SUSPENSION FIRED BOILERS AND THEIR GRATES

.P. W. Levy N. E. I. John Thompson (Aust.), Sydney, Australia

ABSTRACT

Steam for power and process use in raw sugar factories is increasingly being generated by boilers in which bagasse is burnt in turbulent suspen- sion rather than in static piles. Suspension firing offers economies in capital, operating and maintenance costs of the boiler plant.

When firing takes place in suspension, the furnace grate is deprived of the coverage of fuel and ash which, with other firing methods, insulates the grate from radiant combustion heat.

Several forms of grate are employed in suspension fired boilers but, amongst these forms, individual grates are often copies of designs which have been developed over many years for other fuelds and other firing methods, and which are not always well suited to the suspension firing role. Design and operational measures to limit grate component tempera- tures and to ensure uniform air distribution reqyire particular attention.

Each of the various forms of grate offers advantages and disadvantages which should be assessed against the requirements of a particular installa- tion.

A relatively new form the water cooled stationary grate provides techni- cal and economic features which may make it the optimum selection for many applications.

INTRODUCTION

The last twenty years have seen the advent and rapid development of suspen- sion fired, bagasse burning boilers (steam generators) form the cane sugar indus- try. In comparison with older-style boilers in which bagasse is burnt in piles in cell type furnaces or on sloping grates, suspension fired boilers, employing sprea- der stokers, offer the following advantages:

1. Suspension firing provides much highe? burning rates per unit of furnace width with corresponding reduction in boiler size and cost for a given output. For example, current suspension fired furnaces can consume 2,6 kg/s o bagasse per metre width, whereas hearth or cell furnaces are typically limited to around 0,83 kgls per metre of width.

2. Consequently, a suspension fired boiler station occupies less plot space than an equivalent station of older style boilers, thus conserving land or floor area which is often in short supply in a crowded sugar factory.

3. Suspension fired boilers are suitable for construction by the most modern boiler manufacturing methods. In particular, furnaces may be of the fully water cooled type, in which the use of refractories and other massonary materials -traditionally items incurring high maintenance costs- can be redu- ced to a minimum.

4. Suspension fired boilers can be constructed in very large sizes, so that one or

Figure 1. Arrangement of typical, olders style, bagasse fired boiler (John Thompson "Beta" type).

In the Australian industry, suspension firing is highly developed. Several suspension fired boilers with steam outputs around 200 t/h have been installed. One unit, installed by the author's company, has a steam output of 277 tlh.

Figure 2. Arrangement of typical, modern, suspension fired boiler (John Thompson TSM type).

The selectioh of the most appropriate form of furnace grate for bagasse fired boilers has long been a matter of controversy. The, cost of the grate can range from a small to a quite significant fraction of the total cost of a boiler installa- tion. The, costs of grate maintenance can range from insignificantly low to extre- mely high. Consequently one finds a wide range of opinions held by sugar factory managers and engineers about which is the optimum form of grate.

Suspension firing introduces grate design criteria which differ from those ap- plying to other firing methods. Since the trend to suspension firing in the sugar industry is likely to continue, in view of the advantages listed previously, this paper attemps to provide guidance to potencial boiler purchasers by outlining the characteristics of the grate types generally employed in suspension fired boi- lers. The information presented may also be of practical assistance to engineers responsible for the operation of existing boiler plant.

THE SUSPENSION FIRING PRINCIPLE

As reported by Lamb, ' combustion of a fixed bed of bagasse essentially proceeds from the base of the bed upwards. Rate of combustion is strongly influenced by the rate of air flow through the bed but is little influenced by the radiation of heat onto the bed from above. As air flow rate increases, the burning rate also increases but reaches a practical limit when a significant propor- tion of the bagasse particles is lifted from the bed by the air flow. Furthermore, high air flow readily creates blowholes in the fuel bed due to non-ideal bed 'packing. Once such holes appear, air flow tends to favour these low-resistance paths, so that air flow through the effective fuel bed is reduced.

Suspension firing overcomes these limitations by employing up-flow velocities high enough to support virtually the entire fuel mass in turbulent suspension. Suspension firing may be likened to a dulute form of fluidised bed combustion and exhibits high burning rates because of (i) intimate and rapid mixing of oxygen and combustible matter and (ii) enhanced heat transfer rates due to turbulence and particle collisions.

Upflow velocities, indicated in terms of combustion energy released per unit furnace plan area, in the order of 3 M W / ~ ~ and greater are adequate to support practically 100 per cent of reasonably well prepared bagasse in suspension. Streams of high energy secondary air are admitted transversely to the general up-flow direction to create the turbulent mixing effect and to extend particle residence times. Pneumatic distributors are almost universally employed to intro- duce bagasse into the furnace and the air jets from these distributors comprise one of the transverse streams required for mixing. Figure 3 illustrates one of a number of possible air distribution arrangements which will sustain virtually all admitted bagasse in suspension.

One often sees spreader stoker type sugar factory boilers operated in the part-suspension, part-fixed bed mode. This limits the burning rate and can be responsible for smoky combustion. Whilts this practice can be partly explained as adherence to the traditional firing method of older types of boilers, it is often more rationally due to the need to retain a reserve of fuel to cater for inadequa- cies in the bagasse feeding, conveying, storage and reclaim systems. It follows, although detailed discussion is outside the scope of this paper, that suspension fired boilers can operate at their full potential only when supported by well designed and highly reliable bagasse systems which can supply fuel to the boilers precisely at the rate required to match steam generation demand.

FUNCTIONS OF THE GRATE

The main functions of the grate in a suspension fired furnace are:

1. To provide a support surface for the heaviest bagasse particles which may not readily be held in suspension.

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2. To provide a surface upon which heavier ash particles can accumulate for later removal from the furnace.

3. To act as a device to distribute combustion air, incoming from beneath, over the plan area of the furnace.

4. To provide a surface upon which starting-up fires may be lighted. 5 . To provide a platform from which inspection and maintenance work in the

furnace may be conveniently carried out.

Its should be realised that the grate itself is not involved in the combustion process and, therefore, efficient bagasse combustion can be obtained with any form of grate which provides the characteristics listed above. Similarly, should difficulties such as smoky combustion or clinker formation be encountered in suspension firing, this will generally not be the fault of the grate, if well designed, and the source of the difficulty should be sought elsewhere.

OPERATING TEMPERATURE

One of the most important parameters in grate design is the operating tempera- ture of the grate components. The maximum temperature to be catered for will influence the choice of materials of construction and will determine the dimensio- nal tolerances required to accommodate reversible thermal expansions. High temperatures and severe temperature gradients can distort the shape of compo- nents and affect the mechanism of the grate.

Generally, the temperature attained by grate bass is the result of two counte- racting heat transfer processes. The grate bars receive heat radiated from the suspension fire in the furnace above .but the combustion air flowing upward through the holes in and spaces between the bars absorbs this heat by convection and returns it to the furnace. The grate bars attain temperatures which provide equilibrium between these two processes.

It is notable that when suspension firing is properly practised there is negligi- ble ash or fuel coverage to insulate the grate from radiant heat. This is a significantly different situation from, for example, the firing of coal on a grate or stoker, where one relies on the ash coverage to insulate the grate surface.

It is also notable that for a suspension fired boiler of given firing rate, grate components will attain lesser temperatures if higher heat release rates (MW per m2 of grate area) are employed. In other words, a given quantity of air passing through a lesser total grate area, will result in increased convective cooling. Des- pite this effect, it is necessary to select heat release rates to suit combustion requirements rather than for the benefit of the grate.

In general, the lower limit to grate component temperature is the temperature of the under-grate air, but surface components may be some hundreds of degrees hotter than the air. While the heat transferred by radiation can be calculated with fair accuracy, the complex shapes of grate components and their air flow passages prevent theoretical assessment of the convective cooling effect. Therefo- re, experimental procedures are required to determine actual component tempera- tures in grates.

Grate manufacturers haver, sometimes, resorted to the use of higher alloy materials for grate bars, to resist the effects of high temperature. However, this is obviously undesirable in terms of (i) first cost and (ii) procurement of replace- ment components. Less costly measures, aimed at preventing the occurrence of, vepy high, grate temperatures, are available.

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Grate Bar Design

Bars should be designed for high convective heat transfer rates to the combus- tion air. Air velocity should be high through the holes and slots in and between bars. Therefore, the air openings should occupy only a small fraction of the total grate area, should be of similar size throughout and should be uniformly distribu- ted over the grate surface.

The heat absorbing surface of the grate is its fire-side plan area; the cooling surface of the grate is its underside area. Therefore, grate bars incorporating substantial cooling fins on their undersides will generally remain cooler than bars without such fins.

Air Flow Control

In suspensions furnaces with conventional fuel to air ratio control, convective cooling of a grate may decrease more rapidly than its radiant heating as boiler load is reduced. This means that grate temperatures may be higher at part boiler load than at full load. Such a situation may be rectified by adjustment of, or addition to, the combustion corgrols to provide relatively high air flows at lower firing rates.

GRATE TYPES

This section describes the characteristics of three types of grate commonly used in suspension fired boilers: dumping grate, moving grate and water-cooled statio- nary grate.

1) Dumping Grate

This is the least expensive of the grates commonly employed for suspension firing. The grate bars are hinged so that ashes can be dumped from the furnace by opening sections of the grate in sequence via controls outside the boiler. The dumping action may be effected by manual levers, or by pneumatic, steam or hydraulic cylinders, but manul actuation is only suitable for quite small grate sections where the mechanical effort required is small.

For all but a number of seconds each day, during which ash is being dumped, the dumping grate is a stationary device. Also, the dumping action requires only a simple mechanism so that bars can be manufactured to the close tolerances required for good air distribution and grate temperature control. Fig. 4 illustrates a dump grate bar which has substantial skirt-type fins for good convective heat transfer to the under grate air.

It is often stated that dumping grates are only suitable for relatively small boilers. This view is erroneous. For instance, the Australian 277 t/h boiler referred to previously is equipped with a dumping grate of 84,9 m2 plan area. The essential requirement is to divide the grate into the appropriate number of indivi- dual sections, each of which is actuated by a separate dumping mechanism. For larger boilers, a satisfactory arrangement is to provide a fore and aft pair of dumping sections for each bagasse distributor, across the width of the furnace.

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Figure 4. Dump grate bar with underside cooling fins.

This principle is illustrated in Fig. 5. There is virtually no limit to the size dumping grates if this modular form of construction is followed.

Furthermore, when firing takes place in suspension and the grate comprises a number of relatively small sub-sections, there is very little upset to combustion conditions when a grate section is opened for dumping. Thus, there is no need to turn down either the bagasse feed rate or the combustion air flow during dumping, provided the dumping operation per section is achieved rapidly. The optimum arrangement for rapid dumping is to use large, pneumatic actuating cylinders with un-cushioned stroke. Abrupt, jerky action of the bars helps to shake free any tenaciously adhering ash or small pebbles. The bars should be reclosed by rever- se pneumatic action. For tightest re-closure of the bars, the piston should remain under pressure in the closed position rather than be mechanically stopped within the cylinder.

When such features are incorporated in the grate, the complete de-ashing procedure can be accomplished in a few minutes and simply entails the actuation in sequence of a series of manual pilot valves mounted across the boiler front.

Attempts have been made to automate the dumping sequence via a timing device, or to eliminate local attendance by use of remote push buttons. Conside- ring that locally actuated dumping requires so little man-time and since it is pointless to actuate the grate to a time schedule, if this schedule does not coincide with significant ash accumulation, such devices have not proven worthwhile or successful.

Additionally, occasional upsets such as excursions of bagasse moisture content can cause departure from suspension firing conditions and results in significant piles of poor quality bagasse on the grate. Re-establishment of optimun firing conditions is best achieved by early removal of such piles from the grate rather than waiting for them to burn away inefficiently. The batch-wise action of a dumping grate is ideal for rapid removal of such piles. Of course diligent obser- vation of furnace conditions is a pre-requisite for early detection of combustion upsets. This in turn requires that (i) manpower should be available for observa-

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Dump grate sectlon 1 w

Figure 5. Sectionalised arrangement of dumping grate.

tion purposes and (ii) the furnace should be equipped with self-cleaned, safe-to- use inspection ports to facilitate such observation.

Provided under grate, air flow remains generally adequate, the only potential difficulty with a well designed dumping grate is incomplete re-closure of the bars after dumping. Should pieces of tramp iron, hard clinker on stones prevent the reclosure of individual bars, the exposed tips of such bars can rapidly swell and oxidise. The swelling can cause progressive jamming of adjacent bars and the results can range from upset air distribution, to non-operability of a complete dumping section. Design and operational features as recommended above will minimise the chance of incomplete re-closure in most applications.

2) Moving Grate

There are many variants of moving grates but all are essentially assemblies of links, bars, rails, shafts, sprockets and other components, to form a large endless chain mechanism driven by an external motor or engine. Onte type of moving grate is illustrated in Fig. 6. Whatever the variant, this is the most expensive form of grate due to ists multiplicity of parts and complex erection procedures.

Figure 6 . Arrangement of one type of moving grate.

The main advantage offered by the moving grate is continous, unattended ash removal which provides steady rather than batch-wise delivery to the ash handling equipment. In the author's view, this is a dubious advantage with suspension firing of bagasse since (i) the coarse ash content of bagasse and its continuous discharge rate are very low, and (ii) several designs of ash handring plant are available to handle furnace ash in batch-wise manner.

Compared with a dumping grate, a moving grate is less susceptible to jamming by tramp iron and stones, although the moving grate is not completely immune from such damage.

On the other hand, moving grates, which have been developed over many years for bed firing of coal and similar fuels, have several disadvantages when applied to suspension firing of bagasse.

In most designs, the multiplicity of moving parts, with dimensional tolerances which allow for minor misalignments and thermal expansions, results in a reasona- bly large percentage open area on the grate surface. The pressure drop due to air flow through the grate is then too low to ensure uniform air distribution over the furnace plan area unless elaborate compartmentation of the under grate air cham- ber is included. Uniform air cooling of the grate surface is therefore unlikely. Neither do moving grates generally incorporate bars with substantial underside convective cooling surface.

Because of the above and because operation of the grate mechanism is sensitive to thermal expansions, moving grates are! seldom suitable for operation with the high combustion air temperatures (200 degrees C and greater) often used in suspension fired boilers, unless they are constructed from doubly expensive higher alloy materials. Several moving grates in suspension fired boilers with high air temperatures have suffered severe heat distortion.

The continual motion of the grate also introduces the potential for abrasive wear of components, particularly of the grate mat supporting rails. Such wear may be severe where bagasse ash. contains substantial amounts of silica and other hard minerals.

Initial alignment of shafts, wheels and chains must be very accurate and great care is required to ensure that alignments are not lost during operation because of variations between the temperatures of supporting frame members.

Grate speeds are in the order of 15 m/h and shaft speeds are around 0,2 rpm. Adequate lubrication of the heavy duty, low speed shaft bearings is problematical, although self-lubricating carbon block bearings and cast iron bearings with auto- matic lubricators delivering temperature-resistant grease have been used succes- sfully.

Whilst individual moving grates must be size limited to obviate the need for excessively sized shafts and to minimise alignment problems, very large grates can nevertheless be constructed, if required, as assemblies of separately driven subsec- tions. For example, one Australian boiler with a total grate area between shafts of 87,2 m2 has a grate comprising three individually driven sub-sections.

The author contends that grate travel toward the boiler front is superior to travel toward the rear (assuming the bagasse distributors are mounted in the front wall). Because it is important to spread bagasse over the full furnace depth for efficient suspension firing, a proportion of bagasse should be spread as far as the rear wall. The slot opening for ash discharge, if incorporated in the rear wall, will permit the undesirable discharge of some unburnt bagasse directly into the ash hopper.

3) Water-Cooled Stationary Grate

In this type of grate, the bars are clamp bolted onto an array of floor tubes through which water is passed. Typically, the water flow forms part of the circula- tion network of the furnace.

Provided there is good contact between the bars and the tubes, the bars are cooled by conduction to the water in the tubes as well as by convection to the combustion air flow. Figure 7 shows one arrangement of water-cooled stationary grate.

Ash is removed batch-wise, by actuation of a series of steam jets which issue through low-profile nozzles embedded at intervals in the grate surface. The steam jets propel the ash along the grate, which usually has a slight slope for water circulation reasons, and into a receiving hopper.

This form of grate, often confusingly referred to as a "pin hole grate", has several significant ediantages for suspension firing:

1. There are no moving parts whatever and, therefore, no problems associated with the operation of a mechanism in a hostile environment.

2. The efficient bar cooling system allows use of very high temperature combus- tion air without danger of damage to the grate.

3. The uniformity of the air openings ensures long-term uniformity of air distri- bution over the full grate area. ,

Full realisation of these advantages, however, demands good design and accu- rate manufacture. For example, the grate bars must be produced to close dimen- sional tolerances if water cooling is to be effective. The bars should have accura- tely contoured concave surfaces which match the radii of the floor tubes; the water

I Figure 7. Arrangement of water cooled. stationary grate.

cooling effect will be marginal if there is only line contact between bars and tubes. Figure 8 shows a grate bar designed for efficient water cooling, via ade- quate tube contact area, and for efficient air cooling, via deep convection fins.

Figure 8. Stationary grate bar designed for efficient water cooling and air cooling.

Another area requiring careful design with this form of grate is the target zone for the potentially erosive, steam propelled ash stream. Although it adds some structural complexity to the furnace, it is advisable to provide a full-width slot opening in the furnace wall for ash discharge into the hopper and to face the target area in this slot with a dense refractory lining. Also, it is possible to use reduced steam blowing pressure in the final row of nozzles to minimise local erosive action. Such arrangements are preferable to a solid "target" wall incor-

/ porating spaced, mechanical gates which are normally closed but are opened during a grate cleaning cycle.

When the above recommended provisions are incorporated, grate cleaning activities become identical to those described for dumping grates, namely the actuation in sequence of a series of pilot valves across the boiler front.

For the same reason given relative to moving grates, the ash discharge slot is best arranged in the wall which houses the bagasse distributors. This means that the steam jet nozzles should discharge toward this wall. Some tendency exists for dand and small pebbles to enter the blowing nozzles but his can be prevented by constantly purging the nozzles with a reduced flow of steam or air. On one installation, in which the ash discharge direction is away from the bagasse distribu- tor wall, the factory has still found it necessary to purge the blowing nozzles to prevent entry of foreign material; the supposed advantage of rearward ash dischar- ge was not realized.

Similarly to dumping grates, water-cooled stationary grates can be built, if required, in very large sizes by using a modular form of construction. The first such grate installed by the author's company has an area of 62,3 m2.

OTHER TYPES

1) Stationary Air Cooled Grate

The elementary form of this grate, such as is installed in the bases of many oler style cell furnaces, is not well suited to suspension firing, since the bars are usually massive with little emphasis on uniform air distribution. However, one envisages that a suitable stationary grate could be constructed using bars similar to those of a well designed dumping grate but fixed in the closed position. In very small boiler sizes and where labour costs are low, ash removal may be effected by manual raking. In larger sizes, steam cleaning nozzles could be employed for ash removal.

I 2) Vibrating Grate

Without the benefit of confirming experience, the author suspects that with the negligible ash coverage provided by suspension firign and with the high air tempe- ratures often used for bagasse combustion, the mechanical vibrating types of grate would be quite unsuitable for use in modern sugar factory boilers.