Furnaces – Process – Supplying fluid
Reexamination Certificate
1999-08-25
2001-10-16
Ferensic, Denise L. (Department: 3749)
Furnaces
Process
Supplying fluid
C110S297000, C110S343000, C110S234000, C110S238000
Reexamination Certificate
active
06302039
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is directed to a method and apparatus for improving combustion and the operation of new or retrofitted boilers in various ways. The adoption of the proposed method and apparatus can be expected to reduce capital and operating costs. The types of boilers to which the invention applies are boilers burning biomass, wood waste or other solid fuel, and recovery boilers which burn waste liquor from various pulping processes which are employed in the manufacture of pulp and paper. These processes include: the kraft process, the soda process, the sodium-based sulphite process, the closed-cycle CTMP (chemical, thermal, mechanical pulp) process, the magnesium-based sulphite process and the ammonium-based sulphite process. These boilers generate steam for various process requirements.
All these boilers require combustion air and generally have furnaces which are rectangular in horizontal cross-section.
Ineffective combustion air systems result in poor mixing of the combustion air with the combustibles in the furnace. Poor mixing causes inefficient combustion, which can lead to excessive fouling of the heating surfaces, excessive erosion of the boiler tubes and overloading of the electrostatic precipitator. In certain recovery boilers, inefficient combustion also causes excessive emissions of TRS (total reduced sulphur), carbon monoxide and fume, unnecessarily low smelt-reduction efficiencies and may cause problems with char-bed control and smelt run-off. The manner in which the combustion air is introduced at the various elevations in the furnace may generate excessive fume.
Ineffective combustion air systems may also create a central column of rapidly-upward-flowing flue gases which entrains particulate and, in recovery boilers, liquor droplets and particulate, and carries this material out of the furnace. This carryover material can cause fouling of the heating surfaces and overloading of ash hoppers.
Ineffective combustion air systems may have air jets which fail to penetrate sufficiently far into the furnace, thus starving the centre of the furnace of oxygen. Alternatively, the air jets may be too strong and impinge on the opposite furnace wall, causing circulation problems and/or tube damage. Some combustion air systems suffer lack of jet penetration and/or excessive jet penetration when operated at loads other than the design load.
In boilers burning biomass, wood waste or other solid fuel, the fuel is generally burned on a grate, or in a fluidized bed. The combustion air is introduced both undergrate and through multiple air ports in the furnace walls. In this type of boiler, the air introduced through the wall ports is often termed overfire air, but the various air zones may be given the same terminology as the air zones in recovery boilers, as described below.
In recovery boilers firing liquor from the magnesium-based sulphite process, and the ammonium-based sulphite process, most of the combustion air is introduced as so-called primary air through liquor burners located in the walls or roof of the furnace while the remainder of the combustion air is introduced through multiple air ports in the furnace walls. These multiple air ports may be arranged in several zones, or sub-systems of ports, and may be named, successively, from the burner region towards the outlet of the furnace, secondary air and tertiary air, etc. The multiple ports of each air zone may be on one or more walls of the furnace.
In recovery boilers firing liquor from the kraft process, the soda process, the sodium-based sulphite process, and the closed-cycle CTMP process, all the combustion air is introduced through multiple air ports in the furnace walls. The air ports in these recovery boilers, and the air ports in the walls of boilers burning biomass, wood waste or other solid fuel, are arranged in several zones, or sub-systems of ports, named, successively, from the furnace floor elevation, upwards: primary air, secondary air and tertiary air, etc. The ports of each air zone may be on one or more walls of the furnace.
In recovery boilers which fire waste liquor from the kraft process, the soda process, the sodium-based sulphite process, and the closed-cycle CTMP process, conventionally, the primary air is introduced through multiple ports in four walls, such that the quantity of air originating from each wall is approximately the same and the flow through all the individual ports is more or less equal. The primary air jets from these ports on each wall collide with and interfere with the air jets from adjacent walls and are deflected upwards, thus creating the above-mentioned central column of rapidly-upward-flowing flue gases which causes particulate entrainment, fouling, etc. In a similar manner, the other air zones of the boiler can create or reinforce the central column of rapidly-upward-flowing flue gas, or create other regions of unnecessarily high upward velocities , which carry liquor droplets and other particulate out of the furnace.
By employing the proposed method and apparatus, upward velocity extremes are minimized, and gas mixing and combustion are improved. In certain cases, the number of air ports may be reduced. Thus, the operating and capital costs of the installation can be reduced.
PRIOR ART
By way of exemplification and not limitation, several examples of prior art forms of two-wall primary air-jet arrangements and of partially-interlaced air-jet arrangements are described in the following paragraphs. None of these examples presents the concepts embodied in the proposed invention.
The following patents, all entitled “Method and apparatus for improving fluid flow and gas mixing in boilers”, describe a method and apparatus also invented by Blackwell and MacCallum, wherein the primary air in a recovery boiler is introduced substantially from two walls only, and a related method and apparatus in which combustion air is introduced in a partially-interlaced manner:
U.S. Pat. No. 5,121,700: Issue date Jun. 16, 1992
U.S. Pat. No. 5,305,698: Issue date Apr. 26, 1994
Canadian Patent No. 1,308,964: Ser. No. 564,320; Issue date Oct. 20, 1992
Canadian Patent No. 1,324,537; Ser. No. 616,260; Issue date Nov. 23, 1993.
U.S. Pat. No. 5,121,700 describes partial-interlacing of the primary air jets from a first and a second opposing wall, with substantially no air from a third and a fourth opposing wall, partial-interlacing of the secondary air jets with substantially no air from the third and fourth walls, and partially-interlaced secondary air in combination with two-wall primary air which is created by large jets from two opposing walls and essentially no air from the remaining two walls.
U.S. Pat. No. 5,305,698 describes particular arrangements of two-wall primary air in which a small quantity of air is introduced from the third wall, alternatively from the third and fourth walls. This patent also describes secondary air introduced mostly from two opposing walls with somewhat less air, or substantially no air, being introduced from the third and fourth walls at the same secondary elevation, in combination with the afore-mentioned (in this paragraph) two-wall primary air arrangements.
Canadian Patent No. 1,308,964 describes partially-interlaced air jets.
Canadian Patent 1,324,537 describes a secondary air-jet arrangement with the secondary air introduced from two walls only, in combination with a particular arrangement of two-wall primary air in which there are large primary air jets on two opposing walls and small jets on the third and fourth walls.
In all of these patents, the two-wall primary air jets are from air ports which are all at substantially the same, first, elevation while the partially-interlaced air jets of, for example the secondary elevation, are from air ports, all essentially at a common, second, elevation higher than the primary elevation. None of these patents describes the method or apparatus of the proposed invention in which the combustion air at either the primary or secondary elevations, or at any elevation, is introduced in the propos
Blackwell Brian Robin
MacCallum Colin
Boiler Island Air Systems Inc.
Ferensic Denise L.
Rinehart K. B.
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