2 Main definitions used for heat waste assessment

- Waste heat - the energy of heat that is produced in a technological process and could be used in another process, such as heating feedwater or air, power generation, etc, or dumped heat that can still be used;

- The amount of recoverable heat – the total heat that could potentially be recovered from waste heat for space heating, hot water supply or power generation and etc. in heat waste exchanger or other device; it can be calculated using this formula:

where, Q - is the heat content in kcal; V - is the flow rate of the substance in m³/hr; r - is density of the flue gas in kg/m³; C_p - is the specific heat of the substance in kCal/kg ⁰C; ∆T - is the temperature difference in ⁰C.

• The annual heat saving – the amount of recoverable heat that can be saved per year due to waste heat utilization;

- Fuel saving – the amount of gas, oil, etc., that can be saved due to recoverable heat from waste heat utilization.

- Payback period - the period of time required for the return on an investment to "repay" the sum of the original investment.

- Secondary energy resources.

3 Using of waste heat for heating and hot water supply. Equipment for using of industrial waste heat

The industrial sector accounts for approximately one third of all energy used.

Efforts to improve industrial energy efficiency focus on reducing the energy consumed by the equipment used in manufacturing (e.g., boilers, furnaces, dryers, reactors, separators, motors, and pumps) or changing the processes or techniques to manufacture products. A valuable alternative approach to improving overall energy efficiency is to capture and reuse the lost or "waste heat" that is intrinsic to all industrial manufacturing. In some cases, such as industrial furnaces, efficiency improvements resulting from waste heat recovery can improve energy efficiency by 10% to as much as 50%.

Captured and reused waste heat is an emission­free substitute for costly purchased fuels or electricity. Numerous technologies are available for transferring waste heat to a productive end­use. Nonetheless, a great amount of waste heat energy remains unrecovered as a consequence of industrial manufacturing.

Three essential components are required for waste heat recovery: 1) an accessible source of waste heat, 2) a recovery technology, and 3) a use for the recovered energy. Topics investigated for each waste heat source include waste heat quantity and quality, available recovery technologies, and barriers to implementing heat recovery. The results of this analysis are used as the basis for identifying needs that can increase industrial energy efficiency by improving waste heat recovery technologies.

Each waste heat stream is investigated in terms of its waste heat quantity (the approximate energy contained in the waste heat stream), quality (typical exhaust temperatures), current recovery technologies and practices, and barriers to heat recovery. Energy content of waste heat streams is a function of mass flow rate, composition, and temperature, and is evaluated based on process energy consumption, typical temperatures, and mass balances. The work potential (based on Carnot efficiency) is a measure of the maximum energy that could be recovered by using the waste heat to drive a heat engine. Quantifying work potential allows a better comparison of waste heat sources with different exhaust temperatures.

The potential for heat recovery is further scoped out by discussing current waste heat recovery practices and barriers to heat recovery for each unit assessed. Finally, the results from the bottom­up analysis of waste heat sources are used to identify technology development needs for wider implementation of industrial waste heat recovery. Technology needs are discussed in the context of existing technologies, which can be further optimized, as well as developing technologies that may provide new opportunities for heat recovery.

Investigation of current waste heat recovery practices shows that waste heat is generally recovered from clean, high­temperature waste heat sources in large capacity systems. Key opportunities are available in optimizing existing systems, developing technologies for chemically corrosive systems, recovering heat from non­fluid heat sources, and recovering low­temperature waste heat.

Waste heat recovery systems are frequently implemented, but constrained by factors such as temperature limits and costs of recovery equipment. There are a number of cases where heat recovery equipment is installed, but the quantity of heat recovered does not match the full recovery potential. Key barriers include heat exchanger material limits and costs for extending recovery to lower temperature and higher temperature regimes.

Most unrecovered waste heat is at low temperatures.

The waste heat streams analyzed in this study showed that roughly 60% of unrecovered waste heat is low quality (i.e., at temperatures below 232°C). While low­temperature waste heat has less thermal and economic value than high­temperature heat, it is ubiquitous and available in large quantities. Comparison of total work potential from different waste heat sources showed that the magnitude of low­temperature waste heat is sufficiently large that it should not be neglected in pursuing opportunities for waste heat recovery. New technologies are developing that may provide significant opportunities for low­temperature heat recovery.

There are certain industrial subsectors where heat recovery is less common, due to the factors such as heat source’s chemical composition. High­temperature, high­quality heat is wasted in some subsectors due to corrosive/fouling chemicals contained in the waste heat stream.

Losses from nontraditional waste heat sources are difficult to recover, but significant. This study focused on exhaust gas waste heat losses; however, it was found that alternate sources of waste heat are also significant. These include heat lost from hot product streams (e.g., hot cast steel) and hot equipment surfaces (e.g., aluminum sidewalls).

Industrial waste heat can be used in opened, closed and opened-closed circuit schemes.

In closed-circuit scheme waste heat is used for main process equipment (preheating combustion components, load preheating).

Opened-circuit schemes are characterized by using of waste heat for external purposes not related to the main process equipment, which is the source of waste heat (steam generation, space heating, hot water supply for other technological process or consumers, etc.).

The third way - combined facilities in which waste heat is used for both internal and external applications (close-open circuit schemes).