Recovery mission - the search for new sources of oil
The search for new sources of oil is ongoing and Estonia is the current area of interest. Michael Forrest talks to Andreas Orth, Vice-President for Energy, at Outotec, Germany, about oil shale.
Energy security is important for developed nations that rely on imported hydrocarbons, particularly oil, whose price has remained firmly above US$50 per barrel since 2005, apart for a momentary nadir during the 2009 financial crisis. Only six months earlier it had peaked at over US$130/billion barrels (bbl).
Sustained high oil prices have triggered a number of investments in a variety of hydrocarbon resources, including oil sands (see Materials World, pp37-38, October 2009) and coal to liquid (Materials World, pp31-33, June 2008). However, one of the largest hydrocarbon resources can be found in oil shales, a wide variety of fine-grained sediments containing complex organic hydrocarbons in the form of kerogen. Unlike hydrocarbons in oil sands and coal, kerogens cannot be dissolved in organic solvents, such as toluene. Thermal retorting is the only economic method of recovery available.
There are extensive oil shales in the geological record in Australia, China, Estonia, Jordan and the USA, with hydrocarbon resources second only to coal. The latter was featured in the news recently following a report from the Colorado Energy Research Institute that indicated a global resource was 2.8 trillion barrels, half of which are found in the Green River Formation that underlies 20,000km2 of Colorado, Utah and Wyoming, USA.
A long-standing oil shale industry can be found in Estonia, where commercial production began in 1925, although other countries, such as Scotland, had similar industries a century earlier. To a certain extent, the nature and grade (measured in recoverable barrels per tonne) of the shale determines the type of retort used in recovering the contained oil, and, over the years, several types were developed.
‘Outotec’s involvement in oil shale processing began in 2008 when we started a joint venture with Eesti Energy, Estonia, which was officially signed in July 2009,’ says Andreas Orth of Outotec’s Energy Business Line, based in Oberursel, Germany. ‘Our interest was stimulated by the application of our ore handling and processing systems used in the coal and minerals industries around the world. Specifically, our technology in fine-grained material handling and fluidised bed systems have a direct application in the processing of oil shales.’
He adds, ‘The history of oil shale retorting has been one of continually upgrading thermal efficiency in order to maximise recovery, whilst reducing the amount of carbon in the waste product.’
Thermal decomposition of oil shales begins slowly at around 200ºC, but requires temperatures in the range 450-600ºC to complete the transformation, where kerogen breaks down to produce condensible hydrocarbons, uncondensable retort gas, pyrolysis water and a carbonaceous residue. Previously, the quantity of recovered oil was the driving factor, and consequent research and process has focused on maximising oil yields.
Retorting of oil shale requires heating a large mass of rock to pyrolyse the contained kerogen. The organic content of oil shales is around 15-30%, and hence more than 70% of the heating energy is non-productive. Accordingly, an effective retort uses the least amount of energy, preferably by transferring heat from spent shale to the raw input without the use of valuable off-gas condensates.
The first retorts transferred heat through a wall to the raw shale, and consequently suffered low thermal efficiency and small capacity. Later, directly heated retorts were developed that passed hot gas through the shale, normally in a vertical configuration, using off-gases and combustible carbon as the heat source. A second version, created heat via combustion of the off-gas, recycled through the raw shale. In each case, a proportion of the economic product was used in heating the shale. Another disadvantage of the indirect and direct retorts was their inability to process fine-grained shales, resulting in uneven gas flow through the retort and a consequent inability to scale-up.
A third class of retort has been developed in the past 30 years where heat is transferred by combining hot solid heat carriers with raw shale. This requires a more complex material handling system, offset by a number of advantages – high oil yields due to rapid heating rates (flash retorting), undiluted gas and vapour, exploitation of spent shale carbon used as a heat carrier, and no restriction of shale grain size hence scalability to large sizes. Four types are currently in use – Lurgi-Rhurgas, Tocso II, Galoter and Chevron STB.
‘This type of retort is moving towards the materials handling and fluidised bed technology developed by Outotec (formerly Outokumpu Technology) over the past 50 years,’ explains Orth. ‘The most important aspect has been the development of the fluidised bed [which is] ideally suited for processing fine grained materials in a variety applications, from roasting, calcination, reduction, gas cleaning and heat recovery. Lurgi Metallurgy, a company we acquired in 2001, developed the first fluidised bed reactor for roasting sulphide minerals. The system was quickly adopted, replacing multiple hearth furnaces and rotary kilns. [It] reduced emissions and conserved heat while off-gas treatment allowed the production of sulphuric acid.’ Development of the technology has continued. In the classic bubbling bed reactor, most of the particles do not leave the surface, as they are kept in balance by gravity.
Gas velocity has similar particle movement. In the circulating fluidised bed, higher gas velocities allow entrainment of solid particles that can be separated from the gas by a cyclone and recycled through the reactor resulting in a homogeneous temperature distribution.
Orth says, ‘Circulating fluidised bed is an ideal environment for processing oil shales, and our joint venture has already resulted in a flow sheet for treating Estonian shales.’ After crushing and screening, the shale is dried and heated to reaction temperatures. This decomposes the shale to retort gas, oil fumes, and solids. The gases and oil fumes are condensed into oil and fuel gases.
‘One of the principal objectives is to recycle energy using several heat and mass flow cycles to reduce heat loss. The retort is run at the most economical temperature resulting in a small amount of kerogen remaining in the carbonaceous solids. These are used to produce heat in a circulating fluidised bed reactor. The process extracts and uses about 80% of the energy in the oil shale’, claims Orth.
With the announcement in March of the joint venture with Eesti Energia to create new energy sources, Outotec has expanded its portfolio of technology to include oil shale plant engineering. Drawing on several decades of its ore processing technology, the company is now offering its beneficiation technologies to oil shale.