Power Recovery From In-Situ Combustion Exhaust Gases
Authors: Stinson D.L. , Carpenter H.C. , Cegielski J.M. 
Published: December 2007

A field evaluation of the use of a small gas-combustion turbine generator set to recover power from in-situ combustion exhaust gases was conducted. With suitable modifications to the turbine, exhaust gases can be exchanged for compressed air; and the energy of compression, the sensible heat, and the heat of combustion in exhaust gases can be recovered.

Introduction
A worldwide energy shortage and the resulting increase in the price of crude oil has created renewed interest in any process for the recovery of energy. The in-situ combustion of oil to enhance the recovery of crude oil from reservoirs has been the subject of increased study. Either forward or reverse combustion requires the handling of large volumes of air at high pressures. Air-to-oil ratios ranging from 4,000 to 20,000 scf/bbl, with some ratios of more than 40.000 scf/bbl, have been reported. The facilities to compress such volumes of air not only represent significant capital investments and operating costs, but also a very important energy consumption. The cost of the compressed air alone can vary from 40 cents to more than 8 dollars per barrel of oil produced. The energy of compression can represent as much as 25 percent of the heating value of the oil recovered. Any system to reduce these expenses and energy demands would improve the economic feasibility of in-situ combustion projects.
A normal fireflood also produces large volumes of exhaust gases. In most cases these gases are more than 90-percent nitrogen, steam, and carbon dioxide, with minor amounts of methane, carbon monoxide, and higher hydrocarbons. The net heat of combustion of such gases is so low that they will not burn under normal conditions. In some cases, it even has been necessary to incinerate the produced gases with supplemental fuel to reduce air pollution caused by the unburned and partially burned hydrocarbons. partially burned hydrocarbons. The in-situ gasification of coal or retorting of oil shale, and even the high-pressure oxidation of sewage sludge, present many of the same problems. That is, they require large volumes of high-pressure compressed air and produce similar volumes of exhaust gases with low net heats of combustion. Usually the exhaust gases from these processes are combustible under special conditions. A system that would handle crude oil in-situ combustion would also solve the problems of these other processes.
A gas-expansion turbine is the usual device used for the recovery of energy from large-volume, low-pressure exhaust streams. Common examples include turbo-chargers on internal combustion engines and the exhausts gas expansion turbine used with nitric acid plants.
A gas-combustion turbine used for air compression offers a number of unique advantages for energy recovery from exhaust-gas streams. Under typical operating conditions, a gas-combustion turbine requires as much as 400 to 500 percent of excess air to control the temperature of the hot gases entering the turbine. If in-situ combustion exhaust gases are available at pressures slightly above the turbine's combustion-chamber pressure, the exhaust gases can be traded for the excess compressed air without altering the power produced by the expansion turbine. This substitution of one gas for another provides a 100-percent-efficient recovery of the energy of compression contained in the exhaust gases. If the exhaust gases are introduced into the gas-combustion turbine at high temperature, the expansion-turbine section will also use their sensible heat and the energy of expansion of the steam that is usually produced with such gases. produced with such gases. JPT
P. 645