Case Studies
Biomass
Cadogan Consultants has undertaken the feasibility for, specified and acted as engineer for the installation of the Northern Irelands largest heat only, steam raising Biomass Boiler. The project, for a private client in Northern Ireland has really been achieved with the minimum of difficulty, thanks in part to the good working relationship fostered with the client and some careful project planning. The project also represents a continued development in the renewable energy markets. This project coupled with other biomass projects, such as for Emmaus, builds upon our longstanding solid fuels and combustion capability.
The boiler is a 2MWth step-grate boiler producing steam at 8barg. The step grate boiler was selected because of the best relative ability to burn fuels with differing, texture and moisture content. The specific manufacturer was largely chosen on price but after visiting Austria to inspect the construction and in particular to examine the refractory design. These solid fuel boilers incorporate reflecting arch and the refractory must be rebuilt every 6 years or so depending on use. The boiler will run on wet green wood chips with up to 55% moisture by wet weight. Practical experience suggests that starting on green wood is impractical but full temperature operation is quite adequate. The arch height could have been lower but it is still unlikely that starting or turn down could have radically been improved with very wet fuel.
A larger pilot burner and blended start fuel allow easy starting and rapid ramp up to full steam pressure.
The benefits of using bark and greenwood waste are simply cost – at currently less than 20/tonne, the fuel oil savings amount to more than £100,000 per annum – not surprisingly the client is very happy.
The fuel bunker and fuel handling incorporates a hydraulically operated walking floor, which drags the piled wood chips into a series of auger feeds. The step grate shuffles slowly turning the woodchips with a primary airflow from below the bed and a secondary airflow from charge end down the grate. The two-pass boiler generates steam at 8barg and runs virtually flat out which is ideal for this type of boiler plant.
The boiler incorporates state of the art inverter controlled combustion air fans and Lambda O2 trimming system that ensures the optimum over stoichiometric air supply and at temperature the boiler produces, no visible emission. Apart from the faint smell of wood burning, (which is arguably quite pleasant), there is little to tell the boiler is actually running. The fans are all silenced.
Importantly various lessons have been learned despite already having a significant amount of knowledge related to the combustion of various biomass materials. Having actually conducted tests to determine likely bed temperatures and agglomeration temperatures, clinkering or slagging could be reproduced and for the range of wood fuels proposed it is not considered that ash problems will be likely at the relatively low combustion chamber temperatures achieved.
The boiler is capable of running on greenwood chips at 55% moisture content right up to dried willow chips at only 20% moisture content. Extended trials on both have to undertaken but it is likely a blend of fuels will be used to get the best of both cost and operational flexibility. At the tougher end of the scale the boiler is quite capable of burning the wetter fuel – but this could lead to the potential to overload the walking floors. Even the huge hydraulic rams struggled under the weight of nearly 50 tonnes of wet wood!
The project was procured partly with DETI funding and partly with a Carbon Trust interest free loan. A relatively painless procurement route that has massively accelerated payback. Two further plants to provide steam for the near by dairy are now proposed
The boiler has also been designed to burn a range of WID exempt waste fuels including shredded and or pulverised pallet materials. (Principally Sitka with a 15-20% moisture content and a CV approaching 16MJ/kg). The fuel can be accepted as shredded wood or shredded and pulverised. The augers are sturdy enough to destroy the shards associated with shredding. In this way the boiler can use waste wood which very likely would not be recycled. The chain of custody requirements are still to be agreed with DOENI.
