7.3.2 Energy efficiency
IEA (2006a) reports ‘The energy intensity of most industrial processes is at least 50% higher than the theoretical minimum determined by the laws of thermodynamics. Many processes have very low energy efficiency and average energy use is much higher than the best available technology would permit.’ This provides a significant opportunity for reducing energy use and its associated CO2 emissions.
The major factors affecting energy efficiency of industrial plants are: choice and optimization of technology, operating procedures and maintenance, and capacity utilization, that is the fraction of maximum capacity at which the process is operating. Many studies (US DOE, 2004; IGEN/BEE; n.d.) have shown that large amounts of energy can be saved and CO2 emissions avoided by strict adherence to carefully designed operating and maintenance procedures. Steam and compressed air leaks, poorly maintained insulation, air leaks into boilers and furnaces and similar problems all contribute to excess energy use. Quantification of the amount of CO2 emission that could be avoided is difficult, because, while it is well known that these problems exist, the information on their extent is case-specific. Low capacity utilization is associated with more frequent shut-downs and poorer thermal integration, both of which lower energy efficiency and raise CO2 emissions.
In view of the low energy efficiency of industries in many developing counties, in particular Africa (UNIDO, 2001), application of industry-wide technologies and measures can yield technical and economic benefits, while at the same time enhance environmental integrity. Application of housekeeping and general maintenance on older, less-efficient plants can yield energy savings of 10–20%. Low-cost/minor capital measures (combustion efficiency optimisation, recovery and use of exhaust gases, use of correctly sized, high efficiency electric motors and insulation, etc.) show energy savings of 20–30%. Higher capital expenditure measures (automatic combustion control, improved design features for optimisation of piping sizing, and air intake sizing, and use of variable speed drive motors, automatic load control systems and process residuals) can result in energy savings of 40–50% (UNIDO, 2001, Bakaya-Kyahurwa, 2004).
Electric motor driven systems provide a large potential for improvement of industry-wide energy efficiency. De Keulenaer et al., (2004) report that motor-driven systems account for approximately 65% of the electricity consumed by EU-25 industry. Xenergy (1998) gave similar figures for the USA, where motor-driven systems account for 63% of industrial electricity use. The efficiency of motor-driven systems can be increased by improving the efficiency of the electric motor through reducing losses in the motor windings, using better magnetic steel, improving the aerodynamics of the motor and improving manufacturing tolerances. However, the motor is only one part of the system, and maximizing efficiency requires properly sizing of all components, improving the efficiency of the end-use devices (pumps, fans, etc.), reducing electrical and mechanical transmission losses, and the use of proper operation and maintenance procedures. Implementing high-efficiency motor driven systems, or improving existing ones, in the EU-25 could save about 30% of the energy consumption, up to 202 TWh/yr, and avoid emissions of up to 100 MtCO2/yr (27.2 MtC/yr) (De Keulenaer et al., 2004). In the USA, use of more efficient electric motor systems could save over 100 TWh/yr by 2010, and avoid emissions of 90 MtCO2/yr (24.5 MtC/yr) (Xenergy, 1998). A study of the use of variable speed drives in selected African food processing plants, petroleum refineries, and municipal utility companies with a total motor capacity of 70,000 kW resulted in a potential saving of 100 ktCO2-eq/yr (27 ktC/yr), or between 30–40%, at an internal rate of return of 40% (CEEEZ, 2003). IEA (2006b) estimates the global potential to be >20–25%, but a number of barriers have limited the optimization of motor systems (See Section 7.6).
Typical estimates indicate that about 20% of compressed air is lost through leakage. US DOE has developed best practices to identify and eliminate sources of leakage (US DOE, n.d.-a). IEA (2006a) estimates that steam generation consumes about 15% of global final industrial energy use. The efficiency of current steam boilers can be as high as 85%, while research in the USA aims to develop boilers with an efficiency of 94%. However, in practice, average efficiencies are often much lower. Efficiency measures exist for both boilers and distribution systems. Besides general maintenance, these include improved insulation, combustion controls and leak repair in the boiler, improved steam traps and condensate recovery. Studies in the USA identified energy-efficiency opportunities with economically attractive potentials up to 18–20% (Einstein et al., 2001; US DOE, 2002). Boiler systems can also be upgraded to cogeneration systems.
Efficient high-pressure boilers using process residuals like bagasse are now available (Cornland et al., 2001) and can be used to replace traditional boilers (15–25 bar) in the sugar industry. The high-pressure steam is used to generate electricity for own use with a surplus available for export to the grid (see also 7.3.4). For example, a boiler with a 60 MW steam turbine system in a 400 t/hour sugar factory could provide a potential surplus of 40 MW of zero-carbon electricity, saving 400 ktCO2/yr (Yamba and Matsika, 2003). Similar technology installed at an Indian sugar mill increased the crushing period from 150 to 180 days, and exported an average of 10 MW of zero carbon electricity to the grid (Sobhanbabu, 2003).
Furnaces and process heaters, many of which are tailored for specific applications, can be further optimized to reduce energy use and emissions. Efficiency improvements are found in most new furnaces (Berntsson et al., 1997). Research is underway to further optimize combustion processes by improving furnace and burner designs, preheating combustion air, optimizing combustion controls (Martin et al., 2000); and using oxygen enrichment or oxy-fuel burners (See Section 7.3.7). These techniques are already being applied in specific applications.