Some biomass fuels have ash content with a strong tendency to slag and block the heat transfer surfaces. Others contain foreign matter that requires special measures in the fuel handling process to protect the equipment and in the combustion process to protect the environment.
The AET furnace, boiler and combustion system design is based on more than 25 years of experience in combustion of many different types of biomass, varying widely in fuel quality and properties. AET has experience with wood chips, demolition wood, waste wood, sawdust, sander dust, railway sleepers, cuttings from forestry, olive stones, shea nuts, bark as well as straw, poultry litter, compost, sludge, meat and bone meal etc.
AET has many years of design experience with combustion systems for biomass, experience from maintenance of many plants as well as operation and maintenance (O&M) experience at Western Wood Energy Plant.
AET has always had in-house design of the AET Biomass Boiler:
The emissions were previously acceptable, but there were also opportunities for improvement. Therefore, many years ago, AET decided to in-source, with considerable success, the functional and detailed design of the major combustion components:
The AET Combustion System has improved emissions, availability and the maintenance costs significantly. As an example, AET is now able, through these in-house design products to fulfil the most stringent emission criteria for clean wood without secondary measures. At the same time, it simplifies the responsibility to one point only.
Some of the most important qualities of the AET design are:
The fuel is pneumatically transported into the combustion zone using the AET spreader stokers for wood chips and, if necessary, using the AET dust firing system for pulverised fuel. Small particles burn in suspension while larger particles land evenly on the grate, forming a fast-burning layer of fuel.
The grate, which forms a "floor" in the furnace, is an AET BioGrate travelling grate. The continuously-moving grate carpet slowly transports the burning fuel layer towards the boiler front where the ash falls into the ash pit. The grate speed can be adjusted to account for variations in fuel quality and ash content.
Approximately 50% of the combustion air (the primary air) is led to the grate and flows through small air nozzles in the grate bars. The secondary air is injected into the combustion zone through several rows of air nozzles, strategically placed to ensure an efficient mixing of air and combustion gasses. This staged combustion enables low NOx emissions to be achieved.
Unlike other combustion technologies, which has separate zones for drying, gasification, and carbon combustion, the spreader stoker operates with a more evenly distributed combustion over the entire grate.
The optimal combustion temperature is obtained by low excess air ratio, air preheating and/or flue gas recirculation.
A view of the AET Combustion System.
Olive stones can also be utilised as fuel in a biomass-fired plant.
Western Wood Energy Plant in Wales has a very high boiler efficiency and one of the best availabilities in the United Kingdom.