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ALTERNATE INPUTS OF WASTE MATERIALS AND CEMENT MANUFACTURING

The reuse or recycling of waste materials by way of cement manufacturing has been occurring for over a quarter of a century. Early testing focused on the processing of waste materials that required disposal and/or destruction. Some of the earliest recorded efforts came from a Canadian test burn in 1972. St. Lawrence Cement demonstrated that polychlorinated biphenyls (PCBs) could be sufficiently destroyed in a rotary cement kiln¹. A similar demonstration occurred at a Stor Vika cement plant in Sweden in 1978². It was the late 1970s and early 1980s when the emphasis switched to reclamation of energy from waste materials such as oils and solvents, simultaneously conserving natural resources used as primary fuels. Burning waste solvents in cement kilns became increasingly popular throughout the 1980s. That popularity ultimately led to USEPA developing the boiler and industrial furnace regulations, which were published on February 21, 1991 and became effective six months later. These regulations require extensive testing of emissions and feed materials at least every three years until a final permit is issued. The testing required in the United States (US) has resulted in a wealth of data on actual emissions and feed materials that was not available before 1992.

There are numerous ways of inputting waste materials into cement manufacturing processes. The most widely used method is injection of liquid waste-derived fuel into a cement kiln through a lance either in the middle of the primary fuel burner pipe (concentric) or off-set from the primary fuel burner pipe (non-concentric). Initial compliance test burns in the US used non-concentric fuel lances. Some of these tests were even conducted with temporary lances inserted through existing openings in the kiln hood. However, to achieve a more desirable compact flame pattern, the waste fuel lances were subsequently attached to the exterior of the primary fuel coal pipes, usually on the upper half. A number of test burns were conducted in such a manner (i.e. Lone Star Industries, Inc. Greencastle, Indiana COC in 1992⁵). Later developments led to the concentric configuration, which is more common today. Clinker quality standards drive a facility's preference for concentric or non-concentric burners. The flame's shape and resulting kiln temperature profile is critical in cement manufacturing. Long lazy flames extending well into the kiln and unsymmetrical flames that impinge on the load adversely affect clinker quality.

Nozzle design and line pressure are also used to control the shape of the flame with liquid waste-derived fuel lances. It is common to inject air into the liquid fuel line to ensure proper atomisation, prevent plugging, and help control the flame shape. Centrifugal pumps are normally used to supply liquid fuels, however positive displacement pumps have also been used.

Sludges and some types of solids are commonly dissolved or suspended in liquid waste-derived fuel and injected through the liquid feed system described above. Typically, particle size is reduced to less than 7 millimetres to produce an acceptable flame and prevent plugging feed lines. A variety of methods and equipment are used to blend solids and sludges into liquids including ball mills, ribbon mixers, shredders, grinders, in-line mixers, batch mixers and strainers. Alternatively, sludges can be mixed with absorbent wastes like sawdust and injected at the hot end through a separate solid feed system.

Another method, specific to dry combustible waste, used for injecting waste-derived fuels into cement kilns is to mix them with the primary fuel, usually coal or petroleum coke, and input through primary fuel burner, eliminating the need for a separate feed system. This can be done before or after the mill, depending on the waste. Some kiln operators prefer to pneumatically inject dry combustible wastes through a separate system. Both of these methods require injection at the hot end of the kiln. Fuel consistency and particle size reduction are critical challenges to overcome for producing the proper flame shape and temperature profile within the kiln. Large particles may drop out of the flame onto the load or be carried further into the kiln before complete combustion occurs, both of which adversely affects clinker quality.

Some operators inject solid and sludge wastes packaged in plastic containers into kilns. Two methods are used. One utilises a mid-kiln gate that allows one or two containers per revolution to be dropped into the kiln at the calcination zone^3.4. This method offers many advantages and has become the preferred method for feeding whole tyres into kilns. A less common method utilises an air-powered cannon to shoot small containers into the hot end of the kiln. The amount of waste fed into kilns by either of these methods is usually limited to less than 20% in practice. Table 1 identifies the types of wastes and feed systems used by US cement kilns. This Table also gives the percentage of total fuel requirements obtainable from hazardous wastes.

* Values were obtained by calculating percentages from reported mass feed rates for traditional fuels and wastes fuels during compliance test burns. USEPA regulations only limit mass feed rates for hazardous waste-derived fuels to that used during the compliance test, not the percent fuel substitution. These substitution rates may not reflect local and regional regulatory limits or operational limitations.
P - Personal communication with cement plant personnel. Represents maximum practical substitution rate.
CMB 97 - Gossman Consulting Inc., Combustor Database (1997). Percent Substitution values were calculated from data submitted for Re-certification of Compliance Test Burns and represents the maximum allowed hazardous waste feed rate.
BIF 92 - Gossman Consulting Inc., Commercial BIF Compliance Test Results Report (1992). Percent Substitution values were calculated from data submitted for Certification of Compliance Test Burns and represents the maximum allowed hazardous waste feed rate.
L - Pumpable hazardous waste liquid injected at the hot end of kiln through dedicated feed system.
SM - Hazardous waste solids injected mid-kiln in 20 - 25 litre containers.
SC - Hazardous waste solids mixed with coal and injected through coal burner at the hot end of kiln.
SA - Hazardous waste solids injected at "hot end" of kiln with air cannon in 4 to15 litre containers.
S - Hazardous waste solids pneumatic injection at hot end of kiln through dedicated feed system.

There are also a myriad of non-combustible industrial by-products (i.e. coal fly ash, mill scale, foundry sand, filter press cakes) that are suitable for use as raw material substitutes. As with fuel substitution, this practice conserves natural resources while recycling waste material that otherwise requires disposal. These non-organic wastes are normally blended into the raw feed and input into the kiln at the back end or cool end. Cement kilns have rigorous quality control procedures for analysing and precisely controlling constituents in raw feed to produce blends with exact content of calcium carbonate, silica, alumina, and iron oxide. Additional analysis on industrial by-products may be required to determine if they contain elements that could adversely affect product quality or volatile contaminants that could affect emissions.

The emissions data from compliance tests in Tables 2 - 5 are from US cement kilns using a variety of hazardous waste-derived fuels and waste injection methods (identified in Table 1). Attempting to infer cause and effect relationships between emissions and different waste injection methods from these data is not possible. To reach valid scientific conclusions about cause and effects, it is necessary to also consider the concentrations of constituents in traditional raw materials and fuels, differences in operating parameters, differences in kiln design, the condition of the air pollution control equipment, and the type of air pollution control equipment. Most of these controlling factors vary from one plant to another, or even from one test to another at the same plant.

It is important to note that the data collected from US testing was collected under "worst case" operating conditions. The boiler and industrial furnace regulations require compliance tests to be performed with maximum gas flow; maximum feed rates for metals; separate tests for maximum and minimum temperatures; minimum operating parameters for air pollution control devices; and maximum raw material feed rates. In essence, kilns are required to push operating conditions to the limit during compliance tests. Most kilns spike additional metals into the waste-derived fuels to ensure reasonable operational flexibility. The result is that emissions during compliance tests are much higher than normal day-to-day operations. Current US emission limits are also much higher under the USEPA's risk-based regulations than the technology-based limits being considered in the EU.

Injection of liquid waste-derived fuel into cement kilns has been shown by risk assessments in the US to be a safe alternative to disposal for many organic wastes. Comprehensive test results from a cement plant that inputs solids with the coal feed indicate that this method can also meet compliance limits (Continental Trial Burn, 1996⁶). Similarly, comparison of test results in Tables 2 - 5 show that facilities injecting solids either at the hot end or mid-kiln by the methods described above meet US compliance standards.

Numerous tests have been conducted where non-hazardous wastes such as tyre-derived fuel and refuse-derived fuel have been injected separate from the coal input, as well as non-hazardous alternative raw materials, but emissions data is not readily available for all of these tests. However, at least one published test report is available for tyre-derived fuel⁷, refuse derived fuel⁸, and raw material substitution⁹.

It is clear that US cement kilns using a variety of injection methods and wastes types have met the USEPA's risk-based requirements. The USEPA has proposed more stringent maximum achievable control technology (MACT) standards, which are similar to standards under consideration in the EU. The USEPA believes that most facilities using hazardous waste-derived fuel will financially justify the investments required to achieve the proposed MACT standards.

Although risk assessment techniques are controversial, it is interesting that kilns operating in the US have demonstrated compliance with current USEPA regulations whilst operating under abnormal "worst case" conditions, and that both the US and EU are moving toward much more stringent standards. Technology-based standards are easier to enforce because one set of limits can be applied to a group of similar facilities. However, caution is advised in pushing for standards which would increase costs above levels most kiln operators could justify simply because the technology exists to meet those standards. There are many environmental advantages from using waste materials in cement manufacturing but the practice will only continue if kiln operators recognise a financial incentive. The regulatory framework should be designed to favour the use of waste management methods that minimise the chance of polluting the environment, taken as a whole, for any wastes or discharges arising from all relevant industrial processes.

REFERENCES

1. Branscome, M., W. Westbrook, R. Mournighan, J. Bolstad, and J. Chehaske. 1985. Summary of Testing at Cement Kilns Co-firing Hazardous Waste. In Incineration and Treatment of Hazardous Waste: Proceedings of the Eleventh Annual Research Symposium. EPA 600/9-85-028. Pp. 199-205. U.S. Environmental Protection Agency, Office of Research and Development, Hazardous Waste Engineering Research Laboratory, Cincinnati, Ohio.

2. Ahling, B. 1979. Destruction of Chlorinated Hydrocarbons in a Cement Kiln. Environmental Science & Technology. 13(11). Pp. 1377-1379.

3. Cadence Environmental Energy, Inc. Web page. HTTP://www.cadencerecycling.com

4. Report of RCRA Trial Burn for Kiln Nos 1 & 2 March-April, 1994. Volume 1 of VIII. 1995. Ash Grove Cement Company - Chanute, Kansas Waste-Derived Fuel Facility.

5. Trial Burn and Certification of Compliance Test Report. Volume I of VI. 1992. Lone Star Industries, Inc. - Greencastle, Indiana.

6. Trial Burn Report. Volume I of III. 1996. Continental Cement Company - Hannibal, Missouri

7. Stiren, N. 1995. The Utilization of Tire Derived Fuel in Wet Kilns for the Cement Industry. In Waste Combustion in Boilers & Industrial Furnaces: Proceedings of the International Specialty Conference. pp. 67-85. SP-95. Air & Waste Management Association, Pittsburgh, PA.

8. Coles, Charles. 1995. Refuse Derived Fuel as an Alternative Fuel for Cement Kilns. In Waste Combustion in Boilers & Industrial Furnaces: Proceedings of the International Specialty Conference. pp. 86-2. SP-95. Air & Waste Management Association, Pittsburgh, PA.

9. Watson, David M. 1995. The Use of Non-Hazardous By-Products in the Production of Portland Cement Clinker. In Waste Combustion in Boilers & Industrial Furnaces: Proceedings of the International Specialty Conference. pp. 103-115. SP-95. Air & Waste Management Association, Pittsburgh, PA.