Oil shale processing
The key to producing shale oil from oil shale on a commercial basis is a process technology with acceptable mechanical, economic and environmental performance. To date, no process which reliably meets these criteria has been available to unlock the unconventional oil inherent in the world’s oil shale resources. So the very substantial global shale oil resources lie largely untapped.
There are two basic known routes for conversion of kerogens to shale oil: retorting and thermal solution. There are also two basic approaches to retorting: processing of mined oil shale ore in a retort on the surface, and retorting of deep oil shale strata without mining, referred to as in-situ retorting.
The thermal solution route is based on a radically different approach to heating oil shale to achieve pyrolysis in a surface facility. While still an emerging technology, the most advanced thermal solution route for processing oil shale appears to be the Rendall Process.
The fundamental differences between the retorting and thermal solution as alternative approaches to kerogen pyrolysis lead directly to the potential for materially different technical, economic and environmental performances, as explained below.
Surface retorting is long established, having been used in many countries for more than 150 years. Pyrolysis in a retort operating at a temperature of 500-600°C and atmospheric pressure yields hydrocarbon vapours which form unsaturated oil on condensation. The retort also generates a low heating value gas, with significant quantities of coke and heavy tars remaining in the matrix of the spent shale residue. Portions of the coke and heavy tars are burnt to heat incoming oil shale ore.
Oil shale retorts have been built in a wide variety of sizes, configurations and complexities over the years. Retorting oil yields vary from about 35% to 70% conversion of the kerogen, depending mainly on the kerogen type and grade in the oil shale ore. However, oils produced by retorting processes inevitably contain substantial proportions of unsaturated hydrocarbons, which are prone to spontaneous polymerisation and hence are unstable in storage and handling. Accordingly, these hydrogen deficient retorted shale oils require immediate upgrading in a separate hydrotreater before they can be delivered to their markets.
Whether directly or indirectly, retorts are invariably heated by combustion of a significant portion of the kerogen in the oil shale or, more commonly, its breakdown products. In addition, temperature control presents many challenges in the necessarily large vessels in which the oil shale is heated, resulting in both over and under heated zones. Overheating increases gas make at the expense of oil yield and consumes additional kerogen as fuel. Under heating not only increases the tar make at the expense of oil, but also results in the formation of partially-oxidised hydrocarbons such as phenols and cresols, which are both noxious and toxic. With air and hence oxygen in the system, any chloride minerals in the shale are also liable to lead to formation of dioxins. Because the retorting reactions take place in an open system, it is difficult to prevent fugitive emissions into the atmosphere of the toxic side products. Certain of these fugitive emissions come directly from the retort; others in stack gases; or from the shale residue after its discharge from the retort. These emissions often present significant hazards to public health as well as to on site personnel.
Retorts are also characterised generally by heavy demand for water to quench hot spent shale residues on discharge, in order to prevent spontaneous combustion of the accompanying tars and coke. The quenching process itself may also liberate further environmentally damaging emissions. As a result, the latest retort designs have focussed on reducing water requirements.
In Australia, trials by Southern Pacific Petroleum N.L. (“SPP”) of the high efficiency, 8 m diameter, 4,500 bopd ATP retort at the Stuart deposit were concluded in 2004. Following the takeover of SPP by Queensland Energy Resources (QER), this has been followed not by a scaled up 11 m diameter ATP retort as planned earlier by SPP, but by a new program testing Stuart shale in a 1 tph Paraho vertical retort pilot plant in Rifle, Colorado. QER is currently in the last stages of commissioning a small-scale Paraho retort at Gladstone.
Major research has also been continuing on in-situ processing technologies, notably by Shell and Exxon, but none have been tested on a commercial scale.