Heat Recovery With Gas

Heat Recovery with Gas

Dear Reader,

To complete our series of articles focusing on the topic of heat recovery, this issue is intended to give you some information about Heat Recovery with Gases. Besides of liquid heat carrier fluids or vapours, gaseous media can also be used to make waste heat usable as process heat. As with any other heat recovery concept, the aim is to create a benefit for the investor by using the waste heat. The replacement of fossile heat carriers are by already existing thermal energy, fuel costs are saved. At the same time there are even environmental benefits from the reduction of fossile fuel, which results in a reduction of green house gas emissions. The motivation of industrial producers to invest in new equipment without having a clear plan for a reasonable return on investment is limited. Moreover, most of the investment budget is also limited. Therefore, heat recovery projects compete with investments in e.g. more efficient production technologies. Gases have a low density and low specific heat capacity compared to liquid heat carriers or vapours such as steam. Consequently, large duct systems are required to transport a mass flow with sufficient heat capacity. Large ducts are expensive and require a lot of room for installation in combination with strong supporting constructions. Due to the large surface areas, infrastructure such as thermal insulation is also expensive. Furthermore, the blowers required to transport the gases are large and energy intensive. Due to the relatively high costs of transporting gases, it often makes sense to convert the energy into liquid heat carriers if the energy has to be transported over long distances. When talking about heat recovery with gases, we mainly talk about air. Since it is important to keep the distance short and the benefit high, this article focuses on combustion air preheating. This may be one of the most common and most economical applications for heat recovery with gases. Wherever heat is generated by combustion, oxygen is needed for the combustion process. Oxygen is usually supplied by the ambient air, which is mixed to the fuel to generare a combustible mixture. The mixture that is burned in a combustion chamber generates hot gases. These hot gases supply the heat to a heating surface by radiation and/or convection. The heating surface transports the thermal energy into the medium (cold medium) to be heated (there are also directly heated applications, but more about that in later Newsletters). The energy supplied by the fuel is needed to heat a reasonable volume of gas in the combustion chamber to a temperature sufficient to raise the temperature of the cold medium to the target temperature. Heat transfer can only take place if there is a temperature difference within hot and cold media. Therefore, the heat that remains in the exhaust gas after passing the heating surface cannot be used anymore. These heat losses are released through the chimney. It also means that the equivalent part of fuel, which was used to heat the gas in the combustion chamber is lost, as it cannot be used to heat the cold medium. Ocassionally it is possible to use this residual energy by heating warm water or another cold medium. Unfortunately, in many cases, there is no heat sink nearby where the excess heat could be used. A very economical solution to convert waste heat into value is to preheat the air (combustion air), which is supplied to the burner that heats the flue gas. The energy from the fuel must no longer be used to heat the gas in the combustion chamber from the ambient temperature, but only from the combustion air temperature after the combustion air preheater. In this way, fuel savings of 10 % are not exceptional, and therefore the payback period is often times less than a year. To illustrate this, here is the example from the first article in this series, where we have already mentioned examples of the economic result of combustion air preheating.