RESULTS AND DISCUSSION
CHAPTER 6 RESUI/TS AND DISCUSSION
6.3 Spent Liquor Coating
6.5.1 Approach for Experiments
The general approach used in this study was to:
• change one of the process variables of interest
• keep the remaining variables constant and
• study the effect.
This was easily achieved since each variable was independent. Many of the runs were done in duplicate for verification purposes. Duplicate runs were important as it ensured some degree of confidence in the results obtained. A set of average data is located in APPENDIX H for the composition profiles of the expelimental runs conducted. Although, there are many limitations on the data, we should not lose sight of the fact that the gasifier is a useful instrument to study the chemistry of spent liquor gasification.
In the case of the spent liquor to steam ratio (L:S), it was decided that the spent liquor was to be varied and the steam flowrate to be kept constant. The reason for this is that in varying the steam flowrate, fluidisation and mixing wiU be affected. Consequently, heat and mass transfer effects must be accounted for if steam flowrate were to be changed. Therefore, it was decided best to keep the steam flowrate constant and vary the spent liquor flowrate.
6.5.2 Description of Experimental Runs
The general approach to a run was to preheat the bed with air until the temperature in the bulk of the bed was uniform. Thereafter, air was switched over to superheated steam which then takes the bed up to gasification temperatures. This was found to be the most effective method for gasifier operation.
After the operating temperature was reached, a further period of 10 minutes was required before spent liquor was injected. Thereafter, the bed temperature dropped by 5-10°C. Only once steady state with respect to the temperature proftles in the bed, spent liquor and steam flowrates was reached, did analysis of the product gas begin.
Gas chromatography was used in the analysis of the product gas. Fixed volume samples of the product gas were then injected into the gas chromatograph (GC) unit. Prior to injection, water from the wet product gas had to be completely removed and to protect the columns in the Gc. Water was knocked out in a condenser system and the sample gas fllither dried in a dtying column.
CHAP'T'ER 6
6.6 Predicted Equilibrium Gas Compositions
RESUIXS AND DISCUSSION
The equilibrium concentrations were calculated for temperatures of 500-600°C. The following independent reactions were considered In this equilibrium calculation. The calculations are rep01ted in theAPPENDIX B.
C+HzO +--+CO+Hz (A)
CO+HzO
+--+
COz+Hz (B)C+2Hz
+--+
CH4 (C)500°C 550°C 600°C
KA 0.024 0.086 0.269
Kn 5.755 3.669 2.718
Kc 2.586 1.000 0.449
Table 6 - 3: Equilibrium Constants for Reactions A, B, C (Smith et aI., 1996)
Components 500°C 550°C 600°C
H2O 0.326 0.273 0.203
CO 0.032 0.073 0.139
H
2 0.240 0.322 0.392CO2 0.253 0.228 0.196
CH.j 0.149 0.104 0.069
Table 6 - 4: Predicted Composition of Wet Product Gas (mol%)
500°C 550°C 600°C
CO 0.0481 0.1007 0.1747
Hz 0.3560 0.4430 0.4926
COz 0.3750 0.3137 0.2459
CH4 0.2210 0.1426 0.0869
Table 6 - 5: Predicted Composition of Dry Product Gas (mol'%)
The data reported in Table 6-4 and Table 6-5 above show:
I. Hydrogen and carbon monoxide increased with temperature.
2. Methane and carbon dioxide decreased with temperature.
6.7 Experimental Product Gas Compositions 6.7.1 Spent Soda Liquor
Spent soda liquor has low sulphur content (0.03%). Therefore, its contIibution to the fonnation of hydrogen sulphide is negligible. The only source of sulphur that comes into the pulp and papermaking process is from bagasse and not from the pulping chemicals used, In the case of soda pulping of bagasse, sulphur may be referred to as a non-process element.
IOH2 Dca -CH4 6C02!
0:57 1:12 0:43
Timel hr:min o
0:28 0:14
~-. -=
100 .
90
80
III 70
111 Cl
ti 60
:::l
" e
a. 50
C~
~ 40
"0
> 30
20
10
0 0:00
Figure 6 - 13: An example of a typical dry product gas distribution at 500°C with L: S of 0.06 for 23% solids liquor.
Figure 6-13 illustrates the trends that were observed in the chy composition profiles of the syngas from the gasifier. A product gas consisting of hych'ogen, carbon monoxide, carbon dioxide and methane was produced. This product was the result of the following reactions (Lietal.,1991b):
C+H20 ~-+CO+H2
CO+H20 ~-+ CO2+H2 C+2H2
+--+
CH"<6-1>steam-carbon gasification reaction
<6-2>water-gas shift reaction
<6-3>methane fonnation reaction The main reaction was considered to be the water-gas shift reaction.
CHA]JTER6 REsur;fs AND DISCUSSION
In addition, it is clear that the dry product gas species approached equiliblium. When one considers that the gasifier system was continuous with respect to the reactants and product gas, then the abovementioned trend is an acceptable description of the speciesinthe product gas. However, it is surprising that it takes so long to reach equilibrium considering the quick reaction rates and short residence time in the reactor of the steam. A possible explanation is that the gasification was affected by the extent of the coating on the aluminium oxide and perhaps the coating reaches an equilibrium thickness during firing of the liquor.
Furthermore, it is apparent from Figure 6-13 that hydrogen was the principle constituent of the dry product gas at the initial stages of the run. However, with time, the compositions of the remaining gases gradually increased with a decrease in hydrogen. This can be explained by looking at the devolatilisation (pyrolysis) stage, the second stage in the conversion of spent liquor (Figure 2-4). In this stage the organic matter in the liquor degraded to form combustible gases from volatile substances with the formation of hydrogen and carbon monoxide. TIle initial presence of velY high hydrogen concentrations may be due to the fact that the burning stages (spent liquor conversion stages) overlapped to varying degrees (Whitty et aI., 1997). TIllS implied that the char burning stage may have overlapped with the devolitisation stage. The hydrogen concentration was significantly higher than the carbon monoxide concentration at the initial stages of this run. This may suggest that more hydrogen formed than carbon monoxide.
An alternative explanation is that the carbon monoxide was consumed via the water-gas shift reaction <6-2> to form more hydrogen. Towards the end of the run, the shift in the reaction favoured the formation of carbon monoxide rather than hydrogen.
6.7.1.1 The Effect of Temperature
• Hydrogen • Carbon dioxide ACarbon monoxide - Methane 50
45
~C 40III
~35
Eo
U 30 III III Cl
ti 25 :::l
~o
Cl. 20
~~15
~"0 :> 10
5
•
•
1Il
•
oJl---~-~-~-~--~-~--i
480 500 520 540 560Temperaturel Deg.Celc.
560 600 620
Figure 6 -14: Effect of temperature on the product gas compositions
The effect of temperature on the product gas distribution was investigated by increasing the gasification temperature. The effect of temperature in the range of 500-600°C with a constant L: S ratio, solids content and time. The L: S ratio was constant at 0.06, with 23% solids spent liquor used in a 70 minute run. In some instances, the run was a few minutes longer to allow for an extra set of samples, which was required when the reliability of data was questioned.
From the product gas composition profiles, the increase in time by a few minutes had no significant effect on the product gas distribution.
The temperature was limited to a maximum of 600°C and was based on the eutectic considerations of the system rather than a random choice. This decision was based on the NPE's (chlolides and potassium) present in the spent liquor which was reported to reduce the melting points of alkali salts (Backman et al., 1993; Frederick et aI., 1991).
As was discussed in section 6.7.1., a similar trend to Figure 6-13 in the composition proftles at different temperatures was observed. It is quite evident that temperature is an important process variable and its significance on the product gas distribution should be noted.
From the predicted gas compositions, carbon monoxide and hydrogen were expected to increase with temperature; however experimental data proved contrary. In fact, carbon