Combustion of Biomal as additional fuel in the BFB boilers
The following issues were addressed in the project:
- How does the new chemical composition of the fuel mix influence bed agglomeration, deposit growth, ash flows, flue gases and particle size distribution?
- Is it possible to run the boiler at a reduced bed temperature of about 750 ºC due to the increased moisture content originating from the biomal fuel?
The project is based on combustion tests in the two Waste to Energy boilers at 20 MWth each owned by Borås Energy and Environment AB (BEM). Furthermore, results from the Waste Refinery Project “Reduced bed temperature in FB-boilers burning waste – part II” has been used as reference in some cases. At normal conditions the boilers are run on a fuel mixture containing 80 percent sorted industrial waste and 20 percent household waste. This fuel mixture consists mainly of paper, plastics and wood. In Borås the organic part of the household waste is sorted out and used for biogas production. With the addition of biomal, which consists of animal by-products crushed to a pumpable fuel, the chemical composition of the fuel mixture is changed to some extent.
The results from the combustion tests shows that biomal influences the chemical fuel composition, but also that there are large variations in the ordinary waste fuel composition as well. The most evident changes with addition of biomal are:
- Increased moisture
- Reduced heat value
- Increased amount nitrogen, calcium and phosphorus
- Decreased amount lead due to the low concentration in biomal
However, there were no changes in sodium, potassium, sulphur and chlorine, elements important for increased/reduced fuel related problems, derived from biomal.
The increase of calcium and phosphorus with biomal derive from bone and the calcium-phosphates influenced to some extent the sand/ash flows were the largest change was an increase in the amount of phosphorus and calcium. In addition a decrease of the amount of lead was seen in all ash flows. The calcium-phosphates were stable during combustion and no reactions of phosphorus with other elements were detected. The results from the trials in this project show that co-combustion of biomal had the following positive effects on the boiler performance:
- The bottom bed temperature decreased, but > 20 % biomal is needed to reach 750 ºC.
- The risk for bed agglomeration was decreased.
- The NOx emission was reduced at the same time as the addition of ammonia was reduced.
- The deposition rate of the deposits collected on the test probe was reduced, thus no increased risk in deposit formation on the superheater tubes.
The conclusion is that the boiler could be operated in a safe way with addition of biomal.