The cement industry is the building block of a nation's construction industry. Making cement is one of the world’s most energy-intensive industrial activities. It has been estimated that the process consumes about 3% of total primary energy and results in about 5% of the world’s climate-destabilizing carbon dioxide emissions. About half of these emissions are a necessary by-product of the chemical reaction essential in transforming the raw material into the finished product. This means that as long as cement manufacture is based on clinker, the sector will always be an important contributor of GHG. Manufacturers are therefore forced to look elsewhere to reduce their carbon footprint: such as more efficient use of fuel to produce the heat and electricity needed for the process. Investing in power plants integrated into the cement manufacturing process offers unrealized potential to help reduce the energy intensity of the sector as well as displace emissions from the power sector.
According to the Portland Cement Association, as of Dec 2009, U.S. cement production is widely dispersed with the operation of 113 cement plants in 36 states. The consumption of cement was 93.6 million metric tons in 2008, 85% of which is manufactured using dry process technology.
Opportunity
In the dry process cement plants nearly 40% of the total heat input is rejected as waste heat from existing gases of preheater and grate cooler. This waste heat can be effectively utilized for electric power generation. In existing plans cogeneration technologies based on bottoming cycles have potential to generate up to 25-30% of the power requirement of a plant. Cement production involves huge amounts of natural resources, capital, labor and energy.
Because the process requires so much heat, opportunities for heat recovery are abundant. The fact that a reliable supply of electricity is also required to make cement suggests on-site power applications are a natural opportunity that deserves attention. The most important savings opportunity comes from the pre-calciner.
In a report by the Ernest Orlando Lawrence Berkeley National Laboratory, incorporating heat recovery for power generation in a cement plant would save 18 KWH/Ton with a payback period of 3 years. Waste gas discharged from the kiln exit gases, the clinker cooler system, and the kiln pre-heater system all contain useful energy that can be converted into power. Cogeneration systems can either be direct gas turbines that utilize the waste heat (top cycle), or the installation of a waste heat boiler system that runs a steam turbine system (bottom cycle). While electrical efficiencies are still relatively low (18%), based on several case studies power generation may vary between 10 and 23 kWh/ton clinker. Electricity savings of 20 kWh/ton clinker are assumed. In 1999, four U.S. cement plants cogenerated 486 million kWh. Assuming that 34% of the energy introduced into long dry kilns is exhausted as waste gas this suggests a potential generation of 1,200 GWh.
Benefits
An article by Power-gen Worldwide discusses the benefits of CHP in the cement industry. At the plant level, on-site power
- Reduces downtime by guaranteeing against production interruptions from utility failures
- Saves energy costs by improving energy utilization of expensive fuels
- Reduces the amount of refractory’s used (special temperature-resistant materials used to line kilns/furnaces in order to increase their lives
- Allows higher kiln utilization
- Increases quality of the finished product.
Of greater importance are the system-level benefits resulting from increased investment in on-site power. The system-level benefits of on-site power include significantly reduced capital expenditure for transmission, distribution and generation infrastructure, decreased vulnerability to fuel price volatility and vastly reduced smog and climate change-causing emissions.
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