AU2010224353A1

AU2010224353A1 - Concentrated solar energy thermal processing systems

Info

Publication number: AU2010224353A1
Application number: AU2010224353A
Authority: AU
Inventors: Peter James
Original assignee: AGROTEKNIK Pty Ltd
Current assignee: AGROTEKNIK Pty Ltd
Priority date: 2009-09-25
Filing date: 2010-09-21
Publication date: 2011-04-14

Description

P/00/011 Regulation 3.2 AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "CONCENTRATED SOLAR ENERGY THERMAL PROCESSING SYSTEMS" The following statement is a full description of this invention, including the best method of performing it known to us: TITLE "CONCENTRATED SOLAR ENERGY THERMAL PROCESSING SYSTEMS" 5 FIELD OF THE INVENTION THIS INVENTION is of methods for the application of concentrated solar energy as the heat source for the thermal processing of products and/or bulk materials with heated air or other gas. 10 The invention is particularly suited for, but not limited to, use of concentrated solar energy for: a) Drying high moisture content solid bulk material fuels such as: 15 i) sugar cane bagasse; ii) lignite, also known as brown coal; iii) peat; iv) sawmill and other timber processing waste e.g. bark and trimmings, and woodchips to be used for fuel; and 20 v) any other moist biomass or solid bulk material fuel. b) Drying sludges such as sewage sludge; c) Drying harvested agricultural crops including legume fodder crops such as lucerne hay (also known as alfalfa), as well as oaten hay, and hay from other grasses, to preserve their nutritive values; 25 d) Drying any other bulk materials or products requiring drying with heated air or other gas as part of their production process that are amenable to drying by the means herein described.

2 Throughout the specification the term "drying" shall mean any useful, economic, valuable or otherwise desirable reduction in moisture content of the bulk material or product. The invention is also suited for, but not limited to, use of concentrated 5 solar energy as the heat source for other thermal processing, including, but not limited to: e) Heating; f) Roasting; g) Baking; 0 h) Cooking; i) Curing. PRIOR ART The invention includes elements of the separate prior arts of: j) Concentrating solar energy collectors such as parabolic trough collectors 5 being used to heat "heat transfer fluids", including water and proprietary heat transfer fluids. That technology is used in solar thermal power stations, where the water is boiled to generate high pressure steam; or the heat transfer fluid is used to heat water to generate steam. The steam is then used in steam turbines driving electricity generators to generate electrical power. 20 k) The transfer of heat energy from a hot fluid (for example, a heat transfer fluid or steam, including condensing steam) to a gas such as ambient or partially heated air, or particular gas or gas mixture, or waste gas such as gaseous waste product from combustion of fuel (for example flue gas from a fuel fired boiler or heat recovery boiler) by means of a liquid to gas or condensing steam to gas heat 25 exchanger.

3 I) The use of the heated air or gas to dry or otherwise thermally process a solid bulk material fuel, other bulk material or other product to improve its value as an economic commodity such as for example increasing its calorific value by reduction of its moisture content, including various cooking and curing processes. 5 US Patent No 6922908 (Raudales) entitled "Vegetable Product Drying" describes a process utilising solar energy to dry coffee beans. However, in that case: i) The solar energy collection is not concentrated and therefore the operating temperature, e.g. below 100 0 C, is so significantly lower that it is 10 substantively different to the present invention; ii) The solar energy collectors are fixed in their orientation with respect to the Earth and do not track the apparent movement of the sun across the sky; iii) The drying is performed as a sequential batch process rather than .5 as a continuous process; and iv) The heat transfer capacity of the drying process described in US Patent No 6922908 would be of the order of 25kW, being three orders of magnitude (10- or 1/1000th) smaller in capacity than the scale envisaged by the present invention. 20 SUMMARY OF THE INVENTION m) It is an object of the present invention to use one or more concentrating solar thermal energy collectors as the source of thermal energy for processing suitable bulk materials and/or other products, in particular to achieve one or more of the following preferred outcomes: 25 i) To improve the gross calorific value of otherwise moist fuels by drying those fuels prior to their combustion.

4 ii) To dehydrate or dry agricultural fodder crops such as lucerne hay (alfalfa) and the like. iii) To dehydrate or dry other bulk materials. n) The thermal energy is preferably concentrated by the concentrating 5 collector, for example by a set of solar tracking parabolic trough collectors, and transferred to a heat transfer fluid flowing through absorber tubes that are preferably approximately coaxial with the longitudinal focal axes of the parabolic troughs. o) The heat transfer fluid is preferably heated in this way to a temperature above 2000C, and more preferably above 4000C or more as required, and 10 preferably optimally at as high a temperature as economical and practicable for the particular application. p) The heated fluid is preferably then conveyed in thermally insulated piping to a heat exchanger wherein the heat of the heat transfer fluid is transferred to a gas or air stream bringing the temperature of that stream to above 180*C, and 5 preferably above 380*C or more as required, and preferably optimally to as high a temperature as economical and practicable for the particular application. q) The heated gas or air is then preferably employed in a processing system, such as: i) A direct contact gas to solid drying system, as for example: 20 (1) A rotary drum dryer; (2) A pneumatic conveying drying-in-transit system; (3) A flash drying system; (4) A fluidised bed drying system; (5) Any combination of the above.

5 to dry a given bulk material to improve its economic value. ii) A heating system for heating liquid or solid material or an atmosphere in a space; iii) A roasting system for thermal processing of foodstuff, animal feed, 5 minerals, chemical coatings and such like industrial applications; iv) A baking system for thermal processing of foodstuff, animal feed, minerals, chemical coatings and such like industrial applications; v) A cooking system for thermal processing of foodstuff, animal feed, minerals, chemical coatings and such like industrial applications; 0 vi) A curing system for thermal processing of materials that require heat to bring about a reaction to complete the material formation process and such like industrial applications. r) The heat transfer fluid is then preferably recycled back to the concentrating collectors for reheating by the sun's energy. 5 s) Optionally, one or more insulated storage tanks may be interposed in the heat transfer fluid circuit to store hot and cold heat transfer fluid so that the hours of drying operation may be extended beyond the hours of daylight, even to continuous drying operations 24 hours per day. Alternatively heat storage may be in the form of molten salts or steam. 20 BRIEF DESCRIPTION OF THE DRAWINGS To enable the invention to be fully understood, preferred embodiments will now be described with reference to the accompanying drawings, in which: FIG. 1 is a schematic drawing of a first embodiment of the present invention; and 6 FIGS. 2 to 5 are similar views of alternative embodiments of the present invention. NB: The annotations in FIGS. 4 and 5 are by way of example only, and are not limiting to the scope of the present invention. 5 a) FIG 1 Basic System i) The heat transfer fluid is pumped by the circulating pump (4) through insulated piping (6) to the concentrating solar thermal collector (2) absorber tubes. ii) Solar energy (1) is concentrated by highly reflective parabolic trough 10 mirrors (concentrating solar thermal collector (2)) to shine onto the absorber tubes that are carrying the flow of heat transfer fluid. iii) The hot heat transfer fluid is then conveyed by insulated piping (6) to the heat exchanger (3) that transfers the heat to an air or other gas stream, thereby raising its temperature. The air or gas stream may be blown 15 through the heat exchanger by a forced draft fan (9) and/or may be drawn through it by an induced draft fan (10) at the other end of the drying system as shown. iv) Following its exit from the heat exchanger (3), the now cooler heat transfer fluid is carried in insulated piping (6) that discharges into or is connected 20 with the buffer tank (5) which provides capacity in the system for thermal expansion and contraction of the heat transfer fluid and for minor leakage and make-up. v) The hot air or gas stream enters the dryer (7) co-currently with the moist bulk material feed (11). 25 vi) Following exit from the dryer, the now dried solid bulk material is separated from the surrounding moist gas in a separation system (8).

7 vii) Following separation, the dried bulk material (13) proceeds to its end use, storage, combustion or other processing. viii) Preferably, but not necessarily, following separation, the moist gas is exhausted from the system by an induced draft fan (10) 5 b) FIG 2 Vapour Recycle System. i) The heat transfer fluid is pumped by the circulating pump (4) through insulated piping (6) to the concentrating solar thermal collector (2) absorber tubes. ii) Solar energy (1) is concentrated by highly reflective parabolic trough 10 mirrors (concentrating solar thermal collector (2)) to shine onto the absorber tubes that are carrying the flow of heat transfer fluid. iii) The hot heat transfer fluid is then conveyed in insulated piping (6) to the heat exchanger (3) that transfers the heat to the gas stream, thereby raising its temperature. The gas stream may be blown through the heat exchanger by the [5 induced draft fan (10) and drawn through the drying and separation systems by the same induced draft fan (10) at the other end of the drying system as shown. A certain amount of the gas is bled off through a control damper (14) after the induced draft fan (10) and an equivalent volume of air is bled in before the induced draft fan (10) through a control damper (15). 20 iv) Following its exit from the heat exchanger (3), the now cooler heat transfer fluid is carried in insulated piping (6) that discharges into or is connected with the buffer tank (5) which provides capacity in the system for thermal expansion and contraction of the heat transfer fluid and for minor leakage and make-up. 25 v) The hot gas stream enters the dryer (7) co-currently with the moist bulk material feed (11).

8 vi) Following exit from the dryer, the now dried bulk material is separated from the surrounding moist gas in the separation system (8). vii) Following separation, the dried bulk material (13) proceeds to its end use, storage, combustion or other processing. 5 viii) Following separation, the bled off moist gas is exhausted from the system via the damper (14). c) FIG 3 Boiler Flue Gas System i) The heat transfer fluid is pumped by the circulating pump (4) through insulated piping (6) to the concentrating solar thermal collector (2) absorber 10 tubes. ii) Solar energy is concentrated by highly reflective parabolic trough mirrors (concentrating solar thermal collector (2)) to shine onto the absorber tubes that are carrying the flow of heat transfer fluid. iii) The hot heat transfer fluid is then conveyed by insulated piping (6) to the [5 heat exchanger (3) that transfers the heat to the gas stream, thereby raising its temperature. The gas stream may be blown through the heat exchanger by induced draft fan (9) serving the boiler (17) and drawn through the drying and separation systems by another induced draft fan (10) at the other end of the drying system as shown. 20 iv) Following its exit from the heat exchanger (3), the now cooler heat transfer fluid is carried in insulated piping (6) that discharges into or is connected with the buffer tank (5) which provides capacity in the system for thermal expansion and contraction of the heat transfer fluid and for minor leakage and make-up. 25 v) The hot gas stream enters the dryer (7) co-currently with the moist bulk material feed (11), in this case the fuel for the boiler.

9 vi) Following exit from the dryer, the now dried bulk material is separated from the surrounding moist gas in the separation system (8). vii) Following separation, the dried bulk material (13) proceeds to its end use, storage, combustion or other processing. In this case it is shown going to the 5 fuel storage facility (16) and on to the boiler (17) combustion furnace. viii) Following separation, the moist gas is exhausted from the system by an induced draft fan (10). ix) Also shown adjacent to the boiler (17) is a power generation system comprising piping (23), steam turbine (19), electric power generator (20), power 0 transmission (21), exhaust steam condensing plant (22) and boiler feedwater pump (18). d) FIGS. 4 and 5: Separation Systems i) These drawings illustrate the separation systems which may be employed as the separation systems (8) in the embodiments of FIGS. 1 to 3, together with 5 moist bulk material feeds (11). DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS a) BASIC SYSTEM i) It is an object of the present invention to use extended sets of concentrating solar thermal energy collectors (for example parabolic trough 20 collectors) to heat a heat transfer fluid. ii) The collectors are arranged so that their focal axis (i.e. the absorber tubes' long axes) is oriented in the north - south direction and the collectors' reflecting surfaces are arranged with angular positioning drives so that they track the sun as it appears to move across the sky from east to west during the day, in 25 order to maximise the intensity and quantity of the solar energy collected by 10 keeping the reflected rays of the sun focused on the absorber tubes that are coaxial with the parabolic troughs' longitudinal focal axis. iii) The absorber tubes are specially designed and constructed to absorb the reflected concentrated solar radiation to heat the heat transfer fluid and to have 5 a high collection efficiency and low heat loss. iv) The heat in the hot heat transfer fluid is transferred via a liquid to gas extended surface heat exchanger e.g. finned tube heat exchanger to heat the thermal processing gas. v) Where water/steam is used as the heat transfer fluid, the heat transfer to 0 the gas is preferably effected by condensing steam on the inside of the finned tubes. vi) The air or gas stream preferably enters the heat exchanger from the opposite end to the hot heat transfer fluid, that is in a 'counter current' direction, in order to maximise the temperature rise of the air or gas. 5 vii) The bulk material is fed into the dryer by a rotary valve or other feeding mechanism that serves to introduce the bulk material into the gas stream while minimising the leakage of hot gas out of the dryer or cold air into the dryer. viii) The bulk material passes with the gas from one end of the dryer to the other, within a transit time adequate for the drying process to occur by the 20 transfer of heat from the gas to the bulk material and for the concurrent mass and heat transfer of evaporated moisture and its latent heat of evaporation from the bulk material into the surrounding gas. ix) The separation system may include but not be limited to drop out boxes, cyclone separators, filters, electrostatic precipitators and the like. 25 x) The final moist gas exhaust may be further cleaned by a scrubber or other particulate entrainment arrestor system.

II xi) The heat transfer fluid is circulated within insulated piping in those sections of piping where heat transfer is not desired, to minimise the loss of heat to the local environment. xii) Depending on the heat transfer fluid employed, the hot heat transfer fluid 5 temperature can be between 3000C and 4000C or higher with presently available technology. b) VAPOUR RECYCLE SYSTEM i) The circulating gas comprises a mixture of air and water vapour. ii) A certain amount of the gas is bled off through a control damper after the 0 induced draft fan and an equivalent volume of air is bled in before the induced draft fan through another control damper. This is to maintain consistent temperature and humidity profiles in the gas recycle circuit, to optimise the drying process and to prevent any of the water vapour in the gas from condensing within the system. 5 iii) The vapour recycle system minimises the heat required to heat ambient air to the dryer exhaust temperature (approximately 80*C). This heat is wasted. c) BOILER FLUE GAS SYSTEM i) The gas comprises the waste gas from a combustion process for example a biomass, bagasse or lignite (brown coal) fuelled boiler (typically a mixture of 20 nitrogen, water vapour, carbon dioxide and oxygen at around 1200C to 1600C depending on the extent of heat recovery in the boiler system). ii) Because the waste gas is already at a temperature above the dryer exhaust temperature (approximately 800C), there is heat in it that is useful for drying and there is no need to use or to heat ambient air except in those cases 25 where there is insufficient waste gas for the drying duty. The use of waste gas optimises the energy efficiency of the thermal processing system.

12 6) BASIC PROCESS ADVANTAGES a) The advantages of the concentrated solar energy thermal processing system include: i) Renewable energy 5 The source of the thermal energy is a 'renewable energy' and does not in itself contribute to the environmental problems of "greenhouse gas" emissions or "global warming". ii) Low cost energy The solar thermal energy is free and does not incur harvesting, transport or 10 transmission costs or infrastructure to bring it to the collection site. iii) Higher temperature than flat plate collectors Because the solar energy is concentrated, it is transferred at higher temperatures than if collected by a non-concentrating 'flat plate collector' or fixed evacuated tube collector array; this higher temperature enables 15 greater economies in the subsequent heat exchange and drying systems that tend to make the overall system more economically feasible. iv) Higher combustion efficiency of dried fuel When used to dry an otherwise moisture laden fuel such as sugar cane bagasse (50% moisture) or lignite / brown coal (60% moisture), it enables 20 fuel to be dried to such an extent that its combustion efficiency is considerably increased and so produces more steam for a given quantity of combustible matter in the originally moist fuel. v) Increased power generation output The wider implication of the increased combustion efficiency is that sugar 25 mill power cogeneration schemes and brown coal fired power stations can 13 generate more electrical power per unit mass of original combustible matter in their fuel. This means increased electrical power output from these types of power stations from the same fuel resource when using this invention. 5 The increase in electrical power output e.g. MWh/tonne of originally moist fuel, depends on the degree of drying. In a sugar mill cogeneration scheme, the potential increase in total electrical energy (GWh) produced could be up to 22% more from the same crop of sugar cane; after adjusting for power used within the sugar mill site 10 itself, the increase in power available for export or dispatch to the general electrical distribution power grid could be up to 29% more based on a typical sugar mill cogeneration scheme as recently established in the Australian cane sugar industry. The expected practicable and economic increase in dispatchable power is of the order of 22% more. .5 Similar increases in efficiency are expected with lignite / brown coal since it has a similar or higher original moisture content as sugar cane bagasse. vi) Reduced greenhouse gas emissions In the case of sugar mill cogeneration, this invention means increased renewable energy output with no net greenhouse gas emissions; in the 20 case of lignite / brown coal power stations this means more electrical energy output per tonne of carbon dioxide (a "greenhouse gas") emitted or conversely less "greenhouse gas" emitted per unit of electrical energy output. vii) Higher thermodynamic cycle efficiency than direct concentrated solar 25 thermal power It is thermodynamically more effective to use the concentrated solar energy to dry moist fuel rather than use the concentrated solar energy directly to 14 generate steam and then use that steam to generate electric power directly by means of a steam turbine generator because the dried fuel can be burnt at a much higher temperature (from 1000 0 C to over 13000C depending on dryness) and can generate steam at substantially higher corresponding 5 pressures and temperatures. The maximum theoretical efficiency of a "totally reversible engine" is known as its Carnot efficiency and is given by the following equation, where TL and TH are the lower and higher absolute temperatures of the cycle respectively: T OfTH - TLJXI1 th,Carno1 = X 100 = H TH( i 0 It is clear that for any given TL for instance the temperature of exhaust steam condensing from a condensing steam turbine generator, the Carnot cycle efficiency will be higher for a higher TH and so in general higher efficiency conversion of solar energy to electrical energy is achievable using solar thermal energy to dry fuel and thereby increase the efficiency of 5 the fuel to power energy conversion rather than to generate steam and power directly at lower temperatures, given similar steam to power generation technology. A 'totally reversible engine' i.e. an ideally efficient work producing device, operating between 3500C (absolute temperature 623K) and 500C (323K) has a Carnot efficiency of 48% whereas one 20 operating between 550*C (823K) and 500C (323K) has a Carnot efficiency of 61%, an increase in theoretical power output (per unit of heat energy input) of 27%. A similar percentage increase applies with actual, less than ideal, steam turbine generators. 7) POTENTIAL PROCESS MODIFICATIONS AND THEIR ADVANTAGES 25 a) Because oil based heat transfer fluids have relatively higher vapour pressures at high temperatures, it is necessary to blanket any oil based heat transfer fluids in tanks with an inert gas such as nitrogen to reduce the risk of fire or explosion.

15 b) It is of benefit to use any available sources of waste heat laden gas to be the heated drying medium as this will increase the drying effect for a given solar energy collection surface area, or alternatively, will minimise the solar energy collection area necessary for a given drying effect. 5 c) Following separation and as shown in FIG 2 Vapour recycle system, some of the gas stream can in particular applications be recycled back through the heating and drying cycle in the form of superheated water vapour, which has good drying capability due to its relatively high heat capacity. This would be of most benefit in those circumstances where 0 there is not already a source of waste heat laden gas. d) It is possible to incorporate a certain amount of heat storage in the heat transfer fluid system and there are prior art technologies and other proposed methods to achieve greater or lesser amounts of storage. These technologies include storing: 5 i) Heat in the form of steam, in the case of water/steam as the heat transfer fluid; ii) Heat in molten inorganic salt mixtures indirectly heated by the heat transfer fluid; and iii) Hot heat transfer fluid although this can pose considerable and 20 probably unacceptable risk where the heat transfer fluid is flammable and the quantity of storage required is large. e) Other concentrating solar thermal energy collector technologies apart from parabolic trough collectors can be used. These would include but not be limited to: 25 i) Concentrating linear Fresnel reflectors (for example as built by German company Novatec Biosol AG); 16 ii) Compact linear Fresnel reflectors (for example as built by United States corporation Ausra Incorporated). The skilled addressee is to be aware that the preferred embodiments described and illustrated are by way of examples only; and 5 various changes and modifications may be made thereto without departing from the present invention. 0 5 20

Claims

1. A method for the application of one or more concentrating solar thermal energy collectors as the source of thermal energy for processing suitable bulk 5 materials and/or other products, including the steps of: a) concentrating solar thermal energy in the one or more collectors and transferring the thermal energy to a heat transfer fluid flowing though at least one absorber tube operably located adjacent to the one or more collectors; b) heating the heat transfer fluid to a temperature above 200*C; 0 c) conveying the heat transfer fluid in at least one insulated pipe to at least one heat exchanger, wherein the thermal energy in the heat transfer fluid is transferred to at least one gas or air stream at a temperature above 180*C; and d) employing the thermal energy in the hot or at least heated gas or air stream in a processing system for the bulk materials and/or other products. 5

2. The method as claimed in claim 1, wherein: the one or more collectors comprises a set of solar tracking parabolic trough collectors; and the, or each, absorber tube is located approximately coaxial with the respective longitudinal focal axis of a parabolic trough of the parabolic trough 20 collector with which the absorber tube is operably located.

3. The method as claimed in claim 1 or claim 2, wherein: the heat transfer fluid is heated to or above 400*C or more as required for processing of the bulk materials and/or other products, and is heated optimally as high as economical and practicable for the particular application. 18

4. The method as claimed in any one of claims 1 to 3, wherein: the, or each, gas or air stream is heated above 3800C or more as required for the processing of the bulk materials and/or other products, and is heated optimally as high as economical and practicable for the particular 5 application.

5. The method as claimed in any one of claims 1 to 4, wherein: the processing system is operable for thermal processing of the bulk materials and/or other products, such as heating; roasting; baking; cooking and/or curing. 10

6. The method as claimed in claim 5, wherein: the, or each, gas or air stream is employed in a processing system, such as: i) a direct contact gas to solid drying system, such as a rotary drum dryer; 15 ii) a pneumatic conveying drying-in-transit system; iii) a flash drying system; and/or iv) a fluidized bed drying system; and is operable to dry a bulk material of suitable particle size(s) to improve its economic value. 20

7. The method as claimed in any one of claims 1 to 6, including the further step of: e) recycling the heat transfer fluid back to the one or more collectors for reheating to a temperature above 2000C in the, or each, absorber tube. 19

8. The method as claimed in any one of claims 1 to 7, including the further step of: interposing at least one insulated storage tank, or molten salts or steam, in a circuit of the heat transfer fluid to store hot and cold heat transfer fluid, so that 5 the operational hours of the processing system are extended beyond the hours of daylight, to enable up to continuous operation of the processing system for 24 hours per day.

9. A method for the thermal processing of bulk materials and/or other products wherein the thermal energy is provided by the method as claimed in any 10 one of claims 1 to 8.

10. A thermal processing system for bulk materials and/or other products wherein the thermal energy is provided by the method of any one of claims 1 to 8.