EVALUATION OF POTENTIAL IMPROVEMENTS TO BLG TECHNOLOGY P. McKeough, VTT Processes, Finland

Transcription

1 EVALUATION OF POTENTIAL IMPROVEMENTS TO BLG TECHNOLOGY P. McKeough, VTT Processes, Finland AIM to evaluate ways of improving the competitiveness of black-liquor gasification (BLG) technology in combined-cycle applications SCOPE LIMITATIONS conventional causticisation only; i.e. direct (auto) causticisation excluded hot recovery of H 2 S excluded, therefore hot-gas filtration only considered for sulphur-free pulping potential benefits through application with modified kraft pulping, e.g. polysulphide pulping, not evaluated. FUNDING Technology Development Centre of Finland (50%), UPM-Kymmene Ltd (25%), VTT (25%) EVALUATION OF POTENTIAL IMPROVEMENTS TO BLG TECHNOLOGY MAIN TASKS 1. Screening of potential improvements selection of most promising of recently proposed improvements for detailed evaluation 2. Techno-economic evaluation: low-temperature direct gasification processes, in particular, operation at atmospheric pressure dry-quench gas cooling in conjunction with high-temperature gasification dry-quench gas cooling and hot-gas fitration (for sulphur-free pulping, only).

2 PROCESS CONCEPTS EVALUATED AND THEIR RATIONALE I 1. High-temperature (HT) processes: Description Wet-quench cooling; base case; Chemrec type (Figure 1) Base case but gasifier wall cooling included Dry-quench cooling (300 C) + wetquench cooling (Figure 2) Dry-quench cooling (500 C) + heatexchange cooling (Figure 3) Dry-quench cooling (500 C) + hot filtration; for S-free pulping (Figure 4) Rationale Reference case Wall cooling would ease material problems Dry quench would reduce CO 2 absorption in dissolver Further advantage of improved energy recovery in gas cooling Further improvement in powergeneration efficiency Notes: (1) all above HT processes are pressurised, (2) dry-quench cooling would necessitate changes to Chemrec-type process; e.g. increase in spray drop size FIGURE 1. HIGH-TEMPERATURE PRESSURISED GASIFICATION (HT), WET-QUENCH COOLING O 2 Weak wash ~1000 C Wet Quench <200 C H2S Green liquor

3 FIGURE 2. HIGH-TEMPERATURE PRESSURISED GASIFICATION (HT), DRY-QUENCH + WET-QUENCH COOLING O 2 H2O Weak wash ~1000 C Dry Quench e.g. 300 C Separator Wet Quench <200 C H2S FIGURE 3. HIGH-TEMPERATURE PRESSURISED GASIFICATION (HT), DRY-QUENCH + HEAT-EXCHANGE COOLING O 2 H2O HP-steam ~1000 C Dry Quench e.g. 500 C Separator H 2S

4 FIGURE 4. PRESSURISED GASIFICATION FOR S-FREE PULPING, DRY-QUENCH + HOT-GAS FILTRATION H2O Dry Quench e.g. 500 C Separator (complete) PROCESS CONCEPTS EVALUATED AND THEIR RATIONALE II 2. Low-temperature air-blown (LT/air) processes: Description Heat-exchange cooling; atmospheric pressure; base case (Figure 5) Base case but 90 % carbon conversion Base case but pressurised Dry-quench cooling (500 C) + hot filtration; pressurised; for S-free pulping (Figure 4) Rationale Operation at atmospheric pressure offers many technical advantages Complete carbon conversion difficult in practice Improvement in power-generation and overall efficiencies Further improvement in powergeneration efficiency Note: gasification temperature 600 C for all processes

5 FIGURE 5. LOW-TEMPERATURE AIR-BLOWN GASIFICATION (LT/AIR) HEAT-EXCHANGE COOLING Air 350 C HP-steam C H2S Air CW PROCESS CONCEPTS EVALUATED AND THEIR RATIONALE III 3. Low-temperature oxygen-steam-blown (LT/O 2 ) processes: Description Heat-exchange cooling; atmospheric pressure; base case (Figure 6) Base case but 90 % carbon conversion Base case but pressurised Dry-quench cooling (500 C) + hot filtration; pressurised; for S-free pulping (Figure 4) Rationale Operation at atmospheric pressure offers many technical advantages; gasification chemistry close to that of MTCI process Complete carbon conversion difficult in practice Improvement in power-generation and overall efficiencies Further improvement in powergeneration efficiency Note: gasification temperature 600 C for all processes

6 FIGURE 6. LOW-TEMPERATURE O 2 -STEAM-BLOWN GASIFICATION (LT/O 2 ) HEAT-EXCHANGE COOLING O 2 Steam* HP-steam C H2S CW * Superheated steam from HRSG PROCESS CONCEPTS EVALUATED AND THEIR RATIONALE IV 4. Low-temperature indirectly-heated (LT/indirect) processes: Description Heat-exchange cooling; atmospheric pressure; maximum fireside temperature 1700 C; base case; MTCI type (Figure 7) Base case but 90 % carbon conversion Base case but maximum fireside temperature 1000 C Rationale Reference case Complete carbon conversion difficult in practice Lower fireside temperature may be necessary to avoid agglomeration on process side Note: gasification temperature 600 C for all processes

7 FIGURE 7. LOW-TEMPERATURE INDIRECT GASIFICATION (LT/INDIRECT) HEAT-EXCHANGE COOLING HP-steam Air 600 C H2S HPsteam CW Flue gas Steam Combustor Air Gas KEY INPUT DATA FOR THE TECHNO-ECONOMIC ANALYSIS Dry solids rate: 3170 t/d; 36.7 kg/s Dry solids content: 82 % Dry solids HHV: 15.1 MJ/kg Dry solids composition: C 35.7 % H 3.6 % O 37.0 % Na 18.4 % S 5.3 % Gas-turbine process Pressure ratio: 15.7 Combustion temperature: 1095 C Cost parameters Interest: 10 % for 20 a Purchased fuel price: 7.5 euro/mwh Fuel oil price: 150 euro/t Steam process HP steam: 82 bar, 480 C MP steam: 12 bar, 210 C LP steam: 4.5 bar, 148 C Feed water tank: 115 C HP steam from bark: 24 kg/s Final flue-gas temperature: 150 C Reference case State-of-art recovery boiler (Kaukas) with above steam values Pulp mill steam consumption MP steam: 23 kg/s LP steam: 79 kg/s Investments: boiler: 110 Meuro BLG-CC power plant: 145 Meuro Power boiler: 350 euro/kw e+h Lime-kiln (RB case): 15 Meuro

8 ESTIMATED POWER AND HEAT OUTPUTS FROM BLACK LIQUOR ( rate: t/d solids) Process Heat (Primary) Back-pressure Power Energy Output, MW Boiler (Kaukas) HT, LT/indirect, Pressur., Base Base LT/air, Base LT/air, Pressur., LT/O2, Base LT/O2 Pressur., Notes: (1) process heat includes primary heat only; lowest level: 4.5 bar steam (2) output = net output of power plant = (total production - power-plant consumption) ESTIMATED POWER AND HEAT OUTPUTS FROM BLACK LIQUOR ( rate: t/d solids) Process Heat (Primary) Back-pressure Power Energy Output, MW HT, Base HT, Wall cool HT, Dry quench + Wet quench HT, Dry quench, Heat exch. HT, Dry quench, Hot filtration, (S-free pulping) Notes: (1) process heat includes primary heat only; lowest level: 4.5 bar steam (2) output = net output of power plant = (total production - power-plant consumption)

9 ESTIMATED POWER AND HEAT OUTPUTS FROM BLACK LIQUOR ( rate: t/d solids) Energy Output, MW LT/O2, Base Process Heat (Primary) LT/O2, Lower carbon conv. (90 %), Back-pressure Power LT/O2, Pressurised, LT/O2, Pressurised, Dry quench, Hot filtration, (S-free pulping) Notes: (1) process heat includes primary heat only; lowest level: 4.5 bar steam (2) output = net output of power plant = (total production - power-plant consumption) ESTIMATED POWER AND HEAT OUTPUTS FROM BLACK LIQUOR ( rate: t/d solids) Energy Output, MW Process Heat (Primary) LT/indirect, Base LT/indirect, Lower fireside temp. (1000 C) Back-pressure Power LT/indirect, Lower carbon conv. (90 %) Notes: (1) process heat includes primary heat only; lowest level: 4.5 bar steam (2) output = net output of power plant = (total production - power-plant consumption)

10 ESTIMATED PRODUCTION COSTS OF ADDITIONAL ELECTRICITY 35 Electricity Cost, euro/mwh Stand-alone mill Integrated P&P mill HT, Pressurised, Base LT/indirect, Base LT/O 2, Base LT/O 2, Pressurised LT/air, Base LT/air, Pressurised Additional Electricity Output of Power Plant, MW Notes: (1) power output includes power from additional purchased fuel (integrated P&P mill) (2) black liquor rate: t/d solids (3) back-pressure power output of reference recovery boiler: 79 MW ESTIMATES OF INCREASE IN CAUSTICISATION REQUIREMENT ( rate: t/d solids) Gasification temperature Increase in causticisation Increase in limekiln fuel (LHV) 600 C % MW 1000 C 25 % 11 MW Note: Above increases result from H 2 S and CO 2 absorption in conjunction with H 2 S recovery. For the HT base process, any additional CO 2 absorption in conjunction with dissolution of inorganics would further Increase the causticisation requirement.

11 FUTURE IMPROVEMENTS IN RECOVERY-BOILER POWER OUTPUT Examples of modifications and the estimated power increases (based on Raukola, et al, 2000): Case A: - increased preheating of air to 190 C - sootblowing steam from turbine - increased feed-water tank temperature to 145 C - preheating of feed water to 165 C - improved steam values: 505 C, 103 bar Increase in power ouput: 11 MW (compared to base-case output of 79 MW) Case B with further modifications in addition to those of Case A: - application of reheater; steam values 515 C, 110 bar and 515 C, 35 bar Increase in power ouput: 17 MW (compared to base-case output of 79 MW) When the same modifications are made to the steam process of the combined-cycle plant based on BLG, the corresponding power increases would be about half of the above values. CONCLUSIONS I - POWER OUTPUTS Estimated power outputs for low-temperature atmospheric-pressure direct gasification processes (LT/air, LT/O 2 ) are greater than those of the high-temperature (HT) and low-temperature indirect gasification processes (LT/indirect). The advantage over the LT/indirect process is particularly significant if the fireside temperature of the latter is limited to about 1000 C. Pressurisation would naturally increase the power outputs (and overall efficiencies) of the low-temperature direct gasification processes and so would form the key improvement in the second-generation product. In the case of sulphur-free pulping even higher power outputs would be attainable through application of dry-quench cooling and hot-gas filtration. Possible future improvements: A more efficient gas turbine process (higher inlet temperature, higher pressure ratio) would increase the power outputs of all BLG alternatives. In addition, power outputs would be enhanced by steam-cycle improvements; e.g. higher steam values.

12 CONCLUSIONS II - OVERALL THERMAL EFFICIENCIES (POWER + HEAT) The overall efficiency of the black-liquor gasification process is generally somewhat lower than that of the recovery-boiler process. Only in the case of air-blown pressurised gasification would the overall efficiency approach that of the recovery-boiler process. Factors which lower the overall efficiency include: use of O 2 in place of air as gasification medium operation of gasifier at atmospheric pressure application of quench cooling increasing the steam content in gas from gasifier. Possible further improvements: The efficiency of heat recovery in the gasification process can be still improved to some extent. In the case of wet-quench cooling, a significant improvement would be possible by increasing the gasification pressure and/or being able to use lowerlevel heat as a primary heat source in the mill. CONCLUSIONS III - COSTS OF ADDITIONAL POWER For stand-alone pulp mills the power production costs appear to be competitive for nearly all the gasification alternatives. The competitiveness of power production based on black-liquor gasification is lower in integrated pulp and paper mills because of the required additional production of process steam. Nonetheless, the better performing gasification processes should remain competitive in integrated mills. Note: The estimated production costs take into account all additional costs; e.g. those due to increased causticisation requirement and those due to increased steam production from purchased fuel. Possible further improvements: The previously mentioned further improvements in power output and/or overall efficiency would reduce the power production costs. Avoidance of the increased causticisation requirement - e.g through application of a direct causticisation method - offers promise for further cost reduction.