FUEL PROCESSING AND THERMOCHEMICAL/PHOTOCHEMICAL CYCLES
Arlon J. Hunt Lawrence
Berkeley Laboratory Berkeley,
A long-sought goal of energy research has been to find a method to produce hydrogen fuel economically by splitting water using sunlight as the source of energy. Many methods can be employed to produce useful fuels from available raw materials using sunlight. However, implementing these processes on a large scale generally involves significant capital and energy costs. Sunlight is an attractive means of providing a renewable source of energy to drive the process after providing the initial capital outlay. However, the combination of capital costs to provide concentrated solar energy and the elaborate and expensive plants required to carry out the chemical processes puts a heavy financial burden on this approach to a clean and renewable energy economy. Alternately, if sunlight is used in nonconcentrated systems the cost per unit area of the converter must be very low to make a viable system.
Solar driven fuel processing methods include thermal decomposition, thermochemical, photochemical, electrochemical, biochemical, and hybrid reactions. Feedstocks include inorganic compounds such as water and carbon dioxide and organic sources such as oil shales, coal, biomass, and methane. The range of approaches to carry out these processes runs the gamut from well-established chemical engineering practices with near-term predictable costs, to long-term basic photochemical processes, the details of which are still speculative. Thus, the goal remains elusive because near-term systems tend to have high costs, while the costs of advanced long-term systems are not well defined.
The results of the technical session at the workshop reflected the dichotomy between advanced and engineered systems both struggling toward the same goal. The session consisted of four presentations on solar fuels research and chemical processing, followed by a short discussion. The participants discussed the use of solar energy to produce several fuels: primarily hydrogen, methane, and carbon monoxide. One talk concerned the processing of biomass for the production of useful chemicals not necessarily using sunlight as the source of energy.
Two presentations described broad-based efforts, while the other two related experiences with specific technologies. The first talk described the results of a research program funded by the Gas Research Institute (GRI) to investigate low-cost conversion of inorganic materials to gaseous fuels using solar energy. The second talk was a survey of thermochemical hydrogen production processes. The technology-specific talks discussed solar processing of oil shales, the carbon dioxide reforming of methane, and the pyrolysis of wood wastes to produce phenols.
The photochemical dissociation of water, electrochemical cycles, particle-catalyzed reactions, and biological, fuel-producing processes received little or no discussion in this session. The knowledge base for a number of specific chemical cycles for hydrogen production was reviewed, but
technical details regarding a number of other approaches must be supplemented by other means.
With few exceptions, the processes discussed were in the development, engineering design, and scale-up stages rather than the exploratory research stage. Thus, the technology was fairly well understood for many of the hydrogen production and methane reforming cycles; the principles are known for oil shale processing. An examination of the costs of fuel production indicated that solar-derived fuels were currently significantly more expensive than fuels derived from nonrenewable resources. This report briefly discusses the formal contributions, comments regarding those talks, and the general discussion following the talks.
The first talk concerned the GRI experience in solar fuel research and was given by Kevin Krist, the project manager. The GRI funded research was aimed at low-cost conversion of inorganic materials to gaseous fuels. The program ran from 1981 to 1989 and was funded at an annual level of one million dollars or more. The program emphasized photochemical approaches, although thermochemical processes were also evaluated. It is of note that support ended in 1989 and no gas production research is currently underway with GRI support. The key technical factor cited for the lack of success in water-splitting processes was the difficulty in maintaining charge separation in aqueous solutions for sufficient time for fuel-forming reactions to occur. In non-aqueous systems the researchers concluded that effective charge separation was possible, but that the systems work indirectly—generating electricity first, and subsequently producing gaseous fuels via electrolysis. Krist stated that GRI felt that very long-term research was required to develop the type of molecular catalysts that were probably necessary to carry out the reactions efficiently in aqueous phase.
The GRI studies analyzed the cost of solar-produced fuels by assuming a hypothetical nonconcentrating pond system capable of making hydrogen or methane using a membrane separation technique. The costs were based on land area, membrane requirements, and various other costs to arrive at a fuel price. Methane costs for the pond were estimated at $30/MMBtu and for concentrating systems at $82/MMBtu. Methane production costs for mature thermochemical processes were estimated at $40/MMBtu. These costs were contrasted with current wellhead natural gas prices of less than $2/MMBtu. Methane costs were strongly influenced by process efficiency and material costs. The cost of photovoltaic-operated electrolysis to produce hydrogen was estimated at $20/MMBtu for mature plants. By comparison, 1988 prices for hydrogen produced by the reforming of natural gas were about $7/MMBtu. Another conclusion was that high value fuels should be sought to improve the economics of solar fuels processing.
The engineering considerations and costs for thermal water-splitting, thermochemical, and hybrid solar hydrogen production were discussed by Ertugrul Bilgen. He stressed that the plant scale must be very large to supply modest amounts of hydrogen. For example, a typical plant size considered of 500 MW thermal would only supply about 1/3% of Hawaii's energy needs. He concluded that high efficiency and large plant size were necessary for success. Thermal decomposition of water at 2500ºK, for the conditions considered, had a 5% hydrogen composition. The key problem is avoiding the back reaction after dissociation. Separation and recovery may be carried out at low or high temperatures; at low temperatures, rapid quenching is followed by separation by a diffusing membrane; at high temperatures, separation is affected by selectively permeable membranes. However, these means of separating the products have not yet undergone practical testing. Bilgen also concluded that, for better economics, the plants should probably be operated as cogenerators.
Several thermochemical processes were discussed. Most of these processes were based on sulfur cycles involving decomposition of sulfuric acid and sulfur dioxide at temperatures of about 1200ºC. The GA, Mark 16, and Cristina processes were discussed. The Cristina process offered the advantage that sulfuric acid and SO3 decomposition could be operated in reverse at night using air or oxygen as a vector to provide a continuous source of heat for
hydrogen production. The estimated costs for this process were $30 to $70/GJ. Bilgen recommended that solar specific processes be studied and new cycles explored. In addition, more heat transfer studies, bench scale testing, and the construction of a pilot plant were recommended. Further, Bilgen suggested that more research was necessary to find and evaluate materials that could withstand the high temperature sulfur oxide chemistry, especially in the presence of water.
Moshe Levy discussed solar testing experience of oil shale processing and methane reforming reactions for a chemical heat pipe. He pointed out that each country has different potentials from the point of view of needs and resources. Israel has little gas, oil, or coal, but has lots of sunlight and oil shale. Levy also stated that practical demonstration of technologies was necessary. The new central receiver solar thermal test facility at the Weizmann Institute is being used for such tests. Oil shale found in Israel has a 14% organic and about 60% calcium carbonate content. Because rapid heating of the shale produces both CO2 and CH4, it may be possible to induce CO2 reforming of methane in the same reaction as the solar gasification. Two types of reactors were tested: an opaque inconel reactor and a transparent quartz reactor. Currently both reactors are used in batch mode only. There appeared to be a significant improvement in the performance of the transparent reactor over the metal reactor. Levy calculated the fuel value of the product at 4730 kJ/kg including the solar input. If the shale were simply retorted it would yield only 2200 kJ/kg. He speculated that since many complex hydrocarbon compounds exist in oil shales, it was likely that some could initiate photochemical reactions.
Levy discussed the chemical heat pipe in which the carbon dioxide reforming of methane was used to capture energy from sunlight in the form of new chemical bonds for energy storage or transport. Reactors with rhodium catalysts on aluminum oxide substrates were used and successfully survived over 50 cycles of testing. Direct absorption and opaque receivers were used to enable the catalyst to be directly heated by the concentrated sunlight. The work is progressing and construction of a 400 kW reformer is underway.
Craig Tyner of SNL participated in the talk by describing a joint United States-German testing program for direct absorption methane reforming. The project, called CAESAR, tested a 100 kW receiver at a dish test facility in southern Germany. The direct absorption receiver used a reticulated alumina substrate with rhodium catalyst. A receiver efficiency of 85%, chemical efficiency of 55%, and methane conversion of 70% were reported. The tests were considered successful but the effort is currently winding down.
The last paper described a process to provide inexpensive phenol replacements from biomass sources. Helena Chum described the development of a method for the fast pyrolysis of biomass using a vortex reactor. The process involves rapid heating of wood chips, bark, and sawdust to prepare and separate valuable oils. Phenols derived from the process promise to be significantly cheaper than those currently used in the plastics industry. Industry is participating in the transfer of this technology. Phenols represent 11% of the plastics market for adhesive applications including that of binding plywood. Steam is used as the carrier gas to provide a condensable fluid to aid in carrying and separating the products. The reaction takes place at 650ºC and utilizes a low temperature separation process. High heat transfer rates of 8 W/cm2 K are achieved in the reactor. The estimated cost per pound for the product is $0.10 to $0.27, as compared to current prices of $0.42. Several people suggested that the reactor be heated by concentrated sunlight, but Chum replied that a char product of the reaction has sufficient energy content to fire the process on its own and therefore the process requires no additional heat source.
Chum commented that in analyzing chemical markets, it is important to keep a worldwide perspective because of the international nature of the chemical industry. She also commented that the choice of acetone, suggested in the PNL report, as a good application for solar fuel production did not consider the real size of the market and realities of acetone production.
Following the presentations, the discussion dealt with several questions and comments. No clear answer to the question of what technology was ready for commercialization emerged because of the cost issues. Basically, solar-derived fuels are still expensive compared to fossil-derived alternatives. Some participants felt that the systems must be demonstrated on a sufficient scale to provide a credible demonstration of the technology. In the better-defined systems, more data and engineering cost curves were required to determine the potential for economic success. It was stated that there was a need for more data on heat transfer rates in chemical processes. A question arose as to the effectiveness of direct radiant heating of coal or oil shale because the radiation could not penetrate to a significant fraction of the feedstock. In earlier work on solar gasification of coal and shales, a solar driven communition of the feedstock could be used to reduce the material to very fine sizes. This occurred because of the rapid expansion of the steam derived from the water in fine pores of the coal. A new water splitting cycle was discussed (Yokohama) that used photo, thermal, and electric driven processes.
GAS RESEARCH INSTITUTE EXPERIENCE IN SOLAR FUEL RESEARCH
Gas Research Institute
Between 1981 and 1989, the Gas Research Institute (GRI) conducted a fundamental research program aimed at low-cost conversion of inorganic materials to gaseous fuels, using solar energy. Although the program focused on photochemical approaches, thermochemical pathways were also evaluated. Our general conclusions were as follows:
Serious technical problems make direct photochemical production of H2 or CH4 in aqueous solutions long-term, high-risk research. Many of these problems relate to the difficulty of maintaining net charge separation for long enough to cause the desired fuel-forming reactions without also causing a number of competing auxiliary reactions. Also, fuel and oxygen coproducts form too slowly, particularly with low-cost catalysts. Molecular catalysts, which will take a long time to develop, may be required to replace the ineffective surfaces catalysts presently use. Photogenerated charge separation appears to be more reliable in non-aqueous solutions, but these systems work indirectly. They produce electricity first, before producing gaseous fuels by electrolysis. Although a very long research path remains, GRI's program made significant progress in different stand-alone systems for photochemical water splitting and in the conversion of water and carbon dioxide to methane and other hydrocarbons.
Solar photons are an expensive route to gaseous fuels. Based on our research results, TDA Research, Inc. conceptually designed and analyzed a solar methane production plant with a capacity of 10 million SCF per day. Using solar insolation data for Phoenix, Arizona, capital and operating costs-including the cost of CO2—were estimated for different efficiencies and reactor costs. Three types of reactors were evaluated—flat plate, concentrator, and a newly designed, experimental pond reactor. The low areal solar energy density and the low conversion efficiency resulted in large collector area requirements and high capital costs. Projected methane costs were $30/MMBtu for the pond reactor, $82/MMBtu for the concentrator, and $52/MMBtu for the flat plate reactor, by assuming 10% conversion efficiency, 13.5% annual capital and operating costs, and $5/ft2 photocatalyst system cost. PV-electrolysis, currently a much more efficient and reliable route than photochemical fuel generation, was projected to cost above $60/MMBtu in the near term and above $20/MMBtu in the long term.
Dark catalytic chemistry for producing fuels is related to other small molecule redox processes. Since abandoning the solar fuels option, GRI has reoriented this research toward processes with nearer-term potential for benefiting the natural gas industry and its customers. These areas include sulfur recovery, oxygen separation, methane electrochemistry, pipeline and metal component corrosion, NOx removal, and gas sensors.
Thermochemical Fuel Synthesis
The GRI evaluations have estimated that 'optimal' thermochemical processes would cost above $60/MMBtu in the near term and $40/MMBtu in the long-term, if thermal efficiencies of about 40% are assumed.
Losses relating to the operation of thermochemical cycles at practical rates can reduce the thermal efficiencies to between-5 and +20%. These values are much lower than Carnot efficiencies and would not compete with electrolysis, using electricity derived from hydrocarbon sources.
Thermochemical cycles with more than two or three steps appear to have no chance of being economical. However, the reaction entropy requirements are so severe that few, if any, promising two-or three-step cycles exist. Simple hybrids of thermochemical and electrochemical cycles are conceivable. The GRI has patented some concepts for hybrid cycles based on methane reforming. Electrochemical steps in hybrid cycles tend to be very expensive.
We list below a few comments about using concentrated solar energy for applications other than generating electric power.
Photochemical applications for higher value products will probably be realized before the production of gaseous fuels.
Where sunlight produces molecular scale electricity first, through excited-state charge separation, photochemical approaches may not be better than using an appropriate combination of photovoltaic and electrochemical cells. Viable photochemical processes will satisfy a clear need to directly access reactant or photocatalyst electronic excited states with sunlight. One such need could relate to the dispersed nature of certain reactants. Possible examples of such systems include the removal of pollutants or contaminants from process streams and, conceivably, using photocatalysts distributed over large areas for making fertilizers in situ.
The storage efficiency of solar endothermic reactions is usually limited by the energy levels involved. These processes normally cannot use photons below the minimum required energy, and photons above this level usually lose the excess as heat. Further dissipation occurs if exothermic steps follow the initial photoexcitation. These factors affect heat transport and the size of the reactor needed. If direct excitation of the reactants is possible, it is normally difficult to avoid rapid recombination to the thermodynamically favored starting materials. Various photocatalyst systems are under investigation for optimizing efficiency and preventing recombination.
Photons could also supply the activation energy for thermodynamically downhill reactions. Very high valued products would be required to warrant using one or more photons per reactant. Higher quantum yield processes might be possible in some cases.
Processes such as laser pumping where recombination does not have to be eliminated may be especially suited to using concentrated solar photons.
Very high temperature processes such as glass, metals processes, and ceramic sintering might benefit from the use of concentrated solar energy since radiant heat transfer depends upon the difference between the source and sink temperatures. If these systems require a natural gas backup, their cost
could not exceed the value of saving natural gas and any environmental advantages.
Concentrated solar energy may have advantages for providing the high heat transfer rates required for endothermic processes. Solar steam reforming could be evaluated further as H2 and CH4 become more expensive. Applications for H2 as a chemical may precede those for H2 as a fuel because of the lower value of H2 as a fuel, difficulties of transporting and storing H2 effectively, and the low heating value of H2. Synthesis gas might be used to make methanol and other products. Here also, the Cost of the System could not exceed the value of the fuel saved, if a backup system is required, plus the environmental benefits.
There is a need for further research to define the potential for designing lower-cost collector systems and, possibly, systems where solar energy is used in the fabrication of collector units.
THERMAL, THERMOCHEMICAL, AND HYBRID SOLAR HYDROGEN PRODUCTION
Montreal P.Q. Canada
Various types of processes using thermal, thermochemical and hybrid solar hydrogen production methods are reviewed. Basic process evaluation techniques are presented and discussed. Process efficiency and solar hydrogen costs are presented.
First Law Analysis. First law analysis has been carried out using appropriate thermodynamic relations to determine total heat input, total work input and overall process efficiency . Overall first law efficiency is calculated as
Exergy Analysis. Exergy analysis is obtained from the first and second laws of thermodynamics for each module as
where ΔH=Σout nh - Σin nh and ΔS=Σout ns - Σin ns, and Qm is the heat exchanged with the surroundings at T0=298 K. The entropy production is obtained from equations (2) and (3) as
The energy of the solar radiation is calculated from
Overall energetic efficiency is calculated as
Engineering is done based on detailed process flow sheets and the methodologies of chemical engineering [2,3]. Cost evaluation and economics are based on detailed flow sheets and cost calculations . In many cases, a modular cost evaluation method is used . Economical assumptions and methodologies vary slightly from one institute to another, but the differences
are well established . The levelized cost is generally used for comparisons.
HIGH TEMPERATURE SOLAR ENERGY SYSTEMS
Dish Systems. Dish systems are usually double or single reflection parabolic systems with about 2000 or more concentration. The double reflection systems have power of about 103 kW (thermal), while the single reflection systems have about 200–300 kW (thermal) power. The temperature level of thermal energy is 1500–3000 K with optical efficiencies of about 60% for double reflection and up to 90% for single reflection systems . They are most suitable for hydrogen production using thermolysis and two-step cycles. The cost of high temperature solar heat is about $1–3/GJ .
Central Receiver Systems: Central receiver systems consist of a large number of individually tracking heliostats used to concentrate the direct component of sunlight on a receiver at the top of a tower. Thermal energy is produced over a wide range of temperatures and at various power rates [8–10]. The temperature level of thermal energy is below 1500 K with optical efficiencies of about 60%. The power level may be 75–900 MW(thermal). The process heat is most suitable for thermo-chemical and hybrid processes operating with multistep cycles. The cost of high temperature solar heat is estimated at about $5–10/GJ .
HYDROGEN PRODUCTION PROCESSES
Thermolysis. Thermolysis has been studied conceptually , experimentally at bench scale [13,14], and economically  by a small number of authors. Some overall assessment studies are reported  and no pilot plant scale application has been attempted yet. The main problem areas are related to the design and optimization of solar receivers and reactors, gas quenching and separation methods, and construction materials compatible with high temperature operations.
Process Flow Sheet. The process consists of three major subsystems: solar concentrator system, solar receiver-high temperature reactor, and chemical process. Depending on the application, the chemical process may be purely thermal (thermolysis) or thermal combined with an electrochemical step (hybrid thermolysis). The receiver design is expected to meet the total energy requirement . There are basically two types of process:
Process with Quenching and Low Temperature Separation: The product gases are quenched by (i) stirring the gas mixture with the bulk of the impinging jet; (ii) stirring the gas mixture with additional jets of cold gas which may be an inert gas, water mist, steam, etc.; and (iii) bubbling the gas mixture into a cold medium, water for example. As the gas reaches ~500 K, the kinetic activity stops and the gas composition stays fixed at a value close to that of equilibrium at the thermolysis temperature. Hydrogen is then separated diffusing through a membrane made of palladium or an organic polymer such as cellulose acetate .
Process with High Temperature Separation: Either hydrogen or oxygen, or both, are selectively separated at reaction temperature while water dissociation is in progress, by utilizing membranes selectively permeable to each species. Experimental experience includes tests with ZrO2 and ThO2. Membranes made of ZrO2-CaO and ZrO2-CeO2-Y2O3 have been tested so far for oxygen separation . Recently a theoretical approach has also been developed to predict the various interactions in the process .
Thermochemical and Hybrid Processes
Two-Step Cycles. Two-step cycles are thermochemical and hybrid cycles conceived for solar operation. Various cycles have been studied, which may be classified as (1) metal/metal oxide, and (2) metal oxide/metal sulfate (see, for example, ).
Multistep Cycles. There are not many multistep cycles conceived for a solar source. Most of the thermochemical cycles for water splitting using high temperature nuclear heat are adapted for solar operation. They are based on the sulfur family processes where the high temperature thermal decomposition of sulfuric acid into sulfur dioxide, water, and oxygen is the common reaction. The product sulfur dioxide is reacted with water to produce hydrogen and sulfuric acid by using either several thermochemical steps  or an electrochemical step [22–25]. In a new process, hot oxygen was used as a vector in the decomposition process . Further improvements were also recently reported using cogeneration techniques. It is clear that the high temperature thermal decomposition step can be carried out using solar heat and the cycle can be closed as desired [27–32].
Solar Chemical Process. The solar process operates as a H2SO4 decomposer to supply SO2 to the hydrogen production step. In order to reduce temperature cycling, it is reversed during standby or night operation and the decomposer is operated as a sulfuric acid synthesis process. In this manner, the heat generated is supplied to the chemical process itself to keep the equipment near the same level of operating temperature and to reduce thermal inertia problems. The details of this operation are discussed elsewhere [29,31].
H2SO4 decomposition is carried out in three steps: (1) acid boiling and concentration; (2) main acid decomposition; and (3) sulfur decomposition. Step 1 is the acid boiling and concentration step which is carried out at 632 K, step 2 is the sulfuric acid decomposition step at 1029 K, and finally, step 3 is the sulfur trioxide decomposition step which may be accomplished in a catalytic or noncatalytic reactor at 1098 K. The raw materials for sulfuric acid synthesis are sulfur dioxide, water, and oxygen; the product is H2SO4. The thermal energy losses during standby or night operation without energy restitution are about 5.3% of the total sulfur dioxide produced. For example, based on 315,000 mol SO2 per hour and 19.2 hours of operation per day, 2.2 × 109 mole H2 per year or 0.63 × 108 GJ H2 per year is produced. 26,500 mole SO2 per hour is used in sulfuric acid synthesis for standby operation.
Plant Size and Design Criteria. The maximum size of the process is limited with the nominal size of the solar plant if the coupling is not done in a distributed manner. A production rate of 106 Mol SO2 per hour yields a reasonable plant size and it requires a central receiver solar system producing approximately 106 GJ of heat per year [29,32].
Operation. Three types of operation can be envisaged: (1) full power/day charging (decomposition); (2) partial power/day charging (decomposition); and (3) standby/day or night (synthesis).
Overall First Law Efficiency and Cost of Hydrogen. First law efficiency and H2 Costs are shown in Table 1 for various solar processes. The typical costs assume a constant dollar, 5% interest rate (above inflation), and 20 year plant life.
Table 1 First Law Efficiency and Cost of Solar Hydrogen
The thermal and engineering efficiencies of hydrogen producing thermal/thermochemical/hybrid solar cycles and hydrogen costs so far reported show that there is a great potential in these technologies to produce large quantities of hydrogen. Processes specific to solar operation should be further studied, new cycles for this kind of solar source should be searched and experimental studies using bench scale and pilot plants should be carried out. Heat transfer studies and experimental R&D on hybrid receivers should also be done.
Financial support by the Natural Sciences and Engineering Research Council of Canada is acknowledged.
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CHEMICAL REACTIONS DRIVEN BY CONCENTRATED SOLAR ENERGY
Weizmann Institute of Science
Solar energy can be used for driving endothermic reactions, either photochemically or thermally. The fraction of the solar spectrum that can be photochemically active is quite small. Therefore, it is desirable to be able to combine photochemical and thermal processes in order to increase the overall efficiency.
We have been studying two thermally driven reactions: oil shale gasification and methane reforming. In both cases, the major part of the work was done in opaque metal reactors where photochemical reactions cannot take place. We then proceeded working in transparent quartz reactors. The results are preliminary, but they seem to indicate that there may be some photochemical enhancement.
The experimental solar facilities used for this work include the 30 kW Schaeffer solar furnace and the 3 MW solar central receiver in operation at the Weizmann Institute.
The furnace consists of a 96 m2 flat heliostat, following the sun by computer control. It reflects the solar radiation onto a spherical concentrator, 7.3 m in diameter, with a rim angle of 65°. The furnace was characterized by radiometric and calorimetric measurements to show a solar concentration ratio of over 10,000 suns.
The central receiver consists of 64 concave heliostats, 54 m2 each, arranged in a north field and facing a 52 m high tower. The tower has five target levels that can be used simultaneously. The experiments with the shale gasification were carried out at the lowest level, 20 m above ground, which has the lowest solar efficiency and is assigned for low power experiments. We used secondary concentrators to boost the solar flux.
Oil Shale Gasification
Oil shales consist of an intimate mixture of minerals and organic matter. The shales found in Israel contain about 14% organic matter and their energy content is 3700 kJ/kg. They can be burned to produce process heat, with an efficiency of 70%. Alternately, they can be pyrolyzed, in an inert atmosphere to yield an oil that, after processing, can be used as a liquid fuel. The efficiency of this process is about 40%. Another way of utilization of the shales, is their gasification at high temperatures. This process, when used in combination with solar energy, can double the energy content of the initial shales. The gases evolved can be treated with a reforming catalyst to yield synthesis gas, which can either be used as gaseous fuel or can be processed into liquid fuels. We have studied the gasification process in a small-scale pyrolysis/GC system and showed that when the heating rate is high enough, complete decomposition of the kerogen and the carbonates can be achieved.
We then proceeded to work, on a larger scale, with concentrated solar energy from the solar central receiver. A 3-D compound parabolic concentrator (CPC), kindly lent to us by Prof. R. Winston, was used in order to increase the solar flux. It has an inlet aperture of 48.3 cm in diameter, and an outlet aperture of 21.6 cm diameter. The acceptance half-angle is 25°. It is truncated at 47.5 cm and has a calculated concentration ratio of 5.0. The CPC was connected to a box-type insulated receiver, housing an Inconel
reactor tube, 4 cm in diameter. The shales were fluidized by a stream of argon, and the temperatures of the fluidized bed and the wall of the reactor were recorded continuously; 20 g shales were used in each experiment. The heating was very fast and over 90% decomposition of the shales was achieved in 2 to 3 minutes.
In another set of experiments, a 2-D CPC, designed for heating a tubular reactor from all sides, was used. The CPC has an inlet aperture 45 cm wide, and an acceptance half-angle of 10°. It is truncated to 23 cm and has a calculated concentration ratio of 3.5. The reactor, 4 cm in diameter, was made of quartz. The heating rate was very fast; the temperatures reached were over 1000°C and the decomposition was practically complete. From the preliminary experiments it looks as if the reaction is faster in the quartz reactor. However, we still have to carry out control experiments in order to quantify the effect.
The very high photon fluxes reached under these conditions bring about an immediate jump in the temperature of the light absorbing particles. As the shales contain a mixture of very complex organic molecules, it is conceivable that a number of photochemical bond dissociations take place, leading to free radicals with short lives. Free radicals at such high concentrations may lead to different disproportionations that eventually will give smaller molecules. Therefore, the composition of the reaction products could be quite different under thermal and photothermal conditions.
Oil shale is only one of a number of feedstocks that can be used in conjunction with solar energy. Other feedstocks can include coals of different grades, sand tars, heavy residual oils, biomass, and in some cases urban and industrial refuse.
The reaction under study is
CH4 + CO2 = 2H2 + 2CO ΔH = 250 kJ/mole
It serves as a chemical heat pipe, for storage and transport of solar energy at ambient temperatures. We have carried out this reaction, in the solar furnace, in closed loop, for over 50 cycles, with satisfactory results. The process is now being scaled up to a 400 kW unit in the solar central receiver.
The forward reforming reaction, which is the endothermal part, was performed mostly in an opaque metal reactor. It consisted of an Inconel tube filled with a rhodium on alumina catalyst. The reactor tube was suspended in an insulated rectangular receiver with an aperture, 10 cm in diameter, positioned at the focal plane of the solar furnace. In this reactor the maximal power absorbed during the reforming reaction was 8.5 kW.
By using the Inconel reactor the solar flux is first transformed into heat, which is redistributed throughout the walls of the receiver and the reactor and operates only as a high temperature source. Another mode of running the same reaction is by direct illumination of the catalyst . In this mode we have tried a number of reactor configurations with a transparent quartz window. THe catalyst was in the form of a ceramic honeycomb or foam, 14 cm in diameter, coated with Rh metal. The power absorbed in these experiments reached 6 kW. The reaction rates measured were 10 times higher than those obtained with the Inconel reactor. However, this may be at least partly due to the higher activity of the catalyst itself, because of the way it was prepared, and not necessarily due to the photoeffect. It can also be due to the very high temperatures at the reaction site.
A number of other heterogeneously catalyzed reactions may be good candidates for operation with solar energy. They should be found among industrial processes, high in energy consumption, such as in the petrochemical industry.
1. Ingel, G., M. Levy, and J.M. Gordon. 1990. Gasification of Oil Shales by Solar Energy. Int. Symp. on Solar High Temp. Technologies, Davos, Switzerland.
2. Levitan, R., M. Levy, H. Rosin, and R. Rubin. 1990. Closed Loop Operation of a Solar Chemical Heat Pipe at the WIS. Int. Symp. on Solar High Temp. Technologies, Davos, Switzerland.
3. Levy, M., H. Rosin, and R. Levitan. 1989. Chemical Reactions in a Solar Furnace by Direct Solar Irradiation of the Catalyst. J. Solar Energy Engineering 111: 96.
INEXPENSIVE PHENOL REPLACEMENTS FROM BIOMASS: AN ON-GOING TECHNOLOGY TRANSFER EFFORT
Helena L. Chum
Solar Energy Research Institute
The activities of the Chemical Conversion Research Branch of the U.S. Department of Energy (DOE) Solar Energy Research Institute (SERI) include the production of fuels, chemicals, and materials from renewable resources and wastes. This paper describes the collaboration of DOE/office of Industrial Technologies (OIT), Solar Energy Research Institute (SERI), and the Pyrolysis Materials Research Consortium (PMRC). This collaboration is based on the conversion of waste wood and bark into an inexpensive phenolic and neutrals (P/N) product through fast pyrolysis and fractionation. This product replaces a substantial fraction of phenol in phenol-formaldehyde thermosetting resins. The technology transfer mechanism is highlighted.
SERI is a U.S. Department of Energy laboratory, managed by Midwest Research Institute (MRI), a not-for-profit company. SERI has three research divisions, including the Solar Fuels Research Division, which includes the Chemical Conversion Research Branch. The branch has a staff of 21 professionals, 1 postdoctoral fellow, 3 administrative assistants, and 6 part-time chemistry and chemical engineering students. The branch's mission is to develop chemical processes which enhance energy security, resource conservation, and environmental preservation by selecting appropriate technologies for research and development. Specific objectives are:
to develop the science and technology base for the cost-effective chemical conversion of renewable resources and wastes into fuels, chemicals, and materials and for the environmentally sound destruction of hazardous chemical wastes; and
to transfer the developing (developed) technologies to industry in a timely manner, thus improving the economic competitiveness of the United States.
Branch researchers perform exploratory research and bench-scale engineering in these areas in laboratories at SERI and in concert with 64 collaborators (1989–1990) from academia, other research institutions, and industry. One program carried out by the branch and collaborators investigates the area of fast pyrolysis and fractionation to produce inexpensive, reactive replacements for phenol in phenol-formaldehyde (PF) thermosetting resins. An overview of this effort follows.
Approach — Coupling of Basic and Applied Research
The activities described rest on a synergistic interaction between basic and applied research, which allows basic research results to be translated into applied R&D within SERI and collaborating institutions.
The laboratory work at SERI includes process and product screening on a microscale using a unique SERI instrument—the molecular beam/mass spectrometer (MBMS), coupled to a collision-induced dissociation (MS/MS) detector. This instrument can detect, in real time, the products of fractionation, after pyrolysis for effective screening of processes for a wide variety of feedstocks. These studies permit a deep mechanistic understanding of processes and products. In addition, by coupling the data
acquisition to multivariate analyses, we can reduce the number of variables to a few that carry significant chemical information on the processes investigated; this knowledge is expanded by conventional chemical and spectroscopic data analyses. Closely coupled to this small-scale experimentation is the work on an engineering bench scale. Branch engineers have constructed a bench-scale, 75 pounds per hour vortex reactor for ablative fast pyrolysis to develop a variety of processes. A key feature of this reactor is the ability to decouple the residence time of solids and vapors. It does this through a solids recycle loop and a mechanism that guides the solids on the reactor surface with raised ribs, so that fast heat transfer from the reactor wall into the solids can be exploited. It can be operated under conditions suggested by the small-scale technique.
In parallel, we perform technoeconomic evaluations of the various processes and their modifications, so that the limited research dollars can be focused on the key research issues to be resolved. These efforts are performed in collaboration with an independent outside consultant, A. Power (A.J. Power and Associates). The evaluations are updated periodically as the experimental progress warrants.
Inexpensive Phenol Replacements from Biomass
The use of phenolics derived from biomass for use in PF resins has fascinated researchers in the forest products industry for some time. The goal has been elusive because it has been difficult to obtain a reproducible technical product at the necessary low cost. Phenol replacements that cure slower than phenol require a change in press times or temperatures, which are not easily accepted in the United States because they decrease production rates. Process changes are often not desired, because of the relatively high capital cost investment in the manufacturing process. Therefore, the ideal replacements for phenol should not change panel production rate. Ideal replacements also should be cheaper than phenol; abundantly available, preferably from a renewable resource; and reproducible. The literature has excellent reviews of the state of the art, with comprehensive citations. Most of these references include lignin-derived replacements, which have been and continue to be investigated because of their potential low cost, but few have reached commercial applications, without changing one or more parameters from operation with phenol alone.
Wood pyrolysis processes offer a different route to produce low molecular weight phenolic compounds, from lignins or from other sources within the renewable resource. In particular, fast pyrolysis processes offer the advantage of high yields of liquid products from a variety of feedstocks[4,5,9]. In these processes the pyrolysis temperatures are 450–600°C, and the residence times are short, on the order of seconds.
DOE-OIT/SERI Adhesives Program — Case Study in Technology Transfer
Waste resources, such as bark and sawdust, and recovery-furnace-limited mills produced kraft lignins, are the main feedstocks investigated in this program, started in September 1986. As timber resources are depleted, the need for alternative, inexpensive, high-performance adhesives for wood panel and board products will increase. These adhesives will need to be produced from renewable resources with low energy consumption. New markets in composites and molding compounds areas can be developed if inexpensive phenolics can be made with special properties. Significant amounts of energy and petroleum could be saved by replacing these products with renewable adhesive materials. In the United States, about 3 billion pounds of phenolic resins are used annually, of which about half is used in the plywood market. The U.S. phenol production is closely related to the development of engineered plastics, which use high-purity phenol in an integrated way. The use of engineered plastics is expanding rapidly. For this reason, it would be very advantageous if a large user segment of the phenol production, which does not
require a high-purity phenol source, could be replaced with an inexpensive and reproducible feedstock from biomass wastes. This would release high-purity phenol for other uses and complement the feedstock pool with an adequate resource.
The program investigates biomass fast pyrolysis oils; more than 60% of the dry weight of the feedstock is converted into a mixture of controllable products. The SERI vortex reactor (100 pounds per hour) generates oils from a variety of feedstocks. Using sawdust ($10–40 per pound dry ton), the cost of the resulting pyrolysis oil is projected at about $0.02–$0.08 per pound[4,5]. Then the oils are chemically fractionated[10,11]; the various fractions are subjected to a variety of analytical techniques to determine the best extracts for replacement of phenol in PF resins or other high-value applications. The fractionation method developed for fast pyrolysis oils from lignocellulosic materials removes water-soluble carbohydrates and derived polar compounds, and the ethyl-acetate-soluble phenolic and neutrals (P/N) fraction through water and aqueous bicarbonate extractions, respectively. The ethyl-acetate-combined fraction of phenolic and neutrals compounds, the P/N product, is useful in the making of PF thermosetting resins. The P/N products can be separated further into phenolics. From these simple fractionation schemes, we have extracted about 30% of the pine sawdust oil into useful P/N products. In addition, we are investigating bark pyrolysis, because bark oils are much richer in phenolic compounds, and thus appear to be very good candidates for manufacture of adhesives. References 10–12 contain process details and characterization of the P/N product.
Technoeconomic evaluation of these processes indicate that waste sawdust can be converted into phenol replacements at $0.10–0.27 per pound (amortized production cost), compared to $0.42 per pound of phenol (October 1990 list price). In this evaluation, feed cost varied from $10–40 per pound dry ton, plant life from 10–20 years, and return on investment from 15–30%. The capital equipment investment was estimated at less than $10 million for a 250 tons of wood per pound day plant producing 35 million pounds of phenolics per pound year. Simple payback calculations show that about one year would be necessary if the difference between phenol price and phenolics amortized production cost is maintained at $0.20 per pound[5,10]. This process for producing inexpensive phenol substitutes from biomass received an R&D 100 award, given by R&D Magazine, as one of the one hundred most innovative products and processes of 1990.
R. Kreibich, a phenolic resin formulations expert (and SERI consultant), has tested the oil extracts for their ability to serve as feedstocks for various types of resins and for other applications. Formulations with 50% replacement of phenol in PF molding compound resin and plywood are being developed and appear promising. Since its start in September 1986, the OIT/SERI program has worked closely with industry. Representatives from Borden, Inc.; Koppers; Georgia Pacific Resins, Inc.; Weyerhaeuser; Perkins Industries; E. Seidell and Associates; Louisiana Pacific Corp.; and the American Plywood Association participated in industrial advisory boards in 1987 and 1988.
The potential cost and energy savings have been sufficient to attract industrial interest and cost sharing. Technology transfer continues through a consortium of industries that was formed by MRI Ventures, Inc in July 1989. The title of the patent issued in the fractionation process and applied for the base formulations technologies resides with MRI, the not-for-profit company that manages SERI for DOE. MRI has assigned the title of these patents to MRI Ventures Corp., its for-profit subsidiary, to help expedite the transfer of this technology from the government laboratory to the private sector. Intellectual property generated exclusively by the companies' R&D resides with the companies. The Pyrolysis Materials Research Consortium was created to further the development of this technology and allow its timely transfer to and commercialization by industry. The consortium companies were selected after extensive publicity through press releases announcing that this technology was available for continued R&D and commercialization. Preference
was given to U.S. companies because the U.S. government has partially funded the development of the technology (about 33%) since 1986.
Consortium members and their representatives, respectively, are Allied-Signal Corp., C. Gilpin; Aristech Chemical Corp., M. Fields; Georgia Pacific Resins, Inc., A. Ahmad and J. Outman; Interchem Corp., W. Ayres and D. Johnson; Plastics Engineering Co., P. Waitkus and J. Mohr; through MRI Ventures, Inc. (J. Dinwiddie). Both Interchem Corp. and Plastics Engineering Co. are small businesses. The companies collaborate with the OIT/SERI Adhesives program through MRI Ventures, Inc., with direct contributions and in-kind work to expedite technology development and transfer to industry. Because intellectual property is being generated in the program, current work is not reported until the patents are filed. Only these companies will be able to license the technologies developed by the program with U.S. government funds on a non-exclusive basis. Companies will be added to the PMRC if additional R&D work is needed that the present companies cannot perform, and only if all participating companies unanimously agree.
Some lessons have been learned so far. Technology transfer from government laboratories depends primarily on good technologies, with secured intellectual properties, which can be used as incentives for industries to join a higher risk R&D venture with government. It is a people-intensive process, and its characteristics will depend on the specific technology transferred. It provides a way for laboratory personnel to work closely with industries and observe/participate in the development of industrial technology. It provides a mechanism to practice intellectual property and data protection, as well as for assessing more clearly what is likely to become a valuable intellectual property. It also provides the industrial R&D participants a chance to interact with laboratory personnel and gain insights on new technologies and methodologies of analyses. Because the government laboratory personnel can explore higher risk technologies than industry normally would fund, there is a considerable mutual benefit in these joint ventures. The representatives of the companies thus help guide the limited R&D dollars of these programs, in directions that would lead to the fastest transfer of the technology to industry. The benefits are highly satisfactory and motivating to the people involved. The combination of small and large companies is very beneficial, because the former will be more likely to take on more risks than the latter, and the combination is more likely to succeed than individual efforts.
The work described was sponsored by DOE's Office of Conservation and Renewable Energy. We gratefully acknowledge the support by the Office of Industrial Technologies' Division of Waste Materials Management, and the guidance of D. Walter and A. Schroeder in the Adhesives program. The PMRC representatives, C. Gilpin, W. Fischer, M. Fields, W. Krayer, J. Aiken, T. Smeal, K. Henry, A. Ahmad, J. Outman, A. Gibsons, R. Currie, R. McDonald, K. Wirtz, W. Ayres, D. Johnson, V. Marquis, L. Derr, P. Waitkus, J. Mohr, B. Lepeska, W. Kleine, R. Brotz, J. Dinwiddie, D. Kornreich, R. Muir, and K. Howe are gratefully acknowledged for their support of the R&D and commitment to the program. The staff members of the Chemical Conversion Research Branch at SERI are gratefully acknowledged for their contribution to the work described: J. Diebold, J. Scahill, S. Black, R. Evans, D. Johnson, B. Hames, J. Bozell, K. Tatsumoto, F. Posey Eddy, G. Noll, D. Gratson, M. Echeverria, J. Fennel, C. Elam, J. Fodor, and P. Adam. Thanks are also due to our consultants R. Kreibich, A. Power, and to Hazen Research, Inc., staff members R. Kenney and D. Gertenbach.
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10. Chum, H.L., J.P. Diebold, J.W. Scahill, D.K. Johnson, S. Black, H.A. Schroeder, and R. Kreibich. 1989. Biomass Pyrolysis Oil Feedstocks for Phenolic Adhesives. In Adhesives from Renewable Resources, R. Hemingway, and A. Conner eds. ACS Symp. Series 385:135–151.
11. Chum, H.L., and S.K. Black. 1990. Process for Fracionating Fast Pyrolysis Oils and Products Derived Therefrom. U.S. Patent 4,942,269.
12. Johnson, D.K., and H.L. Chum. 1988. In Pyrolysis Oils from Biomass — Producing, Analyzing and Upgrading, E. Soltes and T. Milne, eds. ACS Symp. Series 376:156–166; Chum, H.L., and J. McKinley. 1988. Research in Thermochemical Biomass Conversion, A.V. Bridgwater, and J. Kuester, eds. Elsevier Applied Science, London, pp. 1177–1180; McKinley, J., G. Barras, and H.L. Chum. 1988. ibid. pp. 236–250.