Engineering

At the end of the coal gasification test in Thulin, three boreholes were drilled to provide information on the operating features of the gas generator by analysis of cores and cuttings. Petrological and mineralogical studies of the rocks adjacent to the coal seam provide information on the mineralogical and structural transformations that took place. In particular, analysis of the magmatic rocks indicates that the highest temperatures attained were above 1100°C, whereas other rocks underwent rubefaction at tempertures of only 60-70°C. Petrological studies of the coaly organic matter remaining within the samples show the presence of coke and anthracitic structures. The studies indicate that reaction temperatures of at least 900°C were attained in some locations, and over 1000°C (possibly 1100°C) in others. The enrichment in 13C and D of the coaly organic matter remaining in the cuttings enables zones to be located where combustion and gasification reactions progressed furthest. The information obtained has enabled assessments to be made of the extent of gasification and transformations undergone in each drilled area. The results indicate that steam gasification and methanation processes could predominate over pyrolysis.

The gasifier developed at Thulin within the framework of the Belgo-German project involving underground coal gasification is investigated using mathematical models. These models are based on the simultaneous resolution of the different thermodynamic equilibria of the ...

During the underground coal gasification test at Thulin, heavy water was injected into the gasifier. The increase of the deuterium content in the methane indicates that a part of the methane produced comes from the reduction of injected water and not only from pyrolysis of coal.

During the underground coal gasification (UCG) experiments at Alcorisa, Spain, a series of helium tracer tests were carried out to follow the underground cavity growth. The volume of the cavity increases progressively with the cumulated quantity of oxygen injected. Models ...

During the underground coal gasification experiments at Thulin, a series of tracer tests using 133xenon or helium as injected material were effected. These tests illustrated the hydrodynamics of the flow profile inside the underground reactor during the main ...

Underground coal gasification is presently a powerful means to valorize low grade coal resource either at low or great depth, which cannot be worked by traditional mining. The Belgo-german underground gas reactor project at Thulun (Belgium) was studied through thermodynamics of gasification reactions and the distribution of stable isotopes of hydrogen (H-D), carbon ( 12 C-13 C) and oxygen ( 16 O-18 O) in of the off-gases. The respective contribution of coal distillation and gasification of the residual coal by gasifying fluids was determined through these informations, as was the temperature in each zone of the reactor. Laboratory experiments proved that 13 C isotope measurements in the CO/CO 2 system and

The "El Tremedal" Underground Coal Gasification (UCG) Test at great depth (≈ 600 metres) is scheduled to be conducted in June 1997 and is located near Alcorisa, province of Teruel, Spain. The program is jointly sponsored by Spanish, Belgium and UK organisations, and the European Commission in the framework of the THERMIE programme. The main objective of the project is to demonstrate the technical feasibility of UCG at an intermediate depth of about 600 metres – a significant increase in depth over that of previous successful USA trials of UCG by in-seam drilling.

The "El Tremedal" Underground Coal Gasification (UCG) trial sponsored by Belgian, Spanish and United Kingdom government organisations and the European Community has conducted two gasification phases during the summer-autumn of 1997, of nine and five days duration respectively. A gas of good quality has been obtained on both occasions. During the active gasification phases, which lasted in total 12.1 days, an estimated 237.2 tonnes of coal moisture-ash-free were affected and an average power of 2.64 MW based on the lower calorific value of the product gas was developed underground. The test utilised oxygen and nitrogen as the injection reactants (no steam injection). Access to the 2-3 metres sub-bituminous coal seam situated at an average depth of 560 metres was provided by an in-seam deviated well drilled close to the bottom of the 29 degrees dipping seam. A vertical well was used for the exhaust of the gasification products and the production counter-pressure was maintained in near equilibrium with the underground hydrostatic head (50-54 bars). Three Controlled Retraction Ignition Point (CRIP) manoeuvres were achieved. Analysis of the raw process data was conducted to calculate mass and energy balances, and to determine influences of process conditions on gas composition, shift and methanation equilibrium, water influx and oxygen/coal conversion efficiencies. ABSTRACT The "El Tremedal" Underground Coal Gasification (UCG) trial sponsored by Belgian, Spanish and United Kingdom government organisations and the European Community has conducted two gasification phases during the summer-autumn of 1997, of nine and five days duration respectively. A gas of good quality has been obtained on both occasions.

The "El Tremedal" Underground Coal Gasification (UCG) experiment at great depth was conducted in two phases during the summer-autumn of 1997: (i) from July 21 to July 29 and (ii) from October 1 to October 5. During each phase, a series of Helium injections were realised. These tracer tests using the stimuli-response technique were conducted in order to determine the underground cavity development from the reactor mean residence time. It was shown that the reactor volume increased proportionally to the cumulated quantity of oxygen injected until reaching a maximum maintained afterwards. Based on our previous experience of gasification phases at great depth in Belgium, different models were adjusted to the experimental results. The model presenting the best fitting is the series of stirred tanks exchanging matter with an adjacent porous zone. Fitting results indicated an important dispersion in the flowing fluid and a preponderance of the flowing fluid volume over the porous zone volume.

Underground coal gasification (UCG) or In-situ coal gasification (ISCG) is the gasification of coal in-situ, which involves drilling boreholes into the coal and injecting water/air or water/oxygen mixtures. It combines an extraction (mining) process and a conversion (gasification) process into one step, producing a high-quality, affordable synthetic gas, which can be used for power generation, or manufacture liquid fuels, synthetic natural gas and industrial chemicals. Still in the early stage of commercialisation, UCG is poised to become a future major contributor to the energy mix in countries around the world.

Belgium has a long history of gathering and using gas in association with coal mining operations. In the Southern coal basin (Wallonian coal basin) of Belgium, methane was intensively gathered from gassy seams during coal mining in order to prevent firedamp hazards. In deep coal fields (up to 1450 metre depth in the deepest colliery), gas content of more than 20 m 3 per tonne of coal in place were frequently recorded. Early in the fifties, an important network of gas collection and transport was realised allowing the valorisation of this gas in surrounding factories. Mid-fifties, this network was able to valorise 0.3 millions m 3 of gas (at 35.6 MJ/m 3) per day. In total, during ten years of mining activities, the Southern coal basin of Belgium has produced about 2 billions (US) m 3 of coal bed gas with a proportion of 40 % (0.8 billions m 3) gathered and valorised in surrounding factories. The remaining 60 % were lost in the mine ventilation. Afterwards, an additional 0.5 billions m 3 was gathered from abandoned mines and valorised. The decline of the coal bed gas production that occurred at this moment was due to economical reasons related to coal mining activities in the Southern coal basin of Belgium (nearly all collieries were closed during the sixties in this coal basin) and not due to the depletion of the coal bed gas reservoir. Based on the important experience gained during these years and the new development of the Coal Bed Methane (CBM) industry in the United States, the potential of areas away from previous mining works are presently re-evaluated. A description of the coal basin structure is given with the delineation of potential areas for CBM production. Two targeted areas (≈ 75 km 2) with coal resources at depths estimated between 700 and 1250 meters have the potential for a plateau of production of approximately one million m 3 of gas per day. Potential synergies existing between CO 2 Enhanced Coal Bed Methane (ECBM) production combined with CO 2 sequestration and two zero-CO 2-emission industrial plants (Ammonia and CO 2 power plants) are also presented and discussed in the paper.

The Thulin bituminous coal deposit at 870m depth has been opened by 4 vertical wells (distance 35m). From one of these wells an in-seam drainhole has been drilled toward the production well and a perfect link could be established between both wells. After initiating self-ignition, enlarging the reacting volume and bringing the reactive area nearer to the production well, the injected flow and its oxygen content were gradually raised and then changed into an oxygen-foamy water mixture. A quick and considerable decrease of the permeability was observed at the very beginning. The injection pressure rose to 190bar and the pressure at the production well was kept most of the time at 40bar. We found that mainly due to the high overburden pressure, the in-seam drainhole collapsed and that consequently the following processes were conducted in filtration mode. Manipulation on backpressure and enlargement of the cavity by burning at the production well bottom influenced the permeability. The product gas was characterized by high CO 2-(40-50%) and CH 4-contents (20-25%). This gas composition is related to the high pressure and to the devolatilization process. As the power of the gasifier remained low, the surface plants were adapted in order to inject a higher flow of oxygen-foamy water mixture under high pressure (280bar), resulting in the development of a process with a product gas containing: H 2 : 14%-CO: 24%-CH 4 : 17%-CO 2 : 24%-0 2 : 1%-N 2 : 20%. The maximum recovered power reached about 1 megawatt. Then, the heating value of the recovered gas decreased and its oxygen content increased, due to the development of a bypass bringing directly oxygen near by the recovery well. The oxygen injection was therefore stopped one month later. Scientific trials, in April and May, have proved first, the water foam effectively reacted with the coal or char, and second, that the flow-acceptance of the underground reactor was much higher after flow reversal. Dismantling of the well equipment indicated that the bottom of the injection well suffered serious damage while the recovery well was still in excellent condition.

UCG at great depth makes the collection of data for interpretation and control of the process unusually difficult. We have installed a data acquisition system with more than 200 measuring points which fulfils following requirements: • To respect the rules of safety • To get a high level of reliability, which implies an important redundancy • To organize the data storage in a way which makes future treatment easy • To give immediately a good overview of the process including calculated parameters like equilibrium temperature

The paper summarizes first the technology review of the main generic categories of module configurations currently used worldwide in UCG. The paper develops the guidelines for the future commercialization of UCG. The following best practice guidelines are covered: (i) the site selection; (ii) the gasification operation control; (iii) the instrumentation and monitoring strategy; (iv) the commercial deployment strategy; (v) the site decommissioning; and vi) the environmental impact mitigation. Key results of the technology review and primary conclusions are included.

The gasifier developed at Thulin within the framework of the Belgo-German project of underground coal gasification is investigated using mathematical models. These models are based on the simultaneous resolution of the different thermodynamical equilibria of the gasification reactions and on the relative distribution of three isotopes D, 13 C, 18 O in the gases produced. The contribution of distillation of coal, on the one hand, and gasification of residual char by gasifying agents, on the other, is calculated; the temperature in each zone of the reactor is also evaluated.

The decision about carrying out an experiment of coal underground gasification on the Thulin site has been taken in April 1979. After two years of unsuccessful attempts with a view to realize a linking channel between wells using the reverse-combustion technique, the research team was convinced that this technique was unusable at great depth. Therefore, the decision was taken to execute this connection by a mechanical way, using the technique of directional drilled holes. The linking between both well has been completed in January 1986. The proper gasification experiment has begun in October 1986 and was achieved in April 1987. Nine years of activity on the Thulin site have allowed to make great strides in the techniques which are necessary for the implementing of the coal underground gasification at great depth. We can quote: • A better knowledge of the rocks mechanics and fluid mechanics in the deep deposits, • The use of deviated wells with short or medium bending radius for the gasifier preparation, • The development of well equipment and the use of suitable alloys, • The utilization of fluids under high pressure, and specially, of a gasifying agent made of an oxygen-foamy water mixture. The analysis of the results of the experiment, which took place in Thulin from October 1986 until April 1987 shows up the development possibility of a new underground gasification method which can be applied to the coal deposits at great depth. This method should allow t produce a gas with a methane content higher than those of all the gases which can be now produced by gasification of the mined coal.

Cavity size resulting from UCG has a primordial influence on the process cost price. Global cavity growth modeling is a complex problem, which requires considerable expertise in such different technical areas as rock mechanics, fluid flow, heat and mass transfer, chemistry, hydrology. Global model is made up of several submodels. The first submodel deals with fluid flow. It shows paths taken by injected fluids to the coal face, paths taken by gas from the coal face to the recovery place and computes flow rates of injected fluid meeting the coal face and of recovered gas. The second submodel, combustion submodel, knowing the rate flow of fluid meeting the coal face by the first submodel gives the converted char amount and the rate flow of produced gas. The third submodel, collapse submodel, studies the roof behaviour. Due to coal conversion and combustion heat, rubbles and voids are created and do not support the roof anymore. The so created overburden stress has an important effect on the cavity growth. Since, at great depth, coal is almost not permeable, flow rate of injected flow meeting coal face is low and thus also flow rate of produced gas. Therefore, UCG at great depth depends almost entirely on results of the first model. In this first step, we only consider the fluid flow submodel without taking into account the other submodels. It allows us to predict the influence of such parameters as injection and recovery pressure, injected fluid rate flow, fluid characteristics, well pattern, medium characteristics on the cavity growth. In the following sections, fluid flow in porous media submodel and numerical methods used to solve model equations are introduced. First results are then discussed.

Underground coal gasification (UCG) is a promising unconventional recovery technique for coal deposits, which are too deep, too high in ash or too remote for exploitation by conventional coal mining method. At the difference of surface gasifiers, where the reactor geometry is built in function of the process specifications, the UCG cavity growth behaviour is a consequence of the natural behaviour of the underground reactor: roof collapse, flow distribution through virgin coal, ash, slag, char and rock rubbles, wall recession governed by mass and heat transfer phenomena, etc… Accordingly, we can imagine that UCG cavity growth process is extremely complex involving a variety of chemical and physical phenomena that cut across a number of technical areas. The characterization of the mechanisms affecting cavity growth is largely based on the results of field test experimentations supported by modelling efforts. The approach taken in this work is to pass the most important of them in review. The size and the shape of the cavity are also important economic parameters for underground coal gasification. The so-called " sweep efficiency " of the gasifier is crucial since the drilling and surface piping costs dominate the gasification cost. Generally, the main objective leads to maximize the amount of coal recovered per hole drilled. The importance of this problem increases with the depth and the coal seam thickness. A recent economic study (1) has shown that 30,000 m 3 of coal (thin seam +/-1.5 m) should be gasified per pair of wells at a depth of 1000 m to produce viable electricity.