manufacture of clinker in the kiln. Half of the CO2 generated in cement manufacturing derives from energy use and the other half from the chemical process that converts limestone to lime, the key ingredient of clinker.
While most major energy savings in cement processes require a major upgrade to an advanced dry-kiln process, other technologies that incrementally improve energy efficiency include advanced control systems, combustion refinements, indirect firing, and optimization of components such as the heat shell. Opportunities vary with specific plants; however, the combination of these activities appears to yield energy savings on the order of 10 percent.
The most attractive available energy efficiency technologies, with potential energy savings of 10–20 percent, derive from changing the chemistry of cement to reduce the need for calcination. Blended cements include higher proportions of other cementitious materials, such as fly ash. Steel slag, which is already calcined, is an alternative to limestone for the production of clinker. Technologies that allow production of cement with a lower per-ton share of clinker thus yield multiple benefits: savings in fuel consumption and reductions in greenhouse gas emissions by a factor of two or more above what is associated with energy efficiency alone.
Advanced technologies with a potential to further improve energy efficiency and emissions include fluidized bed kilns, advanced comminution processes, and the substitution of mineral polymers for clinker. A Battelle (2002) study concluded that non-limestone-based binders may yield a reduction of 30 percent in CO2 emissions. Additional advanced approaches to reducing CO2 emissions are hybrid cement-energy plants, currently under investigation in the United States, and the incorporation of carbon capture and storage.
Several illustrations of technologies and approaches that could improve industrial energy efficiency are given in this section. Some have already been introduced but could have much greater application, while others are still in the development stage.
Combined-heat-and-power units transform a fuel (generally, natural gas) into electricity and then use the residual heat for space and hot-water heating or for industrial and commercial processes. Estimates of the economic energy-savings potential of CHP nationwide range from 0.7 quads (McKinsey and Company, 2007) to 2.0 quads (IWG, 2000; Lemar, 2001).