1985. The trials used vertical wells and focused on the options to create the physical link between the boreholes to aid the gasification process. A combination of electro- linkages, hydro-fracturing and reverse circulation produced mixed results, partially due to the design of electrodes and the physical characteristics of the anthracitic coal. The UCG industry now recognises that anthracite is not an ideal coal to use these linking techniques due to its very low volatile matter content.
El Tremedal, Spain, a 7 year trial, which began in 1991, demonstrated the technical feasibility of carrying out UCG at intermediate depths between 500m and 700m using deviated in-seam drilling and the CRIP technology. Three exploratory boreholes established the characteristics of the target seam, which is 2 m to 5 m thick, dipping at 30â£ at a depth of some 530-580 m. The coal was high sulphur (7.6%) sub-bituminous with 22% moisture, 27.5% volatiles and 14% ash with a high heating value of 18 MJ/kg.
The linkage between the injection and production wells was achieved by using directional drilling techniques. The production well was drilled to within 1m of the injection well and connection achieved by using high- pressure water injection.
The trial employed coiled tubing to transport the gasification agents (in this case, oxygen and water) and to position the injection head inside the pressurised in-seam well. The tubing was fully retractable onto a drum at the surface, and the coiled tubing equipment, which is commonly used in the oil and gas industry, was supplied with a wellhead injection assembly, pressure seals and a manifold for connecting the injection gas mixtures to the tubing. The well was cased and there was an in-seam liner which burned through when the CRIP was ignited.
Ignition of the gasifier was started 4 m from the end of the production well by the introduction of a pyrophoric compound, tri-ethyl borane, to ignite methane in a burner located at the end of the coiled tubing. Once the liner was burned through to expose the coal surface, the gasification agents were introduced.
The product syngas was generated as the coal surrounding the ignition point was gasified, creating a caved zone (Figure 1) which post-burn investigations indicated extended horizontally to at least several times the seam thickness. Over a period of days, as the consumption of coal created a cavity, the gasification rate/efficiency declined, and the ignition point was retracted so that fresh coal was accessed. During the trial the coiled tubing was retracted progressively several times and three ignitions made.
The tests took place during July and October 1997, and involved one run lasting nine days, and another lasting four days. The dry product gas consisted roughly of 40% CO2, 12% CO, 25% H, 13% CH and 8% H2S. The test established the potential viability of in-seam drilling, with final linking by high pressure water injection, and of the use of the CRIP in the relatively deep, thin coals seams that are typical of European coal fields.
There have been over thirty trials in the US divided into four sections
The first series 1973 - 1976 were commissioned to evaluate the methods being used in Russia. The projects were carefully researched and extensively monitored and vertical wells formed the basis of the process, usually drilled quite close together.
The second series, conducted by Lawrence Livermore National Laboratory (LLNL) and the US Dept of Energy facility at Hoe Creek.
These tests provided the basis for a number of developments which included the validation of the CRIP (Controlled Retractable Ignition Process) scheme, the validation of subsidence models, and the first oxygen/steam injection experiments in the USA. A wide range of instruments and monitoring tools were used, including the use of chemical (gas) tracers.
The tests also gave rise to the first recognition of possible groundwater hazards. In Hoe Creek I, where explosive fracturing was used, the test continued for 11 days with using air injection. Approximately 7% of the gas was lost into the rock formation. Hoe Creek II used reverse combustion, and gasification lasted for 43 days. Water influx significantly lowered the gas quality. To decrease the water inflow the operating pressure in the burn zone was increased. This resulted in a significant amount of gas (approximately 20%) being lost into the rock formation. Much of this loss is thought to have occurred when the burn zone collapsed, exposing the overlying Felix No 1 coal seam, which was at a lower hydrostatic pressure. This event which happened in strata would now be classed as having high environmental risk (and would therefore not be considered for development of any kind), which gave UCG an unjustified reputation as a potential hazard.
The third series conducted at Rawlins, in Wyoming, using steeply dipping seams and was successful in producing high quality gas, with lower oxygen demand than was the case in horizontal strata.
The fourth series (Centralia and Rocky Mountain trials) occurred from 1984 -1989 was a direct comparison between the CRIP method and the enhanced Soviet (vertical borehole) method. Rocky Mountain Trial - 14,000 tons of coal gasified in 93 days.
It should be noted that a number of trials have occurred in China, mainly using man-made underground galleries. A UCG research centre has also been created at the China University of Mining and Technology in Beijing.