Linde Engineering North America Inc. (formerly Selas Fluid Processing)

Combustion Of Heavy Fuels And Liquid Wastes

This paper describes the design and operating conditions of liquid and gas combustors for heavy fuels and liquid wastes. Lower fuel prices can be achieved by switching from light oil to heavy ones, or combining with unconventional waste fuels. Unconventional waste fuels such as energetic off gases and organic liquid waste require especially designed combustion equipment like a special combustor or waste burner and additional equipment to comply with safety and environmental requirements.

Common liquid waste fuels are Chlorinated compounds, Nitrogen compounds and Sulfur compounds with typical calorific values varying from less than 1200 Kcal/Kg (2160 BTU/Lb) for aqueous wastes to 10000 Kcal/Kg (18000 BTU/Lb) for some organic compounds. Clean combustion without soot or residue may be achieved by the use of proper designed heavy fuel/waste burners or combustors. The combustor type should be selected based on the fuel waste composition, operation requirements and atomizing media available. Hydrocarbons with halogens, bound nitrogen, salts and/or metals are difficult to dispose. Auxiliary fuel may be required to maintain the process requirements and flame stability.

The Combustors for heavy fuels and liquid halogenated wastes are not straightforward. Combustor design is based on Fluid Mechanics, Combustion, Heat Transfer and Emissions requirements. There are several methods used for designing and scaling combustors, both constant velocity and constant flame residence time scaling criterias have limitations.

The combustor functions are to efficiently facilitate the reaction of the air/fuel/waste mixture and to impart the combustion products with flow patterns within the furnace chamber. The Combustor should be designed with very high swirl to facilitate the reaction of the waste fuel and air efficiently with minimum excess air, since any excess air beyond that required to complete combustion will waste fuel. Also the interface between the combustor and the furnace has a heavy impact related to the recirculation of the combustion products. The swirl contributes to another phenomenon taking place in a furnace chamber, the dynamic effect. High swirl also reduces the formation of particulate in the products of combustion.

Liquid fuel/waste must be converted to a gas form, facilitated by increasing the exposed surface area through atomization, before combustion occurs. Good atomization will minimize non-evaporated droplets, improving combustion efficiency and destruction of the liquid wastes. Smaller droplets are easier to vaporize, providing better mixing. Atomized liquid nozzles are used to inject liquid waste through the main burner or into the incinerator chamber. Particulate is also influenced by the degree of atomization.

Combustors are chemical reactors, operating under almost adiabatic conditions, being very profitable to recover waste energy from the heat resultant of the calorific value of both the waste and the auxiliary fuel content in the large volumes and high temperatures of the fluegas. The by-product generation of steam is often used to offset the initial costs.

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