SOLAR-THERMAL PLANT INTEGRATED WITH A FLUIDIZED BED COMBUSTOR

TECHNICAL FIELD

The present invention relates to a solar-thermal plant integrated with a fluidized bed combustor. BACKGROUND ART

As is known, the problems related to the retrieval and the cost of traditional energy sources have made research of new energy sources increasingly persistent, in particular as regards renewable energy sources.

One of the renewable sources that has generated considerable interest is that related to biomass as a combustible. The term "biomass" indicates animal or vegetal - and not fossil - substances which may be used as combustibles for the production of energy. Some sources, such as wood, do not require processing, while others such as vegetal waste material or urban wastes must be processed in a digester.

Another very interesting energy source is solar energy. In particular, in recent years, the technology designated solar-thermal technology has been the subject of exhaustive studies because of its increasingly efficient application in the production of electric energy. Although solar-thermal technology has a minimum environmental impact, it has drawbacks related to the

fact that an effective solar radiation is not always available .

DISCLOSURE OF INVENTION

It is an object of the present invention to provide a solar-thermal plant for the production of electric energy, the technical features of which are such as to ensure the production of electric energy even in the absence of an effective solar radiation.

It is an object of the present invention to provide an integrated plant for the production of electric energy as claimed in claim 1.

BRIEF DESCRIPTION OF THE DRAWING

The following example is provided by way of non- limitative illustration for a better understanding of the invention with the aid of the figure in the accompanying drawing, which is a diagrammatic view of the integrated plant according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the figure, numeral 1 indicates as a whole the integrated plant object of the present invention.

Plant 1 comprises a combustion assembly 2 for the combustion of biomass, a solar-thermal assembly 3, a recirculation circuit 4, in which a process liquid circulates, such as for instance diathermal oil or steam, and a turbine 5 connected to a current source 6.

Combustion assembly 2 comprises a storage silo 7 in which biomass is stocked after having been desiccated by means of a desiccator 8, which, as will be disclosed

later, is heated by the exhaust fumes from a fluidized bed combustor. As shown in the figure, storage silo 7 comprises a radiator 9 such as to ensure a further desiccation of the stocked biomass. Radiator 9 is fed by the cooling water of turbine 5 as will later be disclosed.

Combustion assembly 2 comprises a daily biomass tank 10 fed by storage silo 7, and a fluidized bed combustor 11, which uses the biomass from daily tank 10 as a combustible. An additive, such as for instance CaCO ₃ , present in an appropriate tank 12, is added to the biomass combustible.

As is known to the skilled in the art, the fluidized bed combustor has a series of combustion features such as to ensure low emissions and, therefore, a low environmental impact. As is known, the advantageous peculiar features of the fluidized bed combustion are related to an enormous surface for the combustion and the exchange of heat due to the turbulence generated by the fluidized bed, a good contact between comburent air and the combustible due to the intense mixing generated in the fluidized bed, a very good heat capacity of the sand bed in relation to the amount of fed combustible and an optimal combustion of the effluents in virtue of the free space above the bed in which the combustion of the gases generated during the process occurs. In particular, heavy oil, propane or natural gas may be used as a combustible for

ignition, while the sand may be of the non triturated and dry siliceous river type having an average size of the granules equivalent to 0.32 mm.

In particular, fluidized bed combustor 11 is fed by a fluidification air feeding line 13 and by a secondary air line 14. Furthermore, fluidized bed combustor 11 comprises an ash extraction system 15, and injection ignition means (not shown and disclosed in detail, as they are already known) . The biomass as combustible is provided to fluidized bed combustor 11 once the bed has reached the working temperature (650-700 ⁰ C) by the use of a backup combustible, which is provided by a dedicated feeding line. The temperature of the bed is controlled in order to ensure that the ash melting temperature is not reached. Secondary air feeding line 14 serves to ensure a complete combustion.

Combustion assembly 2 comprises a pair of cyclone separators 16 which in turn comprise a respective ash collection system 17. The exhaust fumes from fluidized bed combustor 11 are first conveyed in two cyclone separators 16 for cutting down coarse particulate, and are then passed through desiccator 8 to carry out the action of desiccation on the biomass. Finally, combustion assembly 2 comprises a disposal assembly 18 of exhaust fumes from desiccator 8. Disposal assembly 18 comprises a heat dissipater 19 to bring the temperature of the fumes to a value below 140 ⁰ C, a bag

filter system 20 for cutting down thin particulate, an extraction fan 21 and a chimney 22.

Solar-thermal assembly 3 comprises a plurality of parabolic linear concentrators 23, a recirculation line 24 for the diathermal oil or the molten salts which are heated by the action of parabolic linear concentrators 23 and a heat exchanger 25, by means of which the process fluid is heated by the heat release by the diathermal oil or the molten salts. Recirculation circuit 4 comprises a first tank 26, a second tank 27, two delivery lines 28a and 28b, each of which allows the passage of the process fluid from first tank 26 to second tank 27, a return line 29, which allows the return of the process fluid from second tank 27 to first tank 26 and a bypass line 30 which connects second tank 27 to return line 29. In particular, in two delivery lines 28a and 28b, the process liquid undergoes the heating required for the operation of the turbine. Namely, delivery line 28a passes through fluidized bed combustor 11 receiving the heat resulting from the combustion of the biomass therefrom, while delivery line 28b passes through heat exchanger 25 receiving the heat from the diathermal oil of recirculation line 24. Furthermore, each of delivery lines 28a and 28b comprises a respective flow regulator 31a and 31b in order to select the respective flow rates.

Return line 29 is connected with turbine 5 to promote the operation thereof and, accordingly, the production of energy.

Bypass line 30 comprises a bypass valve 32 and serves for the conduction of the process fluid from second tank 27 to return line 29 downstream of turbine

5, i.e. without the process fluid itself having transferred heat.

Finally, plant 1 comprises a water recirculation line 33, which, as well as passing through turbine 5 to carry out a cooling action, also passes through storage silo 7 to carry out a heating action through radiator 9 with the purpose of desiccating the biomass within storage silo 7 itself. In use, the process fluid contained within first tank 26 is conveyed in two delivery lines 28a and 28b to be respectively heated through fluidized bed combustor 11 and heat exchanger 25 of solar-thermal assembly 3. The process fluid heated thereby is conveyed within second tank 27 and subsequently, through return line 29, transferred to the steam or organic cycle turbine 5 for the production of electric energy through electric generator 6. Again through return line 29, the process fluid is returned from turbine 5 to first tank 26. Plant 1 may be used in different modes having the possibility of intervening on flow regulators 31a and 31b of delivery lines 28a and 28b.

In particular, on sunny days, solar-thermal assembly 3 may be exploited at its maximum heating power, while fluidized bed combustor 11 will be exploited to integrate the power required for achieving the maximum calorific power of the process liquid. On the other side, on cloudy days or at night, it will be possible to exploit the heating from fluidized bed combustor 11 at the maximum ensuring, even in the absence of a contribution from solar-thermal assembly 3, the maximum calorific power of the process liquid.

In other terms, it will be possible by acting on flow regulators 31a and 31b to select the process liquid flows resulting respectively from solar-thermal assembly 3 or from fluidized bed combustor 11 depending on the conditions outside the plant.

Furthermore, by acting on bypass valve 32, a part of the appropriately heated process fluid may be stocked also in first tank 26. This allows for both heat accumulation in first tank 26 to be used at times when solar radiation is not available and the reduction of the oscillations in the regulation of fluidized bed combustor 11 and the improvement of the performance .

As is apparent in the above description, plant 1 which is the object of the present invention offers the advantage of allowing the use of a solar- thermal assembly 3 for the production of electric energy, ensuring at the same time the production of energy even when the atmospherical conditions do not allow

exploitation of solar energy. This assurance of electric energy derives from the presence of fluidized bed combustor 11.