Energy Convers. Mgmt Vol. 34, No. 9-11, pp. 1023-1030, 1993 0196-8904/93 $6.00+0.00 Printed in Great Britain Pergamon Press Ltd

I N T E G R A T I O N OF FOSSIL AND R E N E W A B L E ENERGY T E C H N O L O G I E S T O M I T I G A T E C A R B O N D I O X I D E

S. W. Gouse, David Gray and Glen Tomlinson

The MITRE Corporation 7525 Colshire Drive, McLean, Virginia 22102, U.S.A.

ABSTRACT

This paper analyzes several integrations of photovoltaics (PV) and integrated coal gasification combined cycle (IGCC) technologies to reduce carbon dioxide. In one of these integrations, liquid hydrocarbon transportation fuels are Imxluced by reacting the carbon dioxide from the IGCC plant with hydrogen produced from PV electrolysis of water. This system appears to be cost competitive with a hydrogen economy alternative for future estimates of system performance and costs.

KEYWORDS

Carbon dioxide mitigation; Fischer-Tropsch synthesis; fossil/PV integration.

INTRODUCTION

Fossil fuels currently supply 88 percent of the world energy demand of 360 exajoules (Gouse et al., 1992), but there are increasing pressures of resource limitations and environmental concerns that are likely to impact their continued use. These pressures will result in a greater reliance on nuclear and renewable en~gy technologies to satisfy world energy demand. If nuclear energy remains environmentally unacceptable, then renewables, particularly PVs, will become of paramount importance. Higher efficiency, lower cost photovoltalc cells (possibly concentrators) will enable solar energy to penetrate the market, initially as peaking and distributed systems, and then in larger central stations (Zweibel, 1990). The application of photovoltalc energy to non-electric loads presents a technological challenge. Although it may be possible to eliminate the use of fossil fuels completely and adopt a hydrogen economy, high energy density liquid hydrocarbon fuels are ideally suited for transportation and the infrastructure for their utilization exists worldwide. They will probably continue to supply the majority of world transportation energy demand until well into the next century. During this period of transition from fossil to renewable technologies, opportunities exist to integrate them in ways that can improve overall system efficiency, minimize environmental impact, and still provide hydrocarbon-based fuels as an alternative to the hydrogen economy.

This paper examines some of these opportunities for integration using as an example an integrated coal gasification combined cycle (IC-CC) plant and a solar photovoltalc facility. These integrations reduce the quantity of carbon dioxide emitted relative to the base case IGCC

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1024 GOUSE et al.: INTEGRATION OF ENERGY TECHNOLOGIES

plant. In the simplest case, the PV plant is used to supplement the daytime power requirement in combination with the IGCC facility to reduce the quantity of coal and hence the carbon dioxide emitted. A further level of integration is analyzed where hydrocarbon transportation fuels are produced. This concept integrates these facilities by reacting the carbon dioxide recovered from the IGCC plant with hydrogen produced by water electrolysis using direct electric current produced from the PV facility. The hydrogen and carbon dioxide are reacted over Hscher-Tropsch catalysts (Anderson, 1984) to produce gasoline and diesel fuel. The oxygen, coproduced in the electrolysis plant, is used in the IGCC plant to gasify the coal. Reaction water, resulting from the Fischer-Tropseh synthesis to produce hydrocarbon liquids is recycled back to the electrolysis process. By using this integration, the carbon dioxide produced in generating power in the IGCC plant is reduced to hydrocarbon fuels and liberated as carbon dioxide again after combustion in mobile internal combustion engines. The final cases examined eliminate the IGCC plant and supply the load entirely using PV with hydrogen storage. Nighttime load is then provided by either combusting the stored hydrogen in turbines or by using fuel cells.

SUMMARY AND CONCLUSIONS

The overall costs of carbon dioxide reduction plotted against the percentage of reduction compared to the base IGCC case for all of the integrations analyzed are shown in Fig.1. These are shown for three sets of assumptions of system performance and costs. The "present" assumptions are for PV costs of $2,400/peak kW, electrolysis costs of $600/kW, and electrolyzer efficiency of 75 percent. The "post-2000" assumptions are: PV cost $1,200/kW peak, electrolysis cost $400/kW, and electrolysis efficiency 85 percent. For "post-2020", PV cost is $600/kW peak, electrolysis $231/kW, and electrolyzer efficiency 90 percent. For the post-2020 time frame, Fig. 1 shows that cost of reducing 100 percent of the carbon dioxide using the IGCC/PV integration with hydrocarbons production is almost identical to the PV-only with hydrogen storage ease. This is for an assumed hydrocarbon liquids value of $50 per barrel. However, the calculated cost of hydrogen for a PV plant with electrolysis under these post-2020 assumptions is $22/GJ, in close agreement with recent estimations of Ogden and DeLuchi (Ogden et al., 1992). This is equivalent to liquid hydrocarbon fuels at $100 per barrel. If it is assumed that the hydrocarbon liquids produced in the IGCC/PV integration are valued at $100 per barrel, then carbon dioxide reduction costs using post-2020 assumptions are reduced to about $25 per tonne. This is significantly less than the PV with hydrogen storage alternative. This implies that liquid hydrocarbon fuels coproduced from Fossil/PV integrations, like those analyzed in tiffs paper, may be a viable, lower cost alternative to the proposed hydrogen economy.

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20 30 40 5O 60 7O 8O 90 100 Percent Of C 0 2 Saved Relative to IGCC

Fig. 1. Cost of CO 2 Saved vs. Percent Saved

GOUSE et aL: INTEGRATION OF ENERGY TECHNOLOGIES 1025

THE BASE CASE FACILITY

To illustrate these integration concepts, we have analyzed the impact on a 1,000 MW IGCC facility sited in an area of the United States that has an average annual insolation of 250 watts per square meter. The assumptions for this facility are given in Table 1. For an average load of 500 MW, annual coal use is 1,324,500 tonnes producing 3.4 million tonnes of carbon dioxide.

Table 1. Base Case

IGCC Plant

Plant Capacity 1,000 MW Average Load 500 MW Heat Rate 8,400 KJ/kWhr Coal Used 1,324,500 Tonnes per year Coal Characteristics 27.91 GJ/Tonne, 70% carbon CO x Evolved 3,400,000 Tonnes/year Annual Output 4,380,000 MWhrs Capital Cost $1,500 MM Operating Cost $ 52.5 MM Coal Cost $ 35 MM Annual revenue required $ 312.5 MM (capital

recovery factor 15%) Electric Power Cost $ 0.071/kWhr

COMBINED IGCC/PV FACILITIES FOR LOAD FOLLOWING

This concept is shown in Fig. 2. In this case the IGCC plant size can be reduced from a capacity of 1,000 MW to 750 MW since it is assumed that the occasional peak load of 1,000 MW occurs during daytime hours. Maximum IGCC turndown is assumed to be 150 MW, therefore, either a combination of PV and turndown fossil can meet the daytime peak load requirement, or a smaller PV plant can be used and peaking can be satisfied as required by temporarily increasing the fossil contribution to output. Figure 3 shows the 24-hour load demand for this combination for a 650 MW PV capacity plant. Average nighttime load provided by fossil is 300 MW, and by 0600 hours the PV component comes on. By 0900, the fossil is reduced to turndown capacity and PV provides the additional load. By 1,500 hours, PV potential is decreasing and fossil is turned up until 1,800 hours when it has to satisfy the complete load again. A summary of this combination is shown in Table 2 for a PV capacity of 650 MW. The IGCC plant provides nearly 60 percent of the the load and the PV plant supplies the rest. The potential load factor for the PV plant is 31.65 percent based on its rated capacity of 650 MW, and its actual load factor in this combination is 30.9 percent; therefore nearly all the potential load is utilized at a PV size of 650 MW. Using this combination, 40 percent of the carbon dioxide produced in the base case IGCC plant can be eliminated for a reduction cost of-$12.53 per tonne assuming a PV cost of $600 per peak kilowatt.

INTEGRATED IGCC/PV WITH HYDROCARBON LIQUIDS PRODUCTION

This integration is shown in Fig. 4. The PV plant is now equipped with electrolyzers to produce hydrogen and oxygen from water. Since the PV plant must also supply the bulk of the daytime electric load, the PV array is oversized to produce the required amount of hydrogen and oxygen

1026 GOUSE et al.: INTEGRATION OF ENERGY TECHNOLOGIES

% OCPower [ . . . . . . . [ ACPower 0 ACPowerto Grid

F/g. 2. Combined PV/Fossil for Load Following

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0 l i l | a ~ i i l i l l t i t I | ~ 1 1 1 1 I |

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Fig.3. Load Analysis for IGCC Plus Photovoitalc Peaking

Table 2. Combination of IGCC and PV

Plant Capacity 750 MW Maximum Turndown 150 MW Coal Used 792,661 TonneL/year CO 2 Evolved 2,034,500 Tonnes/year Annual Output 2,621,234 MWhrs Percent of Load 59.85 Load Factor 39.90 % Capita/Cost $1,125 MM Operating Cost $ 39.4 MM Coal Cost $ 20.97 MM Cost/kWh $ 0.087

Plant Capacity 650 MW Load Factor to grid 30.89 % Potential Load Factor 31.65 % Annual Output 1,758,766 MWhrs Capital Cost $ 390 MM ($600/kW~ Operating Cost $ 7.8 MM CO 2 reduction compared to 1,365,084 tonnes per year base case

Total annual revenue required $295A MM (Capital recovery factor = 0.15%)

Cost differential to base case --$17.1 MM Cost of CO 2 reduction $/tonne --$12.53 % CO 2 reduced compared to 40.15% base case

G O U S E e t al.: I N T E G R A T I O N O F E N E R G Y T E C H N O L O G I E S 1027

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Fig. 4. Integrated PV/IGCC Plant with Hydrocarbon Liquids Production

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Fig. 5. Load Analysis for IGCC Plus Photovoltaic with Liquid Production

for an optimal balance with the IGCC plant. For this integration the optimal excess PV will just supply the oxygen required by the coal gasifiers. Figure 5 shows the contributions to the overall load from the IGCC plant and the PV plant. The breakdown between PV required for grid power and PV required for hydrogen production is also shown. The hydrogen produced is reacted with the captured carbon dioxide from the IGCC plant over reverse water-gas shift Fischer-Tropsch catalysts to give liquid hydrocarbon transportation fuels. Carbon dioxide capture in this system is greatly simplified by slightly shifting the raw coal-derived gas before the acid gas removal system. In this way a pure concentrated carbon dioxide stream is produced without having to extract the carbon dioxide from a nitrogen diluted flue gas. Since oxygen is supplied by the PV plant, this unit can be eliminated from the IGCC facility resulting in lower capital cost and improved overall heat rates for the system. Results from the analysis of this integration are summarized in Table 3. For this optimum case, where the excess PV supplies just enough oxygen for the coal gasification plant, 73 percent of the carbon dioxide is reduced

1028 GOUSE et al.: I N T E G R A T I O N OF E N E R G Y T E C H N O L O G I E S

Table 3. IGCC/PV with Hydrocarbon Liauids Production

Plant ~ t y 750 MW Coal U~uxl 556,118 Tonnes/year CO 2 Evolved 913,743 Tonnes/year

Annual Outlmt , 2,298,770 MWhrs Percent oi ~ 52.5 Load Factor 35% Captial Cost $910 MM (no oxygen plant) Operating Cost $ 31,85 MM

Coal Coat $ 14.71 MM Heat Rate 6,752 KJ/kWhr

Plant ~ t y 1,868 MW Factor to Caid 12.72%

Potential Lo~ Faeto¢ 31.65% Annual Outt~ to Ca'id 2,081,230 MWhrs Annual Energy to Hydrogen 3,096,463 MWiws Total Amount Output 5,177,693 MWhrs Cal~tal Cost $1120.5 MM Operating Cost $ 22.4 MM Electrolysis Plant Capital Cost $246.6 MM

($23 l/kW) Operating cost $4.9 MM

co2 eommed H 2 colmlmed Liquid hydrocalbons

p~uced Liquids barrels pet" day Capital Cost Operating Cost Gas Storage Costs Total annual revenue

required for system

513,627 Tonnes/yeat 70,040 Tonnes/year 163,428 Tonues/year

3,656 (1 barrel = 159 liters) $65 MM $ 1.9 MM $29.2 MM SMM = 183.06 (IGCC) +

190.5 (PV) + 41.92 (H 2) 4- 29.2 (Storage) + 11.72 (Synthesis) = 456A

Cost differential to base case CO 2 reduction coml~red to

base case % CO 2 reduction compexed 73%

to base case Liquid hydrocarbon credit $65 MM

($50/bb0 Cost o£CO 2 reduction $31.75

$/Tonne

$143.9 MM 2,486,257 Tonnes/year

compared to the base case facility at a reduction cost of $31.75 per tonne. This integration can be modified to reduce all of the carbon dioxide by using a closed cycle turbine loop with carbon dioxide as the working medium (see dotted lines on Fig. 4). This integration results in a carbon dioxide reduction cost of $70 per tonne. Liquid hydrocarbon transportation fuels are assumed to be valued at $50 per barrel in these analyses.

PV PLANT WITH HYDROGEN STORAGE

In this case the IGCC plant, and hence all of the carbon dioxide is eliminated and the total load is provided by the PV plant using hydrogen storage. This configuration is shown in Fig. 6. The PV array is sized to provide AC power to the grid and to produce enough hydrogen from electrolysis to satisfy the total load. Figure 7 shows the load diagram for this case. Stored hydrogen can be combusted in gas turbines or reacted in fuel cells to provide the power. Both options were examined in this analysis. Table 4 summarizes this case for the hydrogen-fired gas turbine option.

GOUSE et al.: INTEGRATION OF ENERGY TECHNOLOGIES 1029

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Fig. 7. Load Analysis for Photovolatic Plus Hydrogen Fueled Combined Cycle

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1030 GOUSE et al.: INTEGRATION OF ENERGY TECHNOLOGIES

Table 4. PV Plant with Hvdro~en Storage

EY_Bm

Plant Calmfity Lind Fact~ Tettl Annual Output Anmml Outtmt to Ca'id Output for H 2 Production H 2 Produced Ammally Ctotal Cmt PV opeming Cost pv Electrolysis Plant

Capit~ c,~t (~ating ¢06t

Gas Turbine Plant Plant ~ t y Load Factor Annual Output Heat Rate Capital Co*t Operating Cost

Annual Gas Storage Costs Annual Revenue Required

Cmt dfffereatial to bue case CO2 veductim vomlm~l to base c~e 9t CO2 reduction comtm~ to base case Cost c~ CO 2 reductioa ($/Tonae) Cost t~ Hyth'ogen

2,513 MW 31.65% 6,967,622 MWhrs 2,683,480 MWhrs 4,284,142 MWhrs 96,895 Toanes/year $1507.8 $ 30.2 MM

$ 430.4 MM ($231/kW) $ 8.6 MM

750 MW 25.8 % 1,696,520 MW 6,850 Kl/kWhrs $750 MM $15MM $123.4 MM 256.4 (PV) + 73.2 (H2) + 127.5 (Gas Turbine) +

123.4 (Storage) = $580.5 MM $267.9 3,400,000 Tmnex/year 100 % $78.8o 22 $/GJ

REFERENCES

Anderson, R. B. (1984) The Fischer-Tropsch Synthesis. Academic Press, London. Gouse, S. W., D. Gray, G. C. Tomlinson, and D. L. Morrison (1992). Potential World Development through 2100: The Impacts on Energy Demand, Resources and the Environment. Published in The World Energy Council Journal, December. Ogden ,Joan M. and Mark A. DeLuchi (1992). Solar Hydrogen Transportation Fuels. Paper prepared for the Conference on Transportation and Global Climate Change: Long Run Options. Asilomar, California, August 25-28. Zweibel, Ken (1990). Harnessing Solar Power. Plenum Press, New York.