The John Thompson dump grate is used for burning a wide range of fibrous fuels, from wet bagasse to dry sunflower seed husks. It has also been successfully used for burning wood chips, saw dust and bark.

In common with the John Thompson pin-hole grate, this is often the technology of choice when converting old hearth or ‘self-feeding’ furnaces to spreader firing, where the brick-set boiler construction and limited available headroom obviates the use of other grate types.

The dump grate comprises rows of temperature resistant cast-iron keys mounted on carrier bars that pivot in bearing bushes mounted on the stoker frames. The keys have deep web sections to provide structural rigidity and to aid with cooling. Temperature resistant cast-iron cover castings are employed around the periphery of the grate and as protection for the bearings.

The grate is divided into a number of discreet sections corresponding with the number of fuel feeders / spreaders. During operation, each section of grate is dumped periodically via hydraulic cylinders that actuate the carrier bars and keys via mechanical linkages to remove sand and ash that has deposited on it. A trim damper fitted to each dumping section regulates the distribution of hot primary air to the underside of the grate, whilst facilitating isolation of the air flow to the particular section of the grate being dumped.

The system design is typically such that full boiler load can still be carried whilst individual sections are being dumped.

The dumped ash in the undergrate hopper is removed by water sluicing to quench any burning particles dumped with the ash.

The John Thompson vibrating grate was developed for the combustion of biomass and other low ash solid fuels with up to 50% moisture content.

Either air- or water-cooled, the grate surface is intermittently vibrated with the use of an eccentric drive and a leaf spring mounting arrangement to induce continuous movement of the fuel bed towards the ash discharge end. Based on the fuel preparation, particle size distribution, ash and moisture content, the grate can be sloped and the vibration frequency and duration adjusted to optimise the movement of the fuel and the ash disposal rate.

The air distribution across the depth of the grate is zoned into three sections with the undergrate combustion air supply to each section individually controlled to ensure complete combustion of the fuel prior to reaching the ash discharge end.

 The grates are manufactured from suitable materials to accommodate hot undergrate combustion air at temperatures up to 250°C.

Based on the fuel geometry and characteristics, the grate movement can either be from the front to the rear of the furnace, such as when the fuel is introduced by means of a box ram or similar, or it can be configured to move the fuel from the rear of the furnace to the front, such as when the fuel is introduced by means of pneumatic spreaders.

Although the air-cooled vibrating grates can be horizontal, the water-cooled version in inclined towards the ash discharge end to facilitate natural circulation of the cooling water from the boiler circuit to which it is attached, whilst also enhancing its conveying capability. The movement of the grate is isolated from the water-cooled furnace walls to avoid any fatigue loading on the pressure parts.

The simple mechanical design and few moving parts of the versatile John Thompson vibrating grate ensure low maintenance requirements and high reliability.

Bagasse is essentially a gaseous fuel consisting of water and volatile constituents. That’s why bagasse is best handled by introducing it into the combustion chamber with part of the combustion air, and burning it whilst still in suspension.

To stabilise the heat release rate within the combustion chamber and reduce the amount of excess air required to complete the combustion, this system of combustion (which can be used with most fibrous fuels) requires a uniform spread of fuel across the full width and depth of the combustion chamber.

The first step is to meter the supply of bagasse in proportion to the boiler load. This is achieved with John Thompson three-drum rotary bagasse feeders. Two small inlet rolls in the feeder meter the bagasse supply whilst a third toothed drum positioned below the feeder rolls, runs at a higher peripheral speed. This tears and teases the incoming bagasse supply so that it arrives at the distributors in an ideal form for pneumatic spreading.

The feeders have a linear output in relation to rotational speed and each feeder is independently driven by AC variable speed-controlled motor, gearbox and final chain drive. Independent drives for the metering rolls and carding drum can be provided if required.

We offer three standard bagasse feeder capacities, covering most requirements.

John Thompson combined coal and biomass air-glide fuel distributors were developed through extensive testing and have become the industry standard for modern spreader fired applications.

Manufactured from heat resistant stainless steel, each fuel spreader features an adjustable trajectory plate in the distributor throat, together with air-flow control to enable the required spread to be achieved on coal or biomass.

To optimise combustion efficiency when firing coal, the John Thompson grit re-firing system is prescribed.

This comprises a series of heat resistant cast-iron nozzles and pipes through which fly ash collected in hoppers downstream of the boiler generating bank is re-injected into the furnace so as to recover the energy from any un-burned carbon in the ash. This can equate to a few percentage points of efficiency gain and hence a significant reduction in coal consumption and fuel cost.

A mechanical cyclonic grit interceptor combined with a mud drum throw-out hopper is typically utilised to capture the fly ash after the generating bank, and secondary air is used as the motive force for re-injection.

This system is also suitable for dual fired boilers where it is undesirable to re-inject the fly ash from one of the fuels, such as bagasse. In this case, truncated hoppers and ash diverter flaps are utilised to ensure that only the required fly ash is re-injected in to the combustion zone.