8. Combustion Tests with 12 Mixtures
8.7. Comparison of test results
General evaluation Wood pellets
No problems. Constantly very good combustion, steady heat output and flue gas temperature. No slag, neither fixed in the combustion chamber, nor in the ash. No considerable fouling in combustion chamber and flue gas tubes. The pellet quality was high, i.e. the pellets did not emit dust when filled into the fuel hopper.
R6, R7, R8, R11 and R12
No problems with slag in the ash (except R12), but a considerable formation of fixed slag at the air nozzles. The pellet quality was high, i.e. the pellets did not emit dust when filled into the fuel hopper. The high quality also had a positive effect on the combustion quality. Combustion chamber and flue gas tubes were heavily fouled with dust.
The fuels are probably useful in most small boilers, but using them will require a lot of work regarding frequent cleaning of combustion chamber and flue gas tubes.
R9 and R10
Like R6, R7, R8, R11 and R12 except from some tendency of slag in the ash. It did not cause problems in the current boiler because the slag was crumbly, but it might cause problems in less robust and simpler boilers. There was serious formation of fixed slag at the air nozzles.
R4 and R5
No problems regarding slag in the ash, but a serious formation of fixed slag at the air nozzles. Furthermore, combustion chamber and the flue gas tubes were very heavily fouled. The pellets emitted a lot of dust when filled into the fuel hopper due to the poor pellet quality.
R1, R2 and R3
All the tests were stopped during the first two days due to huge slag problems - the combustion chamber was completely filled with slag. The combustion chamber and the flue gas tubes were very heavily fouled and the pellet quality was poor.
In Table 8-3 the fuels are evaluated according to parameters estimated to be the most important in order to classify the fuels. At each parameter the fuels have obtained a mark from 1 to 10. 1 is best and 10 is poorest. In the column “Total” the overall marks for each fuel are shown. They have been calculated as the average of the evaluated parameters.
It should be noted that the evaluation is subjective and that these comparisons are difficult to make as most of the parameters are evaluated by visual observation.
However, it is estimated that the marks are very useful for evaluation if attention is given to the fact that they are only normative.
Total Slag in
the ash Fixed slag Dust fouling
Combustion quality
Pellet quality
Wood pellets 1 1 1 1 1 1
R1 9 10 - 10 10 6
R2 8 10 - 10 6 7
R3 9 10 10 10 10 5
R4 7 2 10 8 4 10
R5 6 2 10 10 2 7
R6 4 2 9 3 3 3
R7 4 2 9 4 2 2
R8 4 2 9 4 2 2
R9 5 4 10 4 2 3
R10 5 4 10 5 2 2
R11 4 2 8 2 2 4
R12 4 6 8 2 1 2
Table 8-3: No marks were given to R1-R2 in “fixed slag” as the test periods were too short for evaluating this parameter. The evaluation clearly indicates that wood pellets are in a class by themselves just as it is very clear that R1, R2 and R3 are in a class by themselves, but at the other end of the scale. R4 and R5 are evaluated to be poor primarily because of the poor pellet quality and heavy dust fouling in combustion chamber and flue gas tubes. R6, R7, R8, R11 and R12 are evaluated to be the best even despite some differences in the parameters.
Emissions
Below the emission results are presented and commented on.
Figure 8-3. The high CO emission in R1, R2 and R3 is caused by slag formation. Until the slag became extensive, the CO emission was low. The CO emission for the other fuels is relatively low and would have been even better if the boiler was adjusted individually for each fuel. The conclusion is that if the pellet quality is high and there is no slag in the ash, the combustion quality is good.
Figure 8-4. In general, the NOx emissions for the test fuels are high compared to wood pellets. In R1-R6 the base material is straw, and in R7-R12 the base material is grain screenings. The higher NOx emission in R7-R12 is due to the fact that grains have a higher nitrogen content than straw.
NOx-emission
0 50 100 150 200 250 300 350
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
ppm
NOx
CO-emission
0 500 1000 1500 2000 2500
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
ppm
CO [ppm]
Figure 8-5. The SO2 emissions for the test fuels are very high compared to wood pellets. The SO2 emission for wood pellets is approximately zero because the sulphur content in wood is very limited.
Figure 8-6. The difference between wood pellets and the rest of the fuels is considerable.
Dust emission
0 100 200 300 400 500 600 700 800
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
mg/mn3
Dust [mg/mn3]
SO2-emission
0 20 40 60 80 100 120
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
ppm
SO2
Other comparisons
Figure 8-7. The slag temperatures for R1 and R2 is somewhat lower than for the rest of the fuels. This is due to the fact that the slag was very extensive and therefor the combustion was poor. The differences between the rest of the tests are limited.
Figure 8-8. The very low figure for R3 is due to heavy slag formation. A large quantity of slag can ensure that the fuel stays in the combustion chamber until it is completely burned out. The high figures for the rests of the fuels have two main reasons: The first reason is that the step grate moves frequently which means that the fuel is pushed into the ash box before it is completely combusted. The second reason is that the primary air supply is limited because of the fixed slag formation at the nozzles.
Temperatures in combustion chamber
0 100 200 300 400 500 600 700 800 900 1000
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
ºC
Flame Slag 1 Slag 2
Unburned in the ash
0 5 10 15 20 25 30 35 40
Wood pellets
R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12
% w/w a.r
Unburned