Pyrolysis or Gasification of Oil Shale
In the central region of Jordan, the huge and as yet unexploited, oil-shale deposits (i.e. about 5×1010 tonnes) could satisfy the country’s energy needs completely for several hundred years: the shale has an average organic content of between 9 and 13% by weight. Because there has been no interest in utilising this resource, there is little information available about the pyrolysis and/or gasification of Jordanian oil-shales. This is mainly due to the relatively low unit price of conventional competing forms energy, such as crude oil and natural gas, prevailing in the international market. In other words, at present the unit cost of the final energy produced (i.e. shale oil or electricity eventually generated via direct combustion or gasification of the shale) would exceed that achieved by using imported crude oil instead. However, in the long term, oil-shale could prove to be a worthwhile route in supplementing traditional sources of fossil fuels.
There are three main means for developing oil-shale based power systems. The first is based on the direct combustion of the raw shale; in the second, burning shale oil (produced from oil-shale) to stimulate a conventional Rankine cycle system, and the third employs an integrated gasification combined cycle gas turbine. The efficiencies of the first two options would be approximately similar to those achievable with coal or oil-fired systems: based on the limited available experience with a fluidised-bed combustor, which burns oil-shale, these are 
36 (±2)% and may reach 40% for the best scenario. The efficiency is increased dramatically (i.e. to 48%, or even higher) when a combined cycle arrangement is used. Improving the efficiency of a system reduces the fuel consumption as well as the rates of emission of pollutants, which are generated for a specific power output. It also has the knock-on advantage of decreasing the adverse environmental impacts along the whole fuel-supply train (i.e. mining, handling and crushing as well as storage and transportation).
Oil-shale is a complex heterogeneous mixture of kerogen (i.e. organic matter) and a wide range of minerals. The shale oil (produced by pyrolysis) or gaseous fuel (generated from gasification) are the results of several physical and chemical reactions occurring in series and parallel. The kinetics of the thermal degredation of various oil-shales have been investigated and different suggestions as to the decomposition mechanisms have been reported. Although some researchers have tried to explain the complex features of the thermal decomposition of oil-shale, others have reported that it would be sufficient to consider a global first-order kinetic expression to represent the overall decomposition rate, when small shale-particles are used: this may well be acceptable, sufficiently accurate and reliable from an engineering point-of-view.
Many thermogravimetric studies have been undertaken under isothermal conditions, but these involve some inaccuracies (i.e. mainly due to the preliminary heating up of the apparatus). The use of a non-isothermal method for determining the kinetic parameters of the pyrolysis process, by employing a TGA based on heating the sample at a constant rate and recording its weight change, is easier to achieve accurate results than with isothermal methods. This is mainly because of the shorter experimental times and the fewer difficulties, as a result of the initial heating-up period, compared with isothermal methods. Thus, such a technique has been preferred by many researchers in determining the reaction kinetics, such as the activation energy.
From the oil-shale literature, it can be seen that there are several variables that affect the thermal behaviour of the oil-shale during the retorting or gasification processes. The most important factors that may affect the yield of final products from the oil-shale are the type of the sweep gas employed for purging the system, the heating rate, particle size, final temperature and the prevailing gas-pressure. Also, there are other factors, such as the adopted crushing and grinding techniques, which may change the carbon distribution and hence its content in the oil-shale particles, and oxidation of the iron pyrites (present in the shale) which would react with the kerogen, so influencing both the pyrolysis and gasification processes. In order to maximise the outputs and conversion efficiencies of these processes, optimal process conditions should be identified and applied. From previous studies, it can be concluded that the pyrolysis (or gasification) temperature greatly influences the yield of the final products, but there is disagreement concerning the magnitude of the influence of the particle size on the shale-oil yield as the desired product from the pyrolysis process. This confusion, most probably, has arisen because the type of oil-shale and its grade have not been taken into account.
In the present study, the kinetics of the pyrolysis and gasification processes have been investigated, using a TGA apparatus under non-isothermal conditions, for two Jordanian oil-shales in relation to the final temperature, heating rate and particle size.
Oil-shale is the major and most promising indigenous fossil-fuel resource for Jordan, yet it is not used there at present. The representative oil-shale samples (from the Ellujjun and Sultani deposits in the central region of Jordan) were provided by the Ministry of Energy and Mineral Resources, Amman, Jordan, but details about the sampling method used were not forthcoming. The two samples were crushed separately without further treatment, as received, by a jaw crusher, then sieved into the required particle-size ranges. These oil-shales have been examined in a pyrolysis study (using a fixed-bed retort): this gasification investigation (using continuous fluidised-bed reactor) has been reported previously. Table 1 shows the elemental analyses of both samples: the other contents of the raw shale include oxygen and water. More details about these samples and the locations of the oil-shale deposits can be found eleswhere. The Ellujjun oil-shale deposit is, to date, the only one that has been surveyed and analysed for exploitation purposes. The other known deposits (e.g. at the Sultani site, which has almost the same potential of 106 tonnes of oil-shale as that of the Ellujjun deposit) have received much less attention.
Kinetic data were obtained using a Shimadzu Model-50 Series TG Analyser. The employed purge gas was N2, in the case of pyrolysis, and CO2, in gasification tests, supplied at a constant rate of 5×10?5 m3 min?1. The TGA apparatus permits the continuous measurement of sample weight as a function of temperature, and provision is made for an electronic differentiation of the weight signal to give the instantaneous rate of weight loss. In this investigation, TGA data were used to determine the effects of the heating rate, particle size and final temperature on the weight loss of each oil-shale sample.
Pyrolysis or gasification was carried out non-isothermally using a sample, of between 10?5 and 5×10?5 kg, placed in the alumina crucible, which was then put on the sample pan hanging down in the reaction tube, in which the atmosphere could be controlled. The furnace tube was raised to close the system, and the start button depressed. The pre-programmed control-unit regulates all the automatic functions of the recorder (e.g. the continuous change in the mass of the sample is measured), as well as the temperature programming of the furnace. Finally, and after the furnace temperature had achieved its set value, the sample was allowed to cool to normal room-temperature. Table 2 lists the main conditions that have been employed for all the experimental tests of this investigation.
Tags: oil shale
