by Eric R. Hansen

Ash Grow Cement Co. has developed an environmentally sound energy recovery system from solid hazardous wastederived fuel that could ignite a revolution in the cement industry


Introducing hazardous waste fuel directly into the middle of a long kiln, where gas temperatures are sufficiently high to assure complete destruction and utilization of such solid waste fuel, could lower energy costs for cement companies while helping solve the nation's waste-disposal dilemma.

Cement kilns have received considerable attention lately as an environmentally sound solution to the problem of what to do with combustible industrial and municipal hazardous waste. High temperatures in the kilns, long residence times, and the ability to absorb inorganic residue allow complete destruction of combustible hazardous waste while recovering the energy they contain in an environmentally sound manner.

As a result of pressure from an environmentally aware society, there are increasing government regulations requiring more industrial and manufacturing waste to be managed as "hazardous waste" because of its flammable or toxic character. Severe limitations are being placed on the landfilling of this industrial waste. Municipal hazardous waste as well is being placed under increasing regulations to minimize its impact on the environment. The only viable means of safe permanent disposal of this combustible waste is by thermal treatment. Cement kilns are not only ideally suited for the safe disposal of this material, but they also can recover the energy from this type of waste.

Although combustible hazardous waste solids represent a potential source of inexpensive energy for the cement industry, concerns about handling the hazardous waste, plant engineering, product quality, and emission control have deterred kiln operators from taking advantage of this inexpensive source of energy. Extensive energy recovery already is being done with liquid hazardous waste-derived fuel.

Liquid hazardous wastes are easily blended with each other and with conventional fuel to provide uniform fuel that can be burned at the firing end of the kiln with little or no modification of the kiln burner configuration. The use of solid hazardous waste has been limited by the extent to which it can be suspended or dispersed into the flame zone of the kiln. If hazardous waste solids are charged into the sintering zone of the kiln without adequate dispersion and rapid combustion, they will come into contact with the clinker bed. The localized reducing conditions created by fuel falling into the clinker will affect clinker quality and cement color. This problem limits the quantity of fuel utilized by this technique.

Low British Thermal Unit (BTU)-content fuel introduced into the flame adversely affects the flame temperature and therefore reduces the thermal efficiency of the process. If the solid hazardous waste-derived fuel could be introduced at a point in the kiln hot enough to assure complete destruction of the fuel components, yet have sufficient residence time to burn out before sintering begins, the clinker quality would not be adversely affected. Therefore, the addition rate of the fuel would be limited only by the ability of the process to utilize the energy generated by the fuel. Also, the inorganic residues would have sufficient time to completely react with the active cement minerals in the sintering zone.

Ash Grove and Cadence Chemical Co. have jointly developed and received a patent' on their new method for introducing solid hazardous waste-derived fuel directly into the middle of a long wet or dry kiln,

The development of the precalciner was a technological revolution for the cement industry. The major benefits of precalcining include:

- Much improved operational stability;

- Reduction of thermal loading in the sintering zone;

- Increased production for a given kiln size;

- The ability to use low-quality fuel;

The addition of a significant portion (25%) of the process fuel directly into the middle of a long kiln also yielded many of the benefits of precalcining.

Ash Grove's experience in adding 15% to 25% of fuel into the middle of a 12 ft x 450-ft wet process kiln. The kiln is continuously monitored for carbon monoxide and total hydrocarbons, as well as nitrous oxide) is shown in a temperature profile.

Upon adding the 30 x 106 BTU/ hr of fuel to the middle of the kiln, the burner firing rate is reduced from a normal 6.5 tph of coal to 5.25 tph. With the lower firing rate and increased oxygen in the burning zone, the primary flame tightens up and becomes intense. The high excess air and high flame temperature improve clinker nodularity and promote a heavy, durable coating in the burning zone; the coating zone is shortened by about 10 ft.

Frequent firing profile changes would be expected to cause refractory problems in the transition zone. The new spinel-based bricks are performing well in this zone. The reduced thermal loading in the burning zone and resulting improved coating are expected to increase refractory life in the burning zone.

It is expected that wet kilns whose capacity is limited by thermal loading and resulting refractory life may take advantage of the secondary firing by increasing productivity and not changing the burner-firing rate.

The biggest advantage is that low-quality, low-cost fuel can be substituted for expensive fossil fuel. The primary burner must receive reasonably high-BTU fuel (greater than 10,000 BTU/Ib, preferably greater than 12,000 BTU/ lb) in order to maintain high flame temperatures. Low-BTU fuel adversely affects the flame, and this results in increased energy consumption.

The calcining zone does not require high flame temperatures, as exemplified with precalciners where the fuel is dispersed with the meal to achieve flameless combustion, Some precalciners already take advantage of firing a lower- quality fuel than can be fired in the primary kiln burner. Likewise, the heating value of the fuel to be fed to the middle of a long kiln can be quite low, yet full advantage can be taken of the heating value of the fuel. There is even speculation that the energy from the fuel added to the middle of the kiln is more effectively utilized than if it was added at the burner2.

It was of great importance for Ash Grove to demonstrate that the addition of fuel to the middle of a long kiln did not adversely affect the kiln emissions. The system was engineered so that the fuel would be introduced at a point in the kiln where the temperature is above 950ºC. The literature3,4 supports that at temperatures above 950ºC, complete destruction of the organic constituents of the fuel is obtained. Studies on municipal hazardous waste incinerators indicate that introduction of the fuel below 950ºC can result in dioxin and furan formation.

To demonstrate the use of solid fuel by this method, a study was done on solid fuel spiked with trichlorobenzene. Trichlorobenzene was selected because it contains the most difficult to destroy chlorinated hydrocarbons as found in both laboratory studies and field testing by Dellinger5. No trichlorobenzene was detected in the test, and a greater than 99.9999% destruction and removal efficiency was demonstrated.

To prove that the process is continuously sound environmentally, Ash Grove installed a total hydrocarbon monitor. Compliance with a 20-ppm total hydrocarbon maximum is maintained while hazardous waste fuel is being fired.

Older cement kilns now have an opportunity to compete energy costwise with newer technology while providing society with a solution to its waste problems. As with any new technology, it is always difficult to build the first operating unit. Now that there are operating examples of long kilns with secondary firing, there should be a rapid application of this technology.