BIOMASS GASIFICATION SYSTEM AND METHOD, FOR THE PRODUCTION OF COMBUSTIBLE GAS.

DESCRIPTION

The present invention pertains to the field of the gasification of solid combustibles (biomasses) or of gas generators for the production of combustible gas (syngas) suitable for supplying electrical and/or thermal power generating plants.

Gasifiers are boilers in which a solid fuel undergoes a controlled combustion in oxygen deficiency, which provides sufficient energy to trigger a pyrolysis process. In the pyrolysis the tars contained in the dry substance of the solid fuel reach the boiling point, releasing hydrocarbons and combustible gases, so as to obtain a gaseous fuel that can be exploited more easily than the solid fuel.

Large-scale fluid-bed gasifiers are known to the art, in which the material to be gasified is fed continuously. These fluid-bed gasifiers are used in coal power stations to obtain combustible gas from coal or in waste disposal plants to obtain combustible gas from waste.

Small-scale fixed-bed gasifiers are also known to the art, in which the material to be gasified is loaded in batches, by means of an accumulator head. Said fixed bed gasifiers are generally used in the agricultural and livestock farming and in the industrial fields to exploit agricultural, forestry, livestock and industrial waste to obtain a combustible gas that serves to supply the enterprise with energy and/or heat.

The combustible gases obtained with the fixed bed gasifiers, according to the prior art, are of poor quality in terms of heating value. Consequently, electricity generators supplied with said combustible gases undergo a significant reduction in power compared with similar engines supplied with a traditional fuel such at methane, etc.

The energy problem and the growing interest in solid fuels such as the biomasses raises again, in an increasingly recognised way, the need to improve the gasification systems for the production of alternative combustible gases (syngas), with special attention to the energy yield of the electricity generators supplied with them and to the containment of the emissions both during the steady-state operation and during the regulation transient

phases. In fact, by optimising the quality of the syngas produced, it will be possible to use not only traditional internal combustion engines, but also turbine systems and - in the near future - fuel batteries.

Above all, in the agricultural, livestock and forestry fields and in the industrial sector, there is a demand for small-scale gasifiers, which can represent a valid solution for:

- the use of agricultural and forestry production as a supply source of renewable fuels;

- the in-house disposal of waste from agri-foodstuffs and manufacturing industries etc., as an alternative to the waste disposal plants and/or to the conversion of waste to heat in incinerators;

- the creation of co-generation systems by the end users themselves, both in the civil field and in the industrial world; and

- avoiding the reliance on traditional fuels, such as methane gas.

Object of the present invention is to overcome the drawbacks of the prior art by providing a biomass gasification method and system that are able to produce syngas that has a good performance in terms of quality, composition and heating value, so as to maximise the yield of the electricity generator supplied thereby.

This object is achieved in accordance with the invention with the method and the system whose characteristics are set forth in independent claims 1 and 13, respectively.

Advantageous embodiments of the invention are apparent from the dependent claims.

The applicants have surprisingly discovered that the gasifier's performances in terms of quality and of composition of the syngas produced, and consequently the performance of the electricity generator connected thereto, could be improved by enriching the gasification air with oxygen (O2).

The main advantages of this invention can be summed up as follows:

- an increase in the heating value of the syngas produced which leads to an increase in the reliability of the plant, in that the engine of the electricity generator works at a more correct torque speeds (that is, the electrical yield is maximised with the same thermal power input);

- a reduction in investments: in fact, for the same installed power (expressed in kg/h of biomass and/or in kWe), the volume of gas produced is reduced with a consequent resizing of the plant as a whole, including the emission treatment part;

- the greater reactivity of the oxygen-enriched air increases the carbon yield, or rather it reduces the amount of carbon remaining in the ash and therefore lost, as well as increases the purity of the syngas in terms of tar, reducing the pollution load of the scrubbing water;

- possibility of widening the range of biomasses compatible with the process (using biomasses with a lower heating value and a greater moisture content).

The oxygen, under normal temperature and pressure conditions, is a colourless, odourless, tasteless gas, little heavier than air (density of 1.1 compared to air); it is present in the atmosphere at a concentration of 20.9% of volume. The oxygen is an active element, which, although it does not burn, maintains and activates the combustion of the combustible substances. The pure oxygen is not in itself an inflammable gas, but it possesses comburent qualities, so it acts as a support to the combustion; in fact, if it is triggered, it causes the combustion of any solid, liquid or gaseous fuel, helping the development of the flame. If the oxygen concentration in the air increases even a little, the combustion modalities are different and more accentuated.

Oxygen can be made available by:

- compressed storage (for small users);

- cryogenic storage;

- on-site production system.

Further characteristics of the invention will be made clearer by the detailed description that follows, referring to experimental tests and to a purely exemplifying and therefore non-limiting embodiment thereof, illustrated in the appended drawings, in which: Figure 1 is a diagrammatic view illustrating a gasifying device used to carry out the experimental tests according to the invention;

Figures 2-5 are histograms illustrating the results of experimental tests carried out with the gasifying device of Figure 1 ;

Figure 6 is a block diagram illustrating a complete gasification system according to the invention; and Figure 7 is a diagrammatic view in greater detail of the system of Figure 6.

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To perfect the system and the method according to the invention, experimental tests were carried out on a small-scale gasification system with a capacity of about 60 kWe for a biomass consumption of about 70 kg/h.

A fixed-bed gasifier of the downdraft type, illustrated diagrammatically in Figure 1 is used. The gasifier 100 is in the form of a tower that has in its top part an inlet 101 for feeding of the solid combustible (biomass). The gasifier tower has a narrowing, called throat 102, in which an inlet duct 103 is provided for the supply of the oxidiser (oxygen- enriched air).

Beneath the throat 102 the tower widens and has a reduction chamber in which is disposed a grate 104 which supports a packed solid coal bed. Beneath the grate 104 an outlet duct 105 is provided for the outlet of the syngas obtained, while the uncombusted carbonaceous residues and the ashes 106 of the process settle on the bottom of the tower.

The biomass passes towards the lower part of the gasifier 100 through the bed of packed solids, passing through a drying area and a pyrolysis area, until it reaches the throat 102 which forms a combustion area, where the greater part of the gasification reactions takes place.

The gasification products are intimately mixed in the combustion area, which is typically turbulent and at a high temperature. The cracking of the tars 106, which pass through the grate 104 and settle on the bottom of the tower is favoured in the combustion area. Beneath the combustion area a reduction area is formed and the combustible gas generated passes beneath the grate 104 and leaves through the outlet duct 105 to be fed towards an internal combustion engine of an electrical generator (not shown).

The gasifier 100 has a compressor (not shown) that sucks in the air from the outside and introduces it, through the inlet duct 103, into the combustion area upstream from the "reduction bed". According to the invention, the oxygen pre-mixed with the treatment air is sent into the same inlet 103 provided in the original system.

In order to measure the flow of air sucked in by the engine, a special manifold was created through which the air, previously enriched with oxygen, freely flows. The manifold was connected to a system created specially to measure the flows of the air and

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of the oxygen. The regulation of the oxygen flow was obtained by means of a line integrated into the control panel of the air supply system.

At this point various experimental tests were carried out, measuring the higher heating value (HHV) of the gas output by the gasifier according to the percentage increase of oxygen in the air fed into the gasifier's combustion area. Figure 2 shows a histogram plotting the test results. From the histogram of Figure 2 it can be seen that a percentage increase in the oxygen corresponds to a larger percentage increase in HHV (higher heating value).

Next, the KW of electricity produced by the electrical generator were measured according to the percentage increase of the oxygen in the air fed into the gasifier's combustion area. Figure 3 shows a histogram plotting the results of these tests. From the histogram of Figure 3 it can be seen that a percentage increase in the oxygen corresponds to a larger percentage increase in the kW of electricity produced by the system.

In the diagram of Figure 4 it can be seen that, if the oxygen enrichment exceeds 30% (compared with air), a reversal of the trend is seen as far as the electrical power produced is concerned, in that a too rich gas cannot be completely "burned" by engine of the electrical generator (flooding phenomenon). In this case the engine is no longer "carburized" and therefore goes outside the optimal torque speeds; as a result, there is a decrease in the kW of electricity generated.

The composition of the syngas obtained was also measured according to the percentage increase of oxygen in the air fed into the gasifier's combustion area. Figure 5 shows a histogram plotting the results of these tests. From the histogram of Figure 5 it can be seen that a percentage increase in the oxygen corresponds to an overall increase in the CO and in the CH ₄ compared with the H2 contained in the syngas.

This significant variation and difference confirms the fact that the improvement in the composition is not limited to a minor dilution of the syngas due to the fact that a larger amount of oxygen (O2) leads to smaller amount of nitrogen (N2) present in the gasification air, but to the fact that a more active comburent gas (that is the oxygen O2) allows a greater conversion of the carbon from the solid (contained in the biomass) to the syngas. Therefore, compared with a greater "carbon yield", the amount of fuel remaining

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in the ashes and thus lost (because it is not converted), as well as the amount of tars accumulated and thus lost in the syngas filtration system will be smaller.

From these experimental tests the following typical process data have been identified: - flow rate of oxygen in terms of biomass fed to the gasifier: up to 0.5 kg of O2 per kg of biomass treated, preferably from 0.05 kg of O2 to 0.25 kg O2 per kg of biomass treated, with an optimal value of 0.1 kg of O2 per kg of biomass treated;

- oxygen flow rate in terms of kWe size of the gasifier: up to 0.5 kg of O2 per KWe installed, preferably from 0.05 kg of O2 to 0.25 kg O2 per kWe installed, with an optimal value of 0.1 kg of O2 per kWe installed;

- oxygen concentration in the enriched air (air O2 = 20.9%): up to 100%, preferably between 23% and 30% equal to a 10% to 60% increase compared with the O2 contained in air alone, with an optimal value of 24% oxygen concentration in the oxygen-enriched air.

As can be seen from the graphs shown, the outcome of the experimental tests was positive in that:

- the higher heating value (HHV) of the syngas produced was increased;

- the electrical yield of the electrical generator was increased: the greater heating value of the syngas allowed the engine to work at a higher-performance torque speed and with an optimal running curve, reducing the maintenance interventions;

- the "carbon yield" was increased: the greater reactivity of the comburent (oxygen- enriched air) favoured a more complete extraction of the carbon from the biomass, thus reducing the fuel lost in the ashes; - the greater concentration of the syngas facilitates the cleaning (scrubbing + filtration) in that it reduces the volume, the heating value being equal;

- the greater reactivity of the comburent (oxygen-enriched air) prevents the formation of vapours of complex organic compounds (tars);

- the possibility of having available a further process parameter such as the percentage of O2 in the comburent makes it possible to use biomasses with a different heating value (% residual moisture) in a more controlled and efficient manner;

- the availability of a source of oxygen also allows its possible use in the form of O2 and/or of O3 (ozone) for the treatment/regeneration of the scrubbing water in a closed circuit (in-line removal of the pollutants in terms of BOD and of COD).

Figure 6 shows a gasification system according to the invention, indicated as a whole with reference numeral 1.

The system 1 comprises a gasifier 10 into which the biomass and a stream of oxygen- enriched air coming from an oxygen supply system 2 are fed. A combustible gas containing impurities leaves the gasifier 10 and is cleaned and optimised by means of a gas optimisation system 3, which filters, cools and dehumidifies the gas.

The residues of the gasification process produced in the gasifier 10 and filtered in the gas optimisation system 3 are extracted, by means of an auxiliary water circuit, and conveyed to an auxiliary water system 4, which separates the residues and cools the water. The oxygen produced by the oxygen supply system 2 can be used directly for the removal of the pollutants in the auxiliary water system 4. Additionally or alternatively, the oxygen produced by the oxygen supply system 2 can be converted into ozone O3 by means of an ozoniser 9, and then the ozone is sent to the auxiliary water system 4.

The optimised gas leaving the gas optimisation system 3 is sent to an electrical generator 5 for the generation of electrical power, which in turn is sent to the company users 7 or to the electrical main 8.

With reference to Figure 7, the gasifier 10 comprises a tower consisting, starting from the top, of an accumulation chamber 11, a drying and pyrolysis hopper 12, a reactor 13 and a residue collecting hopper 14.

The accumulation chamber 11 is delimited by two motorised shutters l la and 1 Ib, which open and close to allow the sequential loading into and discharging from the accumulation chamber 11 of batches of biomass.

The loading system operates completely automatically. A volume sensor on the hopper 12 opens the respective bottom discharge shutter l ib of the accumulation chamber placed above the hopper 12, carrying out the filling thereof. The discharge shutter 1 Ib is then closed again and its limit switch sets off a timed loading belt and at the same time opens the upper loading shutter l la of the accumulation chamber 11. On completion of timed loading, the upper loading shutter 1 Ia is closed again.

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The hopper 12 contains the amount of biomass necessary for the correct operation of the gasifier 10. The biomass is brought to temperature in the hopper 12 by two simultaneous reactions: combustion and pyrolysis. The controlled combustion in oxygen deficiency of the biomass provides sufficient energy to trigger the pyrolysis process, that is, the overheating of the biomass to about 600°C in hypoaerobic conditions. In the pyrolysis, reaction that is triggered starting from 150°C, the tars contained in the dry substance of the biomass begin to boil, releasing hydrocarbons and combustible gases.

Between the drying and pyrolysis hopper 12 and the reactor 13 there is a throat 13a. The reactor 13, on the other hand, is separated from the residue collecting hopper 14 by a grate that supports a controlled-thickness layer of coal.

The reactor 13 is the core of the system. The reaction bed, consisting of the layer of coal, is passed through by the gases released in the hopper 12 above. Steam, coal and the other hydrocarbons present react at temperature with each other, forming a gas (called "producer gas") with a high content of hydrogen (16-18% of H2), carbon monoxide (16- 18% of CO) and a very low methane (CH ₄ ) content, thus extremely clean and volatile, whose heating value is between 1,000 and 1,300 Kcal per Nm^, according to the type of biomass and to its moisture content.

The tars; the unburnt carbonaceous residue and the ashes, that is the solid residue of the pyrolysis reaction of the biomass, settle in the residue collecting hopper 14 and are disposed of by one of the auxiliary water circuits.

Ducts or nozzles (15, 15') are provided in the pyrolysis hopper 12 near the throat 13a (or, in any case, upstream of the reactor 13) to introduce the oxidant into the gasifier. According to the invention, the oxidant is composed of oxygen-enriched air.

For this purpose the gasification system 1 has an oxygen supply system 2 to supply oxygen to the gasifier 10. The oxygen supply system 2 can include, in combination or as an alternative to each other:

- an on-site oxygen production system 20;

- a cryogenic tank 21 to store the liquid oxygen and then a vaporiser 22 to transform the liquid oxygen into gaseous oxygen; - cylinders or cylinder packs 23 in which the gaseous oxygen is stored.

The oxygen coming from the oxygen supply system 2 is mixed with the air to obtain oxygen-enriched air, which is sent by means of respective delivery lines (25, 25') to the input nozzles (15, 15') of the gasifier 10. Valves (27, 27') controlled through an electrical panel 26 are provided in the delivery lines (25, 25') to vary the percentage of the oxygen contained in the oxygen-enriched air flow.

The electrical panel 26 advantageously receives signals indicating the state of the gas generated in the gasifier 10, so as to vary the percentage of oxygen to be sent to the gasifier, according to the quality of the gas to be obtained.

Beneath the throat 13a, in the upper part of the reactor 13, an outlet duct 16 is provided for the outlet of the combustible gas generated, which is sent to the optimisation system 3.

Thanks to the depression present at the outlet of the reactor 13, the gas is expelled from the gas outlet duct 16. The movement of the gas leads to the extraction of part of the solid combustion residue (ashes) and of steam. The correct working and the duration of the engine of the electric generator 5 are ensured by a good cleaning of the gas. For this purpose a series of different types of filters forming part of the optimisation system 3 is adopted.

The gas is sent to a dry cyclone 30, which carries out a first separation of the fine dusts from the gas leaving the reactor 13. Downstream of the dry cyclone 30 a water filter or scrubber 31 is provided, in which the flow of gas leaving the cyclone 30 is hit by a jet of water (closed circuit system). Because of the difference in heating value of the two fluids, the gas cools instantly to a temperature of about 40-45 ⁰ C and, mixing with the water in a turbulent movement, loses a large part of the ashes, which are decanted with the water in the next cyclone.

Downstream of the scrubber 31 there is a wet compressor 32 which creates the depression necessary for the gasifier 10 to suck in the comburent (oxygen-enriched air) and stabilises the flow of gas to the engine of the electric generator 5. A further water injector is present at the suction to cool and to clean the gas, with a subsequent cyclone 33 for the separation and the collection of the water.

A dehumidifier 34 is provided downstream of the cyclone 33. To avoid the early blockage of the filters and the transport of wet gas, a further decanter cyclone, on whose walls any vapour present in the gas condenses, is included in the line. The condensation water is automatically discharged inside the auxiliary circuit.

Downstream of the dehumidifier 34 there is a fine active filter 35 consisting of two independent lines, each with a biomass filter with a low particle size, serving to ensure the cleanness of the gas by removing the fine particulate. Inside the filter 35 there is a mechanical stirrer, controlled by an electric ratio-motor, which regenerates the filter allowing a less frequent replacement of the biomass.

Downstream of the active filter 35 there is a fine passive filter 36 which consists of two independent lines, each with a biomass filter with a finer particle size than the previous one (sawdust), serving to ensure the cleanness of the gas by removing the fine particles, but without a mechanical stirrer.

Lastly, the optimisation system 3 comprises a HEPA sleeve type safety filter 37 for the removal of any very fine particles and of silicates.

Thus a very clean gas that is not abrasive for the components of the engine of the electric generator 5 leaves the optimisation system 3. The intrinsic characteristics of the gas together with the cleaning/filtering system allow a longer engine life than that expected in biogas systems.

The water of the auxiliary water system 4 tends to heat up and to accumulate solid residues during the operation of the system 1. Therefore, water cleaning and cooling devices have been included in the auxiliary water system 4.

An outward line 40 leads to a first screen 41 disposed downstream of the residue collecting hopper 14. The screen 41 is a large mesh screen (about 400 μm) and serves to separate the slack from the water and to collect it in a suitable container where it is dried. The slack has a high heating value (about 6000 kCal/kg) and can also be re-used in the process to increase its yield.

The outward line 40 of the water circuit is connected to the residual collecting hopper 14, to the scrubber 31 and to the compressor 32. In this manner the finer solid residues and

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the organic elements in solution are filtered from the water. The filtered water coming from the screen 41 then passes through a cooling tower 42, which maintains it at the optimal temperature during the operation of the system. The cooled water is collected in a collection tank 43 from which it is taken through a return line 44 and sent again to the residual collecting hopper 14, to the scrubber 31 and to the compressor 32. The water filtration procedure, together with a periodic pH correction, is essential for obtaining an acceptable "useful life" of the circulating water.

In addition, to facilitate the disposal of the water in the collection tank 43, which is rich in pollutants (COD/BOD) as stated above, the system 1 provides for the input into the collection tank 43 of oxygen and/or of ozone produced by the oxygen supply system 2 and by the ozoniser 9.

The optimised producer gas leaving the optimisation system 3 supplies an engine 53 of the electrical generator 5. Upstream the engine 53 a primary valve 50 and a vent valve 51 are disposed, which are controlled by an automatic gas-regulation system 52 that regulates the gas flow according to the operation of the engine 53. The engine 53 is an endothermic Otto engine, which operates an alternator 54 to produce electrical power.

The heat obtained in co-generation by the cooling system of the engine 53 (roughly equal to the electric power delivered for the whole operating time of the system - 7000 h/year) can be used to integrate the adjacent thermal power plants or for other uses such as the production of cold by means of absorption coolers. It should also be noted that if the biomass is too damp, the heat of the exhaust gas could also be used to dry shredded wood.

A BTU meter 55 is provided downstream of the alternator 54 to meter the power produced by the electric generator 5. Downstream of the meter 55 there is a parallel electrical panel 56 that sends the power produced to the users 7, while the excess power produced is sent to the electrical main 8.

Numerous modifications of detail within the reach of a person skilled in the art can be made to the present embodiment of the invention without thereby departing from the scope of the invention as set forth in the appended claims.