August/17/2005
Waste and Residue Derived Fuels in Cement Kilns
The cement production has experienced great changes during the history, passing from vertical wet-process kilns to short dry-process rotary kilns with cyclone towers, high-efficiency coolers and tertiary air to precalciners. One of the most important changes that can be identified at this moment is the use of alternative fuels.
Nowadays, after the improvement in the production brought by the high-efficiency clinker coolers, which allow the recovery of a significant amount of thermal energy, through the pre-heating of secondary and tertiary air, cement kilns have become more efficient with average heat consumption around 750 kcal/kg of clinker. It can be estimated that a kiln producing 3.500 mtpd (metric tons per day) of clinker leads to an energy consumption of approximately 110 Gcal/h, something around 13,5 mt/h of pet-coke, or 108.000 mt/year of that fuel. At the present, the price of pulverized pet-coke at burner tip is about 65 US$/t, and consequently the total costs related to fuel consumption is approximately 7.000.000 US$/year, what is a huge percentage of the production costs.
The same plant, should it operate on fuel oil alone, would consume around 11,5 mt/h of fuel oil, or 91.000 mt/year. With a cost of US$ 300/mt, the fuel costs could reach up to US$ 27.000.000 per year. After the oil crisis of 1973 and 1979, the industries were forced to seek for alternatives to fuel oils, such as coal, to cut spending. In the 1990’s, petcoke appeared as a cheaper alternative. Previously used only in refineries, today petcoke is widely used as fuel in the cement industry, due to its low cost and process tolerance to its high sulphur content.
At the moment, the plants are increasingly using waste and residue derived fuels (RDFs), not only as thermal input, but as revenue as well, since some suppliers pay the plant to destroy the residue, in addition to the substitution of traditional fuels. This option, formerly restricted only to some European and US plants, is now being applied in Brazil, basically due to stronger environmental regulations, which prohibit dumping of waste in landfills, and thanks to the technological advances of Brazilian plants, allowing the destruction of residues and wastes, without increasing the environment pollution.
The waste and residue destruction, previously done mainly in incinerators, is now passing to cement kilns, due to four basic reasons:
Nevertheless, waste and residues should not be used without certain cares. There are contaminants witch can be extremely damaging to the kiln operation, such as Chloride, Fluoride and moisture. Chloride is a powerful oxidant, which can corrode plates, and in addition cause the blocking of the gas circuit, specially the smoke chamber. Fluoride, on the other hand, is a strong flux, that can bring down coatings. The water evaporated from the residues increase considerably the volume of exhaust gases, and a decrease of production might be expected, to maintain the O2 levels at the smoke chamber and at the exhaust of the cyclone tower.
Dynamis has created a program that allows a simplified simulation of a cement kiln with the most common configuration and calculates approximately the amount of solid residues or waste that could be injected into the kiln. The RDF flow that can be expected will depend on its characteristics, which must be filled at the proper cells of the program. This amount is calculated assuming a high-output operational condition as reference, in which the exhaust fan does not permit any increase in gas flow, and the O2 levels will be lowered to the minimum amounts recommended and observed in a large database of kilns in operation. To download the program, please click on the link below.
RDF Check
The results of this program are preliminary, and a thorough analysis must be executed, in order to better evaluate the injection of residues. In case the kiln or residue does not meet your requirements, please contact us!
Waste and Residue Derived Fuels in Cement Kilns
The cement production has experienced great changes during the history, passing from vertical wet-process kilns to short dry-process rotary kilns with cyclone towers, high-efficiency coolers and tertiary air to precalciners. One of the most important changes that can be identified at this moment is the use of alternative fuels.
Nowadays, after the improvement in the production brought by the high-efficiency clinker coolers, which allow the recovery of a significant amount of thermal energy, through the pre-heating of secondary and tertiary air, cement kilns have become more efficient with average heat consumption around 750 kcal/kg of clinker. It can be estimated that a kiln producing 3.500 mtpd (metric tons per day) of clinker leads to an energy consumption of approximately 110 Gcal/h, something around 13,5 mt/h of pet-coke, or 108.000 mt/year of that fuel. At the present, the price of pulverized pet-coke at burner tip is about 65 US$/t, and consequently the total costs related to fuel consumption is approximately 7.000.000 US$/year, what is a huge percentage of the production costs.
The same plant, should it operate on fuel oil alone, would consume around 11,5 mt/h of fuel oil, or 91.000 mt/year. With a cost of US$ 300/mt, the fuel costs could reach up to US$ 27.000.000 per year. After the oil crisis of 1973 and 1979, the industries were forced to seek for alternatives to fuel oils, such as coal, to cut spending. In the 1990’s, petcoke appeared as a cheaper alternative. Previously used only in refineries, today petcoke is widely used as fuel in the cement industry, due to its low cost and process tolerance to its high sulphur content.
At the moment, the plants are increasingly using waste and residue derived fuels (RDFs), not only as thermal input, but as revenue as well, since some suppliers pay the plant to destroy the residue, in addition to the substitution of traditional fuels. This option, formerly restricted only to some European and US plants, is now being applied in Brazil, basically due to stronger environmental regulations, which prohibit dumping of waste in landfills, and thanks to the technological advances of Brazilian plants, allowing the destruction of residues and wastes, without increasing the environment pollution.
The waste and residue destruction, previously done mainly in incinerators, is now passing to cement kilns, due to four basic reasons:
- Time: The cement kiln has enough solid/gas residence time to allow the complete destruction of residues and wastes;
- Temperature: The process temperature in the cement kiln is adequate to the destruction of residues and waste, specially if injected through the main burner;
- Turbulence: To allow the combustion of solid fuel, residues and waste, turbulence is fundamental. Such characteristic is already very intense on current kilns, allowing the burning of alternative fuels;
- Cost: Incinerators most often charge high amounts to destroy wastes, because their core business is the residue processing, while in cement plants residues and waste are used to reduce the expenses with traditional fuels, and this allows the suppliers to pay less for the destruction of those materials in a kiln.
Nevertheless, waste and residues should not be used without certain cares. There are contaminants witch can be extremely damaging to the kiln operation, such as Chloride, Fluoride and moisture. Chloride is a powerful oxidant, which can corrode plates, and in addition cause the blocking of the gas circuit, specially the smoke chamber. Fluoride, on the other hand, is a strong flux, that can bring down coatings. The water evaporated from the residues increase considerably the volume of exhaust gases, and a decrease of production might be expected, to maintain the O2 levels at the smoke chamber and at the exhaust of the cyclone tower.
Dynamis has created a program that allows a simplified simulation of a cement kiln with the most common configuration and calculates approximately the amount of solid residues or waste that could be injected into the kiln. The RDF flow that can be expected will depend on its characteristics, which must be filled at the proper cells of the program. This amount is calculated assuming a high-output operational condition as reference, in which the exhaust fan does not permit any increase in gas flow, and the O2 levels will be lowered to the minimum amounts recommended and observed in a large database of kilns in operation. To download the program, please click on the link below.
RDF Check
The results of this program are preliminary, and a thorough analysis must be executed, in order to better evaluate the injection of residues. In case the kiln or residue does not meet your requirements, please contact us!
contact@dynamis-ma.com
