Zero Emission Kiln

According to the Intergovernmental Panel on Climate Change (IPCC), cement plants are second only to the power sector in terms of the number of facilities and total greenhouse gas emissions. A successful re-design of the cement kiln could encompass modifications to fuel systems and plant layout with potential for other improvements in electricity generation and feed preparation.
Cement is utilized throughout the world and is second only to water in terms of per capita consumption. Since the world’s population will continue to grow, and more cement will be produced to build urban cities, a zero emission kiln would have great impact on the reduction of carbon dioxide and other pollutants into the atmosphere.

The production of ordinary Portland cement, the binding agent in concrete, is dominated by the production of clinker. This is an energy intensive process that produces ~4% of the carbon dioxide emitted globally. The process of clinker manufacturing involves the release of chemically bound CO2, through the decomposition of limestone, and CO2 resulting from the combustion of fossil fuels to provide the energy. A summary of the CO2 emissions from a cement kiln shows that 40% is from the fuel combustion and 50% is from the chemical reaction with the remainder derived from electricity consumption and transportation. The large proportion of chemically bound CO2 limits the effectiveness of fuel switching as a strategy for CO2 reductions. The large mass of the end product inhibits relocation to countries without emission regulations as an alternative strategy. This suggests a novel approach is needed to reduce CO2 levels.

A successful re-design of the cement kiln could encompass modifications to fuel systems and plant layout with potential for other improvements in electricity generation and feed preparation. Ideally, strategies should maintain the ability to retrofit existing facilities. However, the most efficient designs are likely to aim at new plants. Furthermore, any changes would have to consider the important role of alternative fuels in the cement industry. Cement companies provide an important service by consuming waste materials as alternative fuels. These range from spent solvents and paint residues to sewage sludge. Developing the ability to manage all of the emissions and maintain fuel flexibility would provide economic certainty in a carbon constrained marketplace.

The starting point is to propose combining oxygen combustion, flue gas recycling, and fuel preprocessing to create the zero emission kiln (ZEK). These technologies will be optimized to improve the efficiency of cement production, which reached a plateau for new plants in the early 1990s. The efficiency improves as the actual amount of energy used per unit product approaches the thermodynamic requirement. Newer plants consume 7MJ of primary energy/kg of clinker, which is twice the thermodynamic requirement of 1.75MJ/kg.

Figure 1. Schematic of ZEK plant.

An overview of the design is shown in figure 1. The general concept is similar to conventional plants in that the combustion energy is used to form clinker with the waste heat used for calcination. To avoid diluting the off-gas with nitrogen, the fuel is combusted in oxygen rather than air. An on-site cryogenic oxygen plant will deliver the oxygen required for combustion. The use of high-purity oxygen allows for the recycling of the flue gas, now dominated by CO2, back to the combustion zone. A portion of the flue gas must be removed from the system and sent to storage to maintain mass balance. The remainder, still at temperatures around 900°C, could be used for fuel preprocessing. The fuel and flue gas will be mixed prior to combustion to gasify and preheat the reactants. Gasification will homogenize the feed entering the kiln.

An on-going feasibility study would need to ascertain whether clinker formation in a different atmosphere will still generate a useful product. Therefore, the initial focus of this investigation was a comparison between conventional clinker and that produced in a CO2 environment. More specifically, the experiment will trial oxygen blown calcination and flue gas recycling. The objective was to demonstrate that the lime and clinker produced have similar material properties to those produced by conventional means. There previously was no concerted research into the field of oxygen calcination and clinker production or the quality of the lime/clinker produced. Although there is considerable interest in oxygen combustion for power generation, this information may not be directly applicable to cement manufacturing. With the successful conclusion of this phase, it is possible to design a cement plant that could operate as a zero emission kiln.

The main objective of the investigation is to improve the cement making process from an environmental and economic perspective. Through reduced emissions, less fossil fuel use, generation of carbon credits for sale, and improved access to alternative fuels, the cement manufacturer should be well positioned for future growth. In terms of carbon markets, the large volume of CO2 produced by the cement industry would make it the dominant seller in any trading system.

LCSE completed the initial phase of research into low- and zero-emission cement kiln (ZEK) technologies. This entailed developing a more thorough analysis of the zero-emission cement making process, and producing a number of technical options for the cement industry to counter eventual carbon constraints that will be imposed by pending legislation, and possibly even profit by selling the carbon reductions to other industries harder pressed to collect their carbon dioxide.

Klaus Lackner, Ewing-Worzel Professor of Geophysics, (PI)

Feng (Maple) Lin, Former postdoctoral research scientist

Frank Zeman, Assistant Professor, Department of Chemistry and Chemical Engineering, Royal Military College of Canada,