3.1.Waste heat utilization technologies

Introduction

Primary energy is the energy extracted or captured directly from the environment. Three distinctive groups of primary energy are:

•Nonrenewable energy (fossil fuels): coal, crude oil, natural gas, nuclear fuel.

•Renewable energy: hydropower, biomass, solar energy, wind, geothermal, and ocean energy.

•Waste.

Fig. 1 Primary and secondary energy types

This course is devoted to that sector of the secondary energy resources, which includes heat wastes of energy transformation process. These wastes are used for heating and hot water supply, for power generation.

The definition of ‘heat waste’ is:

Waste heat, sometimes called secondary heat or low-grade heat, refers to heat produced by machines, electrical equipment and industrial processes for which there is no useful application in the same device. Energy is often produced by a heat engine, running on a source of high-temperature heat. A heat engine can never have perfect efficiency, according to the second law of thermodynamics, therefore a heat engine will always produce a surplus of heat. This heat is useful for the majority of heating applications, however, it is sometimes not practical to transport heat energy over long distances, unlike electricity or fuel energy. When produced by humans, or by human activities, it is a component of anthropogenic heat, which additionally includes unintentional heat leakage, such as from space heating.

The largest proportions of total waste heat are from industry, power stations and vehicle engines. The largest single sources are industrial plants such as oil refineries and steelmaking plants. The burning of transport fuels is also a major contribution to waste heat.

1 Sources of waste heat

The largest proportions of total waste heat are from industry, power stations and vehicle engines. The largest single sources are industrial plants such as:

• Gas pipeline compressor stations;

• Oil and gas extraction;

• Petroleum and coal products manufacturing;

• Cement plants;

• Chemical plants;

• Ethanol plants;

• Pulp and paper mills;

• Steel and metal manufacturing;

• Glass manufacturing and melting;

• Incinerators;

• Brick manufacturing;

• Artificial synthetic fibers, rubber, and resin manufacturing.

At a particular industrial site, some of the potential sources of waste heat include:

• Boilers;

• Process heaters;

• Furnaces, ovens, or kilns;

• Conductive, convective, and radioactive losses from hot equipment surfaces;

• Conductive, convective, and radioactive losses from heated product streams;

• Hot combustion gases otherwise discharged to the atmosphere;

• Natural gas pipeline compressor;

• Other natural gas-fired turbines or oil-fired turbines;

• Reciprocating engine exhaust;

• Cooling water from furnaces, air compressors, and internal combustion engines.

Below some examples of heat waste sources and its potential value are presented:

1. Heat in flue gases: the higher the temperature, the greater the potential value for heat recovery;

2. Heat in vapour streams: as above but when condensed, latent heat also recoverable;

3. Convective and radiant heat lost from exterior of equipment: low grade – if collected may be used for space heating or air preheats;

4. Heat losses in cooling water: low grade – useful gains if heat is exchanged with incoming fresh water;

5. Heat losses in providing сhilled water or in the disposal of chilled water:

high grade if it can be utilized to reduce demand for refrigeration;

low grade if refrigeration unit used as a form of heat pump;

6. Heat stored in products leaving the process: quality depends upon temperature;

7. Heat in gaseous and liquid effluents leaving process: poor if heavily contaminated and thus requiring alloy heat exchanger.

The burning of transport fuels is also a major contribution to waste heat.

Energy using systems in buildings and infrastructure (such as electricity substations and air conditioning units) produce heat as a by-product of their operation. This heat is usually rejected to the atmosphere. The heat can be captured, stored and distributed, with the potential to provide low-cost heating to homes and businesses. Initial analysis suggests that utilizing these rejected heat sources is viable at a scale that could attract significant investment. Utilizing the rejected heat would also reduce CO₂ and NO_x emissions.