Analyses of carryover

Tables 2 and 3 present the ash collection and unburned carbon analyses during combustion tests. Generally, the mass balance on the ashes particles accounted for over 90% of the ash input from the fuel. The analyses of the ash collected in all tests for unburned carbon dem­onstrates that with biomass only, there was the least amount of unburned carbon detected in ash collected from the cyclone. However, the unburned carbon content increased when coal was added which suggested that some fine particles were elutriated with the fluidising gas­es. The amount of unburned carbon was, however, quite low, corresponding to about less than 5% of the total carbon input. Such observations seem to suggest that the large particle size and lower heating value of the biomass fuel did not adversely affect combustor per­formance, probably due to the higher volatile matter content of the biomass fuel. The vola­tile matter burns rapidly and the higher volatile matter content of the biomass can also result in a highly porous char, thus accelerating the char combustion as well. In all cases the amount of unburned carbon in the ash increased as the percentage s of coal increased which is due to the low volatility of coal. For the biomass materials the low density of palm fibre and rice husk are also led to increased carbon content in the ash. The initial particle size of the biomass does not appear to be significant.

Analyses of carryover

Figure 4. CO emissions as a function of excess air and Rice husk fraction combustion at heat input 10KW

Fuel

Feed

(kg/h)

Superficial

Velocity

(m/s)

Carbon

feed

(kg/h)

Ash

(kg)

Carbon in

Ash(%)

Efficiency

(%)

Coal (100%)

1.20

0.67

0.900

0.039

23.0

90.25

Rice husk (100%)

2.97

0.56

1.038

0.621

14.5

66.62

Coal (30%) : Rice husk (70%)

2.16

0.99

1.149

0.348

20.9

75.33

Coal (50%) : Rice husk (50%)

1.60

0.85

1.159

0.196

28.7

83.24

Coal (70%) : Rice husk (30%)

1.40

0.81

1.094

0.176

26.6

86.07

Table 2. Ash analysis for single and co-combustion of coal and rice husk at varies percentage of excess air

Fuel

Feed

(kg/h)

Superficial

Velocity

(m/s)

Carbon

feed

(kg/h)

Ash

(kg)

Carbon in

Ash(%)

Efficiency

(%)

Coal (100%)

1.20

0.67

0.900

0.039

23.0

90.25

Palm kernel shell (100%)

1.97

0.59

0.898

0.028

5.0

80.67

Coal (30%) : Palm kernel shell (70%)

1.74

0.74

0.949

0.030

11.7

80.73

Coal (50%) : Palm kernel shell (50%)

1.59

0.65

0.962

0.031

14.9

89.86

Table 3. Ash analysis for single and co-combustion of coal and palm kernel shell at varies percentage of excess air.

2. Conclusion

The conclusions obtained in the present investigation on the temperature profile, carbon combustion efficiency and CO emissions in a 10 kW FBC can be summarised as that biomass combustion behaves differently in comparison to coal due to the significant difference in volatile matter content and variations of particle size and particle density. The carbon com­bustion efficiency was influenced by the operating and fluidising parameters in the follow­ing order: a) settling velocity; b) coal mass fraction; c) fluidising velocity; d) excess air and e) bed temperature (Tb). The maximum carbon combustion efficiency increased in the range of 3% to 20% as the coal fraction increased from 0% to 70%, under various fluidisation and op­erating conditions. Generally, the carbon combustion efficiency increased with increases of excess air and peaks at 50%. The corresponding increasing carbon combustion efficiency with excess air from 30-50% was found to be in the range of 5 – 12 % at 50% coal mass frac­tion in the biomass mixture. Further increase of excess air to 70% reduced the carbon com­bustion efficiency. Increasing the fluidising velocity increases the turbulence in the bed leading to better solid mixing and gas-solid contacting and shows as the amount of carbon in the bed is burnt at higher rate. However, when the combustion is stabilised, increasing fluidising velocity contributed to a greater particle elutriation rate than the carbon to CO conversion rate and hence increased the unburned carbon. Apart from solid mixing, increas­ing fluidising velocity also influenced settling time of fuel particle during the combustion process in FBC. Increasing fluidising velocity brought the lighter fuel particle upward to the freeboard region and completed before they reached the bed surface. The bed temperature had a small effect on carbon combustion efficiency for the biomass fuels. The turbulence cre­ated by increasing excess air related with increases in fluidising velocity had a greater influ­ence than reduced bed temperature. Significant fluctuations of CO emissions observed when coal was added into almost all biomass mixtures depending upon excess air ranges between 200-1500 ppm. The analyses of the ash collected in all tests for unburned carbon demon­strates that with biomass only, there was less unburned carbon detected in the ash collected from the cyclone indicating that the combustion of fixed carbon was almost complete. The percentages of unburned carbon increased in the range 3 to 30% of the ash content with the increases of coal fraction in the coal/biomass mixture. This can be explained by the fact that as the coal fraction increased the higher char combustion and less volatiles combustion oc­curred. Moreover, the elutriated carbon loss increased as fluidising velocity increased result­ing in the lower carbon combustion efficiency. On the contrary, it was found that the bed temperature had no strong influence on carbon loss during the tests. As a conclusion, the combination factors of operating parameters attributed to the resulting effects of biomass co­combustion.

[1]l = Isc_scm’g (і + a(Tc – tc_scm ) (9)

gc _scm

[2]abL1 = (1 — G2) Vkl

[3] DFIG’s Control