Rushmere Heath Nursery - Bedfordshire

Background

The nursery covers an area of approximately 5,000m² and grows predominately hothouse flowers such as poinsettias. Heating of the glasshouses was provided by 3 No. 240kW gas fired boilers consuming on average 1,000,000kWh of energy per year. The owners of the nursery, concerned with ever increasing fossil fuel costs and security of supply were looking for alternative heating sources.

With access to large quantities of free baled cardboard consideration was given to using this as a possible fuel source. Asgard was approached to design, manufacture and install a bespoke boiler plant suitable for burning cardboard.

Although the resulting plant was successfully installed and commissioned by Asgard, changes in the interpretation of the Waste Incineration Directive (WID) made the continued use of this plant not commercially viable. For further information regarding this go to

www.publications.parliament.uk/pa/cm200304/cmhansrd/vo040720/debtext/40720-49.htm

Solution

The installed machinery was a prototype heating system that used baled cardboard packaging waste as a fuel.

A boiler house annex adjacent to the existing boiler house was built to accommodate the system (Fig 1). Pipe work connected the system boiler into the nursery's existing heating pipe work and the system could be isolated from the main heating system by means of 2 manually operated gate valves. A 3-phase electrical supply provided power to operate the 5 motor gearbox units, 3 fan units, 2 linear actuators and other ancillary devices.

Fig 1: Rushmere Heath Nursery Cardboard Fueled Heating System (as viewed from outside)

Bale Feeder

The Bale Feeder unit was designed to accommodate 2 bales of nominal size 1100mm x 800mm x 700mm and weighing approximately 150 Kg. Bales were manually pushed into the unit (Fig 2) from an adjacent conveyor. A carriage plate pushed the bales intermittently into the entry aperture of a shredder unit. The bales travelled at a slow speed no greater than 50mm per minute.

Fig 2: Loading the Bale Feeder Unit

Fig 3: Opening side hinged Shredder Unit

Shredder Unit

The shredder unit was a unique and patented design. Acting as grabbers, a series of intermeshing profiled steel plates were mounted on four vertical shafts. The shafts, by means of a heavy-duty gearbox mounted on top of the unit, all rotated the same way.

An enclosed drive shaft was positioned on the outside of the unit and connected a motor gearbox unit at low level with the gearbox. Steel sheets surrounded the shafts and formed a destruction chamber. Lining this chamber was a series of steel plates, which had welded to them blades. At the bottom of the chamber there was a flighted conveyor belt.

Bales were pushed onto the grabber plates by the bale feeder and material was pulled out of the bale. The grabbers pulled material around the inside of the destruction chamber where it was progressively shredded into smaller pieces by the blades. Eventually this material fell through the blades and grabbers and landed onto the conveyor. The material then passed out of the shredder and dropped into the fuel stoker.

Fuel Stoker

Shredded material was deposited into a fuel transfer hopper positioned at the top of the Fuel Stoker. A load cell mounted at the bottom of the hopper allowed a pre-determined amount of material to fall before both the conveyor and shredder unit were stopped. The material was then moved into the main body of the stoker where it fell onto an air lock plate.

With the transfer hopper in its closed position the air lock plate opened allowing the shredded material to drop onto a stoking auger. This auger directly fed material into the hearth of the boiler. Linear actuators controlled the movements of the fuel transfer hopper and air lock plate.

Boiler

The boiler was a shell type and had been designed and manufactured in accordance with ‘BS 855:1990 Specification for Welded Steel Boilers for Central Heating and Indirect Hot Water Supply (Rated Output 44Kw to 3Mw)’. The heat output from the boiler was no greater than 1 million BTU (300 kW). The boiler featured a water-cooled front door and hearth.

The hearth located at the entry to the boiler and was fed fuel by the stoking auger. Two small fans primary and secondary combustion air to the hearth and a small circulation pump ensured that water was constantly flowing around it. An induction fan mounted on top of the boiler drew the exhaust gases away from the hearth and through the chimneystack to the atmosphere. The I/D fan ensured that combustion took place inside the boiler by providing a negative pressure within it. A de-ash auger was mounted at the bottom of the boiler.

Fig 4: Inspecting Baffles at Front Door of Boiler