GCI TECH NOTES ©
For some time, GCI has been supplying consulting services to international clientel. For most of these countries, this is the first use of waste fuel in cement kilns. Consequently, their questions are relatively straight forward, though they are frequently tainted by environmentalist exaggerations.
When supplying answers to these questions GCI attempts to avoid complicated answers, in some cases because the answers are being read by people for whom English is a second language and in other cases because these answers are being supplied to employees and community leaders who lack a hard technical background. Also, realistically, it is relatively simple to safely destroy wastes in a cement kiln. The difficulty is in documenting compliance to the myriad of U.S. regulations that have grown up around the use of waste fuel in kilns. Why complicate an otherwise elegant concept by including issues that are purely U.S. EPA (or state) regulatory concerns? GCI thought that these questions and answers would be of interest to all our clients and so we present them here.
Question: Waste fuels have a high chlorine content. How will this impact the cement production process and how will this impact be controlled?
Answer: (First we have to deal with the misconception, promoted by the environ-mentalists, that waste fuel contains a high content of chlorine.) There are generally only a few sources of chlorine in kiln fuels. The largest of these is a group of chlorinated solvents, the most common of which are; perchloroethylene, 1,1,1-trichloroethane, tri-chloroethylene, methylene chloride and a variety of chloro-fluoro carbons. Perchloro-ethylene is the most common "dry-cleaning" solvent and is used extensively to remove oil from metal parts. 1,1,1-trichloroethane and trichloroethylene are used for parts cleaning as well, and in speciality coatings and adhesives. Methylene chloride is often called a "super solvent" since it is used in commercial paint strippers and as a solvent in high performance adhesives. The chloro-fluoro carbons (such as DuPont's freons) are used as non-toxic cleaners as well as refrigerants. All of these chlorinated hydrocarbons are being forced off the market due to their potential impact on the ozone layer. The Montreal Accords severely limit their production. Additionally, there are a number of chlorinated greases and resins although the volume is quite small. Some inorganic chlorides end up in wastes as well. Common salt, sodium chloride, often appears in waste oils due to contamination with sea water (ships' bilges). There are often some chloride salts that are by-products of chemical production.
Generally in the U.S. fuel blenders are seeing a decreasing amount of chlorine in the kiln's fuel materials. Currently, there are few cement kilns that accept waste fuels with more than 3% chlorine, fewer still that routinely combust waste fuel at that chlorine level. Based on the information we have seen on the available wastes in various countries there should be little chlorine in the kiln fuel, most probably less than 1-2% by weight.
The tolerance of the individual kiln to chlorine can be determined through simple testing. GCI can analyze plant process data to determine a safe chlorine input limit for a specific kiln. Once an input rate has been established this rate is used to limit the waste fuel input rate.
Question: The heat content of the waste fuels is considerably lower than the normal fuel. How will this affect cement production?
Answer: (Again, due to environmentalist hype, people have a misconception that waste fuel is not really a fuel.) There is no denying that the waste fuel may have a lower calorific value than coal. Additionally these are predominately hydrocarbons, that is when used as fuels the combustion products include water vapor from hydrogen combustion as well as carbon dioxide from the combustion of carbon. Consequently combustion gases from the burning of hydrocarbons occupy more volume of gas than the combustion of the same number of calories of coal. What this means for a cement kiln is that there will be a higher volume of gas flowing through the kiln burning waste fuel than the same kiln burning coal at the same production rate. It is for this reason that waste fuel can not be sold to cement kilns at a price equal to the energy price of coal. To compensate for this the kilns establish a minimum heat content for the fuel (usually about 6,000-10,000 BTU per pound). The supplier agrees to supply fuel of this quality. The savings in the price of the waste fuel versus coal, for the heat content delivered, offsets the revenues lost from a lower production rate should this occur. The use of waste fuel vs. coal is market driven and very competitive, but is based on the cost of the cement produced vs. the sales price of the cement. In the U.S. some of the cement kilns have left the waste fuel business, or reduced the use of waste fuel, because the current cement market is very good. Since there is an oversupply of waste burning capacity this has not affected waste disposal prices, yet. When cement is not selling, using waste fuel, thereby reducing fuel costs, is for some kilns the only way to show a profit.
The heat content of the waste fuel is dependent on the mix of the wastes available for blending. Assuming no change in a country's economy, as the regulations tighten the heat content will at first increase then decrease over time as generators minimize waste production. Waste volumes will follow the same pattern.
Question: What kind of burner design is necessary to burn waste fuel and coal in the kiln?
Answer: (Frequently this question is lumped in with the "low heat content" question above.) Generally liquid waste fuel would only constitute 25 to 40% of the heat input to the kiln. The rest is made up of the regular fuel, usually coal. (This is an important consider-ation when examining the effect of chlorine input to the kiln, i.e. only 25 to 40% of the fuel contains chlorine at 1 to 2%.) Many kilns have done very well with a concentric waste fuel/coal burner design. That is, a liquid fuel and atomizing air burner nozzle is constructed and is installed in the center of the coal burner pipe. Such a design is inexpensive and requires only minor modifications to the existing coal burner. Frequently the use of waste fuel allows the use of a poorer quality of coal or the use of a coke that has a low volatiles content, thereby increasing cost savings for kiln fuel.
Question: Our employees and our sur-rounding community are concerned with the safety of burning wastes in the kiln, how do we address their concerns?
Answer: There are three levels of concern for safety.
Immediate - Can the waste fuel be collected, blended, transported and com-busted safely?
Mid-Term - Does the combustion of the waste fuel impact the health and environment in the immediate area of the kiln?
Long Term - Does the use of waste fuel adversely impact the health or environment by way of the concrete made from the cement or the constituents in the wasted cement kiln dust or over decades to other areas near the kiln?
The "immediate" is covered by such rules as U.S. EPA's RCRA facility rules or U.S. OSHA's hazardous materials handling rules. This is the "common sense" of handling chemicals safely. This is a well known, well researched field, easy to understand and straight-forward in implementation. The "mid-term" deals with emissions and their effect on the area of deposition. This is predominately what U.S. EPA's Boiler and Industrial Furnace regulation is about, the control of the combustion process by controlling what goes into the kiln and how it is combusted. The data from the certification of compliance tests conducted in 1992 on 34 kilns overwhelmingly demonstrates the effectiveness of cement kilns to safely destroy wastes and recover the energy in these wastes. Measured emissions are generally orders of magnitude below U.S. EPA limits. The "long term" is the most difficult part to explain to people. This is embodied in the "risk assessment", a quasi-mathematical process to assess risk based on numerous conservative, often questionable, assumptions. The difficulty is that some people view risk, any risk, as too much risk. Others see the complexity of the risk assessment as the beginnings of a "flim-flam." The task is to honestly present the risk in proportion to the available alternatives as well as "everyday" risks that people willingly incur. The bottom line is, any risk assessment demonstrates possibilities, not certainties. For each of these levels of concern it can be said that the disposal of wastes in cement kilns is the safest method available. One published assessment demonstrated lower long term risks using waste instead of coal as fuel.
Question: We are concerned with how the public will accept the use of waste fuel in the kiln. What should be done to deal with this?
Answer: No matter what is done there will be people who will protest. Those cement companies which have had the least amount of trouble are those that have spent the time and effort to educate their workers and the local people in what is being done. This requires a number of meetings and a lot of explanation. This includes setting up a group of local leaders, with which representatives of the cement company meet regularly to answer questions and to deal with rumors and misrepresentations. Note, however, that these meetings will often include complaints that have nothing to do with the use of the waste fuel but have to do with how the operation of the cement plant impacts the community. The plant must be prepared to address these concerns as well. It is helpful if the government environmental agency actively participates in these meetings. However, if the people perceive that the agency is in the least way favoring business this participation will be of negative value.
Question: What emissions monitoring is needed to ensure that no toxic substances are emitted?
Answer: We need to make a distinction between "reasonable monitoring" to ensure that the process is performing to a standard, and "absolute-continuous-proof-that-nothing-toxic-is-emitted-at-any-time" monitoring. The bottom line is this: As long as the cement kiln makes salable cement at a set of normal operating conditions, the waste fuel is limited to less than a prescribed percent of the heat input, and certain elements in the waste fuel are less than a prescribed level, then the emissions from the kiln will be no greater than if the kiln were using coal and will frequently be less harmful. There is chemically nothing in the waste fuel that isn't already in the raw feed or coal; they are merely there in different proportions. Whatever monitoring is needed to ensure this is all the "reasonable monitoring" that is required. This does not require monitoring the stack for hydrocarbons or other toxic substances. Normally, this means requiring the kiln to exhibit certain operating conditions for waste fuel use to be initiated and maintained. Such operating conditions are: stable kiln operation at normal rates, less than a certain maximum raw feed rate, a certain maximum air pollution control device (APCD) inlet temperature, a certain minimum kiln exit oxygen, a maximum waste fuel feed rate, etc. Any or all of these can be tied into an interlock system that would automatically stop the waste fuel being sent to the burner.
Question: What is the cost to the cement manufacturer and what is the benefit from using waste as fuel in a cement kiln?
Answer: The cost versus the benefit for using waste fuel has been favorably demonstrated at a number of cement kilns in the United States, United Kingdom, Australia, Germany, France, Norway, Mexico, Indonesia and elsewhere; even under the costly environmental com-pliance requirements imposed by U.S. EPA's Boiler and Industrial Furnace regulations. In the late 1970's and early 80's, when many cement kilns in the U.S. were being closed, the use of waste fuel allowed several large "inefficient" wet kilns to continue to operate. Many of these kilns still continue to use waste fuels despite high environmental compliance costs. Each cement company must, however, perform this calculation based on their individual conditions. The task is very similar to the evaluation of an alternative fuel which requires an investment in hardware and routine expenses. An experienced consultant such as GCI can provide assistance in defining the necessary capital equipment required as well as the routine expenses (labor, repairs, etc.). However the possible impacts on cement production and stack emissions from the use of waste fuel requires a demonstration or trial burn using waste fuel in place of 25 to 40% of the current fossil fuel. Such a trial burn will answer a number of operational and environmental questions and demonstrate the safety of the concept of waste fuel combustion.