Chicken powered CHP

On the Delmarva Peninsula, with its many poultry producers, poultry litter disposal has become so pervasive a problem that the waste has contaminated areas of the Chesapeake Bay and is alleged to have actually killed fish because the nitrogen in the waste promotes growth of algae, robbing the water of oxygen.

This is being addressed by the introduction of laws controlling the distribution of poultry litter as a fertiliser based on a phosphorous standard rather than the old nitrogen standard. These new laws (which have now been adopted in Maryland and Ohio) allow only half of the poultry litter to be distributed per acre compared with the old (nitrogen) standards. The EPA currently has a similar law out for comment, which may result in it being adopted nationally in 2003. As some 800 000 tons per year of poultry litter is generated on the Delmarva peninsula, the application of the new standards (which are being phased over some three years) generates many tons of unwanted poultry litter.

The Internally Recirculating Fluid Bed (ICFB) boiler is ideal for the burning of fuels such as poultry litter because of the internal bed circulation coupled with the ability to vary the heat extraction from the fluid bed, thus accommodating the lowest BTU fuel without putting the fire out.

Spinheat already has an ICFB installed at a nursing home near Pottsville, PA, which has been tested on poor grade anthracite and bituminous coals, while tests on poultry litter have been carried out by the Energy Institute of Pennsylvania State University on a fluid bed test rig under identical combustion conditions to the full size boilers, with full emission testing.

To address the poultry litter market a modular CHP plant design – in a range of sizes from 100 kW to 1 MW and steam outputs from 6 360 to 45 000 lb/h – has been developed by Spinheat in which an ICFB is combined with a miniature steam turbine genset (supplied by Coppus-Murray).

The first of these modular units is currently undergoing permitting in the state of Virginia. The planned unit, which is being partially funded by the US Department of Energy, is to be built by Accomac Thermal LLC to supply steam and power to Perdue Farms’ processing plant in Accomac, Virginia, on the Delmarva Peninsula.

The plan is to build a 750 kW plant, with two 22 500 lb/h boilers and miniature steam turbine generator set. The location is significant as there is legislation in Mayland which will subsidise the transport of poultry litter out of state.

Because the miniature turbine proposed only requires saturated steam, no high temperature superheater is needed, thus avoiding the high temperature corrosion, caused by sulphur and alkalis in the poultry litter.

The miniature turbine proposed has only recently been developed as a package for use on small industrial sites or on farms where the emphasis is on low cost rather than high pressure and sophistication. The packaged single-stage back-pressure turbine will accept saturated steam up to 275 psig, and uses advanced, high-efficiency nozzles to ensure high output per pound of steam, so cogeneration with flows as low as 6 360 lb/h (100 kWe) is possible.

These plants all have a truck tipping bay with bucket elevator or clamshell crane (for the larger sizes) which loads the litter in two hoppers with two days or more of litter storage, conveyors to the boiler, a baghouse after the boiler, and ID fan exhausting flue gas to a stack.

The ICFB boiler provides saturated steam at 150 - 250 psig to the turbine, which exhausts the steam at 30-125 psig to the processor’s plant. A DCS controls the plant automatically, so that it can be monitored by one person, on a day shift basis.

The costs of these plants are in the range of $1.5 million (100 kW plant) to $5 million (1 MW plant) and they have a reasonable payback depending on the alternative costs of poultry litter disposal in the district. These cost estimates assume a green field site with a covered fuel reception area and custom made hoppers with a fugitive emission system consisting of a fan and ducting pulling air from the truck unloading and bunker areas, and feeding that air to the primary air fan.

The boiler

The ICFB boiler is essentially a typical “D” shaped package boiler with a unique bubbling fluid bed grafted onto the bottom of the combustion chamber. By using the well established “D” type design and making a modular unit for complete manufacture in the boiler manufacturing plant, the cost of the boiler can be kept down to acceptable levels compared to a field erected unit.

The fluid bed containment is arranged in the bottom of the combustion chamber and is fluidised through non-sifting nozzles of a proprietary design by primary air supplied from a three compartment windbox which, in turn, comes from a primary air fan. The fuel is screw fed (or with some fuels, chute fed) in the middle of the combustion chamber immediately above the top of the dense bed.

Internal circulation is induced in the dense bed by supplying relatively little air to the centre bed and relatively more air to the outer bed nozzles, thus causing the dense bed to move down in the middle, engulfing the incoming fuel and up at the sides where the fuel is mostly burnt out.

The evaporator tubes are arranged so that they bend out of the wall into the fluidised side panels, which are controlled by the two side windboxes. If the fuel that is fed to the bed has a lower BTU than usual (because it is wet, for example) the temperature of the bed will be depressed. This is sensed by the control system which will immediately reduce the air to the side panels, thus reducing the heat transfer to the “nose” tubes, this in turn restores the heat to the main bed, thereby restoring temperature and maintaining good combustion despite the poor fuel. The boiler output will tend to temporarily drop, but the main boiler control loop will sense this and increase fuel rate to the centre bed thus restoring boiler output to the required rate. This control is sensed and responded to with instruments and automated controls.

The fluid bed may be fed with limestone to absorb sulphur or other sorbents such as may be required for control of slagging when burning poultry litter. The poultry litter units are equipped with a separate sand (for start-up) and sorbent hopper (for slagging prevention) and feed screw to the buffer hopper so separate amounts of these materials may be metered into the fluid bed.

The fluid bed is heated up by the over-bed oil fired burner to a temperature at which the solid fuel will continue to burn. In this boiler the oil burner may also be used to obtain at least two-thirds of full load in the case of the solid fuel being unavailable.

The fuel will burn within the dense circulating fluid bed and volatiles will tend to burn in the freeboard above the fluid bed. The freeboard is of generous dimensions in this boiler and NOx control is achieved by adding secondary air immediately above the dense bed. Thus the dense bed may be run at or slightly below stoichiometric air/fuel ratios to inhibit NOx precursors.

At the top of the combustion chamber the gases turn the corner and enter the main evaporator bank. At the bottom of the first pass, the gases go through a 300° turn where solids will tend to drop out of the gas stream. The boiler is equipped with a number of trickle valves at the bottom of this pass, between the mud drum and the combustion chamber back wall, that allows any separated solids to flow back to the fluid bed. In this way any unburnt carbon is recycled through the combustion chamber thereby increasing combustion or carbon burnout efficiency. The tube bank in the back of the boiler is equipped with two sootblowers to remove deposits above each baffle. The reduced temperature boiler flue gas is then ducted to a small economiser, cyclone with recycle and baghouse before being drawn to the stack by the induced draft fan.

Modular approach

The main power plant components – ICFB boiler, turbine-generator, baghouse, deaerator, FD and ID fans, fuel feed hopper, DCS, motor control centre, control room with air conditioner, and the ash dump hoppers – are designed to be delivered to site for erection in one piece. Components having to be delivered in sections are the fuel bunkers, the stack, the drag link conveyor, the main building, and the fuel receiving delivery bay.

In the event of unavoidable litter delivery delays, each plant can be operated at full capacity on waste wood chips or two thirds capacity using the boiler’s start-up burner operating on natural gas or oil.

Litter is delivered by walking beam trucks unloading into a below-grade pit. Two cross screw conveyors from each of the bunker outlets discharge into another conveyor that lifts the fuel into the boiler’s day-bin.

From the day-bin, poultry litter is metered under computer control into the boiler by the fuel feed screw conveyor. Fly ash is filtered and collected by a baghouse operating at a particulate emission rate well below EPA requirements. Bottom ash, expected to be minor, is discharged by a water-cooled screw conveyor. Both fly ash and bottom ash are conveyed by screw conveyors into a pugmill that dampens the ash before it is discharged into the dump hopper for transport to another Perdue fertiliser facility because the ash is high in phosphorus and potassium.

On initial start-up, the boiler’s fluid bed is to be sand that will be heated by the start-up burner to the combustion temperature required by the litter. To maintain a bed sufficient for fluidisation with poultry litter as the sole fuel, a continuous, small flow of sand is required. The sand is to be metered into the day-bin by the sand screw conveyor, under computer control, from the small sand hopper.

The boiler train consists of the boiler, burner, economiser, forced draft fan, burner fan (doubling as the secondary air fan), cyclone and induced draft fan. Included in the plant is the pulse-jet baghouse with 10ft-long Ryton bags, self-supported stack, boiler make-up water treatment, condensate return tank and pumps, deaerator and boiler feed water pumps.

The flue gas goes through an economiser and then the baghouse. The ash is removed from the baghouse hopper by an automatic screw by means of level controls. Bottom ash is removed from the furnace bottom by means of a water-cooled ash screw. The ash is then conveyed to the ash dump truck through the pug mill where the ash is conditioned to avoid fines spillage. The flue gas is exhausted through the ID fan to the stack.

The fuel

Typically the chickens are grown from day-old chicks to five week old chickens, weighing 5-6 lb in large sheds (about 50 ft x 200 ft), which contain around 32 000 chickens. The grown chickens are then sent to a processing company such as Pennfield, Delmarva or Perdue. The floor of the sheds is originally covered with wood shavings (usually pine) which the chickens eventually cover with litter. When the first batch of birds is reared, the top two inches of the mixture of chicken litter and wood shavings is “shaved” and new litter spread ready for the next “flock”. After some six flocks have been reared like this, the chicken shed is cleared right out and produces about 175 tons of poultry litter.

Most of the growers have at least three of these chicken rearing sheds and some many more. As the growers are essentially farmers, most use the poultry litter as fertiliser.

Typically the material has a calorific value of 5 547 - 5 788 Btu/lb, with a moisture content of 20 - 30 per cent. Nitrogen values are quite high, 3.5 - 4 per cent, but measured ammonia has been lower than expected, 0.64 - 0.96 per cent. The potassium and phosphorous are high, at 10.5 - 13 per cent and 4- 6.5 per cent respectively. Alkalies can cause a problem in fluid bed burning as these are low temperature eutectics, which tend to form clinkers in the fluid bed. It was the purpose of the burn tests to solve these problems. One way of curing the formation of low temperature eutectics is to dose the fluid bed with sorbent, which forms high temperature silicates with these metals and thus avoids the formation of clinkers.