Solar Chemical Reactor Technology for the Industrial Solar Production of Lime

Transcription

1 Solar Chemical Reactor Technology for the Industrial Solar Production of Lime Anton Meier a,*, Enrico Bonaldi b, Gian Mario Cella b, Wojciech Lipinski a,c, Daniel Wuillemin a a Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen, Switzerland. b QualiCal Srl, Via Verdi 3, I Bergamo, Italy. c Department of Mechanical and Process Engineering, ETH-Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland. Abstract - We developed the solar chemical reactor technology to effect the endothermic calcination reaction CaCO 3 CaO + CO 2 at K. The indirect heating 10 kw multi-tube rotary kiln prototype processed 1-5 mm limestone particles, producing high purity lime that is not contaminated with combustion by-products. The quality of the solar produced quicklime meets highest industrial standards in terms of reactivity (low, medium, and high) and degree of calcination (exceeding 98%). The reactor s chemical efficiency, defined as the enthalpy of the calcination reaction at ambient temperature (3184 kj kg -1 ) divided by the solar energy input, reached 30-35% for quicklime production rates up to 4 kg h -1. The solar lime reactor prototype operated reliably for more than 100 hours at solar flux inputs of about 2000 kw m -2, withstanding the thermal shocks that occur in solar high temperature applications. By substituting concentrated solar energy for fossil fuels as the source of process heat, one can reduce by 20% the CO 2 emissions in a state-of-the-art lime plant and by 40% in a conventional cement plant. The cost of solar lime produced in a 20 MW th industrial solar calcination plant is estimated in the range $/t, i.e. about 2-3 times the current selling price of conventional lime. 1. Introduction Lime and cement manufacturing are high-temperature energy intensive processes. The minimum amount of energy in the form of process heat that is required to drive the calcination reaction, CaCO s) CaO( s) + CO ( ) (1) 3( 2 g at the decomposition temperature near 1173 K is about 3029 kj kg -1 of CaO, while the heat of dissociation of calcite relative to 298 K is 3184 kj kg -1 of CaO [1]. Total energy usage of modern lime kilns ranges from 3600 kj kg -1 of CaO for vertical double shaft kilns to 7500 kj kg -1 of CaO for non-preheated long rotary kilns [1,2]. Because the process heat is traditionally supplied by the combustion of carbon-based fossil fuels such as oil, coal, or natural gas, a lime or cement plant releases CO 2 both as a calcination reaction product and from the combustion process that supplies the energy for the reaction. According to the World Business Council for Sustainable Development [3], the cement industry produces 5% of global man-made CO 2 emissions, of which 50% is from the chemical process, and 40% from burning fuel. The remainder is split between electricity and transport use. Estimates of the global anthropogenic CO 2 emissions from the lime industry are near 1% [1]. By substituting concentrated solar energy for the fossil fuels used to drive the calcination reaction, one can reduce by 20% the CO 2 emissions in a state-of-the-art lime plant and by 40% in a conventional cement plant. The cleanliness of the solar process also leads to very pure lime or quicklime (CaO) that is not contaminated by combustion by-products. It seems conceivable that a pure product may be advantageous for special niche markets in industries that use high-value quicklime for the production of chemicals. In this paper, we present the development of the industrial chemical reactor technology for the solar production of lime. Based on previous experience with a direct heating rotary reactor prototype [4,5], we * Corresponding author. Tel.: ; fax: address: anton.meier@psi.ch

2 have designed, built, and tested a scaleable indirect heating 10 kw multi-tube rotary reactor prototype. Furthermore, we have developed a conceptual design of a megawatt-scale solar lime plant comprising both the solar lime reactor and the solar concentrating system. 2. Solar Reactor Technology The novel indirect heating multi-tube rotary kiln, for which a patent application is pending [6], comprises a multi-tube reaction chamber made from high-temperature resistant SiC (Fig. 1). The concentrated solar radiation enters through the circular aperture of a water-cooled aluminum front shield and heats the absorber tubes that are regularly arranged along the cylindrical cavity wall. At the rear of the reactor, a solar irradiated SiC plate separates the cavity from the preheating chamber. Small-grained limestone particles are continuously fed into this preheating chamber, where they are heated almost to the reaction temperature. Due to the rotational movement, the limestone is then transported and calcined within the absorber tubes that deliver thermal energy indirectly to the reactants. absorber tubes reactants inlet pre-heating chamber ceramic insulation rotating cavity products outlet Fig. 1: Left: Schematic representation of the multi-tube rotary kiln prototype. Right: 10 kw solar lime kiln prototype mounted on the experimental platform at PSI s solar furnace. The experimental set-up comprises the rotary kiln, which is protected with a water-cooled shield; a white target for solar flux measurements; a reactants charging system (hopper, dozer); and a products discharging system (collector, outlet tube). (Source: PSI) The tilted rotary kiln is sitting on rubber wheels that are driven by a small electric motor. The kiln consists of a 2 mm thick cylindrical steel drum of 418 mm length and 400 mm external diameter. The entire inner part of the reactor was manufactured by INSULTECH AG, Switzerland. The absorber cavity of 225 mm length and 252 mm inner diameter as well as the absorber tubes of 250 mm length and 17 mm internal diameter are made from RSiC (Re-crystallized Silicon Carbide, density kg m -3, maximum operating temperature 1600 C). The reactor is lined with ceramic insulation material consisting of a 26 mm thick inner layer (INSULBOARD 1600: hard, density 170 kg m -3, maximum operating temperature 1600 C) and a 50 mm thick outer layer (INSULTHERM 1000: soft, porous, density 200 kg m -3, maximum operating temperature 1000 C). Figure 1 shows the experimental set-up for the solar calcination experiments in PSI s solar furnace delivering solar power close to 15 kw with a peak solar flux concentration of about 3000 suns (1 sun = 1 kw m -2 ) on a focal spot of 8 cm diameter. The solar power input into the reactor and, therefore, the limestone burning temperature inside the reaction chamber was controlled using a shutter between the suntracking heliostat and the stationary parabolic dish. The solar flux distribution on a white target was measured with a CCD camera before and after each experiment, and the solar power entering the circular aperture was computed using OPTIMAS from MediaCybernetics, USA, and calibration data obtained with a Kendall radiometer. Temperatures within the rotary reactor were measured with type K thermocouples

3 (TC) and transmitted by wireless telemetry to a stationary receiver-decoder connected to a data acquisition system. The MT32 Mini Telemetry system from KMT GmbH, Germany consisted of eight miniature TC modules, an encoder, and a transmitter that were mounted on an air-cooled plate at the rear of the rotary reactor. The thermocouple temperatures, the feeder speed of rotation and the reactor drum speed of rotation, the direct normal solar irradiation, and the shutter position (opening angle) were simultaneously recorded with the data acquisition system MessHaus from Delphin Technology AG, Germany. Figure 2 is a typical representation of a solar experimental day showing solar irradiation and temperature profiles within the rotary kiln. One can observe that it takes about hours until steady state conditions are reached at the desired temperature and that the highest temperatures are measured in the middle of the reactor. Typically, a calcination experiment was running for 30 min with a constant reactant particle flow rate and fixed experimental parameters. During well-defined time intervals, representative product samples were taken and analyzed. The indirect heating multi-tube rotary reactor also may be operated in hybrid mode using any conventional external heat source like oil or gas burners, which may be introduced through the aperture into the cavity in place of concentrated solar radiation. In fact, the type of the heat source does not influence the conditions inside the indirect heating absorber tubes, i.e. the quality of the end product always remains the same. For convenience, we tested the indirect heating multi-tube rotary reactor with an electric heating system consisting of a heating element from KANTHAL International, Sweden. The cavity temperature (maximum 1200 C) could be controlled by adjusting the electric power (maximum 7 kw) supplied to the SiC heating element Day Temperature [ C] Irradiation [W/m 2 ] Int_Front Int_Middle Int_Back Preheat Ext_Front Ext_Middle Ext_Back Irradiation :00:00 10:00:00 11:00:00 12:00:00 13:00:00 14:00:00 15:00:00 16:00:00 17:00:00 Time [hh:mm:ss] Fig. 2: Temperature profiles within the solar lime kiln prototype: 1) inside the absorber tubes (Int_Front, Int_Middle, and Int_Back); 2) in the preheating chamber; 3) between the steel mantle and the outer insulation (Ext_Front, Ext_Middle, and Ext_Back); solar irradiation during a typical solar experimental day at PSI s solar furnace. 3. Results and Discussion 3.1 Solar Reactor Performance The indirect heating 10 kw solar rotary kiln prototype processed 1-5 mm limestone particles, producing high purity lime with a degree of calcination exceeding 98% and any desired t 60 reactivity ranging from 15 s (high reactivity) to 21 min (low reactivity). For details on product quality see [7]. A complete mass and heat balance of the 10 kw solar multi-tube rotary kiln prototype is found in [8]. Here, we briefly outline the procedure for calculating the efficiency of our solar reactor:

4 H out CaCO diss ( Ta ) 3, m& CaO Q& diss ( Ta ) M CaO η = = (2) Q& Q& input input The efficiency η is defined as the enthalpy change of the calcination reaction at ambient temperature, Q & diss ( T a ), divided by the power input Q & input. The heat balance model accounts for non-steady-state conditions by modifying the solar power Q & solar entering the reactor. The effective power input includes the enthalpy change of the reactor wall materials caused by energy being stored ( Q & storage > 0 ) or released ( Q & < 0 ). Only part of the effective power input storage Q & = Q& Q& = Q& + Q& + Q& + Q& + Q& (3) input solar storage diss heat rerad cond is converted to chemical energy per unit time, Q & diss, while a substantial amount leaves the reactor via various heat transfer paths. Q & heat accounts for the loss in the hot product's sensible heat during the cooling phase. It may be partly recovered with a suitable heat exchanger. The value of the re-radiation heat loss Q & rerad through the aperture is determined from the apparent emissivity of the cavity, ε = 1, and the cavity temperature. Q & cond is the conduction heat loss through the cylindrical walls of the reaction chamber. The term Q & other includes all unaccounted heat losses such as free convection through the aperture and the energy contained in non-recovered fine product powder leaving the reactor together with the CO2 exhaust gas. Figure 3 shows the relative contribution of the heat of dissociation and the various heat losses for a variety of solar experiments that have been conducted under specific operating conditions depending on the rotational speed of the reactor (8-18 rpm), the particle feed rate ( g min -1 ), the limestone grain size (2-3 mm), and the temperature ( K) other Relative Power Fraction Q_other Q_rerad Q_cond Q_heat Q_diss Experiment number Fig. 3: Relative power fraction for solar experiments conducted under various experimental conditions. Exp.19 is considered as best solar experiment. See text for details. For a particular solar experiment (Exp.19, see Fig. 3) performed at 1395 K, the degree of calcination was 98.2%, and the CaO production rate was 64.2 g min -1. The solar energy to chemical energy conversion efficiency was 34.8%, and the maximum error associated with this measurement was ±15.2%. Table 1 lists the loss terms of Eq. (3) and their uncertainties, and one can see how each of these terms contributes to lowering the reactor efficiency. Note that Q & ( ) + Q& = Q& ( T ) + Q& according to the H- diss T a product, heat diss r CaCO3, heat

5 T diagram (see e.g. [8]). In the case of Exp.19, heat was stored in the reactor wall materials ( Q & storage > 0 ) due to non-steady-state experimental conditions. Hence, the effective power input was lower than the solar power input, and consequently η eff > η solar as seen from comparing the last two columns of Table 1. The partitioning of the power input to the various heat transfer modes for the reactor is slightly changed if transient effects are included in the calculation. For example, from the effective power input (solar power input), 13.4% (12.4%) is conducted through the walls of the reactor, 22.0% (20.4%) is lost by re-radiation, 22.3% (20.7%) is carried away in the hot product's, and 7.5% (7.0%) leaves the reactor as "other heat losses" (mainly convection heat losses at the aperture and un-recovered calcined powder). Obviously, design changes that reduce any of these terms will lead to better reactor efficiency. In practice, at least part of the sensible heat stored in the products (CaO, CO2, and non-reacted CaCO 3 ) can be used to preheat the limestone raw material. Similarly, the conduction heat losses can be reduced by better insulating the reaction chamber. Assuming only 50% heat recovery, we expect the efficiency of an industrial solar lime kiln reaching 45-55% [8]. Table 1: Heat balance for selected solar experiment (Exp.19 from Fig. 3) performed at 1395 K with DOC>98%. Note that Q & T a ) + Q& diss ( product heat = Q& diss ( Tr ) + Q& CaCO, heat. Parameter Symbol Typical value Max. Error Typical value Typical value Q & parameter Q & parameter Q& solar Q & input W W - - solar & storage 764 ± Q & input diss ( T r ) Q& CaCO 3,heat 2492 ± Q & cond Conduction heat losses 1321 ± Re-radiation heat losses Q& rerad Other heat losses Q& other ±121 ± Heat of dissociation at T & a Q diss ( T a ) 3431 ±293 η solar =0.323 η eff =0.348 Sensible heat of products Q & 2206 ± product,heat, 3 Solar power input Q & ± Stored power Q Effective power input 9867 ± Heat of dissociation at T r Q & 3146 ± Heating of CaCO 3 In this paper, we only present results from solar experiments that have been performed with the indirect heating 10 kw solar multi-tube rotary kiln. It is important to note, however, that corresponding experimental results from tests with the electric heating system are in good agreement with the results obtained in PSI s solar furnace. 3.2 Conceptual Design of an Industrial Solar Lime Plant Figure 4 represents a conceptual design of an industrial solar lime kiln. The dimensions of the scaled-up solar multi-tube rotary kiln are determined by the maximum feed rate of the limestone particles, which in turn yields the minimum cross section of the absorber tubes. Arranging these tubes tightly along the circumference of the absorber cavity defines the minimum diameter of the cavity. The length of the absorber tubes within the cavity is determined by the retention time of the limestone particles. A secondary concentrator is required to enhance the solar radiation entering the kiln, since the concentrated solar flux of large solar concentrating systems is typically limited to a maximum of about 1200 suns (1 sun = 1 kw m -2 ), which is not sufficient to reach calcination temperatures in the range K. Typical dimensions of the absorber tubes, the cavity, and the non-imaging compound parabolic concentrator (CPC, [9]) are shown in Table 2 for rotary kilns of 0.5 MW th, 3 MW th, and 20 MW th solar power input, respectively. Peripheral components of the solar lime kiln comprise state-of-the-art feeding, discharging, and heat recovery systems. For fossil or hybrid mode of operation, both oil and gas burners may be conveniently

6 inserted into the cavity of the kiln. The implementation of these conventional system components is straightforward and therefore is not discussed here. Fig. 4: Conceptual design of a megawatt-scale indirect heating rotary kiln showing the well-insulated kiln including CPC (left) and the arrangement of the absorber tubes (right). For typical dimensions see Table 2. Table 2: Estimated dimensions of rotary kiln and CPC for 0.5 MW th, 3 MW th, and 20 MW th solar power input. Assumptions: Reactor efficiency η = 0. 5 ; solar ~ concentration ratio C = 2000 ; heliostat field rim angle φ rim = 39 ; particle residence time t res = 300s. Parameter Symbol Unit Solar power input 0.5 MW th 3 MW th 20 MW th Feed rate (peak) m& CaCO3 t/h Number of tubes Tube diameter Tube length Cavity diameter CPC entrance diameter CPC exit diameter CPC length n t d t m l t m d cav m d in m d out m l CPC m Principally, two different types of solar concentrating systems are conceivable for the solar lime plant: (1) central receiver tower top (TT) system consisting of a field of heliostats reflecting the sunlight to the lime kiln mounted on top of a tower, and (2) central receiver tower reflector or beam down (BD) system, where the solar lime kiln is sitting on the ground. Although BD systems offer advantages for materials handling, the indirect heating rotary kiln seems not well adapted to concentrated solar radiation impinging nearly perpendicularly on the absorber tubes. Especially, convective heat losses due to buoyancy effects are expected to be much larger than for applications where the solar radiation enters axially through the aperture into the cavity of the kiln. Furthermore, BD systems tend to be more expensive than TT systems [10]. Thus, for both technical and economic reasons, the BD option has not been pursued further. Conventional TT systems with planar heliostat field are well proven technology and no more fundamental planning is required [11]. However, one disadvantage of such large solar plants is the need for extended

7 land area to minimize blocking and shading of the heliostats. This results in large distances from the heliostats to the kiln, requiring extremely high quality mirrors and thus affecting the cost of the solar lime plant. Minor technical concerns are associated with the handling of the material on the rather tall tower and the CPC being off-axis with the kiln in order to accommodate for the rim angle of the heliostat field. For solar lime plants smaller than about 1 MW th thermal input, an alternative arrangement of the heliostat field and the rotary kiln is proposed (Fig. 5). Here, the heliostat field is built on a natural or artificial hill, and the heliostats are arranged in lines on a number of terraces. The advantages of such a configuration visà-vis conventional TT systems are obvious: 1) Less land area is needed; 2) Less blocking and shadowing of heliostats occurs; 3) Distances between heliostats and kiln are smaller and more uniform; 4) The heliostat field may be arranged such that the CPC is on-axis with the kiln and illuminated more symmetrically; 5) Materials handling is easier because the kiln is mounted close to ground level, and no high tower is thus needed. Major disadvantages are due to the additional civil engineering work needed for preparing the trough-shaped slope and the terraces for the heliostats. Furthermore, installation and maintenance of the heliostats is rendered more difficult. Fig. 5: Solar lime plant (<1 MW th thermal input): Alternative natural or artificial arrangement of the lime kiln on a hill and the heliostat field on a south-facing slope. Indicated are the extreme positions of the heliostats and the sunrays at both winter and summer solstice for typical Mediterranean regions. In summary, the TT system is a viable technical concept for an industrial solar lime plant. For solar lime plants with power input less than about 1 MW th, it may be worth considering the alternative plant configuration with heliostats on a slope and the lime kiln on a hill. For a 20 MW th industrial solar calcination plant, cost of solar produced lime are estimated in the range $/t, about 2-3 times the current selling price of conventional lime [10]. The solar production of high purity lime might be competitive with conventional fossil fuel based calcination processes at current fuel prices. 4. Conclusions and Outlook We have developed the solar chemical reactor technology that aims at reducing CO 2 emissions in the lime and cement industry by replacing fossil fuels with solar energy. Concentrated sunlight is used as process heat for driving the calcination reaction: CaCO 3 CaO + CO 2. Our work was primarily focused on developing a scaleable solar calcination reactor for efficiently processing limestone (CaCO 3 ) particles and producing high purity quicklime (CaO) for applications in specific market sectors of the chemical and pharmaceutical industry. The question of technical feasibility was addressed by first designing, constructing, and then experimentally evaluating the performance of an indirect heating 10 kw solar multi-tube rotary reactor prototype for effecting the calcination reaction at temperatures up to 1400 K. Experimental results obtained at the small scale in PSI s high-flux solar furnace confirm that limestone particles in the range of 1-5 mm can be efficiently calcined with concentrated sunlight and that a high degree of chemical conversion can be achieved. The solar produced high purity lime is not contaminated with combustion by-products and meets the highest industrial standards. The reactor s efficiency, defined as the enthalpy of the calcination reaction

8 at room temperature (3184 kj kg -1 ) divided by the solar energy input, reached 30-35% for solar flux inputs of about 2000 kw m -2 and for quicklime production rates up to about 4 kg h -1. We are convinced that our solar lime technology has the potential of meeting an industrial standard for reactor performance. We expect the efficiency of an industrial reactor to be higher than that of the smallscale experimental version if conduction losses were reduced and sensible heat were recovered for preheating the limestone particles. Such a reactor with a thermal efficiency near 45-55% that also produces high quality quicklime would demonstrate that an Industrial Solar Lime Plant could be an economically viable path for reducing CO 2 emissions in specific market sectors of the lime industry. Acknowledgements This work has been funded in part by the Swiss Federal Office of Energy (BFE) and performed at the Solar Furnace, Paul Scherrer Institut, Villigen, Switzerland. We thank R. Palumbo, A. Steinfeld, and C. Wieckert for fruitful discussions. We also thank M. Brack for help with calibrating the flux measurement system and P. Häberling for technical assistance. References [1] Oates J.A.H. (1998), Lime and Limestone: chemistry and technology, production and uses, Weinheim: Wiley-VCH Verlag GmbH, Germany. [2] Boynton R.S. (1980), Chemistry and Technology of Lime and Limestone, New York: John Wiley & Sons Inc., USA. [3] World Business Council for Sustainable Development (WBCSD), (2002), The cement sustainable initiative our agenda for action, [4] Bonaldi E., Cella G.M., Lipinski W., Palumbo R., Steinfeld A., Wuillemin D., Meier A. (2002) CO 2 Mitigation in the Lime Industry: Replacing Fossil Fuels with Concentrated Solar Energy, In Proc. 10 th International Lime Association Congress, Washington D.C., USA, May 7-10, [5] Meier, A., Bonaldi E., Cella G.M., Lipinski W., Palumbo R., Wuillemin D. (2004), Design and Experimental Investigation of a Horizontal Rotary Reactor for the Solar Thermal Production of Lime, Energy The Int. J. 29 (5-6), [6] Meier A., Bonaldi E., Cella G.M., Lipinski W., Wuillemin D. (2003), Reactor for indirect utilization of external radiation heat for thermal or thermochemical material processes, European Patent Appl., EP , May 9, [7] Meier A., Cella G.M., (2004), Harnessing the Power of the Sun, World Cement Magazine. Volume 35, Number 8, August [8] Meier A., Bonaldi E., Cella G.M., Lipinski W. (2004), Multi-Tube Rotary Kiln for the Industrial Solar Production of Lime, J. Solar Energy Eng. Submitted for publication [9] Welford W.T., Winston R. (1989), High collection nonimaging optics, San Diego, CA: Academic Press. [10] Meier A., Gremaud N., Steinfeld A. (2004), Economic Evaluation of the Industrial Solar Production of Lime, Energy Conv. & Mgmt. In press. [11] Romero M., Buck R., Pacheco J.E. (2002), An Update on Solar Central Receiver Systems, Projects, and Technologies, J. Solar Energy Eng. 124,