|Gossman Consulting, Inc|
The high temperatures and long residence times in the combustion zones of cement kilns have for over fifteen years been used to burn flammable liquid wastes, such as solvents, as fuels. Increasing government regulation and control of this technology has actually resulted in expanded use as both waste generators and cement manufacturers have grown more comfortable and confident of this technology. Liquid petroleum and petrochemical wastes have been a part of this liquid fuel stream from its inception. Solid and sludgy petroleum and petrochemical wastes present greater handling difficulties. Nevertheless, the desirability of not landfilling many of these wastes has resulted in the motivation to develop solid and sludge handling processing technologies to allow their use as fuel. These processing options extend back to the point of generation. Changes in filter press media and drying technologies are allowing certain petroleum and petrochemical wastes to be pneumatically introduced into cement kilns. Quality control technologies, including laboratories at both cement kilns and the generating/processing location, have also been a critical part of these developments. The cost effectiveness of this option coupled with the significant, positive environmental impacts provides a significant opportunity in developing countries for infrastructure improvement in a way that maximizes sustainable development.
The integration of two seemingly diverse technologies, management of wastes and production of cement, appears to be having a profound effect on both industries:
The process of using large rotary kilns for manufacturing cement is generally understood by the waste management industry because of the large quantity of liquid waste fuels currently burned in 25 cement plants in the U.S. as well as others in Europe and elsewhere. Nevertheless, a review of the cement manufacturing process and chemistry is warranted, since critical characteristics are significantly different from incinerator technology.
Figure 1 is a schematic of a basic straight rotary cement kiln. Raw materials such as limestone, shale, clay, sand, fly ash, etc. are fed, either wet or dry, in specific proportions into the back end of the kiln. This material travels toward the front end of the kiln as the kiln turns. Initially, these raw materials give off water vapor (dehydration) and then give off CO2 (calcination). Finally, in the hottest end of the kiln, the final chemical reactions occur and the material falls out of the kiln into a cooler where it is quenched.
Figure 2 is a schematic of the more complex preheater type of cement kiln. In this system, the dehydration step occurs in the preheater cyclones which offer much better heat exchange efficiencies than the chain section of a straight kiln. A flash furnace may also be present. A flash furnace, or precalciner as it is also called, allows a good portion of the calcination reaction to occur prior to material entering the kiln and also allows up to 70% of the fuel to be used in the flash furnace rather than at the hot end of the kiln. Although the use of a flash furnace does not save energy, it does allow higher production rates since more materials can travel through a kiln which has a smaller volume of hot gases flowing through it. In both designs, the gas and material flow is counter current and exit gas temperatures from the process are generally quite low, 150C to 300C.
There are four major components that make up the clinker that exits the kiln. They are shown with their short hand notations in Figure 3. The various types of Portland Cement generally require different proportions of these four major components. This is largely done by controlling the proportions of raw materials entering the kiln. Figure 4 further illustrates the chemical reactions which occur during the process of forming portland cement clinker. The presence of oxidizing conditions during these clinker forming reaction steps is critical to the production of portland cement.
Major Components of Portland Cement Clinker
The Hazardous Waste Incinerator
Figure 5 provides a very generalized schematic flow diagram of a rotary kiln incinerator. Features of this process, which are critically different form cement kilns and can impact emissions, are noted below.
There are no chemical reactions in an incinerator other than those induced directly by incineration (ie. oxidation). Therefore, gases exiting the incinerator directly reflect what is happening in the combustion zone of the kiln.
In an incinerator, the gases are moving the same direction as solids and liquids. This tends to drive any phase equilibrium to the gaseous side of the equation thus increasing emissions such as metals. Vapor pressure becomes the critical factor.
The thermal capacity and therefore thermal stability of an incinerator is relatively low. Process upsets can occur within a matter of minutes or even seconds that can allow uncombusted organics to escape from the process. The use of an afterburner is largely prompted by this potential.
In a cement kiln, the dehydration and calcination steps produce large quantities of gases that largely mask gaseous emissions from the combustion zone of the cement kiln. Nearly half the mass of raw materials that enter a wet process cement kiln leave the kiln as gases, primarily water vapor and carbon dioxide, yet they are not combustion by-products. Hydrocarbons found in the raw materials may also be released which are unrelated to fuel combustion.
In a cement kiln, the counter current flow design tends to entrain in the cement clinker via the development of recirculating loads. Only if metals reach the cooler chain section prior to condensation are they likely to be enriched in the kiln emissions and even then primarily as a particulate because of the relatively low exit temperatures.
In a cement kiln, at any given moment there is typically greater than 1,000 times as much solids undergoing chemical reactions at 1200C as there is waste fuel being combusted. This provides enormous thermal stability in the kiln. A cement kiln upset can take hours and generally reflects a small decrease in production capacity rather than any changes which might affect combustion. The only critical short term factors in maintaining complete combustion in a cement kiln while clinker is produced are excess oxygen and correct draft fan operation.
Table 1 provides a comparative look at combustion zone conditions in a cement kiln versus an incinerator. Those factors with the most significant difference, gas temperatures and retention times, play an important role in insuring the destruction of hazardous wastes.
|Parameter||Typical Cement Kiln||Typical Industrial Waste Incinerator|
|Maximum Gas Temperatures||>2200C1||<=1480C|
|Maximum Solid Temperatures||1420-1480C||<=1370C|
|Gas Retention Times at >=2000oF||6-10 Seconds||0-3 Seconds|
|Solid Retention Times at >=2000oF||0-30 Minutes||2-20 Minutes|
|Turbulence (Reynolds' number)||>100,000||>10,000|
Waste Processing Options
The use of cement kiln technology for using a wide variety of wastes is only beginning to be tapped. It is theoretically possible to produce quality clinker from 100% raw material and fuel substitution. Some kilns already substitute 100% of their fuel requirements from wastes. Figure 6 illustrates that liquid wastes are only a small portion of the wastes that might be eventually used in cement kilns.
Solids and Sludges
such as contaminated
soils or filter cake.
Requires some form
of thermal separation
or direct feed for
such as Aluminum
Suitable for blending into
Wet Process slurries.
Likely not suitable
for Dry Process Kilns.
Difficult to handle.
Can be blended into liquids or otherwise
|<0.1% Organics||<3000 kcal/hg||>3000 kcal/kg|
Because the gross chemical makeup of many wastes is similar and compatible with the raw materials used to manufacture portland cement, the opportunity to expand on this concept and help to meet national goals for minimizing waste disposal is significant. The water, lime, silica, alumina, and iron which make up the primary constituents in a large quantity of wastes are the raw materials required to manufacture cement. So long as any harmful constituents are controlled and then destroyed or rendered inert in the cement kiln manufacturing process, the advantages are clear. For any type of waste, one or more processing options may be available, or in fact required, prior to reusing waste in the cement manufacturing system. Table 2 outlines some of the processing options and briefly comments on each. There are a number of points in the cement manufacturing process where waste materials can be introduced for recycling and reuse. Figure 7 provides a schematic of the wet process with the following introduction points noted:
|1. Mixing||Mixing of different wastes or waste types is performed generally to provide a more uniform feed and meet specifications.||This very common form of processing is currently used with liquid wastes and for getting solids into liquids. It has broad application to almost any waste category.|
|2. Neutralization||The neutralization of acid or caustic waste with other wastes or new materials is sometimes required prior to further processing.||This is particularly applicable for certain aqueous inorganic waste streams, but may have application in any category.|
|3. Drying||Drying to remove moisture may be needed for certain solid waste||This process may have particular application for dry process cement kilns.|
|4. Particle Sizing||Particle sizing is frequently needed to handle and use a variety of waste streams. Both grinders and mills may be used along with separators to produce the desired particle size.||Particle sizing has been and will continue to be needed for a wide cross-section of waste streams prior to used as fuel and may be needed for certain inorganic wastes.|
|Thermal separation or pyrolysis is typically used to separate the volatile and semivolatile organics from an inorganic matrix. Contaminated soil is a good example.||This process may prove to be valuable for cement kiln resource recovery projects since it would allow the organic fraction to be placed in the hot end of the kiln and the inorganic fraction in the cold end.|
|6. Pelletization||Pelletization is used to produce more or less uniformly sized pellets from the sludges and solids.||It may be possible to use pelletization for processing sludges and solids to provide a uniformly sized pellet that could then be injected into the hot end of a cement kiln.|
The Effects of Using Waste Fuels on the Cement Manufacturing Process
The use of waste fuels in cement kilns has the potential for impacting the cement manufacturing process in four different areas: kiln control and operation, cement clinker (product) quality, cement kiln dust (byproduct) quality, and stack emissions. Both positive and negative effects have been noted in these categories while using waste fuels. Appropriate levels of quality control on waste fuels can prevent significant negative impacts and enhance positive effects on the process.
Kiln control and operation is generally enhanced when using even small quantities of waste fuels because the high level of volatiles stabilizes and aids combustion, particularly while simultaneously using low grade coals or coke. Some operators have found that they can use a less expensive, lower grade coal or larger quantities of coke while using waste fuels. Higher substitution levels for coal, in excess of 20-25%, may require some retraining of kiln operators and adjustments to atomizing air in order to prevent overheating the front end of the kiln. On the other hand, excessive chlorine levels in waste fuels and/or a lack of compensating adjustments in kiln operations can result in large recirculating loads of alkali chlorides. In a straight kiln, this can result in rings which reduce production. In a preheater or precalciner, complete plug-ups have occurred. Excessive fluxing of the mix has also occurred resulting in bad product and ruined clinker cooler grates. Under the most extreme circumstances, very high chlorine levels have stripped the coating and brick from the hot end of a kiln. Other problems in kiln operations have occurred with excessive fluorine or phosphorous levels. Fuel that has been allowed to stand unagitated has occasionally phase separated into layers with different physiochemical properties causing severe kiln upsets when used. For these reasons, quality control and proper storage of the waste is a critical part of the process.
Cement clinker, which is the product of the kiln, is frequently enhanced when using waste fuels. Chemically, cement clinker may exhibit lower alkali caused by the presence of chlorine in the waste fuel. A number of plants have completely eliminated the need to purchase and add calcium chloride to produce this low alkali cement. Some operators have also observed that subtle changes in kiln temperature profiles produced while using waste fuels enhance the quenching of cement clinker, producing a product with both better long term strengths and characteristics which allow for easier grinding. The greatest potential negative impact on clinker quality comes from the ash in waste fuels. Generally, ash levels are lower in waste fuels than in coal. This may require minor changes in raw mix ratios, particularly at higher substitution rates of waste fuels. Most of the time, waste fuel ash is chemically very similar to coal ash and therefore does not have any negative impact. Excessive levels of lead and/or zinc could, however, reduce cement strengths if specifications and quality control are inadequate. Excessive levels of chromium, which can also be present in waste fuel could also negatively impact the safety characteristics of mortar cement.
Cement kiln dust is a byproduct of the cement manufacturing process generated by most cement plants. Because kiln dust is primarily generated in the cool end of the kiln or preheater tower by the interaction of the ground raw materials with the high velocity gas stream exiting the system, there is very little effect from the combustion of waste fuel in the hot end of the kiln. Minor increases in certain volatile heavy metals such as lead and cadmium have been observed while using waste fuel. These increases are not generally beyond typical levels found in CKD, nor will they leach at levels in excess of established leachate limits so long as adequate specifications and quality control are provided for waste fuels.
Extensive research by the U.S. EPA on the use of waste fuels in cement kilns and the associated impact on stack gas emissions reveals little, if any, negative impact. Generally speaking, waste fuel burns cleaner than coal in a cement kiln and can reduce S0x emissions. Trace level increases in lead emissions have been observed while using waste fuel, but these increases are well below the levels which might indicate health and safety concerns. The only other impact that has been observed involves the use of waste fuel in kilns with older marginal electrostatic precipitators (ESPs). In these situations, chlorine levels in the waste fuel must be controlled to prevent the formation of very fine particulate chloride salts which can be more difficult for these older ESPs to capture.
With adequate specifications and quality control, the use of waste fuel in cement kilns can have significant positive impacts in product quality, operations and the environment. In contrast, incinerators have greater negative impacts on the environment because they represent a new source of emissions that need not exist if cement kilns can burn the waste. Existing cement kilns burning waste fuels have consistently shown compliance with air quality standards. The development and implementation of material handling and processing technologies is significantly extending the use of solids and sludges as waste fuel in cement kilns.