San Jacinto-Tizate geothermal field,Nicaragua: Exploration and conceptual model

The results of the exploration of the San Jacinto-Tizate geothermal field during 1992–1995 included geological, hydrogeological and geophysical investigations (magnetotelluric, frequency soundings, subsurface temperature, and soil-gas surveys), and the drilling and testing of seven deep wells (728–2339 m). The geothermal field, located within a composite volcano-tectonic depression, can be divided into two main areas: San Jacinto and Tizate. The San Jacinto area shows evidence of a high-temperature (250–300°C) fossil geothermal system that at present has reservoir temperatures in the 180–190°C range. In the Tizate area there is an active geothermal system with temperatures of 250–285°C. An upflow zone with an excess pressure gradient exists in the central part of this area. Two hydraulically connected reservoirs exist: a shallow one at 550–1200 m depth, and a deeper one below 1600m. Two-phase conditions exist in the upper part of the shallow reservoir. Production tests demonstrate the commercial potential of both Tizate reservoirs.     

Geothermal resources in the Asal Region,Republic of Djibouti: An update with emphasis on reservoir engineering studies

Three independent geothermal systems have been identified, so far, in the Asal region of the Republic of Djibouti (i.e. Gale le Goma, Fiale and South of Lake). Six deep wells have been drilled in the region, the first two in 1975 and the others in 1987–88. Well A2 was damaged and wells A4 and A5 encountered impermeable yet very hot (340–365 °C) rocks. Wells A1, A2, A3 and A6 produce highly saline (120 g/L TDS) fluids leading to mineral scaling. Well test data indicate that the reservoir might be producing from fractured and porous zones. The estimated permeability-thickness of the deep Gale le Goma reservoir is in the 3–9 darcy-meter range. Lumped-parameter modeling results indicate that well A3 should be operated at about 20 kg/s total flow rate and that injection should be considered to reduce pressure drawdown. The estimated power generation potential of well A3 is 2.5 MWe, and that of all Asal high-temperature hydrothermal systems is between 115 and 329 MWe for a 25-year exploitation period.     

Chemical and physical reservoir parameters at initial conditions in Berlingeothermal field,El Salvador: a first assessment

A study has been madeto obtain the main chemical and physical reservoir conditions of the Berlin field (El Salvador)before the commencement of large-scale exploitation of the geothermal resource The upflowzone and the main flow path within the geothermal system have been determined from the arealdistribution of chemical parameters such as Cl concentrations ratios such as Na/KK/Mg,K/Ca,and temperatures computed from silica concentrations and cation ratios Gas compositions havebeen used to calculate reservoir parameters such as temperature steam fraction and P_CO2 The computer code WATCH (new edition 1994) hasbeen used to evaluate the temperature of equilibration between the aqueous species and selectedalteration minerals in the reservoir The fluid in Berlin flows to the exploited reservoir from thesouth entering it in the vicinity of well TR-5 Along its flow-path (south–north direction) thefluid is cooled by boiling and conductive cooling The chloride-enthalpy diagram indicates theexistence of a parent water with a chemical composition similar to well TR-5 that boils and theresidual brine produces the fluid of well TR-3 which is very concentrated in salts The fluid ofTR-5 is probably produced from this parent water generating the fluids of wells TR-2 and TR-9by boiling and the fluids of wells TR-1 and TR-4 by conductive cooling The computed values forthe deep steam fraction clearly indicate that this is a liquid-dominated system with computedtemperature values decreasing from 310°C (upflow zone) to about 230°C from south to north© 1999 Published by Elsevier Science Ltd on behalf of CNR All rights reserved     

Reservoir engineering assessment of Dubti geothermal field,Northern Tendaho Rift,Ethiopia

Following on from surface exploration surveys performed during the 1970s and early 1980s, exploration drilling was carried out in the Tendaho Rift, in Central Afar (Ethiopia), from October 1993 to June 1995. Three deep and one shallow well were drilled in the central part of the Northern Tendaho Rift to verify the existence of a geothermal reservoir and its possible utilisation for electric power generation. The project was jointly financed by the Ethiopian Ministry of Mines and Energy and the Italian Ministry for Foreign Affairs. Project activities were performed by the Ethiopian Institute of Geological Surveys and Aquater SpA. The main reservoir engineering data discussed in this paper were collected during drilling and testing of the above four wells, three of which are located inside the Dubti Cotton Plantation, in which a promising hydrothermal area was identified by surface exploration surveys. Drilling confirmed the existence of a liquid-dominated shallow reservoir inside the Dubti Plantation, characterised by a boiling-point-for-depth temperature distribution down to about 500 m depth. The main permeable zones in the Sedimentary Sequence, which is made up of lacustrine deposits, are located in correspondence to basalt lava flow interlayerings, or at the contact between volcanic and sedimentary rocks. At depth, the basaltic lava flows that characterise the Afar Stratoid Series seem to have low permeability, with the exception of fractured zones associated with sub-vertical faults. Two different upflows of geothermal fluids have been inferred: one flow connected to the Dubti fault feeds the shallow reservoir crossed by wells TD-2 and TD-4, where a maximum temperature of 245 °C was recorded; the second flow seems to be connected with a fault located east of well TD-1, where the maximum recorded temperature was 270 °C. A schematic conceptual model of the Dubti hydrothermal area, as derived from reservoir engineering studies integrated with geological, geophysical and geochemical data, has been tested by numerical simulation, using the TOUGH2/EWASG code. Preliminary simulations, using a simple 3-D numerical model of the Dubti fault area, showed that measured temperature and pressure distribution, as well as evaluated non-condensable gas pressure at reservoir conditions, are compatible with the rise of geothermal fluid, at about 290 °C, along the sub-vertical Dubti fault from beneath the surface manifestations DB1, DB2 and DB3 located at the south-eastern end of the fault. According to the proven shallow field potential, development of this field could meet the predicted electricity requirements of Central Afar until the year 2015.     

Changes in silica chemistry and hydrology across the Rotorua geothermal field,New Zealand

A comprehensive review of field data from Rotorua geothermal field, New Zealand, was undertaken to evaluate the mixing relationships of cooler groundwaters with the primary fluid. Unlike previous studies the reservoir well compositions were calculated using only the most reliable measured downhole fluid temperatures. Inconsistencies in gas chemistry showed that the hot fluid in the southeast could not be derived from a postulated 250°C parent by simple boiling, a mechanism proposed by previous authors. Mixing relationships showed conclusively that across the field from southeast to northwest, all the fluids were related along a mixing line with low-chloride water at 150°C. Between 158 and 175°C the diluting fluid changed to a low-chloride groundwater at 15°C.An SiO₂-temperature plot of all the well data showed that silica values for the hot wells fall around the quartz solubility line. The data for the majority of the cooler wells scatter about the α-cristobalite solubility line. Plots of the Cl/SiO₂ ratio were used to evaluate changes in silica chemistry across the reservoir by comparing measured data with theoretical mixing lines, assuming partial equilibrium with respect to the three silica polymorphs, quartz, α-cristobalite and amorphous silica. The only assumptions made in the calculation were that the 150 and 15°C diluting fluids were in equilibrium with cristobalite and amorphous silica, respectively. Surprisingly, the trends may be explained by mixing with cooler silica-rich diluting fluids without requiring equilibration with respect to any of the silica polymorphs.     

Thermal anomalies in lanzarote (Canary Islands)

About a hundred thermometers have been installed 3 m deep in an area of about 20 km² in the ? Montañas de Fuego ? (island of Lanzarote). The temperatures measured oscillate between 16°C and 350°C. In other wells 150 to 250 m deep a gradient of 0.2°C/m has been measured. This gradient is always found in a region of about 200 km² around the zones of the highest superficial anomalies.The local surface anomalies are related to the outflow of hot fluids through tectovolcanic fractures. At great depths these fluids probably form a convective system under impervious layers (hyaloclastites?). The heat transfer takes place by means of convection in a deep reservoir and in the superficial levels, but an intermediate zone with impervious layers must exist where heat is transmitted by conduction.     

Industrial-scale investigations on effects of tertiary-air declination angle on combustion and steam temperature characteristics in a 350-MW supercritical down-fired boiler

Industrial-scale experiments were conducted to study the effects of tertiary air declination angle (TDA) on the coal combustion and steam temperature characteristics in the first 350-MW supercritical down-fired boiler in China with the multiple-injection and multiple-staging combustion (MIMSC) technology at medium and high loads. The experimental results indicated that as the TDA increased from 0° to 15°, the overall gas temperature in the lower furnace rose and the symmetry of temperature field was enhanced. The ignition distance of the fuel-rich coal/air flow decreased. In near-burner region, the concentration of O₂ decreased while the concentrations of CO and NO increased. The concentration of NO decreased in near-tertiary-air region. The carbon in fly ash decreased significantly from 8.40% to 6.45% at a load of 260 MW. At a TDA of 15°, the ignition distances were the shortest (2.07 m and 1.73 m) at a load of 210 MW and 260 MW, respectively. The main and reheat steam temperatures were the highest (557.2°C and 559.4°C at a load of 210 MW, 558.4°C and 560.3°C at a load of 260 MW). The carbon in fly ash was the lowest (4.83%) at a load of 210 MW. On changing the TDA from 15° to 25°, the flame kernel was found to move downward and the main and reheat steam temperatures dropped obviously. The change of TDA has little effect on NO_x emissions(660–681 mg/m³ at 6% O₂). In comprehensive consideration of the pulverized coal combustion characteristics and the unit economic performance, an optimal TDA of 15° is recommended.     

Water chemistry of San Marcos area,Guatemala

Two well-equilibrated NaCl geothermal liquids are recognizable in the San Marcos area. Both have the same Cl concentration (540 mg/kg) and the same isotopic composition (δD of −66.5‰ and δ¹⁸O of −9‰) under reservoir conditions, but they come from two distinct aquifers with different temperatures, i.e. 240°C below La Cimarrona and 185°C below La Castalia. The numerous thermal NaCl to NaClHCO₃ springs located in the San Marcos area originate through dilution and boiling of these two geothermal liquids and different degrees of re-equilibration at lower temperatures. Silica and K contents are useful in discriminating between dilution, boiling and re-equilibration phenomena. Thermal NaHCO₃ waters, generated through conductive heat transfer or input of geothermal vapor or gases from below, delineate the extent of the geothermal reservoir(s) at depth.     

The Tanggu geothermal reservoir (Tianjin,China)

The Tanggu geothermal system is an extensive, highly permeable, horizontal sandstone reservoir, situated within the North China Sedimentary Basin. Twenty-three successful production wells, yielding water with an average temperature of about 70°C, have been drilled into this reservoir since 1987, distributed over an area of some 330 km². The hot water is mostly used for space heating. In 1995 the annual production exceeded 5 million tons. Hot water extraction has caused the water level to drop to a depth of 80 m in the production wells, and it continues to decline at a rate of 3–4m per year. This has raised the question as to whether the reservoir may be overexploited. The main objective of a reservoir evaluation carried out in 1996 was to estimate the long-term production potential of the Tanggu reservoir. Two simple models were developed for this purpose. The potential is determined by specifying a maximum allowable pump setting depth of 150m. On this basis the potential of the Tanggu reservoir is estimated to be about 10 million tons per year, for the next ten years. A comprehensive reservoir management program must be implemented in Tanggu. The first priority of such a program should be to improve the energy efficiency of space heating in the district, which should result in about 50% reduction in hot water consumption. Another management option is reinjection, which would counteract the water level draw-down.     

Fluid chemistry and temperatures prior to exploitation at the Las Tres Vírgenes geothermal field,Mexico

Generation of electricity at the Las Tres Vírgenes (LTV) geothermal field, Mexico, began in 2001. There are currently nine geothermal wells in the field, which has an installed electricity generating capacity of 10 MW_e. The chemical and temperature conditions prevailing in the field prior to its exploitation have been estimated, including their central tendency and dispersion parameters. These conditions were computed on the basis of: (i) geochemical data on waters from springs and domestic wells, and on geothermal well fluids (waters and gases); most of the sampling took place between 1995 and 1999; (ii) fluid inclusion studies; (iii) geothermometric data; and (iv) static formation temperatures computed using a modified quadratic regression Horner method.Fluid inclusion homogenization temperatures (in the 100–290 °C range) suggest that there is a high-temperature fluid upflow zone near wells LV3 and LV4 in the southern part of the field. Computed average chemical equilibrium temperatures for the geothermal fluids are ∼260 °C, based on the Na/K and SiO₂ geothermometers, and ∼265 °C, based on the H₂/Ar, and CO₂/Ar geothermometers. In general, the fluid inclusion homogenization temperatures are consistent with geothermometric data, as well as with static formation temperatures. Some of the observed differences could be related to well interference effects and different fluid production/sampling depths. The deeper geothermal waters show higher concentrations of Cl, Na, K, B, Ba, but lower concentrations of SO₄, Ca, and Mg than the shallower waters. Fluid inclusion salinities are also higher in the deeper rocks. The measured Na/Cl ratios of the geothermal well waters are more or less uniform throughout the field and are very similar to that of seawater, strongly suggesting a seawater component in the fluid of the LTV system.The heat stored in the LTV geothermal system was estimated to be at least 9 × 10¹² MJ, of which some 4 × 10¹¹ MJ (equivalent to about 148 MW_e for 30 years of operation, assuming a conversion efficiency of ∼35%) might be extracted using wells. These results indicate that the installed capacity at LTV could be safely increased from the current 10 MW_e.     

Geochemical monitoring of the Krafla and Námafjall geothermal areas, N-Iceland

The Krafla and Námafjall high-temperature geothermal areas in N-Iceland have been exploited for steam production since the late and early 1970s, respectively. Power generation at Krafla was 30 MW until 1998, when it was increased to 60 MW. At Námafjall the steam has been utilized for operating a 3 MW back-pressure turbine unit, drying of diatomaceous earth and heating of fresh water for space heating. A total of 34 wells have been drilled at Krafla, of which 18 are producing at present. At Námafjall 12 wells have been drilled but only three are productive. The highest temperatures recorded downhole are 320 and 350 °C at Námafjall and Krafla, respectively. Geochemical monitoring in the two fields during the last 20–25 years has revealed decreases in the Cl concentrations in the water discharged from most of the wells that have been producing for more than 10 years. The cause is enhanced colder water recharge into the producing aquifers of these wells due to depressurization by fluid withdrawal from the geothermal reservoir. Such recharge is particularly pronounced in the central part of the Leirbotnar wellfield at Krafla but it is also extensive in the only producing well in the Hvíthólar wellfield. At Námafjall incursion of cold groundwater into the reservoir was particularly intense subsequent to the volcanic-rifting event in the area in 1977. Solute (quartz, Na/K, Na/K/Ca) geothermometry temperatures have decreased significantly in those wells where Cl concentrations have decreased but only to a limited extent in those wells which have remained constant in Cl. This indicates that the changes in the concentrations of the reactive components, on which these geothermometers are based, is largely the consequence of colder water recharge and not partial re-equilibration in the depressurization zone around wells where cooling of the fluid occurs in response to extensive boiling. Aqueous SO₄ concentrations increase as Cl concentrations decrease. Except for the hottest wells, which are low in SO₄, sulphate concentrations are controlled by anhydrite solubility. Increase in SO₄ concentrations is a reflection of cooling as anhydrite has retrograde solubility with respect to temperature. H₂S-temperatures are similar to the solute geothermometry temperatures for wells with a single feed. They are, on the other hand, higher, for wells with multiple feeds, if the feed zones have significantly different temperatures. H₂-temperatures are anomalously high for most wells due to the presence of equilibrium steam in the producing aquifers. The equilibrium steam fraction amounts to 0–2.2% by wt. of the aquifer fluid (0–47% by volume). CO₂ temperatures are anomalously high for some Krafla wells due to high flux of CO₂ from the magma intruded into the roots of the geothermal system during the 1975–1984 volcanic-rifting episode. During the early phase of this episode the Leirbotnar wells were the ones most affected. The new magma gas flux has migrated eastwards with time. Today some wells in the Sudurhlídar wellfield are the ones most affected whereas the Leirbotnar wells have recovered partly or fully. The depth level of producing aquifers in individual wells at Krafla and Námafjall has been evaluated by combining data on temperature and pressure logging and geothermometry results. The majority of wells at Krafla receive fluid from a single aquifer, or from 2–3 aquifers having similar temperature. The same applies to two of the three productive wells at Námafjall.     

Chemical and isotope characteristics of the Chachimbiro geothermal fluids (Ecuador)

The parent geothermal water proposed for the Chachimbiro geothermal area has calculated values of 2250 mg/L Cl and approximately 5 bar P_CO2. It comes from a reservoir having an estimated temperature of 225–235 °C, although temperatures somewhat higher than 260 °C may be present at the roots of the system. The geothermal reservoir at Chachimbiro is recharged mainly by meteoric water (about 92%) and secondarily by arc-type magmatic water. Carbon and sulfur isotope data support a magmatic origin for the C and S species entering the geothermal system from below, consistent with indications provided by He isotopes.The thermal springs of Na–Cl to Na–Cl–HCO₃ type located in the Chachimbiro area originate through dilution of the parent geothermal water and have reached different degrees of re-equilibration with country rocks at lower temperatures.     

Geothermal prospecting by geochemical methods on natural gas and water discharges in the vulsini mts volcanic district (central Italy)

The Latera and Torre Alfina geothermal fields were discovered in the Vulsini Mts district (central Italy) in the 70s. The fluid produced by the two geothermal systems is a high pCO₂ (around 7 MPa) sodium chloride solution (T.D.S. is 9200 ppm at Latera and 7800 at Torre Alfina), with high SiO₂ and H₃BO₃ contents. The fluid temperature taken at well bottom is about 155°C at Torre Alfina, whereas at Latera it ranges from 200 to over 350°C. In spite of these temperatures, recorded in producing wells, previous geochemical prospectings using geothermometers in natural thermal manifestations had predicted temperatures no higher than 140°C in all the Vulsini district. This contrasting feature between real temperatures and those evaluated during prospecting is caused by the fast circulation of large amounts of meteoric waters in the aquifer located in the shallow parts of the carbonate reservoir formations, and by the short interaction between the latter and the deep geothermal fluids.In the present study a new geochemical survey on thermal and cold springs, stream samples, as well as natural gas emissions has been carried out. A critical review of the main geothermometers, some considerations about the hydraulic behavior of the reservoir formations, and the cross comparison between NH₄⁺/B ratio, pCO₂ and SiO₂ content in both cold and thermal waters, have led to the conclusion that in the Vulsini Mts there are no shallow anomalous areas apart from those already discovered at Latera and Torre Alfina.The present method could be successfully applied in other geothermal systems, where the potential reservoir is represented by carbonate formations.     

Geochemical study of fluid inclusions from the western upflow zone of the Matsukawa geothermal system,Japan

Prior to development, the Matsukawa geothermal field was partially vapor-dominated. The youngest mineral assemblage consists of early pyrophyllite, diaspore and pyrite, and later anhydrite and quartz, implying deposition from an acidic, high-temperature fluid. Fluid inclusions in anhydrite and quartz from core and cutting samples collected in wells drilled in the western upflow zone of the field were studied to characterize the temperatures and compositions of these late fluids.The results of fluid inclusion studies indicate that the temperatures during the deposition of anhydrite and quartz were up to several tens of degrees lower than the reservoir temperatures at the time of exploitation. Fluids trapped in anhydrite had temperatures of up to 257 °C, CO₂ concentrations in the 0.4–2.6 mol% range and salinities of 1.9–11.3 wt.% NaCl. This compositional variation is related to vapor loss occurring during boiling. The data suggest that the geothermal reservoir is currently being reheated by subvolcanic intrusions.     

High-temperature measurements in well WD-1A and the thermal structure of the kakkonda geothermal system, Japan

The New Energy and Industrial Technology Development Organization (NEDO) drilled well WD-1a between 1994 and 1995 in the Kakkonda geothermal field as part of their Deep Seated Geothermal Resources Survey project. High-temperature measurements were carried out in WD-1a. Logging temperatures above 414°C were confirmed at 3600 m and 3690 m depth after 82 h standing time. Simple Horner extrapolations based on observed temperatures up to 82 h after shut-in suggested a temperature of about 500°C at 3500 m depth. Temperatures between 500°C and 510°C were also confirmed at 3720 m depth after 129–159 h standing time, using calibrated melting .tablets. These are the highest temperatures measured in a geothermal well. These results suggest a thermal structure consisting of three layers. Layer one is a shallow permeable zone of the reservoir, at less than 1500 m depth, at 230°C to 260°C. The second layer is a deep zone of the reservoir, which is less permeable and has a temperature of 350°C to 360°C from 1500 m to about 3100 m depth. The third layer is a zone of heat conduction. The transition between the hydrothermal-convection zone and the deeper heat-conduction zone is at 3100 m depth in well WD-1a.     

Experimental study on rapid cold start-up performance of PEMFC system

The cold start-up of a proton exchange membrane fuel cell is considered one of the main factors affecting the commercialization of fuel cell vehicles. In this study, an automotive fuel cell system was designed and tested for cold start-up at low temperatures. In the absence of PTC (Positive Temperature Coefficient) heating device, the stack was directly loaded to generate heat, which provided the cold start-up characteristics of system at low temperatures. Cold start-up process and purging control strategies were analyzed at −20 °C and −30 °C. It was found that the fuel cell system could produce 50% power in 25 s at −20 °C, the coolant temperature's heating rate was 0.78 °C/s, the coolant outlet temperature could reach 20 °C within 40 s and no apparent low voltage of single cell occurred. While, the cell close to the end plate had low cell voltage and reverse polar phenomena throughout the −30 °C cold start-up process. The heating rate of the coolant temperature was 0.44 °C/s, and the temperature of coolant outlet reached 20 °C within 90 s. The purging time ranged from 180 to 260 s according to the voltage drop value of stack and the ohmic resistance of stack was 360–470 mΩ after the high-volume air purging at different tests. After 30 cold start-up tests, the rated point performance of the stack declined by about 1%, and the consistency of cell voltages did not change significantly. Future work will focus on optimizing cold start-up strategy and speeding up purging time to minimize the performance impact of the cold start-up.     

Temperature–depth relationships based on log data from the Los Azufres geothermal field,Mexico

Equilibrium temperatures based on log data acquired during drilling stops in the Los Azufres geothermal field were used to study the relationship between temperature, depth and conductive heat flow that differentiate production from non-production areas. Temperature and thermal conductivity data from 62 geothermal wells were analyzed, displaying temperature–depth, gradient–depth, and ternary temperature–gradient–depth plots. In the ternary plot, the production wells of Los Azufres are located near the temperature vertex, where normalized temperatures are over 0.50 units, or where the temperature gradient is over 165°C/km. In addition, the temperature data were used to estimate the depth at which 600°C could be reached (5–9 km) and the regional background conductive heat flow (≈ 106 mW/m²). Estimates are also given for the conductive heat flow associated with the conductive cooling of an intrusive body (≈ 295 mW/m²), and the conductive heat flow component in low-permeability blocks inside the reservoir associated with convection in limiting open faults (from 69 to 667 mW/m²). The method applied in this study may be useful to interpret data from new geothermal areas still under exploration by comparing with the results obtained from Los Azufres.     

Effect of heat pre-treatment temperature on isolation of hydrogen producing functional consortium from soil

A functional hydrogen producing consortium was isolated from soil by heat pre-treatment technique and hydrogen production at different substrate concentration was evaluated. The forest soil was heat pre-treated at 65, 80, 95, 105 and 120 °C temperature for 1 h. As revealed by PCR-DGGE analysis and hydrogen yield, the hydrogen producing microbial community changed with increase in heat pre-treatment temperatures giving potential hydrogen producing consortium at 95–105 °C soil pre-treatment. The maximum hydrogen production rate, hydrogen yield and cumulative hydrogen with 15–20 g glucose were 1390–1576 mL/L/day, 1.83–1.93 mol H₂/mol glucose, and 2966–3146 mL H₂/L, respectively. The metabolic pathways shifted from ethanol-type to acetate–formate type as soil pre-treatment temperature increased from 65 to 120 °C. The soil heat pre-treatment approach is effective for isolating hydrogen producing natural Clostridium consortium from the soil as enumerations of the functional strains need specific temperature range to florish.     

Coke formation in the co-production of hydrogen and phenols from pyrolysis-reforming of lignin

The phenolics derived from pyrolysis of lignin are important fractions of bio-oil, which could be reformed to generate hydrogen. Nevertheless, some phenolics are value-added chemicals and they might not have to be utilized as source of hydrogen. In this study, we have explored the concept of co-production of hydrogen and phenolics via a pyrolysis-reforming process at the low to medium temperatures of 450–650 °C over Ni/Al₂O₃ catalyst. The results indicated that, below 500 °C, pyrolysis of lignin was almost the exclusive reaction route to form phenolics of varied side chains. The effective reforming of the phenolics initiated at 600 °C and became remarkable at 650 °C. Meanwhile, cracking of the methoxy group and other side chains of the phenolics was also intense at these higher temperatures, producing phenol as the main phenolic compound. Polymerization of the phenolics on Ni/Al₂O₃ catalyst occurred from 450 to 650 °C, while the gasification of the precursors of coke accelerated at 650 °C, forming remarkably lower amount of coke together with enhanced yield of hydrogen. The coke formed from the phenolics was generally the polymeric type with abundant aliphatic structures like C–O–C, –OH and -C-H, low thermal stability, low carbon crystallinity, hydrophilic surface and amorphous morphology at 450–650 °C. The polymerization or cracking of the phenolics could form multiple carbon layers in the vicinity of nickel species, and also on alumina.     

Oxygen isotopic ratios of sulfate ions-water pairs as a possible geothermometer

In this paper a brief review is given of the dependence of the oxygen isotopic fractionation of the sulfate ions-water system on temperature and the pH. From the available experimental data some relationships have been elaborated, which show that the isotopic exchange time is strongly temperature and pH dependent. The times for 97 per cent of isotopic exchange (near equilibrium conditions) at pH 7.0 are about 9 years at 200°C and 0.6 years at 330°C, while at pH 3.8 and at the same temperatures the times of exchange are 1.5 years and 0.08 years respectively. Thus, at the temperatures and pH of geothermal reservoirs the sulfate could be in isotopic equilibrium with environmental water, and the oxygen isotopic fractionation factors of sulfate-water geothermal pairs, being temperature dependent, can be used as geothermometers.Also reported here are some results on the O¹⁸ content of sulfate-water pairs from some wells on the edge of and outside the Larderello geothermal basin. The estimated isotopic temperatures are not very significant for the deep reservoir temperatures due to the geological features of the Larderello area which show important outcropping and deep anhydrite layers. Furthermore, as regards the wells outside the Larderello basin (Travale wells) some mixing of the geothermal water with colder underground water has been proved. However, the isotopic temperatures are generally higher than those measured at the well-head, and the highest ones are close to those estimated for the geothermal reservoir.In other geothermal areas more convenient from a geological point of view, the O¹⁸ content of the sulfate-water pair can be a useful and accurate thermometer.The O¹⁸/O¹⁶ ratios of several other sulfates (surface and deep anhydrite samples, sulfate ions in thermal springs) from the same area were also determined and differ substantially from borehole sulfate values.