Combustion System for Biomass – Travelling Grate Spreader Stoker

The most important decision in the design of a well-functioning biomass fired plant is the choice of combustion system.

The fuel challenge

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 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 forest, olive stones, shea nuts, bark as well as straw, poultry litter, compost, sludge, meat and bone meal etc.

Full control of the design at AET

AET have many years of design experience with combustion systems for biomass, experience from maintenance of many plants as well as operational experience at Western Wood Energy Plant.

AET has always had inhouse design of the Boiler Concept:

• furnace
  
• overfire air system
  
• superheaters
  
• economiser
  
• ducting
  
• etc.

The emissions were previously acceptable, but there was also opportunties for improvement. Many years ago AET therefore decided to insource the functional and detailed design of the major combustion components with success:

• Dosing bin
• Rotary valves for fuel
• Spreader stoker (single fuel or more fuels - combi spreader)
• Dust firing system
• Travelling grate
• SNCR DeNOx. SNCR DeNox is also supplied to other boiler types.
• Re-injection of fly ash
• Integrated Coarse ash seperator

The AET Combustion System has improved the emissions, availability and maintenance costs remarkably. As an example AET can by these in-house design products full-fill the most stringent emission criteria for clean wood with any secondary measures. At the same time it simplifies the responsibility to one point only.

Our references all show long-standing high availability

Some of the most important qualities in the AET design are: Very low flue gas emissions Very high availability Very long operating time (>8000 hours) between shutdown for manual cleaning Robustness towards foreign objects (stones, metal, glass) in the fuel Low power consumption Good fuel burnout High boiler efficiency Low maintenance costs AET has chosen the right technology for biomass utilisation

The fuel is pneumatically transported into the combustion zone using AET-BioSpreaders for wood chips and, if necessary, using 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 a mass burning grate which has separate zones for drying, gasification, combustion and fuel burnout, the travelling grate 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.