Technology "What can the chemical engineer do
Process heat from solar energy still unproven Solar energy has found encouraging success in localized applications, particularly for space heating. It is still looking for acceptance as a major source of industrial process heat. The major barrier to industrial application is the difficulty of collecting the energy and transporting it to the processing plant. Another problem is storing the energy to accommodate the intermittent nature of sunshine. Hardware studies now in progress suggest that suitable hardware would be available on demand if industrial process heat by solar en^ ergy were to catch on. Much of the future of the solar energy option rests on how much of the current development effort can be salvaged from budget pruning by the federal government. The major exhibit in the new program for industrial process heat from solar energy is a steam generator in Barstow, Calif., that will feed an electric generating station (see page 27). Elsewhere the pickings are getting slim, but a skeleton effort persists. This picture sums up the current status of the development of industrial process heat from solar energy as outlined at the 1982 Winter Meeting of the American Institute of Chemical Engineers, held last week in Orlando, Fla. Lorin Vant-Hull, a physics professor at the University of Houston, emphasized the importance of collection systems for industrial process heat that feature the central receiver concept. Solar towers (central receivers) that accept solar energy from heliostat (sun-tracking mirrors) arrays have become something of a standard method for high energy fluxes. Systems covering a range from 25 kW to 35 MW (thermal) have been built and operated and other systems of up to 1600 MW have been designed and their costs estimated. The advantages of central receivers center on the savings in energy transport. A high-temperature collecting loop is not required, but the receiver must be elevated above the heliostat field. The business end of the central receiver is the tower, which serves as the collector and processing plant all in one. Vant-Hull counts at least 10 operating heliostat fields now in operation around the world. Most of them are used as development test facilities. The smallest is a 0.1-MW (thermal) system at Genoa, Italy. The largest is 26
C&EN March 15, 1982
with 5 MW?"
the system about to begin operation at Barstow. As the number of heliostats rises, the cost decreases. Well-designed solar energy systems can convert about 60% ôf the incident solar radiation to heat. In the Southwest, each square meter of surface will produce about 3.6 kWh per day (13,000 Btu per day) at an investment of about $130. At a 20% fixed charge rate, Vant-Hull says, that is equivalent to a solar energy cost of $5.75 per million Btu. In the more elaborate systems, the receiver itself would be the processing unit. Such is the case envisioned for photocatalytic reactions or for flash pyrolysis and the direct heating of fluid beds. The key material for enclosed reactors is a window material that can tolerate high temperatures and corrosive atmospheres. Fused silica glass is one possibility. Even though most of the emphasis on the present program of central receiver development is on steam generation, proposals for chemical conversion are gaining in priority. Other than the Barstow receiver, the most prominent is the 5-MW unit at Sandia National Laboratory, Albuquerque, N.M. Sandia researcher John T. Holmes is now seeking projects to test on the 5-MW unit and has begun a project with the question
Direct use of the solar spectrum for photochemistry demands that all of the available wavelengths be useful. Although some of the wavelengths are appropriate for some applications, for practical purposes the solar radiation will have to be used as a heat source only for endothermic reactions. To maximize energy conversion efficiency, the absorption will have to be maximized and that, in turn, means cavity absorbers will be required that can use up to 90% of the incident energy. Under these constraints, says Holmes, the obvious reactor configuration is the rotary kiln. For simple ore processing, such a kiln can be open to the atmosphere, but there are some potential applications that would require closed units. Holmes cites some general criteria for selecting chemical projects for the 5-MW receiver. It would be preferable that the product have a high value, something always desirable. It would also be desirable to replace electrical heating with solar heating directly, and for the process to have an intrinsically high operating temperature. With these criteria in mind, the U.S. Department of Energy is supporting development of solar chemistry through a proposed Sun Fuels program. The program is still in the planning stages and will involve universities, national laboratories,
Sun Gas process has two stages of fluidized bed Product gas
Blower
Cooler
Filter
Gasifier Carbonizer Stage II
Stage Gas
Coal feed
SteamSolar simulation by electric heaters
Ash
and industry. The Central Receiver ment. The recirculated gases act as the cost of production considerably Test facility at Sandia will be the both heat transfer ' and fluidizirxg and probably would make the process center of the program, which DOE media. more accessible to phosphorus raw hopes will begin in 1985. The em So far, carbonization experiments materials. At present, production is phasis will be on upgrading renewable have been conducted in batch at 823 confined to areas of cheap hydro and nonrenewable chemical feed Κ with subbituminous coal. To sim electric power and these usually are stocks to conventional forms of fuel. ulate the solar radiation unit, dry ni far removed from the raw materials. Other than the Sun Fuels program, trogen is heated in an electric furnace The basic idea of the IGT process Holmes suggests that a 5-MW unit and used as the heat carrier for the is to react an intimate mixture of would be appropriate to develop cer carbonizer. The main objective is to phosphate rock, silica, and carbon in tain metals by iodide or carbonyl py- assess the amount of solar energy re a furnace at temperatures up to 1770 rolysis. He says that titanium, vana quired for carbonization and which K. Experiments were completed with dium, chromium, ironj zirconium, could be stored in the form of product the aid of the U.S. Army's solar fur hafnium, tantalum, thorium, and gas. For the particular coal samples nace at White Sands Missile Range in tungsten already have been shown to used, the carbonization heat was 2185 New Mexico. Solar radiation was ad be derivable from such processes, Btu per lb and the energy that was mitted to the reactor through a quartz particularly the iodide process. Car transferred to the product gas is window. An inert atmosphere was bonyl pyrolysis is still more of a curi present as calorific heat and sensible provided by sweeping with argon gas, osity. In both processes, a solar- heat. which also acted as a carrier for the heated fluid-bed reactor would be The energy transferred to the char phosphorus vapor generated in the used in the prospective development. is reflected in its higher heating value. experiments. Four runs all produced elemental Another potential application of The product gas had a heating value solar energy pertains to coal gasifi of about 500 Btu per standard cu ft. phosphorus at an initiation temper cation. At the Orlando meeting, a The heating value of the char was ature of 1525 K. The temperature development group from the Uni 25,000 Btu per kg. Thermal efficiency profile was erratic over the course of versity of New Hampshire discussed of the fluid-bed carbonizer was the runs. Whaley submits that these initial experiments demonstrate that the Sun Gas process, in which solar 97.7%. These experiments have been elemental phosphorus can be pro energy is used to provide much of the heat to raise the coal to pyrolysis conducted with bench-scale appara duced by the method. However, con temperatures. In conventional gasi- tus. Experiments now in progress will siderable refinements are necessary fiers, most of the energy required for utilize the char to produce synthesis to establish the concept, which ap pyrolysis is used to heat the coal to gas and will monitor the continuous pears to have merit on the basis of the pyrolysis temperatures. About 30 to operation of the carbonizer and gasi- early results. The use of solar energy for process 40% of the total heat is consumed for fier. The eventual aim is to use the this purpose. Air also is required for data obtained on the bench to design heat is still largely a matter of faith combustion, thereby introducing ni a pilot plant for the solar gasification and hope. The Barstow steam gener ator could go a long way in proving or trogen into the process, frequently of coal with this process. A nonfuel application of solar en disproving the utility of solar power. where it is not desired. The alterna tive is gasification with oxygen, which ergy is the generation of elemental In the meantime, there are several phosphorus in a solar furnace. This R&D facilities for conducting exper requires an expensive oxygen plant. One solution to those problems, has been tried, with initial success, by iments of value. Sandia's Holmes, for suggests the New Hampshire group, a group directed by Thomas P. example, has 5 MW available for would be to use solar-derived heat. Whaley of the Institute of Gas Tech those who can also pay the operating Other advantages would be a de nology. All of the elemental phos expenses of the facility. There may be creased carbon dioxide output and phorus in the world, Whaley notes, is some reason to believe that private the elimination of nitrogen in the made by an electric furnace process, funding may pick up some of the product gases. The Sun Gas process with electric power representing slack left by the drastic cuts in solar diminishes the problem of the inter nearly half of the operating costs of energy R&D funding. But the imme diate prospects for further develop mittent nature of solar radiation by production. using an auxiliary fossil-fired unit for The substitution of solar energy for ment of solar energy, particularly for night duty. some or all of the electricity would cut industrial process heat, look dim. D The Sun Gas process uses two stages of fluid beds. Coal is devolatilized in the first stage where solid Solar electric power plant due to start up fuel is heated to carbonization tem can be harnessed by utilities to pro peratures. The volatile products and Rudy M. Baum duce electricity on a commercial char from the first stage go to the C&EN, San Francisco scale, will begin producing power. second with steam and hot recircu lated product gases. Gasification oc The first commercial solar-powered With a rated maximum power out curs in the second stage at about 1200 motor was built in 1900 by Aubrey put to the utility grid of 10.8 MW, K. The condensables from the first Eneas and installed at Watson's Os Solar One is the world's largest solarstage are cracked to carbon monoxide, trich Farm, a tourist attraction in powered electrical generating facili hydrogen, and low-molecular-weight Pasadena, Calif., to pump water to ty. Although Eneas' solar-driven hydrocarbons. This eliminates the irrigate oranges and provide water motor and Solar One operate on need for cleanup of tarry residuals in for the ostriches. It drove a 15-hp vastly different scales, the basic prin ciple of their operation is the same: the product gas stream. Sulfur in the motor. In early April of this year, Solar Sunlight concentrated by mirrors can coal is converted to hydrogen sulfide and can be removed by conventional One, a central receiver pilot plant boil water. Solar power, on a large or small means. There is no oxygen require designed to show that solar energy March 15, 1982 C&EN
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