Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
2000-10-06
2002-11-26
Lazarus, Ira S. (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S342000, C110S345000, C122S235260
Reexamination Certificate
active
06484651
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
FEDERALLY SPONSORED RESEARCH
Not Applicable
TECHNICAL FIELD AND INDUSTRIAL APPLICABILITY OF THE INVENTION
The present invention is directed to the operation of slag tap-type combustion apparatuses for the production of steam, generation of electric power, or for any other purpose. Application of the present invention to solid fuel-fired slag tap-type boilers will allow a reduction in noxious emissions of these boilers. More particularly, the present invention is directed to the addition of iron-bearing compounds to solid fuel-fired slag tap-type boilers for the purpose of fluxing the ash of fuels that would otherwise produce ash slag with high ash fusion temperatures and/or high viscosity, or that would otherwise be incompatible, for environmental or other reasons, with such boilers. As described in detail below, many different industrial products and by-products may be used as the iron-bearing compounds utilized in the method of the present invention. The method of the present invention may be used in heat, steam, and power production, and in any other applications of solid fuel-fired slag tap-type boilers.
BACKGROUND OF THE INVENTION
The combustion of solid fuel releases energy that may be harnessed for the production of heat, steam, electricity, or for other purposes. Solid fuels presently combusted for the production of heat, steam, electricity, or other purposes include, but are not limited to, coal, biomass, petroleum coke, and other synthetic fuels. The most widely used solid fuel for electricity generation is coal. Coal is a macromolecular network composed of groups of polynuclear aromatic rings, to which are attached subordinate rings connected by oxygen, sulfur, and aliphatic bridges. Combustion of the organic matter of the macromolecular network generates heat that typically is used to generate steam that drives steam turbines and generates electricity. Along with the generation of heat, combustion of coal produces flue gases and fuel ash slag. The ash slag generally consists of the mineral matter remaining after combustion of the organic matter in the coal. Thus, the slag may include, for example, silicas, aluminas, iron oxides and other inorganic compounds.
Coal is abundant in the United States, more abundant than oil, for example, and coals from different regions of the United States and the world have different characteristics, such as chemical composition, heat value, and physical properties. Much of the coal from the eastern region of the United States has a high sulfur content and when combusted produces an ash that can be handled as a liquid at conventional combustion chamber temperatures. Conversely, low sulfur coal from the eastern region of the United States produces an ash that cannot be handled as a liquid at these temperatures, due to elevated melting temperatures. Handling low-sulfur coal from the western United States may present handling problems similar to those encountered with eastern region low sulfur coals, or they may present operational difficulties due to unfavorable slag viscosity characteristics.
Certain boiler systems that are fired with solid fuels, and more particularly fired with coal, are designed to operate with a continuous flow of liquid fuel ash or “slag” out of the bottom region of the boiler. Boilers of this type are variously referred to as “slag tap” or “wet bottom” boilers. The operation of these boilers is limited to coals that combustion produce an ash slag having a low ash fusion temperature (AFT), low viscosity, or other characteristic that allows the ash slag to flow from the combustion chamber of the boiler during operation of the boiler. The ash fusion temperature properties of the resultant fuel ash slag may be characterized in a number of ways, including initial deformation temperature (IDT), softening temperature (ST), hemispherical temperature (HT), and fluid temperature (FT). The AFT properties are commonly determined by the procedure outlined in ASTM Standard D-1857. This procedure defines the IDT as the temperature at which the first rounding of the tip of a cone formed from the ash of the coal being evaluated occurs; the ST is defined as the temperature at which the cone has fused down to a spherical lump having a height equal to the width of the base; the HT is defined as the temperature at which the cone has fused down to a hemispherical lump having a height which is one half the width of the base; and the FT is defined as the temperature at which the fused mass has spread out in a nearly flat layer with a maximum height of one sixteenth of an inch. In this specification, these temperatures are collectively referred to as the Ash Fusion Temperature or AFT characteristics of a particular ash.
Slag tap boilers fall into two general categories, both requiring production of a liquid ash slag from combustion of the solid fuel to operate properly. The first category is characterized by boilers that fire the fuel in pulverized form. Such boilers are referred to herein as “pulverized fuel” boilers. The second category includes boilers commonly referred to as cyclone boilers. In a cyclone boiler, crushed coal is fed into the burner end of a water cooled, horizontally oriented cylinder. Combustion air is introduced into the cylinder tangentially to impart a whirling motion to the coal. Coal fired boilers in both of these broad categories require the use of coal that produces an ash with properties characteristic of low AFT and favorable viscosity properties over the temperature range experienced in the combustion environment. Such AFT and viscosity properties are necessary to allow the ash slag to flow from the boiler. Typically, coals and other fuels with the low AFT characteristics or favorable ash viscosity properties required for use in slag tap boilers also have high sulfur concentrations. High sulfur content coals and other fuels produce greater sulfur dioxide and noxious gases when combusted in fuel-fired boilers.
Slag tap boilers are typically individually designed to bum coal from the local area. Therefore, the combustion chamber is designed to operate with the particular ash produced from a certain coal type. Conversely, coal meeting certain predetermined specifications must be used in each slag type boiler to ensure proper operation. These specifications include, but are not limited to, specifications for AFT characteristics, ash slag viscosity temperature profile range, ash slag base-to-acid ratio, as well as others. Coals which do not meet the specifications engineered into a certain designer's slag tap boiler cannot be used in the boiler.
Phase I of the Acid Rain Program of the 1990 Clean Air Act placed a limit on the amount of sulfur dioxide (SO
2
) that can be emitted annually by certain sources. SO
2
emissions from coal fired electric utility plants located in the eastern and midwestern states were typically limited to a greater degree than other sources. Sources of SO
2
air pollution are allocated allowances, or “rights to pollute”, based on their historic level of SO
2
emissions. Each allowance permits one ton of SO
2
to be released into the air and cannot be used again. At the end of each year, polluting sources must hold enough allowances to cover their SO
2
emissions for that year. Any remaining allowances can be sold, traded, or banked for future use. If a source of air pollution does not have enough allowances to cover its SO
2
emissions, it can buy allowances on the open market. Every year the limit on the amount of SO
2
that can be emitted annually is reduced. Allowances are, therefore, a valuable commodity and emissions of SO
2
are becoming more expensive every year. Phase II of the Acid Rain Program, which began in the year 2000, further tightens the annual SO
2
emission limits imposed on coal fired utilities and on smaller, cleaner burning plants.
Therefore, burning of high sulfur coals has become or will shortly become cost prohibitive for many electric or steam generation facilities. Since slag tap or cyclone boilers we
Rozelle Peter L.
Shepard, Jr. Robert N.
Crown Coal & Coke Co.
Kirkpatrick & Lockhart LLP
Lazarus Ira S.
Rinehart K. B.
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