The thermal efficiency and cost of producing hydrogen and other synthetic aircraft fuels from coal

International Journal of HydrogenEnergy, Vol. 1, pp. 365-377. Pergamon Press, 1977. Printed in Northern Ireland

T H E T H E R M A L E F F I C I E N C Y A N D COST OF P R O D U C I N G H Y D R O G E N A N D O T H E R S Y N T H E T I C A I R C R A F T FUELS FROM COALt ROBERT D. WrrcOFSKJ NASA Langley Research Center, Hampton, VA 233@5, U.S.A. Al~lraet--A comparison is made of the cost and thermal efficiency of producing liquid hydrogen, liquid methane and synthetic aviation kerosene from coal. These results are combined with estimates of the cost and energy losses associated with transporting, storing, and transferring the fuels to aircraft. The results of hydrogen-fueled and kerosene-fueled aircraft performance studies are utilized to compare the economic viability and efficiency of coal resource utilization of synthetic aviation fuels. INTRODUCTION NASA's Langley Research Center is actively engaged in a liquid hydrogen-fueled aircraft technology program. The program addresses the areas of aircraft performance, aircraft fuel system requirements, impact of such aircraft on air terminal facilities, fuel safety and fuel production. Earlier results [1] have indicated that liquid hydrogen-fueled aircraft exhibit potential benefits over conventionally-fueled aircraft from the standpoint of both performance and environmental aspects. The onboard energy consumption of hydrogen-fueled aircraft has been determined [1] to be significantly less than their conventionally-fueled counterparts, for aircraft having long ranges. For short range aircraft hydrogen-fueled aircraft have been determined [1] to consume more onboard energy than their conventionally-fueled counterparts. Results have also indicated [1] that higher fuel prices, per Btu or joule, can be paid for liquid hydrogen than for conventional fuels, while maintaining the same level of direct operating cost of the airplane, again, particularly for aircraft having a high fuel fraction. The onboard fuel does not account for the energy required to produce the fuel and this must be taken into account if the remaining U.S. resources of energy are to be used in an efficient and judicious manner. Total fuel cost, including the cost of the prime source of energy, cost of fuel production, distribution and storage must be considered when assessing the economic viability of one aviation fuel versus another. Hydrogen can be produced by utilizing almost any primary source of energy. The production technology status, thermal efficiency, and cost aspects of the various hydrogen production processes [2] vary greatly as do the magnitudes of the resources of primary energy used to produce the hydrogen. In the far term solar and nuclear energy are envisioned as primary sources of energy for hydrogen production. In the interim period between the present time and the time when conventional energy resources in the U.S. are essentially depleted, coal has been identified as one of the more plentiful U.S. energy resources. The gasification of coal has been identified [3, 4] as one of the more inexpensive and thermally efficient methods for producing hydrogen. Three fuels, suitable for use in aircraft, can be produced from coal; liquid hydrogen, liquid methane and synthetic aviation kerosene. It is the purpose of this paper to present an overview of recently completed studies aimed at defining the thermal efficiency and cost of producing liquid hydrogen, liquid methane and synthetic aviation kerosene from coal. The fuel production studies were conducted for NASA's Langley Research Center by the Linde Division of Union Carbide [5-7] and the Institute of Gas Technology [8-10]. Results of the fuel production studies are then combined with available fuel transmission and storage data, estimates of fuel transfer losses, and recently obtained aircraft performance data to better analyze the overall thermal efficiency and cost of coal-derived aircraft fuels. Detailed analyses of the fuel production studies are covered in separate papers given at this conference [7, 10]. t Presented at the 1st World Hydrogen Energy Conference, Miami Beach, Florida, U.S.A., March 1976. 365

366

THERMAL EFFICIENCY AND COST OF PRODUCING FUELS FROM COAL FUEL P R O D U C T I O N

In the work done for NASA by the Institute of Gas Technology [8], which is under way at the time of this writing, the thermal efficiency and cost of producing gaseous hydrogen and methane and synthetic aviation kerosene, manufactured from coal are being analyzed. The coal selected for the hydrogen and methane analyses is Montana subbituminous coal and that selected for the synthetic aviation kerosene analysis is a Pittsburgh seam coal. The compositions of the coals are listed in Tables 1 (a and b). Nominal plant size was 250 billion Btu per day product for all processes analyzed. In keeping with the customary procedure of the gas industry, the higher heating valuest of all fuels are used in all calculations.

Basic coal gasification A schematic of the basic process for producing hydrogen or methane from coal is shown in Fig. 1. Coal and air or oxygen react in a gasifier to produce heat which in turn drives the reaction of steam with coal to produce a synthesis gas, the constituents of which are noted in Fig. 1. The composition of the synthesis gas can be controlled by controlling the temperature and pressure within the gasifier. High temperature and low pressure operation of the gasifier favor the production of a synthesis gas rich in HE and CO. High pressure and low temperature operation of the gasifier favor the production of a synthesis gas rich in CH4. The steps that follow depend upon whether the desired end product is HE or CH4. In the case of hydrogen production the CO is reacted with steam to produce more H2 and the methanation step is used basically as a cleanup step to get rid of any excess CO. In the case of methane production the CO is reacted with steam to produce HE in order to adjust the ration H 2 to CO so that more methane can be produced in the methanation step.

Hydrogen production Three coal gasification processes were analyzed [8] for hydrogen production, Koppers-Totzek, U - G A S TM, and the steam-iron process. The Koppers-Totzek gasifier operates at essentially atmospheric pressure, while the U - G A S TM gasifier operates at 335 psig (2.31 MN/m2). Both processes utilize oxygen to avoid nitrogen dilution of the produce gas and first produce a synthesis gas which is then upgraded to produce hydrogen, generally following the procedure shown in Fig. 1. Detailed descriptions of the Kopper-Totzek and U-GAS TM processes are given in [8]. The steam-iron process does not produce hydrogen in t h e general manner described in Fig. 1. The steam-iron process (Fig. 2) is based on the decomposition of steam by iron oxide which occurs in the oxidizer: 3 F e O + H 2 0 --> F e 3 0 4 d- H 2. H 2 or C H 4 H 2 , H20,

Coal

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FIG. 1. Basic process for producing hydrogen or methane from coal.

t Low and high heat values. Any fuel containing hydrogen yields water as one product of combustion. At atmospheric pressure, the partial pressure of the water vapor in the resulting combustion gas mixture will usually be sufficiently high to cause water to condense out if the temperature is allowed to fall below 120-140°F. This causes liberation of the heat of vaporization of any water condensed. The low heat value is evaluated assuming no water vapor condensed, whereas the high heat value is calculated assuming all water vapor condensed. (Marks' Handbook).

367

ROBERT D. WITCOFSKI Coal Air

El ectricol power

Steam

ElectricGI power

Producer Spent producer ~ I 1t00 K I 2515 kN/m2

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3 88 i4.25

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FTG. 2. Schematic of steam-iron process of producing hydrogen from coal. Coal is gasified with steam and air to provide a producer gas, the composition of which is shown in Fig. 2. The purpose of the producer gas is to supply the CO and H2 for regeneration of the iron oxide which occurs in the reductor: Fe304 + CO -~ 3FeO + C O

2

Fe304 + H2 --~ 3FeO + H20. Air can be used in the gasifier instead of oxygen because the hydrogen product gas is not formed in the gasifier, but in the decomposition of the iron o x i d e - - t h e iron oxide acting as a barrier against the nitrogen. No CO shift is required in the steam-iron process since the product gas is not made directly from the synthesis gas. Not all the CO and H2 in the producer gas are expended in reducing the FeaO4 to FeO and the remaining spent producer gas, the composition of which is shown in Fig. 2, has a heating value plus sensible heat at 1520°F (1100K) corresponding to 54% of the input coal value. The reductor off-gas is burned in a combustor and expanded through gas turbines to produce electricity and shaft power for air compression. The expanded gas is then used in a steam-power cycle to generate process steam and more electricity. After all plant energy requirements have been fulfilled, 1,229,500kW of by-product electrical power is produced from the reductor off-gas. Energy balances and the resulting thermal efficiencies for all three hydrogen production processes are presented in Table 2. All product gases in Table 2 are dried and compressed to 1000 psig before leaving the plant. Thermal efficiencies for the Koppers-Totzek and U - G A S T M are seen to be 57 and 66.4%, respectively. The thermal efficiency of the steam-iron process is shown in Table 2 as 81.5%. This calculation was made by taking as by-product energy the heating value plus sensible heat at 1520°F ( l l 0 0 K ) of the spent producer gas emerging from the reductor. Had the electrical by-product obtained in [8] been converted to Btu's or joules, the resulting thermal efficiency would be 62.5%.

Methane production The thermal efficiency of producing methane gas from coal via the H Y G A S ® Process was also determined by the Institute of Gas Technology [8]. The H Y G A S ® Process gasifier operates at about 1000 psig (6.98 MN/m 2) and is representative of advanced pipeline gas technology. Process details are covered in [8-10]. As shown in Table 2, the thermal efficiency of the H Y G A S ® Process is 74%.

368

THERMAL EFFICIENCY AND COST OF PRODUCING FUELS FROM COAL

TABLE l(a). Montana subbituminous coal used in the process of designs for hydrogen and methane

TABLE l(b). P i t t s b u r g h s e a m coal u s e d in CSF coal liquefaction process design Proximate analysis

Proximate analysis

Wt % MF (moisture free) coal

Composition (wt %)

Moisture Volatile matter

22.0 29.4

Fixed carbon Ash

42.6 6.0

Volatile matter Fixed carbon Ash

39.07 47.71 13.22 100.00

Ultimate analysis (dry) 100.0

Hydrogen Carbon Nitrogen Oxygen Sulfur

Ultimate analysis (dry) Carbon Hydrogen Nitrogen Oxygen Sulfur Ash

67.70 4.61 0.85 18.46 0.66 7.72

Total

100.00

4.90 68.97 1.28 7.34 4.29 100.00

MF gross heating value = 29,390 kJ/kg (12,640 Btu/lh) As received HHV = 25,158 kJ/kg (10,820 Btu/lb)t

Dry heating value (Btu/lb) 11,290 (26,251 kJ/kg)

t Based upon maximum moisture content of 14.4 wt %

It is interesting to note that the gasification of coal to produce methane via the HYGAS ® Process requires the addition of hydrogen to the gasification process, since coal does not contain sufficient hydrogen. At the Institute of Gas Technology's HYGAS ® pilot plant in Chicago, a steam-iron pilot plant is being constructed to provide the necessary hydrogen for the HYGAS ® plant. The H Y G A S ® analysis herein utilizes the hydrogasification of char to produce the required hydrogen.

Liquid hydrogen The Linde Division of Union Carbide has performed, under contract for NASA, an analysis of the thermal efficiency and cost of liquefying hydrogen [5-7], considering both current liquefaction technology and liquefaction technology which may be available in the 1985-2000 time period. The liquefaction energy requirements from the Linde study have been utilized herein to TABLE 2. Thermal efficienciest for manufacturing gaseous hydrogen and methane from coal (250 × 109 B t u / d a y p r o d u c t ) (263.5 T J / d a y p r o d u c t ) Hydrogen product Gasifier type

Koppers-Totzek

U-GAS T M

Methane product Steam-Iron

HYGAS ®

Total coal input, 106 Btu/hr (GJ/hr) 18,355 (19,346) 15,736 (16,586) 23,375 (24,637) 14,876 (15,679) Product gas, 106 Btu (GJ/hr) 10,428 (10,991) 10,425 (10,988) 10,423 (10,986) 10,417 (10,980) Product gas composition, H 2 or CH4 (MOL % (dry basis) 93.1 94.3 95.7 94.7 By-product energy, 106 Btu/hr (GJ/hr) 34 (36) 28 (30) 8,625 (9,091)(a) 588 (620) By-product energy converted to Electrical power, kW --1,229,500 (b) Overall plant efficiency coal to product gas, % 57.0 66.4 81.5 74.0 t All calculations based on high heating values of coal and product gases (a) Heating value plus sensible heat at 1520°F of spent producer gas (b) By-product energy in spent producer gas converted on site to electricity [8, 10].

ROBERT D. WITCOFSKI

369

TABLE 3. Thermal efficienciest for producing liquid hydrogen and methane from coal (250x 109 Btu/day product) (263.5 TJ/day product)

Liquid hydrogen product Gasifier type

Koppers-Totzek

U-GAS

TM

Steam-Iron

18,355 (19.346) 15,737 (16,587) 23,375 (24,637) Coal to gasifier, 106 Btu/hr (GJ/hr) Power for liquefaction, kW current liquefaction technology 682,003 697,549 771,015 1985-2000 liquefaction technology 558,233 570,957 631,092 Fuel for liquefaction, 106 Btu/hr (GJ/hr) current liq. tech., electric gen = 50% 4655.4 (4907) 4761.5 (5019) -1985 liq. tech., electric gen = 50% Eft. 3810.5 (4016) 3897.4 (4108) Fuel gas recovered, 106 Btu/hr (GJ/hr) 2361.7 (2489) 2068.3 (2180) 1054.7(1112) Excess electrical power, kW current liquid energy req. --458,485 (b) 1985-2000 liquid energy req. --658,543 (b) Excess elec. power equivalent, 106 Btu/hr current liquid --3216.5 (a) 1985-2000 liquid --4620.0 (a) Liquid product fuel value, 106 Btu/hr (GJ/hr) 7709.9 (8124) 7883.6 (8309) 8713.9 (9184) Overall plant efficiency, % current liquid energy req. 37.3 42.8 55.6 1985-2000 liquid energy req. 38.9 44.9 61.6

Liquid methane product HYGAS ® 14,867 (15,670) 102,800

701.7 701.7 (740)

w

9635.2 (10,156) 64.8 (plus 3.9% - - by-product)

t All calculations based on high heating values of coal and products (a) Heating value plus sensible heat at 1520°F of spent producer gas (b) By-product energy in spent producer gas converted on site to electricity d e t e r m i n e t h e t h e r m a l efficiencies of p r o d u c i n g liquid h y d r o g e n f r o m c o a l - d e r i v e d h y d r o g e n a n d t h e results are s h o w n in T a b l e 3. T h e fuel gas r e c o v e r e d r e f e c t s t h e r e c o v e r y d u r i n g purification of t h e C O a n d c a 4 in t h e feedstock, 2.6% of t h e H2 feedstock lost d u r i n g purification, a n d flashoff losses d u r i n g t r a n s f e r to storage. In t h e L i n d e study t h e gaseous f e e d s t o c k was delivered to the liquefaction p l a n t at a p r e s s u r e of 200 psig (1.38 MN/m2), w h e r e a s the gaseous f e e d s t o c k f r o m the w o r k d o n e by t h e Institute of G a s T e c h n o l o g y e m e r g e d f r o m t h e f e e d s t o c k p l a n t at 1000 psig. It is a s s u m e d h e r e i n t h a t t h e h y d r o g e n gas is delivered to t h e liquefaction p l a n t at 1 0 0 0 p s i (6.89 M N / m 2) a n d t h e e n e r g y r e q u i r e m e n t s for liquefaction have b e e n a d j u s t e d accordingly. A 12.3% loss of the h y d r o g e n f e e d s t o c k d u r i n g liquefaction was a s s u m e d as was the case in the L i n d e study. W i t h the e x c e p t i o n of t h e s t e a m - i r o n process it has b e e n a s s u m e d t h a t t h e electrical e n e r g y n e c e s s a r y for fuel liquefaction will b e p r o d u c e d at a t h e r m a l efficiency of 5 0 % . H y d r o g e n or m e t h a n e fueled aircraft are n o t e x p e c t e d to see significant service until after 1985 a n d as indicated in [11] electrical g e n e r a t i o n t h e r m a l efficiencies in the r a n g e of 5 0 % would s e e m r e a s o n a b l e for t h a t t i m e period. T h e electrical e n e r g y g e n e r a t e d by t h e s p e n t p r o d u c e r gas in the s t e a m - i r o n process analysis was g e n e r a t e d at a t h e r m a l efficiency of 4 8 % . B e c a u s e of t h e large electrical b y - p r o d u c t of the s t e a m - i r o n process, m o r e t h a n t h a t r e q u i r e d to liquefy the h y d r o g e n p r o d u c e d , t h e s t e a m - i r o n process is s e e n in T a b l e 3 to h a v e the h i g h e r t h e r m a l efficiency for p r o d u c i n g liquid h y d r o g e n f r o m coal, 5 5 . 6 % a s s u m i n g c u r r e n t liquefaction t e c h n o l o g y e n e r g y r e q u i r e m e n t s a n d 6 1 . 6 % a s s u m i n g 1 9 8 5 - 2 0 0 0 liquefaction t e c h n o l o g y e n e r g y requirements.

Liquid methane B e c a u s e of the very low liquefaction e n e r g y r e q u i r e m e n t s for m e t h a n e (Table 3), the fuel gas

370

THERMAL EFFICIENCY AND COST OF PRODUCING FUELS FROM COAL

which is recovered during purification and liquefaction, along with a small part of the methane feedstock, provides sufficient energy for liquefaction [9]. The fuel gas is composed of H2, CO, and CH4. Again, assuming a 50% efficient conversion of the fuel to power for the liquefaction cycle the thermal efficiency of producing liquid methane from coal via the H Y G A S ® Process is 64.8% plus a 3.9% by-product or 68.7%. This value of thermal efficiency is significantly higher than for any of those for liquid hydrogen.

Synthetic aviation kerosene One observation made in the study done by the Institute of Gas Technology [8] was that from a standpoint of thermal efficiency and cost, it makes sense to produce gasoline from coal-based synthetic crude oil as a feedstock, thereby freeing a greater fraction of our remaining resources of natural crude oil for jet fuel. This deduction stems from the fact that synthetic crude oil produced from coal is higher in aromatics (deficient in hydrogen) than is conventional crude oil. This aromatic crude lends itself quite readily to the manufacture of gasoline but not for aviation kerosene, where copious quantities of hydrogen must be added to lower the aromatic content. Detailed discussion of this aspect is covered in [8]. A coal-based aviation kerosene was considered in the study of [8]. Because of the availability of data, a Pittsburgh seam bituminous coal was selected. As shown in Fig. 3, the consol synthetic fuel liquefaction process was used to produce Naptha, high Btu gas, heavy oil, sulphur and ammonia. The Naptha, sulphur and ammonia were credited as a by-product. Fifteen percent of the high Btu gas was a by-product, 9% was used for power generation, 12% for plant fuel and 64% was used to manufacture hydrogen for hydrocracking and aromatic hydrogeneration of the liquid product. A thermal accounting is provided in Fig. 3 indicating an overall thermal efficiency of 54.8%.

Fuel production costs The study done by the Institute of Gas Technology [8] has also resulted in cost estimates for hydrogen gas produced via the U - G A S T M and Steam-Iron Processes and methane gas via the H Y G A S ® Process. Costs are representative of mid-1974 costs and private industry financing was Product output 109Btu/SD (TJ/SD)

SD = Stream day Naptha 12, 200 B/SD product to sole

Energy input I O--Bfu/SD (TJ/SD)

Sulphur and ammonia

525.5 Coal = (553.9) 24, 283T/SD

CSF coal liquefaction

I

High Btu gas 74.49 x 109 Btu/SD

By-product high Btu Gas

479X109 I Btu/SD ~

I ~.~-- I nyu[vHeavy oil

35,400 B/SD

I

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144xI(7°SCFD~, ~]

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62,45 (65.82)

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9.12 (9.61)

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~._~ capacityl I l I

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1 /

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(

136, 1708/SD

2051 (216.2)

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efficiency=

28778xlOgBtu/SD 0 5 4 8 525 5 xlO9Btu/SD =

Total output = 287 78 (303.$)

FIG. 3. Schematic of process for producing aviation kerosene from coal.

ROBERT D. WITCOFSKI

371

used, the basic assumptions o f which are as follows: 100% equity capital 25-yr project life 16-yr sum-of-the-years' digit depreciation of total plant investment 12% discounted cash flow (DCF) return rate 48% Federal income tax rate The same financial rules were also used by Linde [5] in their h~drogen liquefaction study. Based on the results of the aforementioned studies the cost of both hydrogen and methane in both gaseous and liquid forms are shown in Fig. 4 as a function of the cost of coal. Liquefaction costs include the cost of the loss of 12.3% of the gaseous hydrogen feedstock. A credit is taken for the fuel gas recovered during the liquefaction process and the value of this fuel gas is taken to be equal in cost per million Btu to the coal feedstock. Two cost estimates are shown for liquid hydrogen from the U - G A S T M Process and three from the steam-iron process. The U - G A S T M derived liquid hydrogen costs reflect current and 1985-2000 liquefaction technology energy requirements and it is assumed that electrical power costs 2 C/kWh. Two of the steam-iron process derived liquid hydrogen costs reflect current and 1985-2000 liquefaction technology energy requirements and assume that electrical power costs 2 C/kWh. This is particularly important in the case of the steam-iron process which has a great deal of by-product electrical power and the cost of the hydrogen gas product is a strong function of the value of that electrical by-product [8, 10]. It is assumed in Fig. 4 (with the exception of the curves labeled "b") that the electrical power by-product of the steam-iron process has a value of 2 ~[kWh. In the steam-iron cost curves labeled (b) in Fig. 4, it h a s been assumed the electrical by-product of the steam-iron process has a value corresponding to the cost of electricity generated by a coal-fired electrical generating plant at any particular coal price on the abscissa. It is likewise assumed that electrical energy for liquefaction can be purchased for the same price as the by-product electric power is sold. The assumptions made in the calculation of electrical power

--3 0

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Coal cost,

FIG. 4.

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120 140 t60

180 200

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Synthetic aviation fuel costs as

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372

THERMAL EFFICIENCY AND COST OF PRODUCING FUELS FROM COAL I0 FAssurnes. I - o Liquid hydrogen initiolly produced o? / 55.6% efficiency 81-- o 1974 liquefaction energy requirements

% of voporized hydrogen recovered ond reliquefied

~

~

25 50

~=

75

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FIG. 5. The effect of liquid hydrogen vaporization and reliquefaction in thermal efficiency. costs are as follows: Installed cost of plant = $470/installed kW electric 15.5% fixed charge 40% thermal efficiency, coal-to-electric 90% utilization factor 0.65 mills/kWh operation and maintenance costs Presentation of the data in this manner produces no drastic change in the economic comparison. Although liquid hydrogen produced via the steam-iron process is less expensive than that produced via the U - G A S T M Process, it is still about 50% more cosily than liquid methane produced via the H Y G A S ® Process. The major factor in the cost differential between liquid hydrogen and liquid methane is the fact that it costs about $3.00/106 Btu ($2.85/GJ)(GJ = 109 J) to liquefy hydrogen and less than $1.00/106 ($0.95/GJ) Btu to liquefy methane. A n estimate of the cost of aviation kerosene via the method shown in Fig. 3 is shown in Fig. 4 as a function of coal cost. At the higher coal costs the synthetic aviation kerosene is seen to be in a comparable price range with liquid hydrogen from the steam-iron process and about 50% more expensive than liquid methane. B O I L O F F / F L A S H O F F LOSSES Between the time a cryogenic fuel is liquefied and finally enters the engine to do useful work it must be stored, pumped through transfer lines and placed into the fuel tanks of the aircraft. Depending upon how well each of these steps is accomplished, a certain amount of boiloff and flashoff will occur. How much of this boiloff and flashoff can be captured (on the ground) and reliquefied and how much this will cost from the standpoint of thermal efficiency and economics is a moot question which must be answered by further study. In order to give some insight into the magnitude of boiloff and flashoff losses a brief analysis of the problem is made by considering the penalty in thermal efficiency and cost accrued when varying percentages of the fuel is vaporized and varying percentage of of the vapor is captured and reliquefied. The analysis considers hydrogen produced by the steam-iron process from $1.20/106 Btu ($1.14/GJ) coal, liquefied by current technology, having a thermal efficiency of 55% and costing $6.00/106 Btu. It is assumed that from 0 to 20% of the fuel is vaporized and that from 0 to 100% of the vapor is captured and reliquefied. That which is not captured is assumed lost. Feedstock losses during reliquefaction (12.3%) are accounted for. The penalty in thermal efficiency is shown in Fig. 5 as a function of the percent of the fuel vaporized, for varying

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FIG. 6. The effect of liquid hydrogen vaporization and reliquefaction on the cost of liquid hydrogen. degrees of fuel captured and reliquefied. If as much as 20% of the fuel is vaporized the penalty in thermal efficiency will vary between about 5.0 and 7.5% depending upon the degree to which capture and reliquefaction can be achieved. Taking steps to capture and reliquefy for the sake of only 2.5 percentage points in thermal efficiency might hardly seem worthwhile until one examines the cost picture. The increase in the cost of liquid hydrogen due to vaporization losses is shown in Fig. 6 as a function of the percent of the fuel vaporized, for varying degrees of fuel captured and reliquefied. If as much as 20% of the fuel is vaporized, the increase in the cost of the liquid hydrogen will vary from $0.76 to $1.53/106 Btu (0.73-$1.46/GJ), depending upon whether 100 or 0% of the vaporized fuel is captured and reliquefied. A $1.53/106 ($1.46/GJ) increase in the cost of the fuel can hardly be dismissed and even if 100% of the fuel is captured and reliquefied, the additional cost of $0.76/106 Btu ($0.73/GJ) for reliquefaction is sizeable. In addition, these calculations do not include the cost of collecting the vaporized fuel. This simple exercise has demonstrated that it is essential that vaporization be minimized and that, depending upon the degree of vaporization which occurs, it may be cost effective to capture and reliquefy the vapors. Whether the vapors could be collected and utilized for some other purpose has not been determined, but this possibility would depend greatly upon the cost of producing the gas. HYDROGEN FUELED AIRCRAFT Published results [1] of studies conducted for N A S A by the Lockheed-California Company under contract NASI-12972 have indicated that for aircraft sized to carry 400 passengers distances 5560 and 10,190 km, conventionally fueled aircraft require 5 and 12% more onboard energy (based on lower heating values of fuel) to accomplish their respective missions than would hydrogen fueled aircraft sized to perform the same missions. Further studies being conducted under the same contracted effort have considered aircraft sized to carry fewer passengers shorter distances as well as an aircraft sized to carry 400 passengers 9260 km and return without refueling. Results obtained from the study relating to the energy utilization of hydrogen fueled versus conventionally fueled aircraft are presented in Fig. 7. The ratio of the kilojoules of onboard energy required per available seat kilometer for the liquid hydrogen fueled aircraft to the kilo joules of onboard energy required per available seat kilometer for the conventionally fueled aircraft are shown as a function of the fuel fraction of the conventionally fueled aircraft (fuel fraction is the ratio of onboard fuel weight to gross take-off weight). The range and number

374

THERMAL EFFICIENCY AND COST OF PRODUCING FUELS FROM COAL 13--

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of seats available on each particular aircraft are shown in Fig. 7 for each data point. Each data point represents an analysis made of two aircraft sized to accomplish the same mission, one hydrogen fueled and one conventionally fueled. Results are shown in Fig. 7 based on both the higher (shaded symbols) and lower (open symbols) heating values of the fuels. If only the onboard energy is to be considered, the data based on the lower heating values of the fuel are appropriate since the latent heat of vaporization of water in the combustion products is not recovered in the actual combustion process. However, all previous calculations herein are based on the higher heating values of the fuels and if a consistant comparison with the fuel production efficiency is to be made, the energy utilization of the aircraft and the data based on the higher heating values of the onboard fuel must be used. As can be seen in Fig. 7, conventionally fueled subsonic aircraft having fuel fractions greater than about 0.2 and 0.4, based on the lower and higher heating values respectively, consume more energy per seat kilometer than would hydrogen fueled aircraft sized to accomplish the same mission. The greater the fuel fraction, the greater the advantage of hydrogen. For conventionally fueled aircraft having fuel fractions less than the above values, hydrogen fueled counterparts would tend to use more onboard energy. The vertical lines in Fig. 7 correspond to the fuel fractions representative of todays commercial jet transport aircraft. The vertical lines indicate that significant onboard fuel savings are expected to occur only for aircraft having ranges equal to or greater than that of the Boeing 747.

Overall thermal efficiency A comparison of the overall energy efficiency of utilizing coal derived hydrogen, methane, and synthetic aviation kerosene is shown in Table 4. Hydrogen and methane gas are manufactured, transported 805 km via pipeline and liquefied. Synthetic aviation kerosene is manufactured and transported 805 km via pipeline. In the case of both liquid hydrogen and liquid methane, it is assumed that 10% of the fuel is vaporized during transfer from storage to aircraft and that 75% of the vapor is recovered and reliquefied. Finally, the thermal efficiencies are adjusted by dividing by the energy utilization factors for two of the aircraft shown in Fig. 7. The energy utilization factors based on the higher heating values are used to be consistant with previous calculations. Although dividing the thermal efticiencies of the fuels by the energy utilization factor of the particular aircraft does not represent a true thermal efficiency it does provide a method for comparing how well each type of coal derived fuel will utilize our coal resources if the coal resources are to be used to produce aircraft fuel.

ROBERT D. W1TCOFSKI

375

TABLE 4. The overall thermal efficiency of coal-derived aviation fuels Thermal efficiency(%) Liquid Liquid Aviation hydrogen methane kerosene Coal to gas or aviation kerosene Coal to gas or aviation kerosene piped 805 kin Coal to gas or aviation kerosene piped 805 km and gas liquefaction Tmusfer losses (cryogens 10% loss and 75% recovered and reliquefied) Coal to fuel in aircraft Adjusted for fuel consumed by aircraft (130 passengers, 2780 km range) (400 passengers, 10,186 km range)

81.5 (a)

74

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(a) See Tables 2 & 3. (b) Ref. [12]. (c) Ref. 13. Within the framework of the assumptions herein, Table 4 indicates that if coal were to be used to produce synthetic aviation fuels, more coal would be consumed if the coal were converted to liquid hydrogen rather than synthetic aviation kerosene. In the case of the 130 passenger, 2780 km range aircraft 21% (54.5/45.0) more coal would be consumed to perform the mission. In the case of the 400 passenger 10,186 km range aircraft, 4% (54.5/52.5) more coal would be consumed in performing the mission. From Table 4 and Fig. 7, if the fuel fraction of the kerosene fueled aircraft was greater than that of the Boeing 747-200B, the coal consumption would be greater for the synthetic aviation kerosene fueled aircraft than for hydrogen-fueled aircraft. The thermal efficiency of producing and delivering liquid methane, is seen in Table 4 to be superior to that of both liquid hydrogen and synthetic aviation kerosene. If coal is to be the energy source for future aircraft fuels there is a need for an analysis of the performance of methane fueled subsonic aircraft.

Overall cost aspects In order to shed some light on the overall cost of aviation fuels, from coal to fuel onboard the airplane, a comparison is made of in Table 5 of the cost of liquid hydrogen, liquid methane, and synthetic aviation kerosene, including estimates of the cost of fuel transmission, storage and transfer to the aircraft. The calculations in Table 5 are based on a coal cost of $1.20/106 Btu ($1.14/GJ) or about $21/ton. Estimates of the cost of transmission of the hydrogen and methane gas a distance of 805 km via pipeline are taken from [12] and the estimate of the cost of the 805 km transmission, via pipeline, of the synthetic aviation kerosene is taken from [13]. Though transmission costs are taken from two different sets of data the overall impact on cost appears quite small. Liquid hydrogen storage costs represent data from [3] increased 25% to allow for cost escalations. Liquid methane storage costs are estimated as half that of liquid hydrogen as was indicated in [12]. Storage costs for the aviation kerosene were not available but should certainly be significantly less than that for either hydrogen or methane. Losses occurring during the transfer of fuel from storage to aircraft fuel tank were taken to be 10% for hydrogen and methane with 75% of the losses recovered and reliquefied. The additional cost of capturing the vaporized fuel is not included. Losses for the aviation kerosene were assumed negligible.

376

T H E R M A L E F F I C I E N C Y A N D COST OF P R O D U C I N G F U E L S F R O M C O A L TABLE 5. The overall cost of coal-derived aviation fuels for coal cost of $1.20/106 Btu ($1.14/GJ) Cost of fuel, $/10 6 Btu Liquid hydrogen

Fuel preparation phase

Liquid methane

Coal to gas or aviation kerosene 2.51 (2.38) 3.07 (2.91) Coal to gas or aviation kerosene piped 805 km +.08 (.08) (a) +.03 (.03) (a) Coal to gas or aviation kerosene piped 805 km and liquefaction of gases 6.11 (5.80) 3.95 (3.75) Storage of liquids +0.20 (0.19) (c) +0.10 (.09) (b) Flashoff/boiloff during delivery to aircraft tank (10% loss & 75% recovery & reliquefaction) + 0.46 (.44) +0.14 (0.13) Total cost of fuel onboard aircraft 6.77 (6.42) 4.19 (3.98) (a) Ref. [12]. (b) Ref. [13].

($GJ) Aviation kerosene

5.85 (5.55) +.05 (.05) (b) 5.48 (5.20) (?) 0 5.48 (5.20)

(el Ref. [3].

Based on the above assumptions, synthetic aviation kerosene costs 31% ($1.29/106Btu) ($1.22/GJ) more than liquid methane and liquid hydrogen costs 62% (54.58/106 Btu) (52.45/GJ) more than liquid methane. Liquid hydrogen is seen to cost 24% (51.29/106 Btu) ($1.22/GJ) more than aviation kerosene. The implications of fuel cost may be carried one step further by comparing the direct cost of operating an airplane (DOC) with each fuel. Data indicating the effect of fuel cost on the DOC of subsonic transport aircraft carrying 400 passengers 10,186 km using hydrogen and kerosene fuels have been adjusted to reflect fuel costs based on the higher heating values of the fuels have been adjusted to reflect fuel costs based on the higher heating values of the fuels (again, previous calculations are based on higher heating values) and are shown in Fig. 8. When fuel cost is based on the higher heating value of the fuels, the DOC's for the subsonic airplanes are 1.65C/seat mile (0.89¢/seatkm) for the hydrogen fueled aircraft (fuel at 56.77/106 Btu) (56.42/GJ) and $1.46¢/seat mile (0.79C/seat km) for the kerosene fueled aircraft (fuel at $5.48/106 Btu) ($5.20/GJ), the kerosene fueled aircraft being 12% cheaper to operate. Data on the DOC of subsonic methane fueled aircraft are not available.

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ROBERT D. WITCOFSKI

377

CONCLUSIONS Based on, and within the scope of the data presented herein, the following conclusions are drawn: Of the three processes considered for producing hydrogen from coal, the steam-iron process has the highest thermal efficiency and lowest product cost, whether gaseous or liquid. Although gaseous methane from the HYGAS ® Process is slightly more costly and less thermally efficient to produce than is gaseous hydrogen from the steam-iron process, liquid hydrogen costs about 50% more than liquid methane and is about 20% less thermally efficient to produce. This is due to the higher cost and energy requirements of hydrogen liquefaction. If coal is to be the energy source for aircraft fuel in the future, a study of the performance potential of subsonic methane fueled aircraft is warranted. The thermal efficiency and cost of producing synthetic aviation kerosene from coal are similar to that of producing liquid hydrogen from coal. Flashoff and vaporization losses associated with the transfer of liquid hydrogen must be minimized and may warrant capture and reliquefaction of the vapors. From the standpoint of total energy consumption (coal), only aircraft having fuel fractions greater than that of the 747-200B will benefit by the use of liquid hydrogen fuel. Coal is the only source of energy considered herein and conclusions drawn from this paper with regard to the viability of hydrogen fueled aircraft in the future of aviation should be done so with that fact in mind. Emerging hydrogen production technologies, based on other energy sources and improved liquefaction techniques, may greatly change the perspective of hydrogen fueled aircraft in the future. REFERENCES 1. G. D. BREWER,R. E. MORRIS,R. H. LANGE& J. W. MooRE, Study of the application of hydrogen fuel to long-range subsonic transport aircraft. NASA CR 132559, prepared by Lockheed-California Company and Lockheed-Georgia Company under Contract NASI-12972, January (1975). 2. J. C. GILLIS,D. P. GREGORY& J. P. PANGBORN,Survey of hydrogen production and utilization methods. Prepared by the Institute of Gas Technology under Contract NAS8-30757, August (1975). 3. J. E. JOHNSON,The economics of liquid hydrogen supply for air transportation. In Advances in Cryogenic Engineering, Vol. 19, p. 12. (Edited by K. D. Timmerhaus) Plenum Press, New York (1974). 4. J. E. JOHNSON,Economic perspective for hydrogen fuel. Presented at the Hydrogen Economy Miami Energy (THEME) Conference, Miami Beach, Florida, 18-20 March (1974). 5. Anon. Survey study of the efficiency and economics of hydrogen liquefaction. NASA CR-132631, prepared by the Linde Division of Union Carbide under Contract NASI-13395, 8 April (1975). 6. C. R. BAKER,Efficiency and economics of large-scale hydrogen liquefaction. Presented at the National Aerospace Engineering and Manufacturing Meeting, Culver City, Los Angeles, California, 17-20 November (1975). 7. C. R. BAKER,R. L. SHANER,A study of the efficiency of hydrogen liquefaction. Presented at the 1st World Hydrogen Energy Conference, Miami Beach, Florida, 1-3 March (1976). 8. C. L. TSAROS,J. L. ARORA& K. B. BURNHAM,A study of the conversion of coal to hydrogen, methane, and liquid fuels for aircraft. Prepared by the Institute of Gas Technology under Contract NASI-13620, expected publication date, February (1976). 9. C. L. TSAROS,J. L. ARORA& K. B. BURNHAM,The manufacture of hydrogen from coal. Presented at the National Aerospace Engineering and Manufacturing Meeting, Culver City, Los Angeles, California. 17-20 November (1975). 10. C. L. TSAROS,J. L. ARORA& K. B. BURNHAM,The manufacture of hydrogen from coal. Presented at the 1st Worm Hydrogen Energy Conference, Miami Beach, Florida, 1-3 March (1976). 11. I. STAMBLER,EPRI gas turbine outlook. Gas Turbine World 5, (5), 19 (1975). 12. D. P. GREGORY, A hydrogen-energy system. Prepared by the Institute of Gas Technology for the American Gas Association, Catalog No. L21173, August (1972). 13. A. E. UHL, Fuel energy systems: conversion and transportation efficiencies. Presented at the 9th IECEC Conference, San Francisco, California, 26-30 August (1974).