AE&E takes on another fowl job

That contract is now near to bearing fruit. The new plant will provide an environmentally friendly way for the region’s poultry farmers to dispose of approximately 400 000 tonne per year of their poultry litter and at the same time produce 36 MW of ‘green electricity’. Aside from its value in energy recovery this kind of combustion process also makes it possible to turn minerals from the poultry litter into a re-usable material for agricultural industries.

It will not be the only power plant on this busy site, adjacent to the small Dutch town of Moerdijk, which already contains a major municipal waste incineration plant, AVI Moerdijk, a Shell petrochemicals complex and a 339 MW CHP plant consisting of three Siemens 60 MW V64.3 gas turbines each with an unfired HRSG, and a 175 MW steam turbine.

The contract includes the fuel handling and storage systems, the boiler plant with its auxiliaries, the flue gas cleaning system, all power plant equipment including the water–steam systems, condensing steam turbine generator set, electrical and control equipment and associated civil works.

The erection phase of the main power plant systems started in January 2007. Commissioning was due to take place at the end of 2007 and commercial operation is scheduled to start in May of 2008. The Moerdijk plant will then be operated and maintained by AE&E under an O&M contract with BMC for at least 3 years. AE&E will also take on a ten per cent share of the project. Other shareholders in BMC Moerdijk are DEP (a poultry producers' co-operative) the multi-utility company DELTA, and ZLTO, an organisation for producers of agricultural products in the region.

Handling biomass

Material supplied to the plant is weighed and then delivered to a reception/tipping hall. After unloading, the material, mainly chicken litter, is transferred to a large storage bunker via an automatic crane, where the different loads are mixed and stored. From the bunker the material is fed to several conveying and dosing systems to be transported into the boiler.

Combustion takes place in the hot bubbling sand bed of an AE&E EcoFluid BFB boiler. The heat released to the flue gas is recovered in the walls of the boiler and in the heating surface bundles to produce superheated steam. The steam generated (67 bara, 478°C) supplies a condensing turbine which drives a generator whose electrical energy is fed into the public grid as ‘green energy’.

Intensive cleaning of the flue gas is managed by a 4-stage system that allows safe compliance with the very stringent Dutch regulations applicable to this kind of power plant.

Downstream from the boiler, the exit flue gas cleaning starts with de-dusting in an electrostatic precipitator. The ‘Turbosorp’ process that follows consists of a turbo-reactor where additives (lime, activated carbon) and water are injected into the flue gas to clean from the gas its primary pollutants of chlorine, sulphur, heavy metals and so on. The third stage is the bag filter, in which all the products formed by the reactions between additives and pollutants are separated from the flue gas, while the final cleaning stage is an SCR (selective catalytic reduction) plant to reduce NOx emissions.

The combustion technology

AE&E bubbling fluidised bed EcoFluid boilers typically cover a range from 20 MWt to more than 200 MWt and in several design variations are intended to meet the special requirements of biomass power plants and the pulp and paper industries. The main feature of this boiler type is its extreme tolerance regarding fuel varieties and net calorific values, which can be in the range 3 to 20 MJ/kg.

This is achieved by special design features based on substoichiometric combustion in the bed and a controlled, staged air supply to the combustion chamber. With a flue gas recirculation system for temperature control and good mixing of the flue gases at the inlet of the post-combustion chamber, low emissions and a very good burnout is achieved. This allows the operation of these boilers on very demanding fuel types such as chicken litter, sludges etc. In fact AE&E’s experience from a previous chicken litter project, the Westfield plant in Scotland, was, they say, a major factor in the success of this project.

Boiler design

The EcoFluid unit selected for this application is a compact modular BFB boiler designed primarily for the incineration of biomass and biogenous residues for an output rating up to 40 MWt. The compact footprint is achieved with a vertical 4-pass arrangement with three passes integrated into the evaporator membrane walls, a BFB combustion and post-combustion chamber in the first pass, with a second pass that is either empty or contains additional (platen) superheater surfaces. The third pass contains a convection bundle and the fourth, an economiser. The second pass arrangement that allows for extra superheating confers considerable flexibility of design.

Graduated incineration and flue gas recirculation in this boiler is intended to facilitate precise temperature management in the combustion chamber, resulting in optimum matching to the fuel calorific value and therefore complete burning out. Its main characteristics are:

• Adherence, in a standard solution, to the incineration stipulations contained in the EU directive EC/2000/76 regarding residue incineration plants

• An integrated design wherein the combustion chamber forms part of the pressure section of the boiler and its frame is formed by the membrane boiler walls

• Availability in a modular design concept providing tailor-made solutions for steam outputs of up to 250 tonne/h

• Membrane wall linings of erosion-resistant refractory materials

• Fuel feeding via an air feeder in the underpressure zone above the bed

Combustion and slag characteristics are determined by an in-house developed open nozzle type combustion grid, a facility for the continuous removal of foreign bodies during plant operation, but without any detrimental effects, integrated cooling of the removed bed material and separation into fine and coarse fractions.

Generic ‘Ecofluid’ design showing combustion chamber, superheater, and economiser passes