7.3.4 Heat and power recovery
Energy recovery provides major energy efficiency and mitigation opportunities in virtual all industries. Energy recovery techniques are old, but large potentials still exist (Bergmeier, 2003). Energy recovery can take different forms: heat, power and fuel recovery. Fuel recovery options are discussed in the specific industry sectors in Section 7.4. While water (steam) is the most used energy recovery medium, the use of chemical heat sinks in heat pumps, organic Rankine cycles and chemical recuperative gas turbines, allow heat recovery at lower temperatures. Energy-efficient process designs are often based on increased internal energy recovery, making it hard to define the technology or determine the mitigation potential.
Heat is used and generated at specific temperatures and pressures and discarded afterwards. The discarded heat can be re-used in other processes onsite, or used to preheat incoming water and combustion air. New, more efficient heat exchangers or more robust (e.g., low-corrosion) heat exchangers are being developed continuously, improving the profitability of enhanced heat recovery. In industrial sites the use of low-temperature waste heat is often limited, except for preheating boiler feed water. Using heat pumps allows recovery of the low-temperature heat for the production of higher temperature steam.
While there is a significant potential for heat recovery in most industrial facilities, it is important to design heat recovery systems that are energy-efficient and cost-effective (i.e., process integration). Even in new designs, process integration can identify additional opportunities for energy efficiency improvement. Typically, cost-effective energy savings of 5 to 40% are found in process integration analyses in almost all industries (Martin et al., 2000; IEA-IETS, n.d.). The wide variation makes it hard to estimate the overall potential for energy-efficiency improvement and GHG mitigation. However, Martin et al. (2000) estimated the potential fuel savings from process integration in US industry to be 10% above the gain for conventional heat recovery systems. Einstein et al. (2001) and the US DOE (2002) estimated an energy savings potential of 5 to 10% above conventional heat recovery techniques.
Power can be recovered from processes operating at elevated pressures using even small pressure differences to produce electricity through pressure recovery turbines. Examples of pressure recovery opportunities are blast furnaces, fluid catalytic crackers and natural gas grids (at sites where pressure is reduced before distribution and use). Power recovery may also include the use of pressure recovery turbines instead of pressure relief valves in steam networks and organic Rankine cycles from low-temperature waste streams. Bailey and Worrell (2005) found a potential savings of 1 to 2% of all power produced in the USA, which would mitigate 21 MtCO2 (5.7 MtC).
Cogeneration (also called Combined Heat and Power, CHP) involves using energy losses in power production to generate heat for industrial processes and district heating, providing significantly higher system efficiencies. Cogeneration technology is discussed in Section 4.3.5. Industrial cogeneration is an important part of power generation in Germany and the Netherlands, and is the majority of installed cogeneration capacity in many countries. Laurin et al. (2004) estimated that currently installed cogeneration capacity in Canada provided a net emission reduction of almost 30 MtCO2/yr (8.18 MtC/yr). Cogeneration is also well established in the paper, sugar and chemical industries in India, but not in the cement industry due to lack of indigenously proven technology suitable for high dust loads. The Indian government is recommending adoption of technology already in use in China, Japan and Southeast Asian countries (Raina, 2002).
There is still a large potential for cogeneration. Mitigation potential for industrial cogeneration is estimated at almost 150 MtCO2 (40 MtC) for the USA (Lemar, 2001), and 334 MtCO2 (91.1 MtC) for Europe (De Beer et al., 2001). Studies also have been performed for specific countries, for example Brazil (Szklo et al., 2004), although the CO2 emissions mitigation impact is not always specified.