Testing Methodologies and Parameters 152

The reactor was filled with 12-17 kg bed materials in each test, which was fluidized by the primary air (around 185 L/min) and was heated up to above 750ºC for 1-2 hours using propane gas before introducing the solid fuels. A combustion mode was initially performed at an air-to-fuel ratio of 1.4 with a feed rate of about 8 kg/h of the fuel to warm up the whole system before the unit was switched to gasification mode. When a relatively stable bed temperature profile was reached, the gasification mode was started by increasing feed rate to 10-25 kg/h and decreasing total air flow rate to around 300 L/min to match the desired equivalence ratio (ER). In oxygen/air-blown gasification, ER has been commonly used as an important operating parameter, defined as the ratio of oxygen content of air supply to oxygen required for complete combustion (Devi et al., 2003).The value of ER has been observed to strongly influence the gas product compositions and gasification efficiency for air-blown biomass gasification (Kinoshita et al., 1994; Narváez et al., 1996). Usually, the ER is in a range from 0.2 to 0.4 for biomass gasification, to avoid incomplete gasification and excessive char formation at an excessively low ER (<0.2) as well as to prevent formation of incombustible gases like CO2, and H2O at an

extremely high ER (>0.4) (Narváez et al., 1996). Specific operational conditions for each gasification test can be found in Table 7-2. The gasification tests were performed with the four aforementioned bed materials (dolomite, olivine, limestone and iron ore) at varying ER from 0.20 to 0.35. By adjusting both fuel feeding rate and total air flow rate, the desired ER can be maintained. For instance, as shown in Table 7-2, an higher ER was usually achieved by decreasing the fuel feeding rate while keeping the total air flow rate controlled at around 290-300 L/min for most runs, However, in the tests with a same ER

for different bed materials, the fuel feeding rate was kept approximately the same on a thermal-input basis.

Table 7-2 Specific operation parameters for each run

Fuel Bed Material Equivalence Ratio

(ER) Feed Rate (kg/h)

Total Air Flow Rate (L/min)

Pine Sawdust Dolomite 0.20 20.7 290

Pine Sawdust Dolomite 0.25 16.6 290

Pine Sawdust Dolomite 0.30 13.9 290

Pine Sawdust Dolomite 0.35 12.0 292

Pine Sawdust Dolomite 0.40 10.8 300

Pine Sawdust Olivine 0.20 19.0 270

Pine Sawdust Olivine 0.25 17.7 300

Pine Sawdust Olivine 0.30 16.0 300

Pine Sawdust Olivine 0.35 15.0 370

Pine Sawdust Olivine 0.40 15.0 400

Pine Sawdust Limestone 0.20 20.4 290

Pine Sawdust Limestone 0.25 16.4 290

Pine Sawdust Limestone 0.30 14.1 300

Pine Sawdust Limestone 0.35 12.2 300

Pine Sawdust Limestone 0.40 10.4 300

Pine Sawdust Iron Ore 0.20 21.0 290

Pine Sawdust Iron Ore 0.25 16.3 270

Pine Sawdust Iron Ore 0.30 16.0 315

Pine Sawdust Iron Ore 0.35 12.2 290

Pine Sawdust Iron Ore 0.40 10.4 290

Crushed Peat Olivine 0.20 17.4 290

Crushed Peat Olivine 0.25 14.5 300

Crushed Peat Olivine 0.30 12.0 300

Crushed Peat Olivine 0.35 10.9 300

At the steady state, the gasification temperatures (in the dense phase of the fluidized bed) were maintained in the range of 700- 900C (depending on the applied ER) by the partial combustion of the fuel without external heating. A stable temperature profile

along the bed height could be obtained through adjusting the secondary air and primary air ratios (while maintaining a constant total air flow rate). In this study, because the secondary air was introduced above the dense phase of the fluidized bed, the heat loss was inevitable along the reactor column. The average flue gas temperature in the freeboard and in the vicinity of the ash deposition probe was thus in a relatively wide range of 500-700ºC, a temperature that still prevented condensation of tar formed in the gasification process. Additionally, a significant loss of bed material due to physical attrition and thermal fragmentation was observed during some tests, so that additional bed materials had to be added to keep a constant bed level, which was monitored via three pressure sensors located at different heights of the bed as shown in Figure 7-1.In a typical run, at least 3-4 hours of stable operation was performed before cooling down the reactor to room temperature using nitrogen.

A cleaned probe was installed vertically in the freeboard zone of the reactor, right before the whole unit attained a steady state of operation of the gasification mode. When the whole unit was cooled to room temperature, the ash deposition probe was carefully removed from the freeboard and the deposited ash was completely recovered for weighing to calculate the ash deposition rate (DA, g m-2 h-1), as defined below:

(h) Duration ) (m probe the of area Surface (g) deposit ash collected of Mass 2 _  A D (7-1)

The collected deposited ashes were submitted for various characterizations by using XRF for chemical compositions in accordance to the ASTM D4326 standard and SEM in order to have a clear view regarding the effects of the different operations (i.e. bed materials and ER) on the morphology.

Figure 7-3 Schematic of the tar sampling system

Additionally, a non-iso-kinetic tar sampling system was used in this study, using a train of impingers containing an isopropanol solvent, as illustrated in Figure 7-3. Produced gas was drawn using a vacuum pump through a particulate filter into electrically heated lines (maintained at > 350°C to prevent the condensation of tars), the

tar was condensed at the impinge train, and the incondensable product gas flowed through a wet-gas meter and vacuum pump before it was finally vented. The impinger system was composed of six solvent-containing vessels, three in a water bath (20°C) and

three in an ethylene glycol bath at -10°C, plus a final droplet trap. Tar sampling was

started after reaching steady state operation, and continued for 45-90 minutes. Total gas volume and smpling time were recorded with a wet gas meter. Following gasification, the solvent/tar mixture was collected. The impinger system and any piping below 350°C were washed with isopropanol, and the solvent/tar mixture filtered to remove any residual particulate matters. The isopropanol was evaporated at 50°C under reduced pressure with

a rotary evaporator, and the tars were weighed for calculation of the tar concentration in the producer gas.

Because of the relatively large operating scale and the complexity of the fluidized bed facilities, it normally took 2-3 days to complete a successful gasification test (including fuel/facility preparation, operation, after-run cleaning/maintenance). As such in this study duplicate tests were carried out only for the reference test (gasification of pine sawdust using limestone as the bed material at ER=0.3) to examine the reproducibility of the ash deposition rates. The relative standard deviations of the ash deposition rates were within 15.0%.