Coal Gasification Experiments Research Articles

Proposal of a new soot quantification method and investigation of soot formation behavior in coal gasification

Coal gasification is one of the key technologies for utilization of coal. Coal gasification mainly consists of coal pyrolysis, char gasification, and gas-phase reactions. Soot is made up of fine solid carbon particles formed by volatile matter decomposition and has lower gasification reactivity than char. Since the solid products of coal gasification contain both char and soot, it is difficult to quantify the amount of soot produced by this process. In this study, a novel soot and char quantification method utilizing a laser diffraction particle size analyzer is proposed. Coal gasification experiments were performed using a pressurized drop tube furnace (PDTF). The soot and char yields were quantified by the new method. During CO2 gasification, char was selectively gasified and the amount of soot barely decreased. Hence, the carbon conversion initially increased, but remained around 0.8 even at 1673K. In the case of other gasifying agents, O2 or H2O, different behavior was found. During O2 gasification, the soot yield considerably decreased as the O2 concentration increased. The main reason was not soot gasification, as presumed, but rather the reaction of tar components with O2. In the H2O gasification, the soot yield slightly decreased as the H2O concentration increased.

Research on the surface movement rules and prediction method of underground coal gasification

With the continuous development of the technology of underground coal gasification (UGC), it gradually moves into commercial development. However, there is limited measured data and related research on the overlying strata and surface movement rule caused by underground coal gasification, which has severely restricted the further application of underground coal gasification. In order to solve the problem of surface subsidence prediction of underground coal gasification, this numerical simulation software, FLAC3D, is adopted to study the differences of overlying strata and the surface movement rule of UGC strip mining (UCG mining) and strip mining, and the differences of prediction parameters under different recovery ratios. The model is established according to the geological and mining conditions of the underground coal gasification experiment area in China. The research results show that overlying strata and surface movement rules of UCG mining and strip mining are similar. So, the surface subsidence prediction of UCG mining can refer the prediction method of strip mining. Meanwhile, the difference values of surface movement extrema under different recovery ratios are less than 5 mm and the difference values of surface subsidence boundary are not more than 10 m. Therefore, the surface subsidence prediction parameters of UCG mining can be chosen as the corresponding prediction parameters of strip mining. Finally, the surface subsidence prediction method of UCG mining is proposed based on the above research results and prediction software is compiled for engineering application. Research results have important theoretical and practical significance for promoting the application of underground coal gasification.

Underground coal gasification (UCG): An analysis of gas diffusion and sorption phenomena

This article describes the results of measurements of the gas transport phenomena in coal and strata samples. The samples were taken from the surroundings of an underground georeactor in the “Barbara” Experimental Mine before and after an in situ underground coal gasification experiment. Measurements of the coal and strata samples indicate that the UCG process influences the porosity and structure of the coal seam and the strata surrounding the underground georeactor. Determining parameters such as the porosity, intrusion volume and diffusion velocity of the gases is crucial to assess the potential for environmental pollution during the UCG process. Moreover, measuring UCG gas transport in the strata and the post-process coal may provide useful information on how gas migrates into the surroundings of the georeactor.The analysis of the sample properties presented in this study included the total amount of desorbed product gases and the comparison of adsorption ability of the strata surroundings. The conducted analyses estimated the amount of the absorbed gases and defined the gases’ potential transport paths in the georeactor and the surroundings.

Analysis and assessment of a critical event during an underground coal gasification experiment

The main goal of the study presented in this paper was to analyse the mechanisms affecting an Underground Coal Gasification (UCG) process and to identify possible deviations of the system from normal work to limit, or even avoid, losses. The UCG process is one of the most innovative technologies connected with the exploitation of coal deposits that are currently being tested and developed all around the world. It allows the conversion of a coal seam into gas under in situ conditions of high temperature with the use of gasifying agents such as air, oxygen, steam or with a mixture of them.The paper presents the results of the analysis and assessment of a critical event during the process: a dangerous gas accumulation that occurred during an underground coal gasification experiment in the Experimental Mine “Barbara” of the Central Mining Institute (Poland). The UCG experiment using the shaft method is described, together with its monitoring system and the problems that appeared during the process. The application of the Fault Tree Methodology allowed the establishment of the main factors that may lead to the explosion and to present possible scenarios of its occurrence.Moreover, calculations were carried out to evaluate the risk level of explosion for the gas mixture and the minimum level of oxygen in the mixture that is necessary to initiate an explosion. These calculations were based on a modification of the formula proposed by Le Chatelier. During the course of the underground experiment, original information of the process behaviour has been acquired that can be used in the preparation of other UCG experiments in operational mines to guarantee the safety and the stability of the process.

Chemical and toxicological evaluation of underground coal gasification (UCG) effluents. The coal rank effect

The effect of coal rank on the composition and toxicity of water effluents resulting from two underground coal gasification experiments with distinct coal samples (lignite and hard coal) was investigated. A broad range of organic and inorganic parameters was determined in the sampled condensates. The physicochemical tests were supplemented by toxicity bioassays based on the luminescent bacteria Vibrio fischeri as the test organism. The principal component analysis and Pearson correlation analysis were adopted to assist in the interpretation of the raw experimental data, and the multiple regression statistical method was subsequently employed to enable predictions of the toxicity based on the values of the selected parameters. Significant differences in the qualitative and quantitative description of the contamination profiles were identified for both types of coal under study. Independent of the coal rank, the most characteristic organic components of the studied condensates were phenols, naphthalene and benzene. In the inorganic array, ammonia, sulphates and selected heavy metals and metalloids were identified as the dominant constituents. Except for benzene with its alkyl homologues (BTEX), selected polycyclic aromatic hydrocarbons (PAHs), zinc and selenium, the values of the remaining parameters were considerably greater for the hard coal condensates. The studies revealed that all of the tested UCG condensates were extremely toxic to V. fischeri; however, the average toxicity level for the hard coal condensates was approximately 56% higher than that obtained for the lignite. The statistical analysis provided results supporting that the toxicity of the condensates was most positively correlated with the concentrations of free ammonia, phenols and certain heavy metals.

Development of High-efficiency Oxy-fuel IGCC System

CO2 emission control is a significant issue as a countermeasure for a global warming problem. Carbon dioxide capture and storage technology is regarded as one of the methods of reducing CO2 emission, but it leads to a drop in a thermal efficiency. It is important to maintain thermal efficiency as high as possible for an efficient use of limited fossil fuel resources. CRIEPI started a joint project with Kyushu university to develop “High-efficiency oxy-fuel IGCC”, a high thermal efficiency integrated coal gasification combined cycle power generating system with CO2 capture, which can keep its efficiency more than 40% (HHV). This system gasifies coal with mixed gas of O2 and recirculated exhaust gas mainly composed by CO2, just as oxy-fuel combustion boiler. This paper is to introduce latest achievements done by CRIEPI on this project by quoting latest papers. In order to clarify the effect of CO2 enrichment on coal gasification reaction, coal gasification experiments were executed using pressurized entrained flow coal gasifier whose fuel capacity is 3TPD. And in order to evaluate characteristics of O2- CO2 gasifier by numerical simulation, pressurized drop tube experiments were performed to obtain reaction data necessary for constructing high precision reaction model for numerical simulation code. The data clarified difference between O2 gasification and CO2 gasification on soot behaviour. When we start this project, this system had a problem to be solved. That is carbon deposition on the surface of hot gas clean-up sorbent, installed to improve thermal efficiency. As CO concentration of syngas is so high that carbon particles are supposed to deposit on desulfurization sorbent. But fundamental experiments showed an appropriate countermeasure for carbon deposition, whose impact on thermal efficiency is quite small. Now it is time for us to summarize achievements and propose the next steps toward realization of this system.

Environmental aspects of a field-scale underground coal gasification trial in a shallow coal seam at the Experimental Mine Barbara in Poland

A study on the environmental impacts of underground coal gasification (UCG) was conducted during a field-scale, in situ coal gasification experiment at the Experimental Mine “Barbara” in Poland. This two-week experiment was a part of a technological effort aiming to produce hydrogen-rich product gas using the UCG technology. As groundwater pollution is recognised to be the most serious environmental risk related to UCG, extensive investigations on the formation, release, and migration of contaminants were conducted. The study revealed a wide range of organic and inorganic contaminants that were characteristic for the UCG. The main organic contaminants were phenolic compounds and aromatic hydrocarbons (mono- and polycyclic). Among the inorganic species, heavy metals, ammonia and cyanides were the most serious group of contaminants. These compounds were identified in the post-processing effluents and in groundwater near the test site. The impact of the gasification process on the groundwater, however, was small and tended to effectively decrease over the time and distance from the cavity zone. A possible negative environmental impact of the gasification by-products, including ash and char left underground, was also evaluated. This study revealed that these post-gasification residuals are the main source of toxic trace elements, but the most volatile species, such as Hg, As and Se, tended to escape the cavity zone with the product gas during gasification phase. Monitoring activities detected no chemical hazards on the surface during the course of the experiment and after its termination; however, UCG-induced thermal effects on surface were observed.

Chemometric Study of the Ex Situ Underground Coal Gasification Wastewater Experimental Data

The main goal of the study was the analysis of the parameters of wastewater generated during the ex situ underground coal gasification (UCG) experiments on lignite from Belchatow, and hard coal from Ziemowit and Bobrek coal mines, simulated in the ex situ reactor. The UCG wastewater may pose a potential threat to the groundwater since it contains high concentrations of inorganic (i.e., ammonia nitrogen, nitrites, chlorides, free and bound cyanides, sulfates and trace elements: As, B, Cr, Zn, Al, Cd, Co, Mn, Cu, Mo, Ni, Pb, Hg, Se, Ti, Fe) and organic (i.e., phenolics, benzene and their alkyl derivatives, and polycyclic aromatic hydrocarbons) contaminants. The principal component analysis and hierarchical clustering analysis enabled to effectively explore the similarities and dissimilarities between the samples generated in lignite and hard coal oxygen gasification process in terms of the amounts and concentrations of particular components. The total amount of wastewater produced in lignite gasification process was higher than the amount generated in hard coal gasification experiments. The lignite gasification wastewater was also characterized by the highest contents of acenaphthene, phenanthrene, anthracene, fluoranthene, and pyrene, whereas hard coal gasification wastewater was characterized by relatively higher concentrations of nitrites, As, Cr, Cu, benzene, toluene, xylene, benzo(a)anthracene, chrysene, benzo(b)fluoranthene, benzo(k)fluoranthene, and benzo(a)pyrene.

Emissions of particles and trace elements from coal gasification

Some of the trace elements in coal are easily emitted with the syngas obtained from coal gasification in the gaseous phase and condensed as fine particles. Such impurities in syngas could result in corrosion and/or degradation of electrode materials in fuel cells if the coal gasification process is applied to Integrated Gasification Fuel Cell (IGFC). Thus, to realize the coal gasification process combined with fuel cells, it is extremely important to control the emission behavior of hazardous trace elements in coal gasification. In this study, particles in the coal gasification gas were sampled and analyzed in detail to determine their particle size distributions and elemental compositions. A drop tube type furnace was utilized for the coal gasification experiment. Pb and Se were evaluated in terms of their distribution behavior to particles. Pb is one of the most typical volatile heavy metals, and Se is known as a corrosive substance for Solid Oxide Fuel Cells (SOFC). Thermal equilibrium calculations were also conducted to derive the chemical forms of Pb and Se during their emissions. As a result, particles entrained in syngas from the coal gasification process were confirmed to have bimodal peaks in their particle size distributions. The peak larger than 7.8μm in the super-micron size range consists of ash with unburnt carbon, while the smaller peak at 0.5μm in the sub-micron range consists of soot with mineral deposits. Se and Pb were found to react together during coal gasification process. In addition, almost all of them were vaporized in the reactor and deposited on soot particles in the sub-micron range during the gas cooling process.

Development of a Bench-Scale High-Pressure Fluidized Bed Reactor and Its Sequential Modification for Studying Diverse Aspects of Pyrolysis and Gasification of Coal and Biomass

This paper reviews the design, operation, and sequential modifications of a bench-scale high-pressure fluidized bed reactor system, capable of diverse pyrolysis and gasification applications, all based on the same hardware platform. The reactor is small: 34 mm i.d. and 504 mm high. The mechanical design is relatively simple and inexpensive, featuring direct electrical heating to avoid the use of a separate furnace. The reactor body is made of a high-strength alloy, with 1000 h creep resistance at maximum design conditions (1000 °C and 3.0 MPa). The design thus obviates the use of a “cold” pressure casing. The system can be operated by a single person and has demonstrated ability to generate fuel reactivity and product distribution data rapidly and cheaply, using a wide range of solid fuels. In batch operation, conversions achieved during coal pyrolysis and gasification experiments were comparable with results from a high-pressure wire-mesh reactor, indicating nearly single-particle behavior. The system has been modified for continuous feeding (∼3 g min −1), to study the effect of reaction conditions on the fate of fuel-N, during the gasification of coal and sewage sludge (to 3.0 MPa). The work showed that steam plays a primary role in the formation of NH 3 from both volatile-N and char-N. HCN concentrations were found to depend strongly on the residence time at temperature and on the extent of contact with heated bed solids. A quartz lining allowed the determination of extents of trace element emissions during gasification. Above 900 °C, enhanced depletion of Ba, Pb, and Zn in bed solids was accompanied by enrichment of fines, collected in a downstream filter. The system was subsequently modified to enable a slug of coal to be injected and bed solids discharged (1−3.0 MPa) from the fluidized bed under controlled conditions and precise solid residence times in the bed. The reactivities of the chars were measured as a function of “residence time at temperature” and were observed to decrease with increasing temperature, time, pressure, and particle size. At 1000 °C, coal char reactivities were found to diminish by nearly a factor of 4 within 10 s. The study confirmed and extended prior work carried out in a high-pressure wire-mesh reactor. The system was subsequently configured for pyrolyzing waste plastic material at low temperatures. Altering the electrode positions enabled changing the position of the heated zone, enabling trouble-free injection of polymer samples at temperatures up to 600 °C. The aim of these experiments was to investigate the potential to produce liquid fuel precursors from pure and mixed waste plastics. In its latest incarnation the system has been further modified for performing oxy-fuel gasification experiments.

Reaction swing approach for hydrogen production from carbonaceous fuels

The paper presents the results of the investigations on a reaction swing approach for the production of high purity hydrogen from syngas exploiting the Boudouard at temperatures ranging from 550 to 700 ∘ C and pressures ranging from 0 to 150 psig. The concept of the process is the sequential use of a catalyst, that enhances the CO disproportionation reaction in conjunction with the water gas shift reaction to completely eliminate the CO in the product stream, and a CO 2 removal agent for in situ capture of the produced CO 2 . The thermodynamic evaluation of the above system shows the potential of achieving greater than 99% hydrogen purity values in the product stream even at atmospheric pressures. The effects of temperature, pressure and steam addition on the hydrogen enrichment are reported. Experiments were also conducted to evaluate the effect of catalyst and CO 2 removal material loadings on the product gases. Increasing the pressure improved the purity and the duration of the enrichment cycle. The separation efficiency was maximized at a temperature of 650 ∘ C . A 1:2 ratio of the catalyst to CO 2 removal material was found to be sufficient to produce nearly carbon-free hydrogen for over an hour before regeneration. The use of 25% steam was found to increase the hydrogen yield by nearly 50% (due to the contribution of the water gas shift). The results from simultaneous coal gasification and hydrogen enrichment experiments in a single reactor are also presented. It was found that for a Fe 2 O 3 : coal ratio of 22:1 for effective gasification and over 99% pure hydrogen stream.

Release behavior of trace elements from coal during high-temperature processing

High-temperature coal utilization processes (combustion, gasification and pyrolysis) emit toxic trace elements into the atmosphere, where they affect the environment and human health. We carried out coal combustion and gasification experiments using several kinds of coals at various temperatures and atmospheric conditions to examine the release behavior of trace elements. Results showed that Group 1 and 2 metals such as Hg, Se, Sb, and Zn were partly released from coal during combustion, gasification and pyrolysis. Oxygen or steam in the operating atmosphere affected the behavior of Hg, Se, Sb, and Zn release from coal.

Biomass and fossil fuel conversion by pressurised fluidised bed gasification using hot gas ceramic filters as gas cleaning

Gasification of biomass and fossil fuels, hot gas cleanup using a ceramic filter and combustion of LCV product gas in a combustor were performed using a 1.5 MWth test rig (pressurised bubbling fluidised bed gasifier) at Delft University and a 10– 50 kWth system at Stuttgart University (DWSA) in the framework of experimental research on efficient, environmentally acceptable large-scale power generators based on fluidised bed gasification. The influence of operating conditions (pressure, temperature, stoichiometric ratio) on gasification (gas composition, conversion grades) was studied. The gasifiers were operated in a pressure range of 0.15– 0.7 MPa and maximum temperatures of ca. 900°C. The Delft gasifier has a 2 m high bed zone (diameter: 0.4 m ) followed by a freeboard approximately 4 m high (diameter: 0.5 m ). The IVD gasifier has a diameter of 0.1 m and a reactor length of 4 m . Carbon conversions during wood, miscanthus and brown coal gasification experiments were well above 80%. Fuel-nitrogen conversion to ammonia was above ca. 50% and the highest values were observed for biomass. The results are in line with other investigations with biomass bottom feeding. Deviation occurs compared with top feeding. Measurements are compared with simulation results of a reaction-kinetics-based model, using ASPEN PLUS, related to emission of components like fuel-nitrogen-derived species. Data from literature regarding initial biomass flash pyrolysis in the gasification process are used in the gasifier model and will be compared with simulation results from the FG-DVC model. Measurements and model predictions were in reasonably good agreement with each other.

COAL GASIFICATION AND THERMODYNAMIC CALCULATION OF MOVING BED COAL GASIFIER WITH DRAFT TUBE

The coal gasification experiment and thermodynamic calculation of the newly developed moving bed coal gasifier with a draft tube are done according to the thermodynamic calculating model built on the basis of chemical reaction equilibrium, mass balance and heat balance. The obtained experimental results are quite approaching to the thermodynamically calculated values. The effects of steam/coal ratio, air/coal ratio, reaction temperature, reaction pressure, steam temperature, air temperature on the thermodynamic indexes of the gasifier are examined through thermodynamic simulating calculations. Under the optimal simulating conditions, gas calorific value, cold gas thermodynamic efficiency and available energy efficiency can reach 12 MJ/Nm3, 88.6% and 95.4% respectively, when air is used as the oxidizer. Compared with the directly-burning-coal-gasifier, the gasifier exhibits good characteristics and prospects for industrial application.

Pyrométamorphisme induit par la gazéification souterraine de niveaux charbonneux du Westphalien dans le bassin de Mons (Belgique) [Pyrometamorphism induced by underground coal gasification of Westphalian beds in the Mons basin (Belgium)

During the underground coal gasification experiment conducted at Thulin (Belgium), partial melting occurred in the surrounding rocks (Westphalian shales and sandstones). Samples were collected at the roof of the coal bed. Minerals which crystallized from the partial melts were identified under the polarizing microscope, with electron probe microanalysis, by X-ray diffraction and scanning electron microscopy. They include cordierite, orthopyroxene, olivine, spinel, cristobalite, mullite, osumilite, K-feldspar, ilmenite, hematite, wollastonite, calcite, andradite, monticellite, pyrrhotite, and chalcopyrite. In these paralavas, the glass proportion ranges from 20 to 40%. In some cases, melting and subsequent crystallization have produced a magnetite-bearing gabbro. This gabbro contains an inclusion of sedimentary rock (relic) in which sillimanite, mullite and corundum were recognized. Metamorphic textures suggest that sillimanite is replaced by mullite, and mullite by corundum. Temperatures in the range of 1083 °C up to 1500 °C have been estimated for the underground coal gasification process by several methods: phase diagrams in the system (FeO, MgO)-Al2O3-SiO2 numerical modelling of gabbro crystallization, cordierite polymorphs crystallization, and metamorphic reactions.

Results of the tracer tests during the El Tremedal underground coal gasification at great depth

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 based on exchange of matter between the flowing fluid and a transverse dead zone were used. Results indicate that the gasifier behaves almost like a small number of stirred tanks in series with a high level of back mixing.

Modelling of flow at Thulin underground coal gasification experiments

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 operating periods: the series of reverse combustion tests and the gasification phase. The models developed to simulate the flow conditions are based on the material exchange between flowing fluid and the dead zone. The dead zone is represented either by a stagnant zone exchanging material with the flowing fluid in accordance with film theory or by a homogeneous porous zone exchanging material with the flowing fluid by pure molecular diffusion. With these models, excellent fits were obtained and physical significance of the parameters is possible in correlation with the gasifier operating conditions. The results show a little evolution of the flow conditions inside the underground reactor during the series of reverse combustion tests. On the other hand, during the gasification phase, the reactor volume calculated from the tracer results is proportional to the cumulative quantity of coal consumed.

Effects of Aquifer Interconnection Resulting from Underground Coal Gasification

ABSTRACTAn investigation was conducted to evaluate the effects of aquifer interconnection caused by the collapse of cavities formed in coal seams by two small underground coal gasification experiments in the Powder River Basin, Wyoming. The main objective of the work was to assess the magnitude and extent of changes in the ground‐water flow patterns near the sites of the two experiments. Hydraulic head measurements in the three affected aquifers were used to calibrate a steady‐state ground‐water flow model of the interconnection zone at each site. Flow modeling and field measurements show that water from one or both of the upper aquifers enters the collapse rubble and flows down to the lowest aquifer (the gasified coal seam) where it flows away from the collapse zones. The hydraulic conductivity of the collapse rubble is less than that of the aquifers and provides only a very moderately permeable interconnection between them. A marked reduction in the hydraulic conductivity of the gasified coal seam near the collapse zones causes restriction of flow in the seam, away from them. Changes in hydraulic head and flow patterns caused by aquifer interconnection extend generally only 200–300 ft (60–90 m) away from the experiment sites. Flow in the uppermost aquifer at one of the sites may be influenced as far as 400 ft (122 m) away. At both experiment sites, aquifer interconnection allows water from the uppermost (sand) aquifer, which contains the poorest quality water of the three aquifers, to enter one or both of the underlying coal aquifers.

Application of Geological Studies to Overburden Collapse at Underground Coal Gasification Experiments: ABSTRACT

Detailed geologic and mineralogic studies were conducted on the Hanna, Wyoming, and Hoe Creek, Wyoming, underground coal gasification sites. These studies demonstrate the importance geologic factors have on controlling overburden collapse into the reactor cavity during and after coal gasification and on subsequent environmental problems. Parameters that control the collapse of overburden material into the reactor cavity include: duration of the burn; maximum span of unsupported roof rock; lateral and vertical homogeneity, permeability and rock strength; and thickness of overburden materials. The duration of the burn, maximum span of unsupported roof rock, and total thickness of overburden rock are, in large part, determined by the technical engineering aspects of the burn The remaining parameters are determined by geological factors including original composition, depositional environment, diagenesis, and structural deformation. The coals and overburden units at both sites are Eocene in age and are virtually flat lying. The three burn cavities studied at the Hoe Creek site are directly overlain by laterally discontinuous, poorly indurated, kaolinite-cemented sandstones, siltstones, and mudstones of channel and overbank origin. At the Hoe Creek I experiment, a small reactor cavity and a correspondingly short maximum span of unsupported roof rock consisting of fine-grained, low permeability overbank deposits resulted in minimal collapse. At the Hoe Creek II experiment, a significant amount of collapse occurred due to an increased span of unsupported roof rock comprised of poorly consolidated, more permeable channel sandstones and a limited amount of overburden mudstones and siltstones. Roof rock collapse extend d to the surface at the Hoe Creek III experiment where the maximum span of unsupported roof rock was largest and where the roof rock consisted of highly permeable, poorly consolidated channel sandstones. The unit comprising the reactor cavity roof rock at the Hanna II experimental site is a laterally continuous lacustrine delta deposit, which primarily consists of sandstones with lesser amounts of interbedded siltstones and claystones. The overall strength, specifically the bridging capacity, of this unit is enhanced by abundant calcite cement. This cement reduced permeability and interstitial waters which probably kept spalling of the roof rock to a minimum. Consequently, roof rock collapse at the Hanna II experiment was much less extensive than at the Hoe Creek II and III experiments. End_of_Article - Last_Page 1336------------

Mutagenic and toxic activity of environmental effluents from underground coal gasification experiments.

Using bacterial bioassays, we have screened for the presence of mutagens and toxins in extracts from groundwater, and in tar from product gas, at the sites of two Lawrence Livermore National Laboratory (LLNL) in situ experiments: Hoe Creek II and Hoe Creek III. The sites exhibited different potential biological hazards, suggesting that different gasification processes may represent different human health concerns. We found that mutagens are present in groundwater, persist for at least 2 yr after gasification has been terminated, and show a change in activity with time-possibly in parallel with changes in chemical composition. Preliminary evidence suggests that the mutagens in groundwater are quinoline and aniline derivatives, while the toxins in groundwater may be phenolic compounds. In tar from the product gas, the organic bases and neutrals were found to be genotoxic in both bacterial and mammalian cells; the neutral compounds appear to be the major mutagenic health hazards. Neutral compounds constitute most of the tar (85-97 wt%) and were mutagenic in both the bacterial and mammalian cell assays. Tar in the gas stream may be a problem for the aboveground environment if gas escapes through fractures in the overburden. Because it is mutagenic and induces chromosomal damage to mammalian cells, the tar may represent a disposal problem as well. However, it is difficult to assess tar quantitatively as a health hazard because its mutagenic activity is low, possibly due to contaminants in the neutral fraction that act to suppress mutagenicity.