GB2056030A - Controlling Solid Fuel & Air Supply to Boilers - Google Patents

Abstract

A solid-fuel fired boiler has a fire grate (18), first air supply means for supplying primary air to pass through the fire grate, a feeder device (12, 13, 14, 15) for feeding solid-fuel onto the fire grate, second air supply means (19) for supplying secondary air to the firegrate, a control device operable as a function of boiler demand to control the operation of the feeder device and said first and second air supply means, and a banking device operable under conditions of zero boiler demand to maintain a fire bed on the fire grate. The banking device is coupled with the control means and includes a timer which is operable, upon initiation of zero boiler demand, to set a predetermined time interval. The banking device is operable, after the predetermined time interval, to operate the control device so that the latter effects operation of the feeder device for a predetermined period, and also operates the second air supply means only so as to burn the fuel fed onto the fire grate by the feeder device during said predetermined period and to maintain the fire bed on the fire grate. <IMAGE>

Description

SPECIFICATION Banking Device for Use with A Solid Fuel Fired Boiler This invention relates to a banking device for use with a solid-fuel fired boiler.

According to the invention there is provided a solid-fuel fired boiler having a fire grate, first air supply means for supplying primary air to pass through the fire grate, a feeder device for feeding solid-fuel onto the fire grate, second air supply means for supplying secondary air to the fire grate, a control device operable as a function of boiler demand to control the operation of the feeder device and said first and second air supply means, and a banking device operable under conditions of zero boiler demand to maintain a fire bed on the fire grate; in which the banking device is coupled with said control means and iricludes a timer which is operable, upon initiation df zero boiler demand, to set a predetermined time interval;; and in which the banking device is operable, after said predetermined time interval, to operate the control device so that the latter effects operation of said feeder device for a predetermined period, and also operates said second air supply means (but not said first air supply means) so as to burn the fuel fed onto the fire grate by the feeder device during said predetermined interval and to maintain the fire bed on the fire grate.

By maintaining the fire bed on the fire grate, during conditions of zero boiler demand, by means of the banking device, a fresh charge of fuel supplied to the fire grate by the feeder device, under the control of the control device when a demand is placed on the boiler, will undergo almost immediate emission of volatiles (under properly controlled fuel/air ratio conditions) which will burn without emission of customary "dark smoke" which tends to be generated with conventional boilers immediately after a fresh charge of fuel is applied to a fire grate which has been operating under zero boiler demand.

The predetermined time interval set by the timer may be, for example, fifteen minutes, though preferably the interval is adjustable up to, say, thirty minutes. Also, the predetermined period of operation of the feeding device by the control device (under control by the banking device) may be of any desired short duration, say five seconds, but this period also may be adjustable to suit different conditions of use.

The boiler may have a suitable apparatus for feeding solid fuel thereto, though one preferred embodiment provides a storage hopper, a substantially horizontal feeding passage arranged to receive fuel from the hopper, a feeder device mounted so as to carry out a linear reciprocating cycle of movement in said passage and having a delivery surface which is substantially smaller in area than the cross-sectional area of said passage, and a control device operable as a function of boiler demand in order to control the cyclical speed of operation of the feeder device.

Further, the apparatus may include a grit recycling arrangement for returning grit to the fire grate so as to undergo further combustion.

One embodiment of solid-fuel fired boiler according to the invention will now be described in detail, by way of exmple only, with reference to the accompanying drawings, in which: Figure lisa schematic side view of fuel fesding apparatus of the boiler; Figure 2 is an end view of the boiler and the apparatus; Figure 3 is a longitudinal sectional view, to an enlarged scale, of part of the apparatus; Figure 4 is a view, similar to Figure 3, of a further part of the apparatus; and Figure 5 is an end view of the apparatus.

Referring now to Figures 1 and 2 of the drawings, the apparatus is designated generally by reference numeral 10 and is illustrated installed in a boiler 11. The apparatus 10 comprises a storage hopper 12, a feeding passage 1 3 arranged to receive fuel which falls under gravity from the hopper 12, a ram 1 4 which is mounted so as to carry out a linear reciprocating cycle of movement in the passage 13, and a downwardly extending discharge pipe 1 5 which is arranged at the delivery end of feeding passage 1 3 and extends downwardly through the top of the shell 16 of the boiler 11.

A control device (not shown) is provided which is operable as a function of boiler demand in order to control the cyclical speed of operation of the ram 1 4. The construction and operation of the ram 14 will be described in more detail below with reference to Figures 3 and 4. However, it will be noted that the ram 14 constitutes a feeder device which is linearly reciprocable within the feeding passage 13, and which has a delivery surface constituted by the leading end of the ram 14 which is substantially smaller in area than the cross-sectional area of the feeding passage 1 3.

The fuel which falls under gravity from the hopper 1 2 into the feeding passage 1 3 will have a natural angle of repose, and the dimensional parameters of the hopper and the feeding passage will be determined, for a particular fuel, so that the fuel on the bottom of the passage 1 3 will stop short of the delivery end of the passage 13 at which fuel is discharged into discharge pipe 1 5. Thus, depending upon the type of solid fuel which is stored in the hopper 12, the position of the discharge outlet of the hopper 1 2 relative to the longitudinal axis of the feeding passage 1 3 will be varied accordingly so that the fuel on the bottom of the passage 13 will not be discharged into the pipe 1 5 when the ram 14 is inoperative.

Adjacent the bottom of the discharge pipe 15, a diverter 1 7 is provided which takes the form of a cone or other divergent shape in order to direct the fuel which fails down the pipe 1 5 laterally. It will be noted from Figure 1 that the bottom end of pipe 1 5 is spaced vertically above the fire grate 18, and the diverter 1 7 is arranged to divert the solid fuel laterally over the surface of the fire grate 1 8 to as to obtain a substantially even deposition of fuel. Any conventional means may be provided in the boiler 11 in order to supply primary air which is directed upwardly through the fire grate 18, though this is not shown in the drawings.

However, in order to supply secondary air, which is usually provided in a boiler and is blown over the surface of the fire grate, the apparatus according to the invention is provided with a secondary air supply system. In the illustrated arrangement, this comprises an outer pipe 1 9 which is arranged concentrically with respect to the discharge pipe 15, and secondary combustion air is supplied thereto via an inlet manifold 20 connected by a pipe 21 to a fan 22.

It will be evident from Figures 1 and 2 that the apparatus may be readily installed as replacement equipment in existing boilers, or may be supplied as original equipment with newly constructed boilers. To install the apparatus, it is only necessary to mount the apparatus above the boiler 11, and to take the pipes 15 and 19 downwardly through the shell 16 of the boiler.

This may involve some displacement of the steam pipes normaily present in the boiler, but relatively simple assembly can be obtained, in that the fuel discharge arrangement (pipe 1 5) and the secondary air supply system (19) are taken through a common hole formed in the shell 1 6.

However, other means for supplying secondary air to the boiler may be employed if desired. In such event, pipes 1 5 and 19 may be installed separately from each other, in which case relatively smaller holes will be formed in the shell 16 in order to accommodate the pipes, and this may result in less displacement of the steam pipes.

Referring now to Figures 3 and 4 of the drawings, the apparatus is shown to an enlarged scale and in greater detail. The ram 14 is mounted substantially mid-way between the top and bottom of the feeding passage 13, and the ram 14 is shown in solid lines occupying its retracted position, and in dash-dot vines occupying its fully extended position. The ram 14 is mounted for reciprocation in a cylinder 23 which is supplied with hydraulic fluid under the control of the control device (not shown). The cyclical speed of operation of the ram 14 is controlled by the control device as a function of boiler demand, and the control device is arranged so that the ram 14 has a constant and relatively high speed retraction stroke, whereas the delivery stroke is at a lower but variable speed depending on demand.

With the illustrated arrangement of hopper 12, feeding passage 13 and ram 14, it has been found that very accurately controlled delivery of fuel can be obtained and without any appreciable degradation of the fuel. This is believed to arise due to the relatively small area of the delivery surface of the ram 14 as compared with the cross sectional area of the passage 13, and the mounting of the ram 14 substantially mid-way in the passage 1 3.

Preferably, although not shown, the apparatus is provided with a banking device and a kindling device which control the operation of the apparatus during periods of low or nil demand.

The apparatus as described above with reference to the drawings provides for over-firing of fuel (coal) onto a grate with improved efficiency and boiler response time, coupled with relatively low equipment and maintenance costs plus increased equipment aesthetics. However, in order that advantages can be realised, the burning rate of coal, usually washed singles, is relatively high, so that tiny particles of unburnt material (commonly called grits) are lifted-off the grate and carried forward through the boiler tubes.

In order to improve the operating efficiency of boiler provided with the apparatus, a grit recycling arrangement may be provided for separating grit from the combustion gases and connected to the discharge pipe 1 5 for delivering separated-out grit to the pipe 1 5 to be discharged with a fresh charge of fuel onto the fire grate 18.

The grit re-cycling arrangement comprises a grit arrester which may take any convenient conventional form, such as a conventional high efficiency cyclone of the multi-cell, cyclone or multi-tubular type (not shown). The base of the grit arrester may be formed in the shape of a cone, so that the grits will fall to the bottom, at which point a venturi may be fitted. A separate external fan (not shown) is provided in order to blow air through the venturi and transport the grits via a pipe line to be re-fired in the boiler. A part of a pipe line 24 is illustrated in Figure 1, which comprises the delivery end of the gritreturn pipe and is arranged to discharge grits into the upper end of the discharge pipe 15.

Preferably, as illustrated in the drawings, secondary air is supplied via concentric pipe 19, and this creates a curtain of high velocity secondary air which contains the coal and recycled grits as they fall under gravity onto the fire grate.

As indicated briefly above, a banking device (not shown) is provided, which is intended to prevent, or at least reduce the tendency of the boiler to generate "dark smoke" when a fresh charge of fuel is applied to the fire grate following operation under zero boiler load.

The type of fuel feeding arrangement, as described above with reference to the accompanying drawings, provides high overall operating efficiency, excellent response to load demands, and lends itself readily to automatic controls and operation. However, one of the prime difficulties (which also applies to other boilers which provide for downward discharge of fuel through a discharge pipe onto the fire grate), has been the tendency to produce "dark smoke" under low load conditions, and an inability to restart firing conditions from over-night shut down.

As will be described in more detail below, a banking device is provided to overcome these problems, and avoid the emission of dark smoke on low loads, and enables the boiler to be brought on line automatically without producing dark smoke for periods of up to forty-eight hours or more.

Before describing the banking device, there will first be described the mode of operation of conventional "sprinkler" stoker operation, e.g. as provided by the GWB Vekosystem. Fuel from on overhead feeding and distribution device is dropped onto a conventional fire grate, with high resistance bars and having approximately square configuration. The primary air supplied from under the fire grate causes combustion of the fuel, and of the coke produced. A supply of high velocity secondary air, which enters the boiler from an annular tube surrounding the fuel dropping pipe, ensures that the volatiles are burntoff thus giving high combustion efficiencies. Because of the even distribution of fuel, and relatively high burning rates of up to 60/65 pounds per square foot per hour, there is little or no residual unburnt fuel retained on the bed.Thus, when the water or steam pressure conditions are satisfied, the boiler shuts down and within a period of, say, ten minutes, the fire bed temperature has dropped substantially. After a period of half an hour, the bed temperature is too low for recovery to take place, when fresh coal is applied on boiler re-start.

It can also be seen that it is impossible to restart from over-night shut-down, and the conventional system of "kindle" control, whereby the boiler is re-started every hour, or half hour, for a five minute period, merely increases the problem by creating an uneven fire bed, and adding to the grit problem.

A banking device according to the invention overcomes this problem by creating a thin, but even fire bed of partially burnt fuel. On re-start, the fresh solid fuel is distributed onto a more reactive bed, from which the temperature recovery is rapid, and thus green coal cannot rest on the bed without immediate emission of the volatiles under properly controlled fuel/air ratio conditions. Emission of "dark smoke" is thereby avoided.

The banking device is coupled with the main control device of the boiler (which is operable as a function of boiler demand to control the operation of the feeder device and the sources of supply of the primary and secondary air flows) and is operable under conditions of zero boiler demand to maintain a firebed on the fire grate by exercising control over the operation of the control device. Thus, the banking device has a separate cycle control circuit, built into a control panel incorporated with timers, and a separate electrical circuit. When the boiler ceases to call for heat or steam, the timer circuit is energised, and after a period of about fifteen minutes, the normal burning firing sequence is started by the timer. The timer therefore provides a predetermined time interval, which may be adjustable up to a maximum of half an hour.

On commencing the burning cycle, the control device operates the fuel feeder device for a predetermined period of short duration, say some five seconds, this period also being adjustable to suit particular conditions. The control device also operates the source of secondary air e.g. a secondary air fan, but the primary air fan is not allowed to start. The control device also allows the secondary air to be supplied for the conventional "after-burn" period, so as to burn-off the volatiles which are produced, so that the fire bed is in a condition ready to accept full firing without the emission of dark smoke.

The cycle arrangement is continued for operation every fifteen minutes, or as set by the operator, until the boiler again calls for firing to recommence. This can, if required, be connected to a conventional kindling system, if a degree of background heat or steam is needed.

Claims (5)

Claims

1. A solid-fuel fired boiler having a fire grate, first air supply means for supplying primary air to pass through the fire grate, a feeder device for feeding solid-fuel onto the fire grate, second air supply means for supplying secondary air to the fire grate, a control device operable as a function of boiler demand to control the operation of the feeder device and said first and second air supply means, and a banking device operable under conditions of zero boiler demand to maintain a fire bed on the fire grate; in which the banking device is coupled with said control means and includes a timer which is operable, upon initiation of zero boiler demand, to set a predetermined time interval;; and in which the banking device is operable, after said predetermined time interval, to operate the control device so that the latter effects operation of said feeder device for a predetermined period, and also operates said second air supply means (but not said first air supply means) so as to burn the fuel fed onto the fire grate by the feeder device during said predetermined interval and to maintain the fire bed on the fire grate.

2. A boiler according to claim 1, in which the predetermined time interval is adjustable.

3. A boiler according to claim 1 or 2, in which the predetermined period of operation of the feeding device is adjustable.

4. A boiler according to any one of the preceding claims, in which the feeder device comprises a storage hopper, a substantially horizontal feeding passage arranged to receive fuel from the hopper, a feeder element mounted so as to carry out a linear reciprocating cycle of movement in said passage and having a delivery surface which is substantially smaller in area than the cross-sectional area of said passage, and a control device operable as a function of boiler demand in order to control the cyclical speed of operation of the feeder element.

5. A boiler according to any one of the preceding claims, including a grit re-cycling arrangement for returning grit to the fire grate so as to undergo further combustion.