REG is capable of using heat from a variety of sources, such as steam and/or flue gases, and then combining these to yield a greater output of electricity generated. Dhunlall says that the amount of money or energy that could potentially be saved by using REG can only be quantified once more information on the potential heat streams available is known. However, he notes, with low-pressure steam in the order of 1,3 bar and a flow of 10 t/h, one would be able to generate about 1 MW of electricity.

Dhunlall says that stack-exhaust losses are inherent in all fuel-fired processes, and increase with the exhaust temperature, and the amount of excess air the exhaust contains. He says that at stack gas temperatures greater than 540 ºC, the heat going up the stack is likely to be the single biggest loss in the process, and, at temperatures above 985 ºC, stack losses will consume at least one-half of the total fuel input in the process.

However, Dhunlall says that the energy that is recovered from waste heat streams could displace part, or all, of the energy input needs for a unit operation in a plant. Therefore, he says, waste heat recovery offers an opportunity to use energy productively and reduce overall plant energy consumption and greenhouse-gas emissions.

REG involves capturing unused waste heat from industrial processes and converting it into electricity that can be sold to power purchasers or used on site without any additional fuel consumption, and with zero additional emission. REG uses an organic Rankine cycle to generate electricity that is essentially similar to a traditional steam turbine, with the exception that this system can be used with low-pressure steam.

The Rankine cycle is a thermodynamic cycle used to produce electricity in many power stations, and is the practical approach to the ‘ideal’ Carnot cycle. Superheated steam is generated in a boiler, and then expanded in a steam turbine. The turbine drives a generator, to convert the work into electricity.

A disadvantage of using the water-steam mixture, says Dhunlall, is that superheated steam has to be used, otherwise the moisture content after expansion might be too high, which would erode the turbine blades. Instead of water, an organic fluid can be used and this is called an organic Rankine cycle. The major advantage of this, he says, is that these fluids can be used below a temperature of 400 ºC.

Dhunlall believes that many companies are unaware of REG as the technology was initially developed for geothermal power plants, and as South Africa has no, or limited, geothermal activity, it has not been widely publicised. Further, he says, the historically low price that was paid for power has limited com- panies to being power purchasers and not power producers, with the exception of a few of the larger entities, such as Sasol.

Waste-heat recovery methods used with industrial process heating operations intercept waste gases before they leave the process, extracting some of the heat they contain, and recycling that heat back into the process.

Dhunlall admits that the economic benefits of waste-heat recovery does not justify the cost of these types of systems in every applica- tion. Heat recovery from lower temperature waste streams such as hot water or low-temperature flue gas is thermodynamically limited, while equipment fouling, occurring during the handling of ‘dirty’ waste streams, is another barrier to more widespread use of heat recovery systems.

Dhunall says that no particular challenges exist in implementing or using REG.

“As long as there is an available heat source, the unit will produce electricity. “The typical challenge is the type of the heat source, the quality of the flue gases, the availability of the source, and the consistency of the heat flow. “Most challenges are in the design of the tapping of these sources and the availability of space for the system,” he says.

Dhunlall says that REG is suited to a wide variety of industries, such as cement, petrochemicals and metals.

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