Recommendations by Sector

CEMENT MANUFACTURING

Process and Energy Use

Colorado consumes about 3 million tons of cement each year, about two-thirds of which are produced in the state. Cement is a very energy-intensive industry which consumes over one percent of primary energy use in the U.S. Raw materials for cement making, limestone or chalk and clay, are usually mined close to plants then are sifted, crushed, and ground, a process that uses about 23-32 kWh/short ton of material. Around 70% of cement plants use a dry process for preparing raw material for firing, the remainder a wet process, which uses substantially more energy. After initial preparation, the material is heated in large kilns with flames that are at 1800 to 2000 degrees C (3272 to 3632 degrees F) to produce clinkers, a process that is typically fueled by coal. Clinkers are then ground along with small quantities of such materials as gypsum and anhydrite to produce the final product, a process that uses 29 to 45 kWh/short ton of cement depending on the technology used for grinding and the hardness of additives used in the process.

The energy consumed in kilns to produce clinkers represents about 90 percent of the total for manufacturing cement, while the processing of raw materials and grinding of the final product account for most of the rest.

There are two general approaches to lowering energy use in the cement manufacturing process. The first, in widespread use in Europe, involves blending other materials such as fly ash and slag from blast furnaces into the clinker-grinding process. The result is a strong cement whose embodied energy per unit volume is about seven percent lower than that of unblended cement. Blending has the advantage of recycling waste products of other processes, thereby lightening the load on landfills and diminishing both energy use and greenhouse gas emissions from the process of cement production.

The second approach to diminishing energy use involves a number of energy efficiency and maintenance measures undertaken at various stages of the cement production process. It is estimated that cost-effective measures can lower the energy consumption of the process of producing cement by 11%. Combined with blending, overall savings of about 18% are possible with cost-effective, off-the-shelf technologies.¹

Measures whose payback periods are routinely well less than three years include the following:

• Undertake a preventative maintenance program. This requires systematic maintenance measures undertaken by personnel trained to be attentive to energy consumption and efficiency.
• Reduce kiln heat loss by using pre-heaters, installing better-quality, high-temperature insulation, and possibly shortening the length of the kiln.
• Use waste fuels such as tires to supplement conventional fuel used to fire the kiln. This measure is employed by about 2% of the cement producers in the U.S.
• Convert wet kilns to semi-wet. This involves a major retrofit of the more wasteful wet kiln process but paybacks are quite short.
• Install a clinker cooler grate. There are at least four technologies used to rapidly cool clinkers, which is necessary in order for the cement to retain important hardening properties. Grate technology is more energy-efficient than the others in current use.
• Use high-efficiency motors. It is almost always cost-effective to use premium-efficiency motors since energy costs far outweigh the initial investments.
• Upgrade the kiln combustion system. Modern equipment and controls can optimize burning parameters to achieve good throughput while maximizing efficiency.

In addition, substantial heat is released from the kiln which may be used in preheating material or in cogeneration of electricity to power motors in other operations in the cement plant. More details on these energy efficiency measures and others may be found in Martin et al, 1999.²

¹ “Efficiency Opportunities for the U.S. Cement Industry,” Nathan Martin, Ernst Worrell, and Lynn Price, Lawrence Berkeley National Laboratory, Proceedings of the 2001 ACEEE Summer Study on Energy Efficiency in Industry.
² “Energy Efficiency and Carbon Dioxide Emissions Reduction Opportunities in the U.S. Cement Industry,” Nathan Martin, Ernst Worrell, and Lynn Price, LBNL Report 44182, September 1999.

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