However, there are limits to this as the speed of reaction is also temperature-dependent. For example, if an equilibrium reaction between AB and CD produces AC and BD and the equilibrium can be shifted rightward by increasing temperature, continuously removing AC or BD from the reaction can serve to reduce the temperature requirements (c.f. In equilibrium reactions where temperature is one of the factors influencing the equilibrium, temperature requirements can be reduced by removing the desired products in a continuous process. As higher grade heat is more cumbersome and expensive to produce and as materials have limited heat resistance, there are efforts to reduce working temperatures wherever possible through the use of catalysts and fluxes. This is because heat naturally flows from hot to cold and it is thus always possible to use a high temperature source of heat for lower temperature applications but not vice versa. Heat is usually described by "grade" with higher temperatures having a higher "grade". Some processes such as molten salt electrolysis provide the required process heat by the same electricity that is also needed to keep the endothermic reaction going. Heat pumps which are commonly employed for home heating, warm water and other heat applications below 100 ☌ (212 ☏) have too low a Carnot efficiency at high temperature differences between "hot" and "cold" end to be worthwhile. Thus this source of heat is only used where electricity from non-thermal sources (such as hydropower) is cheap and plentiful. Resistive heating would in theory be a possible source of process heat but even as it converts nearly 100% of the supplied electricity to heat, it is obviously less efficient to burn a fuel in a thermal power plant to produce electricity only to use that electricity for process heat than to use the fuel directly. Those processes operating below the median can draw on a much wider variety of sources, including waste heat from other processes in the same industrial process. These higher-temperature processes can generally only be supplied by dedicated supplies like natural gas or coal, although pre-heating from other sources is also common in order to reduce fuel use. The required temperature of the process varies widely, with about half the industrial process heat having operating temperatures above 400 ☌ (752 ☏).
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