Portland cement is made by mixing and reacting raw materials in a high temperature rotary kiln. The raw materials, i.e. clinker precursor material, is typically composed of a mixture containing predominantly limestone and shale. The pulverized material may be supplied to the kiln either in a dry form (dry process) or as a slurry with water (wet process). The composition of the clinker precursor material is carefully controlled to ensure the proper proportions of the desired minerals, namely CaCO.sub.3 (calcium carbonate), SiO.sub.2 (silica), Al.sub.2 O.sub.3 (alumina), Fe.sub.2 O.sub.3 (iron oxide), and MgCO.sub.3 (magnesium carbonate). Upon entry into the furnace system, the clinker precursor material first undergoes a drying and heating process. Next, the material undergoes calcination in which the carbonate minerals are converted to oxide minerals through the evolution of CO.sub.2 (carbon dioxide). At still higher temperatures, the minerals chemically react with each other to produce primarily calcium silicates and calcium aluminates. This process is called clinkering, and it occurs in the burning zone of the rotary kiln. The resulting clinker is then cooled and pulverized and mixed with additional ingredients to form portland cement.
There are several different types of cement plants, including wet process rotary kilns, long dry process rotary kilns, preheater kilns, and precalciner kilns. The difference between these systems is primarily in the method used to dry, preheat and calcine the clinker precursor material. In all of these systems the process of forming clinker is accomplished in the same way, using a counterflow rotary kiln with direct firing in the burning zone.
It is known that the clinker production rate may be increased by injecting oxygen into the rotary kiln to improve the main combustion reaction which is typically an air-fuel flame. However, due to the highly variable operating conditions which characterize cement kiln practice, fuel-rich conditions or undesirable excess oxygen conditions are difficult to avoid. The unsteady state operating conditions of a cement kiln which may cause significant changes in the oxidant demand are due to many factors such as changes in the solids throughput, fuel flow rate, induced draft fan performance, or pressure drop through the system.
It is important in the operation of a cement kiln to maintain oxidizing conditions in the kiln because excess fuel or reducing conditions will cause inefficient kiln operation thus reducing the clinker production rate. Moreover, reducing conditions around the clinker will increase the release of sulfur dioxide from the clinker and may lessen the clinker quality. In addition to causing increased emissions of sulfur dioxide, insufficient oxygen can also cause emissions of carbon monoxide and unburned hydrocarbons. Conversely, if too much excess oxygen is present in the kiln, fuel efficiency is compromised, nitrogen oxides (NO.sub.x) emissions may become a problem, and any added enrichment oxygen is only being wasted.
One way to deal with fluctuating oxidant demand is to adjust the flow of the oxygen which mixes with the main combustion reaction in concert with the fluctuating oxidant demand. However, such oxygen flow changes will cause changes in the flame characteristics, such as flame shape, intensity, stability and length of the flame of the main combustion reaction. Such changes in the main combustion reaction flame characteristics reduce the stability of the burning zone which leads to increased difficulty in controlling the main combustion reaction. Moreover such changes cause refractory coating within the kiln to build up in new places and fall off in others. The repeated building and shedding of refractory coating causes the refractory brick to wear down quicker than if the main combustion reaction were more constant, resulting in higher maintenance costs.
The problems caused by adjusting the flowrate of the oxygen which intermixes with the main combustion reaction may be avoided by maintaining such flowrate constant. However, the fluctuations of oxygen demand in the kiln lead to situations where there is either too little or too much oxygen. Too little oxygen leads to emissions problems and poor quality clinker. Too much oxygen may cause elevated NO.sub.x emissions and is expensive because oxygen is wasted. The high oxygen cost due to wasted excess oxygen has been a significant factor in keeping the use of oxygen from becoming widespread in the cement industry.
Accordingly it is an object of this invention to provide a method for producing clinker which can advantageously employ oxygen to produce high quality clinker at a high production rate.
It is another object of this invention to provide a method for producing clinker which can effectively avoid excess emissions caused by reducing conditions while maintaining relatively constant flame characteristics of the main combustion reaction.
It is a further object of this invention to provide a method for producing clinker which can produce good quality clinker at a high production rate without incurring excessive maintenance costs or high emission levels.