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GCI TECH NOTES©
Liquid Alternate Fuel Combustion
David L. Constans
An alternate fuel combustion burner need not be difficult to design nor expensive. An adequate torch can be as simple as a fuel pipe and mixing chambers inside of an air pipe shoved through an opening into the kiln hood. Many initial test firings in the early days of alternate fuel use were executed with such simple designs fabricated on-site. These frequently did as well as later, more complex designs. Clearly the alternate fuels used today have different physical characteristics than those fuels burned in the early 80's. Except for minor internal changes, this has little effect on the burner design. Indeed, many of the burner assemblies used today at cement kilns burning hazardous waste fuels, with the exception that they are located concentrically within the coal pipe, are virtually the same as those first designs.
You can understand our confusion then when GCI is told that a combustion burner designer/manufacturer has quoted $100,000 or more for an alternate fuel torch. Or worse, refuses to quote a fixed price suggesting instead an expensive program to develop a burner assembly. Why so expensive? The extra cost to maintain the overhead requirements of a design/manufacturer firm? Perhaps that is part of it. But in most cases it is the physical characteristics of the proposed fuel that drive the costs so high.
Alternate fuel with the following characteristics can be delivered to the kiln and a burner assembly designed and constructed at a moderate cost. Specific gravity: <1.2, viscosity: <100 centipoise, solids content: 30%, particulate size: 5 mm, heat content: at least 5000 Btu/lb preferably >8000 BTU/lb. There are several kilns utilizing relatively inexpensive burner assemblies that consume 50% or more of their heat requirement using such a fuel.
If the viscosity of the fuel exceeds 100 centipoise, delivery of the fuel to the end of the burner assembly and proper dispersion of the fuel into the burning zone becomes more difficult to achieve and hence more expensive. Centrifugal pumps become less efficient as the viscosity rises, pressure losses in the piping systems quickly increase. Depending on the piping length and pipe diameter, the fluid line losses soon exceed the pump's capabilities. The designer must then either increase the pipe diameter, decrease the line length, choose a more powerful pump and a higher pressure rated piping system or some combination of these to deliver the fuel to the end of the of the burner. Then proper dispersion must somehow be affected. It is at this point that the combustion burner designer/manufacturer becomes wary. If they have been told that this fuel may have a viscosity of 300 centipoise, this may be well outside their experience especially if particle size is 5mm or more. At the present time there are only a small number of kilns that have burner assemblies capable of properly firing such a high viscosity/high solids fuel. These systems have been constructed as joint efforts between the fuel suppliers and the cement facility, often the result of sequential improvements following limited success, the old "cut-and-try" development method.
In other cases it is the desire of the operations group that drive up the cost of the burner assembly. To get effective dispersion and combustion of the alternate fuel, energy in some form must be imparted to the alternate fuel stream to break it into droplets so that it will burn. Generally this is done with compressed air blown into a mixing chamber to create an air/fuel droplet spray out of the end of the burner. If too much air is used, this will disrupt the coal/air stream that surrounds the alternate fuel/air stream. This may create problems with the flame impinging on the brick in the burning zone or simply create a large bushy flame that is less efficient in a cement kiln. Also, too much air may create a high fuel/air velocity. Normally this would not matter except that large particulates will travel much further than desired up the kiln before being consumed, this will make the flame too long creating higher temperatures in a part of the kiln not designed for such temperatures. To reduce the amount of air generally requires tightening the pattern of the burner outlet by reducing the opening. However, with high solids, large particulate fuels this will soon lead to plugging or partial plugging of the outlet. Another way to put energy into the alternate fuel to effect dispersion is the heat from the surrounding coal flame and /or clinker load. This intense radiant heat will cause the fuel to explosively vaporize, especially waste fuels that contain significant volatile content.
Whether you call it hazardous waste fuel, supplementary fuel,
fuel or secondary fuel, such liquid fuels can be burned in cement kilns
utilizing burner assemblies that can be of relatively simple design and
fabricated locally at modest cost. The key is to understand the
between what the involved parties desire and what is realistic.
Obviously there are many physical characteristics and situations that impact combustion burner design in addition to these few discussed above. Each situation is unique. However, this GCI Tech Notes should give you a feel for the first few steps you need to take to achieve a successful burner design at a modest cost. GCI has advised many of our clients on burner design as well as specifying physical and chemical characteristics of alternate fuels such that fuel use is optimized and environmentally safe.