Adding fuel to the fire
Every day, people in the booming metropolitan area of Dallas/Fort Worth spend hours driving on highways, sitting in office buildings and running to catch airplanes at DFW International Airport. What many of these people do not realize is that the cement holding many of these structures together is being manufactured approximately 40 miles outside of Dallas in the small town of Midlothian, Texas. Texas Industries' (TXI) Midlothian cement plant is the fifth largest in the United States;15 different types of cement are produced there. The processes used - from collecting runoff water to restoring the adjacent quarry site with kiln dust to burning waste fuel - are turning what could pose environmental risks into environmental benefits, but people in nearby communities do not believe the processes are beneficial.
Beginning the process
The manufacturing of cement at the Midlothian plant begins in the quarry. The primary raw material, limestone, is quarried from land that was once a sea bed and crushed on site. After crushing, the limestone is transported approximately 1.5 miles on an overland conveyor, then stockpiled in a raw material storage building. Raw materials are fed proportionately to two 1,500-horsepower raw grinding ball mills. The finely ground material, mixed with water and called slurry, is pumped to blend tanks for blending and quality-controlled chemical analysis.
The slurry is then transferred to a kiln feed tank. Quality is monitored and controlled in the central laboratory. The kiln feed slurry is burned in four 12-foot-by-450-foot rotary kilns, which are fired with finely ground coal, natural gas and liquid waste fuel. The kilns are controlled and the emissions continuously monitored from the kiln control room.
The kiln product, called cement clinker, must reach temperatures above 2,700°F. This requires a flame temperature of 3,500°F to 4,000°F. The cement clinker discharges into the clinker cooler and then onto a conveyor system, which carries it to storage.
The final stage of the process is grinding the cement clinker. There are five grinding mills for the production of the finished product. The finished product is pneumatically conveyed to banks of storage silos. From this storage location, the cement can be conveyed to truck or rail loadout stations or to the package loadout area.
EPA has the final word
Although this process seems simple, a part that has raised many questions is the burning of liquid waste fuel. Several lawsuits have been filed against the plant by residents of Midlothian and nearby towns, who claim the air emissions generated from burning waste fuel present a danger to their health. TXI Director of Communications Harold Green said that in all of these cases, plaintiffs' lawsuits were dismissed after tests performed by both the Texas Department of Health and U.S. Environmental Protection Agency did not find any evidence the plant was at fault. "The reason it is controversial is because it is called hazardous," Green said. "If you did not use the word hazardous, people would understand that by using this waste we are helping reduce emissions, because it would be burned somewhere else. Instead, it is helping small business people inexpensively get rid of the waste and helping to replace coal, a depleting natural resource."
Studies have been conducted in the Midlothian area by several regulatory agencies to find out if the burning of waste in the kilns at TXI is causing health problems, and each has concluded that there are no additional health risks to the citizens of those areas. The burning process emits nitrous oxides, carbon monoxide, sulfur dioxide and particulate.
In January 1996, EPA Region 6 released its findings based on two studies. According to EPA, the studies showed that the citizens of Midlothian and surrounding communities are not projected to experience health risks above regulatory guidelines. "The risk assessment's theoretical modeling predicted a potential for non-cancer health effects after a period of 30 to 40 years of plant operation from low level fugitive dust emissions. EPA believed this potential to be negligible in light of actual environmental data," the report concluded.
Also in 1996, the Texas Natural Resource Conservation Commission (TNRCC) stood behind its five-year monitoring study. "The TNRCC study was reviewed by EPA, monitoring and sampling data were reviewed by the Texas Department of Health and both concurred with our findings," said TNRCC Executive Director Dan Pearson. "The EPA even conducted an independent risk assessment, which concurred with our findings."
Pearson also said that the TNRCC conducted more sampling, more monitoring and more evaluation in the Midlothian community than any other community in Texas. "In analyzing our data we have used worst case assumptions and added additional margins of safety, which means that even the low estimates presented in our risk assessment represent an overstatement of the actual exposures and risks for area residents," said Pearson.
Watching the waste
The Midlothian plant's management of the waste fuel is called the resource recovery program. Under this program, every step of the process is carefully administered. Most of the waste comes from commercial and industrial processes. These materials are similar to many ignitable household products such as paint thinner, nail polish remover and rubbing alcohol.
Before accepting a load of waste fuel, the plant's laboratory performs a sample test of heat value, chemical composition, specific gravity, solids, viscosity, extract from polychlorinated biphenyls (PCB) and other physical characteristics to determine if the substance meets all environmental safety requirements, permit regulations and operational criteria. If the sample does not pass the test it is rejected. The test samples are saved for three months.
Darrell Lewis, a resource recovery analyst, said, " We use the best technology available in this lab. The reason we do not outsource it to another lab is because we get better results in a more timely manner here in our lab."
The TXI-owned tankers that transport the fuel that comes from both big and small businesses such as oil companies, chemical companies, body shops, paint shops, dry cleaners and printers include vapor recovery systems and are operated by drivers who have received all required U.S. Department of Transportation training and 40 hours of hazardous materials training. The facility where the fuel is unloaded and processed includes:
- A contained unloading area for two rail cars and five trucks;
- A closed-loop vent-back system confining all vapors to tankers or holding tanks;
- Inert gas pressurization to eliminate vapors;
- Six 25,000-gallon process tanks to blend incoming materials into more consistent fuels;
- Two 110,000-gallon burn tanks where blended fuel is pumped into the kilns;
- Concrete-walled containment area with capacity of 278 percent of the largest tank;
- Seamless 60-millimeter poly liner under the slab;
- Microwave radar and two-stage alarm to prevent over-filling of the tanks;
- Tank agitators capable of turning the maximum tank volume every minute and keeping 1/8-inch particles in suspension;
- A computerized monitoring system that tracks fuel from its arrival to its destruction, even if it is processed through several different tanks and is directly linked to the TNRCC regional office; and
- Fire protection equipment, including four 350 gpm water cannons in the tank area and foam system in the truck and rail unloading area.
For thermal destruction of organic compounds in hazardous waste, the material must be exposed to temperatures in excess of 2,000°F for a minimum of two seconds. In order to manufacture the cement, though, the kilns reach temperatures that approach 4,000°F and exceed 2,000°F for more than five seconds. At this temperature, the kiln flame consumes the organic material in the fuel. It also vaporizes all the inorganics. The raw material fed into the kiln is heated to approximately 2,700°F. As it passes through and cools, inorganic compounds break down and partition at various temperatures.
The kilns, which run 24 hours a day, seven days a week, are continually monitored and computer controlled. If any of 11 operating conditions move outside EPA-prescribed parameters, the flow of waste derived fuel is automatically and immediately shut off without operator intervention. "This process is a safeguard, and it proves that this system is well controlled and well maintained," Green said.
During manufacturing, inorganic materials in the fuel chemicals bond with the other inorganic raw materials, which make up the cement clinker or kiln dust. This dust is used by TXI to rebuild the area where the limestone is quarried. The company places the dust on top of a liner and then plants grass on the land. Once the grass has grown, cattle are raised on the reclaimed land.
TXI operates as an interim status boiler and industrial furnace facility, with a Part B Permit currently being processed. Stack samples taken from the plant during the burning of recycled fuel showed a destruction removal efficiency (DRE) of organic material of 99.9999 percent - 100 times greater than the EPA requirement. The systems removal efficiency (SRE) of inorganic material is greater than 99 percent for most metals.
Although residents of the Midlothian area may not be completely satisfied with the findings of the studies, TXI will continue to use waste in its incinerators. Despite controversy, the company stands behind its belief that its processes are doing more good than harm to the environment.
This article originally appeared in the 03/01/1999 issue of Environmental Protection.
Of his 22 years with DuPont, Jim Zeigler, PhD, has spent the past 10 working on fabrics for the protective apparel market. Located at the DuPont-Richmond, Va., site, he is the senior member of the market support and product development team for DuPontä Tyvek® protective apparel and DuPontä Tychem® chemical barrier fabrics, Richmond, Va. Dr. Zeigler is the inventor of many of DuPont's protective apparel fabrics, including Tychem® 7500, Tychem® 9400, Tychem® BR, Tychem® LV, Tychem® 10,000 and Tychem® TK. A member of the F23 Protective Clothing Committee of the American Society for Testing and Materials (ASTM), Zeigler also sits on the National Fire Protection Association (NFPA) Technical Committee for Hazardous Materials Protective Clothing and Equipment. He has particular knowledge in protective apparel technology; industry regulations and standards; and on matters involving weapons of mass destruction.