EP3188805A1

EP3188805A1 - Prevention of combustion in storage silos

Info

Publication number: EP3188805A1
Application number: EP15757500.2A
Authority: EP
Inventors: Neil Simpson
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2014-09-04
Filing date: 2015-09-03
Publication date: 2017-07-12
Also published as: GB201415623D0; WO2016034663A1

Abstract

A storage silo for storing flammable material. The silo comprises a furnace configured to receive the flammable material, which material is combusted to produce a flue gas; and flue gas feed means for feeding the flue gas to the storage silo to thereby create an inert atmosphere therein such that combustion of the flammable material is prevented.

Description

Prevention of combustion in Storage Silos

The present invention relates to apparatus and methods for preventing unwanted combustion in silos for storing flammable materials. In particular, the invention relates to preventing fires in silos in which biomass fuels are stored prior to combustion.

The burning of biomass as a fuel in power stations has become more prevalent in recent years and the volume of biomass used and stored at power stations has correspondingly increased. In general terms, biomass comprises plant matter, this may be in the form of wood, a fluff material or pellets formed from material which has been shredded and compacted. The biomass material is stored in large silos to keep the material dry and reduce loss of the material prior to being conveyed for use in the boilers. Such silos can range from hundreds to thousands of cubic meters in volume. Biomass dust may be generated from the biomass during storage and handling. The dust is drawn off in an air stream which is filtered to remove the dust. The dust is then pneumatically conveyed to dust silos where it is stored prior to being burnt in a furnace.

Fires may occur in both biomass storage silos and dust storage silos, and the factors which cause fires in both cases are broadly the same. Fires in biomass storage silos can come about as a result of bacterial and fungal activity which generate heat and produce methane, carbon monoxide and carbon dioxide. Heat accumulates to over 50°C leading to thermal oxidation of the biomass. As the temperature continues to rise, dry matter is lost, fuel quality deteriorates and eventually the biomass ignites. Although water is the best medium for removing heat from smouldering fires, the use of water sprinklers would cause damage to the silos and cause wood dust to set, resulting in large costs and downtime.

It is known in the art that smouldering fires can be controlled and extinguished by providing an inert atmosphere within the silo. This is commonly achieved by providing a carbon dioxide and/or nitrogen atmosphere within the silo, although other gases such as argon can also be used. Such an atmosphere is typically provided by purchasing large volumes of inert gases such as argon, carbon dioxide and/or nitrogen for use as an inert atmosphere. Carbon dioxide and nitrogen are typically stored in a liquid gas store, and are then injected into the silo as necessary. This practice involves the use of a large volume of inert gas, and since the gas has to be obtained specifically for this purpose, this can be expensive and means a large area of a power station has to be set aside for the storage of the gas. Additionally, care has to be taken to ensure that stocks are constantly replenished as the gas is used to create an inert atmosphere.

Alternatively, the inert gas can be obtained from a Pressure Sawing Adsorption (PSA) unit or a membrane filter. However, there are problems inherent with both these methods as a PSA unit will need to be purpose built and needs high pressures and expert handling resulting in increased cost. The purity of gases obtained from a membrane filter will be lower than those obtained from liquid gas stores or a PSA unit. There is therefore a need to provide improved systems for preventing unwanted combustion and fires in silos for storing flammable materials.

Thus, in a first aspect, there is provided an apparatus comprising: - a storage silo for storing flammable material;

- a furnace configured to receive the flammable material, which material is combusted to produce a flue gas; and

flue gas feed means for feeding the flue gas to the storage silo to thereby create an inert atmosphere therein such that combustion of the flammable material is prevented.

The phrase "inert gas atmosphere" can mean an atmosphere in which the concentration of oxygen is too low for combustion of the flammable material to take place. As such, fires are prevented. Advantageously, the apparatus of the first aspect effectively feeds the flue gas that is produced as a result of combustion in the furnace to the storage silo in order to inert the flammable material, and so does not need large quantities of inert gases (e.g.

nitrogen or carbon dioxide) to be kept on site to provide an inert atmosphere in a silo. Preferably, the means for feeding the flue gas to the storage silo comprises a conduit, a first end of which is configured to be operably connected to the storage silo, and a second end of which is configured to be operably connected to the furnace, more preferably a flue gas outlet thereof. The first end of the conduit may be connected to at least adjacent the top or the bottom of the silo. Alternatively, the first end may be connected to a region of the storage silo between its top and bottom. Preferably, the apparatus is configured to treat the flue gas before it is fed to the storage silo. For example, the apparatus preferably comprises means for removing water vapour from the flue gas before it is fed to the silo. Preferably, the means for removing water vapour from the flue gas is disposed within the flue gas feed means. It is preferred that the means for removing water vapour comprises a condenser.

Advantageously, provision of the condenser effectively reduces the water content in the flue gas, thereby rendering it more amenable for producing the inert atmosphere in the silo. Preferably, the condenser is a water-cooled condenser comprising a cold water inlet, an outlet, and a passageway therebetween, wherein, in use, the flue gas flows passed the passageway such that water vapour condenses thereon. Preferably, the condenser comprises a drain through which the condensed water removed from the flue gas flows. Preferably, the water outlet of the condenser is operably connected to a boiler which is configured to generate steam or low grade hot water.

Preferably, the apparatus comprises means for extinguishing any flame or spark which may be present in the flue gas. Preferably, the means for extinguishing a flame or spark is disposed within the flue gas feed means, more preferably downstream of the means for removing water from the flue gas. Preferably, the means for extinguishing a flame/spark comprises a flame trap. Advantageously, provision of the flame trap reduces the risk of a fire starting in the silo.

The flammable material acts as a fuel source. The flammable material may comprise any material that can be combusted in the furnace to produce heat and energy.

However, the flammable material preferably comprises a biomass substance, for example plant material. The flammable material may be in the form of pellets and/or dust. Preferably, the apparatus comprises means for feeding the flammable material from the storage silo to the furnace where it is combusted. The flammable material feed means may comprise a mechanical or pneumatic conveyor, an auger, or the like.

Preferably, the apparatus comprises an air pump, which is configured to pneumatically deliver or inject the flammable material into the furnace. Preferably, the air pump uses atmospheric gas to deliver the flammable material into the furnace. In one embodiment, the air pump is configured to control the amount of atmospheric gas injected into the furnace to allow complete combustion of the injected flammable material under stoichiometric conditions. However, in a preferred embodiment, the air pump is configured to deliver an excess of atmospheric gas into the furnace for combustion purposes. Preferably, an excess of atmospheric gas is defined as the amount of atmospheric gas that is needed to result in at least i%, 2%, 3%, 4%, 5%, 10%, 15% or 20% more oxygen being present in the furnace than would be necessary for complete combustion to occur under stoichiometic conditions. The furnace may be configured to heat or melt a furnace load. However, in a preferred embodiment, the apparatus comprises a boiler which is disposed either inside the furnace, or is in operable communication therewith, wherein the furnace is configured to heat the boiler and thereby generate steam. The apparatus may be configured to feed substantially all of the flue gas generated in the furnace to the storage silo. However, in a preferred embodiment, the apparatus is configured to feed a first portion of the flue gas to the storage silo, and a second portion of the flue gas is released to the atmosphere. The apparatus preferably comprises an exhaust chimney through which flue gas that is not fed to the storage silo is released to the atmosphere.

Preferably, the apparatus comprises a first damper, which is configured to control the flow rate of flue gas to the storage silo. Preferably, the first damper is disposed within the means for feeding the flue gas from the furnace to the silo. The apparatus may comprise a second damper disposed within the exhaust chimney, and which is configured to control the flow rate of flue gas into the atmosphere and thereby control the pressure in the furnace.

The apparatus preferably comprises an induced draft fan which is configured to control the flow of the flue gas being fed to the silo. Preferably, the induced draft fan is disposed within the flue gas feed means, and more preferably downstream of the means for extinguishing any flame or spark present in the flue gas (i.e. the flame trap).

Preferably, the apparatus comprises a valve which is configured to control the flow of the flue gas being fed to the silo. Preferably, the valve is disposed within the flue gas feed means, and more preferably downstream of the induced draft fan. Optionally, the apparatus comprises a frequency invertor associated with the valve. The valve is preferably used in conjunction with the fan to control the flow of the flue gas along the conduit. However, it will be readily appreciated that either the valve or the fan may be used independently to control the flow of the flue gas.

Preferably, the apparatus comprises a supplementary (i.e. back-up) inert gas source for feeding inert gas to the storage silo to thereby create an inert atmosphere therein. This supplementary inert gas source is in addition to the flue gas being fed to the silo. The supplementary inert gas preferably comprises nitrogen, carbon dioxide or argon. The supplementary inert gas source may comprise a liquid nitrogen store, a liquid carbon dioxide store, a Pressure Sawing Adsorption (PSA) unit, a membrane filter, or any other appropriate inert gas source. Preferably, the apparatus comprises one or more gas inlet by which the supplementary inert gas is introduced into the silo. Advantageously, the supplementary inert gas source may be used for a period of furnace shutdown and/or startup.

The apparatus may be configured to allow an operator to manually select the amount of the inert gas from the supplementary inert gas source which is fed into the silo.

Preferably, however, the apparatus comprises control means for automatically controlling the amount of the inert gas from the supplementary inert gas source which is introduced into the silo depending on the concentration of oxygen in the flue gas. The apparatus preferably comprises an oxygen sensor which is configured to sense the concentration of oxygen in the flue gas, and means for varying the amount of the inert gas from the supplementary inert gas source which is introduced into the silo depending on the oxygen concentration in the flue gas. Preferably, the oxygen sensor is disposed within the flue gas feed means downstream of the condenser and flame trap.

Preferably, the treated flue gas comprises less than 10% oxygen (v/v), more preferably less than 5% (v/v), and most preferably less than 3% (v/v).

Preferably, the apparatus comprises control means for automatically controlling the concentration of carbon monoxide in the flue gas. The apparatus preferably comprises an oxygen sensor which is configured to sense the concentration of oxygen in the flue gas, and means for varying the amount of oxygen which is injected into the furnace depending on the oxygen concentration in the flue gas. Preferably, the oxygen sensor is disposed within the flue gas feed means downstream of the condenser and flame trap.

Alternatively, or additionally, the apparatus may comprise a carbon monoxide analyzer which is configured to sense the concentration of carbon monoxide in the flue gas, and means for varying the amount of oxygen which is injected into the furnace depending on the carbon monoxide concentration in the flue gas. Preferably, the carbon monoxide analyzer is disposed within the flue gas feed means downstream of the condenser and flame trap.

The inventors believe that the conduit which feeds the flue gas exiting the furnace to the storage silo, and which treats the gas to render it more suitable for creating the inert atmosphere, is an important part of the invention. Thus, in a second aspect, there is provided a flue gas treatment apparatus for preventing combustion of a flammable material contained within a storage silo, the apparatus comprising a flue gas conduit comprising a first end which is configured to be operably connected to a storage silo in which flammable material is stored, and a second end, which is configured to be operably connected to a flue gas outlet of a furnace in which the flammable material is combusted, and means for removing water vapour from the flue gas and/or means for extinguishing any flame or spark, which may be present in the flue gas.

Preferably, the means for removing water vapour from the flue gas before it is fed to the silo comprises a condenser. Preferably, the means for extinguishing any flame or spark, which may be present in the flue gas, comprises a flame trap.

Preferably, the apparatus comprises an induced draft fan which is configured to control the flow of the flue gas being fed to the silo. Preferably, the apparatus comprises a valve which is configured to control the flow of the flue gas being fed to the silo. Preferably, the apparatus comprises an oxygen sensor which is configured to sense the

concentration of oxygen in the flue gas. Preferably, the apparatus comprises a carbon monoxide sensor which is configured to sense the concentration of carbon monoxide in the flue gas. The features of the flue gas treatment apparatus are as defined with respect to the apparatus of the first aspect. The inventors believe that that they are the first to have ever used flue gas to prevent combustion or fire of a flammable material contained in a storage silo, i.e. inerting a biomass fuel. Hence, in a third aspect, there is provided use of flue gas for preventing combustion of a flammable material contained in a storage silo.

Preferably, the flue gas is treated prior to introduction into the silo, which treatment comprises removing, from the flue gas, water vapour and/or any flame or spark present therein.

The inventors have also developed a novel method for reducing the risk of a fire occurring in a storage silo. Accordingly, in a fourth first aspect of the present invention, there is provided a method of preventing combustion, in a storage silo, of a flammable material contained therein, the method comprising:

a) combusting a flammable material in a furnace to thereby create a flue gas; and

b) feeding the flue gas into a storage silo containing flammable material to thereby provide an inert atmosphere therein, and thereby prevent combustion of the flammable material.

Advantageously, the method of the invention does not need large quantities of inert gases to be kept on site to provide an inert atmosphere in a silo.

Preferably, the method comprises treating the flue gas before it is returned to the storage silo. Preferably, the method comprises removing water vapour from the flue gas before it is fed to the silo. Preferably, the water is condensed out of the flue gas.

Preferably, the method comprises extinguishing any flame or spark, which may be present in the flue gas.

Preferably, the method comprises feeding the flammable material from the storage silo to the furnace where it is combusted. Preferably, the method comprises pneumatically delivering or injecting the flammable material into the furnace. Preferably, atmospheric gas is used to deliver the flammable material into the furnace. Preferably, the amount of atmospheric gas injected into the furnace is controlled in order to allow complete combustion of the injected flammable material under stoichiometric conditions. However, in a more preferred embodiment, the method comprises delivering an excess of atmospheric gas into the furnace for combustion purposes. Preferably, an excess of atmospheric gas is defined as the amount of atmospheric gas that is needed to result in at least i%, 2%, 3%, 4%, 5%, 10%, 15% or 20% more oxygen being present in the furnace than would be necessary for complete combustion to occur under stoichiometic conditions.

The method may comprise feeding substantially all of the flue gas generated in the furnace to the storage silo. However, in a preferred embodiment, the method comprises feeding a first portion of the flue gas to the storage silo, and releasing a second portion of the flue gas to the atmosphere.

The method preferably comprises controlling the flow of the flue gas being fed to the silo using an induced draft fan and/or a valve. In embodiments in which the concentration of oxygen in the flue gas is too high, preferably the method comprises feeding inert gas to the storage silo from a

supplementary (i.e. back-up) inert gas source. The supplementary inert gas preferably comprises nitrogen, carbon dioxide or argon. The method may comprise manually selecting the amount of the inert gas from the alternative source which is fed into the silo. Preferably, however, the method comprises automatically controlling the amount of the inert gas from the alternative source which is introduced into the silo depending on the concentration of oxygen in the flue gas. The method preferably comprises sensing the concentration of oxygen in the flue gas, and varying the amount of the inert gas from the alternative source which is introduced into the silo depending on the oxygen concentration in the flue gas.

Preferably, the method comprises automatically feeding inert gas from the alternative source into the silo when the concentration of oxygen in the flue gas exceeds a pre- determined set-point or amount. Preferably, the method comprises calculating the amount of the inert gas from the alternative source which is fed into the silo so as to maintain the concentration of oxygen in the inert atmosphere in the silo to an acceptable predetermined maximum. Preferably, the predetermined maximum of oxygen in the silo is a concentration of oxygen of less than 10% (v/v), more preferably less than 5% (v/v), and most preferably less than 3% (v/v), 2% (v/v) or 1% (v/v).

The method preferably comprises sensing the concentration of oxygen in the flue gas, and varying the flow of oxygen into the furnace depending on the oxygen concentration in the flue gas. Preferably, the method comprises automatically increasing the flow of oxygen into the furnace when the concentration of oxygen in the flue gas falls below a pre-determined set-point or amount.

Alternatively, or additionally, the method comprises sensing the concentration of carbon monoxide in the flue gas, and varying the flow of oxygen into the furnace depending on the carbon monoxide concentration in the flue gas. Preferably, the method comprises automatically increasing the flow of oxygen into the furnace when the concentration of carbon monoxide in the flue gas exceeds a pre-determined set- point or amount.

All features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying Figures, in which:-

Figure 1 depicts a prior art apparatus for providing an inert atmosphere within a storage silo;

Figure 2 depicts an apparatus for providing an inert atmosphere within a storage silo in accordance with one embodiment of the present invention; and

Figure 3 is a logic flow diagram showing how the system shown in Figure 2

automatically regulates the flow of inert gas from an alternative inert gas source.

Example

Referring to Figure 1, there is shown a prior art system in which biomass 1, in the form of plant matter pellets, dust, fluff, wood and/or chips, is stored in a silo 2. The burning of biomass 1 as a fuel in power stations has become more prevalent recently and the volume of biomass l used and stored in silos 2 at power stations has correspondingly increased. While only a single storage silo 1 is shown, it will be readily understood that multiple storage silos 1 in warehouses can also be used. It has become good practice to provide an inert gas atmosphere 3 in the space immediately above the stored biomass 1 in the silo 2. This is achieved by introducing inert gases, such as nitrogen, carbon dioxide, argon or a combination thereof, into the atmosphere 3 forming the headspace above the biomass 1. In the prior art illustrated in Figure 1, the inert gas atmosphere 3 is provided by connecting the silo 2 to an inert gas source 4. Typically, the inert gas source 4 may comprise a liquid nitrogen gas store 5 of nitrogen gas 6. The inert gas source 4 is connected to the silo 2 by a first conduit 7, and flow of the inert gas 6 from the gas source 4 into the silo 2 is controlled by a first valve 8 disposed within conduit 7. It will be understood that inert gas may also be provided by a liquid carbon dioxide gas store, a Pressure Swing Adsorption (PSA) unit, a membrane filter, or any other appropriate inert gas source.

It will be readily understood that the inert gas atmosphere 3 will have to be to be replaced as the inert gas 6 is lost due to leakages. Additionally, the volume of biomass 1 will vary continuously as the biomass 1 is used and the stock replenished, and so the flow of the inert gas 6 will have to be controlled accordingly.

In both the apparatus of the invention and the prior art, the biomass 1 is transferred from the storage silo 2 to a furnace in which it is combusted, by a number of methods, including a mechanical/pneumatic conveyor 9, auger, bin, etc. The biomass 1 is injected pneumatically into the furnace 11 using a forced draught fan 10. The heat which is generated by the combustion reaction of the biomass 1 may be used to heat or melt a furnace load, but typically is used to heat a heat load boiler 12, which is connected to the furnace 11, in order to generate steam. Exhaust or flue gas 14 is then expelled via an exhaust chimney 22.

The forced draught fan 10 pumps atmospheric gas into the furnace 11, and the flow is controlled via a valve 13. The atmospheric gas comprises 20.9% oxygen (0₂) and 79.1% nitrogen (N₂), and the biomass fuel contains hydrogen (H) and carbon (C). Under stoichiometric conditions, the oxygen in the atmospheric gas will react with the hydrogen in the furnace 11 to form water vapour (H₂o), and with the carbon, to form carbon dioxide (C0₂). While undesirable, if there is insufficient air or poor mixing between the atmospheric gas and the biomass 1 delivered to the furnace 11, then there is a potential for partial combustion to occur and carbon monoxide (CO) would be generated as a result.

The nitrogen in the atmospheric gas is essentially inert and will normally pass unreacted through the furnace n as flue gas 14. However, while undesirable, at elevated temperatures and in excess oxygen conditions nitogen oxides (NO_x), including NO and N0₂, may be formed. During normal use, to prevent carbon monoxide from forming an excess of atmospheric gas is fed to the furnace 11. The excess is typically an excess of about 5-20% atmospheric gas, which results in an excess of 5-20% more oxygen than would be necessary for stoichiometric conditions in which the reactants are perfectly mixed. This results in a small amount of oxygen being present, together with the products of the combustion reaction, water and carbon dioxide, and the unreacted nitrogen in the exhaust gas 14.

For example, if the biomass 1 comprised wood it would have the chemical formula CeHiaOe, and would combust according to the following reaction:

C₆H₁₂0₆ + 60₂ = 6C0₂ + 6H₂0

Nitrogen would also be present due to the oxygen having been provided in atmospheric gas. Accordingly, assuming stoichiometric combustion (i.e. no excess oxygen) the exhaust gas 14 would comprise 17% carbon dioxide, 17% water and 65% nitrogen.

However, where an excess of atmospheric gas is provided, the exhaust gas will contain oxygen and a higher concentration of nitrogen. This is shown in Table 1, below:

Table l: Concentration of carbon dioxide, water, oxygen and nitrogen in the exhaust gas for the combustion of wood with a varying excess of atmospheric gas fair)

Referring now to Figure 2, one embodiment of an apparatus 32 according to the invention is shown. The apparatus 32 is based on the prior art system shown in Figure 1, but has been modified such that, instead of the exhaust flue gas 14 being expelled to the atmosphere via the exhaust chimney 22, it is instead fed to the storage silo 2 along a second conduit 17, such that it produces the inert atmosphere 3 above the biomass 1 to prevent it catching fire.

Not all of the exhaust flue gas 14 may be needed to provide the inert atmosphere 3. For this reason, first and second dampers 15, 16 are provided. The first damper 15 is disposed within the exhaust chimney 22 and is configured to control the flow of the exhaust flue gas 14 therethrough, as it is released into the atmosphere. The second damper 16 is disposed within the second conduit 17 and is configured to control the flow of exhaust gas 14 therealong towards the storage silo 2. As shown in Figure 2, the second conduit 17 is provided with various treatment units which treat the diverted flue gas 14 before it reaches the silo 2, as described in detail below.

Firstly, it is known that water can damage the silo 2 and cause the biomass 1 to set firm, thereby resulting in large costs and downtime, as the silo 2 is cleaned. Accordingly, it is desirable to remove any water vapour from the exhaust flue gas 14 prior to it being fed to the silo 2. Therefore, the exhaust flue gas 14 is first passed through a water-cooled condenser 18, which removes the water vapour therefrom. The condenser 18 comprises a cold water inlet 18 and a water outlet 19, which are connected by a pipe 33 along which cool water flows. Due to the heat passing from the exhaust gas 14 to the cool water flowing through the condenser 18, the temperature of the exhaust gas 14 will decrease, which causes the majority of the water vapour therein to condense on the surface of the pipe 33. The water which has condensed out of the exhaust flue gas 14 is removed from the condenser 18 via a drain 21. The concentration of carbon dioxide, oxygen and nitrogen in the dehydrated exhaust gas 34 is shown in Table 2 assuming all the water has condensed out.

Table 2: Concentration of carbon dioxide, oxygen and nitrogen in the dehydrated exhaust gas for the combustion of wood with a varying excess of atmospheric gas, where all the water vapour has been condensed out of the exhaust gas

In reality, however, a small amount of water vapour still remains in the dehydrated exhaust gas 34. However, the concentration of the water remaining in the exhaust gas 34 is sufficiently low for it not to cause any problems in creating the inert atmosphere 3 above the biomass 1 in the silo 2.

As explained above, it is necessary to use an excess of atmospheric gas to ensure complete combustion of the biomass 1, and thereby prevent carbon monoxide being created. However, the excess will have to be carefully controlled to prevent too high a concentration of oxygen being present in the treated flue gas 34.

It will be readily understood that due to the heat exchange process between the flue gas 14 and the cold water in pipe 33, the condenser 18 will recover some of the latent heat of vapourisation. Accordingly, the water flowing through outlet 20 of the condenser 18 will be warmer than it was when it flowed through inlet 19. While not shown in Figure 2, in another embodiment of the apparatus 32, the warmed water in outlet 20 is pumped into the heat load boiler 12, thus reducing the energy needed to generate steam.

The reason an inert atmosphere 3 is necessary is because the biomass 1 held in the silo 2 might otherwise combust and catch fire. For this reason, it is important that a flame or spark is not introduced into the silo 2. Accordingly, a flame trap 23 is disposed in the second conduit 17 downstream of the condenser 18 in order to extinguish any flame or spark that may be present in the exhaust gas 34. The flame trap 23 comprises a planar element 35 which has a plurality of channels 36 extending therethrough. The channels 36 may be regular or irregular in shape, and their size is selected based upon the flammability of the biomass 1. The flame trap 23 absorbs the heat from any

flame/ spark that may be present in the exhaust gas 34, thereby resulting in the flame being extinguished. The flow rate of the treated exhaust gas 34 in conduit 17 is carefully controlled in order to replace the inert atmosphere 3 in the headspace above the biomass 1 as it is lost due to leakages, or as the biomass 1 is used and the stock replenished. An induced draught (ID) fan 24 is therefore disposed in the second conduit 17 downstream of the flame trap 23, by which the flow rate of the exhaust gas 14, 34 can be controlled. The ID fan 24 controls the flow of the exhaust gas along the second conduit 17 and through the condenser 18 and flame trap 23, and thereby controls the volume of the treated exhaust gas 14, 34 which flows into silo 2. In the embodiment shown in Figure 2, the ID fan 24 is disposed downstream of the flame trap 23 since the speed of the fan's rotation will additionally act as a flame arrestor or flame trap.

As shown in Figure 2, a valve 25 is also disposed within the second conduit 17 downstream of the ID fan 24. The valve 25 functions in conjunction with the ID fan 24 to control the flow of the exhaust gas 14, 34 along the second conduit 17. However, it will be readily understood that either the valve 25 or the ID fan 24 can be used independently to control the flow of the exhaust gas 14, 34 along the conduit 17 to silo 2.

A pressure balance system allows excess products to vent traditionally to the atmosphere via the exhaust 22. Accordingly, the flow of the gas along the conduit 17 and out of the exhaust chimney 22 may be carefully controlled to maintain a constant pressure in the furnace 11. In addition to the system described above, in which flue gas 14 is fed to the silo 2 to produce the inert atmosphere 3, it may also be beneficial to provide an alternative gas source 4, to act as a supplementary supply of the inert atmosphere 3. As in the prior art apparatus shown in Figure 1, the apparatus 32 of the invention comprises an inert gas store 4 connected to the silo 2 by a first conduit 7. Flow of the inert gas 6 into the silo 2 can be controlled by a first valve 8 disposed in conduit 7. It is possible for an operator to manually select how much inert gas from the alternative gas source 4 is fed into the silo 2.

However, in a preferred embodiment of the apparatus 32, this step is automated due to the provision of an oxygen sensor 37 which is disposed in the second conduit 17 downstream of the condenser 18 and flame trap 23, and upstream of the ID fan 24. The oxygen sensor 37 measures the oxygen concentration in the treated exhaust gas 34. In a second preferred embodiment of the apparatus 32, an oxygen sensor 38 is disposed downstream of both the first and second conduits 7 and 17 and upstream of the silo 2. Both these embodiments have been illustrated in Figure 2, however, in practice it is likely that only one oxygen sensor would be provided. If the oxygen sensor 37 or 38 senses that the oxygen concentration is above a pre-set safety level for example a value of 3% oxygen (v/v), either due to the combustion conditions or a shutdown of the system, then inert gas 6 is injected from the inert gas source 4. However, a much lower volume of inert gas 6 from the alternative gas store 4 will be necessary for the apparatus 32 of the present invention, in contrast to the prior art. This will significantly reduce the operating costs and will result in less inert gas 6 having to be stored on site, thereby improving safety. When the oxygen concentration in the treated flue gas 34 is below the safety level, there is no need for inert gas 6 to be delivered to the silo 2. While not shown in Figure 2, it will be appreciated that the treated exhaust gas 34, and where necessary the inert gas 6 from the alternative source 4, may be introduced to the silo 2 through a plurality of inlets. The or each inlet may be provided at the top or bottom of the silo 2, or in between. Referring to Figure 3, there is shown a logic flow diagram showing how the flow of inert gas 6 from the alternative gas source 4 may be regulated automatically when the oxygen sensor is 37 is disposed in the second conduit 17. When a change in the oxygen concentration of the treated exhaust gas 34 is detected 26 by the sensor 37, the system checks to see if the oxygen concentration exceeds a pre-set maximum 27. For example, the pre-set maximum 27 may be 3% oxygen (v/v). If the oxygen concentration does not exceed a pre-set maximum, the system continues monitoring the concentration of oxygen in the treated exhaust gas 28. However, if the oxygen concentration does exceed a pre-set maximum, the system calculates the flow rate of the inert gas from the alternative store 6 necessary to maintain the inert atmosphere at or below the pre-set maximum 29.

The system then initiates the flow of the inert gas from the alternative source 6 into the silo 2 at the calculated rate 30. The system then continues monitoring the

concentration of oxygen in the treated exhaust gas 28. When a change in the oxygen concentration of the treated exhaust gas is sensed 26 by the sensor, the system checks to see if the oxygen concentration exceeds the pre-set maximum 27. If the oxygen concentration no longer exceeds the pre-set maximum, the system stops the flow of gas from the alternative source 31 and continues monitoring the concentration of oxygen in the treated exhaust gas 28. However, if the concentration of oxygen still exceeds the pre-set maximum, then the system continually performs the step of calculating the flow rate of the inert gas from the alternative store necessary to maintain the inert atmosphere at or below the pre-set maximum 29.

The step of calculating the flow rate of the inert gas from the alternative store necessary to maintain the inert atmosphere at or below the pre-set maximum 29 may be readily carried out as long as the concentration of oxygen in inert gas 6 from the alternative source 4 and the flow rate of the treated exhaust gas 14 are known. For instance, if the pre-set maximum is a concentration of 3% oxygen and the concentration of oxygen in the inert gas 6 from the alternative source 4 was 0% and the flow rate of the exhaust gas 14 into the silo was 100 cm³/s, then if the system registers that the concentration of oxygen in the exhaust gas 14 was 6%, the system can calculate that a flow rate of 100 cm³/s of the inert gas 6 from the alternative source 4 would be necessary to ensure that the concentration of oxygen in the inert atmosphere 3 stays at or below the preset maximum. It will be appreciated that when the oxygen sensor 38 is disposed downstream of both the first and second conduits 7 and 17 and upstream of the silo 2 then the oxygen sensor 38 will measure the concentration of oxygen in the treated exhaust gas 34 mixed with the inert gas 6 from the alternative gas source 4. Accordingly, the system may increase or decrease the flow of inert gas 6 from the alternative gas source 4 as the concentration of oxygen sensed by the oxygen sensor 38 either rises above a pre-set maximum or falls below a pre-set minimum.

As mentioned above, it is desirable that complete combustion of the biomass 1 occurs in the furnace 11 otherwise uncombusted and/or partially combusted gaseous fuel, such as carbon monoxide (CO), could be fed along the second conduit 17 and into the storage silo 2. This would result in a combustible stream of gas being fed into the silo 2, and if the concentration of this gas increased to a high enough level then a flame front may be capable of travelling from the furnace 11 all the way to the silo 2 and igniting the biomass 1 stored therein. In order to prevent a build up of combustible gas occurring, the system is configured to increase the flow of atmospheric gas (containing oxygen) injected into the furnace 11 by the forced draught fan 10 when the concentration of oxygen measured by the oxygen sensor 38 falls below a preset minimum. This will increase the amount of oxygen in the furnace 11 and ensure complete combustion occurs. Similarly, when the concentration of oxygen raises above the preset maximum, the system can be configured to decrease the flow of atmospheric gas injected into the furnace 11 by the forced draught fan 10.

In an alternative embodiment, the system may be provided with a CO analyzer (not shown). The CO analyzer may be provided together with the oxygen sensor 38 or separately thereform. In this embodiment, when the CO analyzer detects levels of CO which exceed a preset maximum, then the system will increase the flow of atmospheric gas injected into the furnace by the forced draught fan 10.

Advantages of the apparatus 32 of the invention reside in a much lower volume of inert gas 6 from the alternative gas store 4 being necessary for the apparatus 32 of the present invention, in contrast to the prior art. This will significantly reduce the operating costs and will result in less inert gas 6 having to be stored on site, thereby improving safety. Additional advantageous of the apparatus 32 reside in the provision of a condenser 18 capable of removing excess water from the flue gas 14, thereby preventing damage to the silo and preventing the biomass l from setting firm, thereby saving large costs and downtime which would otherwise be necessary for the silo 2 to be cleaned and repaired. Additionally, provision of the flame trap 23 allows all sparks and flames present in the flue gas 14 to be extinguished, thereby preventing an initial spark causing a fire in a silo 1.

Another advantage is the provision of a feedback loop illustrated in Figure 3 capable of controlling the oxygen concentration of the gas atmosphere in the silo, by monitoring the concentration of oxygen in the treated flue gas 34, and injecting inert gas 6 from an alternative source 4 when it is required.

Claims

1. An apparatus comprising:- a storage silo for storing flammable material;

2. An apparatus according to claim l, wherein the means for feeding the flue gas to the storage silo comprises a conduit, a first end of which is configured to be operably connected to the storage silo, and a second end of which is configured to be operably connected to the furnace, more preferably a flue gas outlet thereof.

3. An apparatus according to claim 2, wherein the first end of the conduit is connected to at least adjacent the top or the bottom of the silo.

4. An apparatus according to any preceding claim, wherein the apparatus is configured to treat the flue gas before it is fed to the storage silo.

5. An apparatus according to any preceding claim, wherein the apparatus comprises means for removing water vapour from the flue gas before it is fed to the silo.

6. An apparatus according to claim 5, wherein the means for removing water vapour from the flue gas is disposed within the flue gas feed means.

7. An apparatus according to either claim 5 or claim 6, wherein the means for removing water vapour comprises a condenser.

8. An apparatus according to claim 7, wherein the condenser is a water-cooled condenser comprising a cold water inlet, an outlet, and a passageway therebetween, wherein, in use, the flue gas flows passed the passageway such that water vapour condenses thereon.

9. An apparatus according to claim 8, wherein the water outlet of the condenser is operably connected to a boiler which is configured to generate steam.

10. An apparatus according to any preceding claim, wherein the apparatus comprises means for extinguishing any flame or spark, which may be present in the flue gas.

11. An apparatus according to claim 10, wherein the means for extinguishing a flame or spark is disposed within the flue gas feed means.

12. An apparatus according to either claim 10 or claim 11, wherein the means for extinguishing a flame/spark comprises a flame trap.

13. An apparatus according to any preceding claim, wherein the flammable material comprises a biomass substance, for example plant material.

14. An apparatus according to any preceding claim, wherein the apparatus comprises means for feeding the flammable material from the storage silo to the furnace where it is combusted.

15. An apparatus according to any preceding claim, wherein the apparatus comprises an air pump, which is configured to pneumatically deliver or inject the flammable material into the furnace.

16. An apparatus according to claim 15, wherein the air pump is configured to deliver an excess of atmospheric gas into the furnace for combustion purposes, preferably wherein an excess of atmospheric gas is defined as the amount of

atmospheric gas that is needed to result in at least 1%, 2%, 3%, 4%, 5%, 10%, 15% or 20% more oxygen being present in the furnace than would be necessary for complete combustion to occur under stoichiometic conditions.

17. An apparatus according to any preceding claim, wherein the apparatus is configured to feed substantially all of the flue gas generated in the furnace to the storage silo.

18. An apparatus according to any preceding claim, wherein the apparatus is configured to feed a first portion of the flue gas to the storage silo, and a second portion of the flue gas is released to the atmosphere.

19. An apparatus according to any preceding claim, wherein the apparatus comprises a first damper, which is configured to control the flow rate of flue gas to the storage silo, preferably wherein the first damper is disposed within the means for feeding the flue gas from the furnace to the silo.

20. An apparatus according to any preceding claim, wherein the apparatus comprises a second damper disposed within an exhaust chimney, and which is configured to control the flow rate of flue gas into the atmosphere and thereby control the pressure in the furnace.

21. An apparatus according to any preceding claim, wherein the apparatus comprises an induced draft fan which is configured to control the flow of the flue gas being fed to the silo.

22. An apparatus according to claim 21, wherein the induced draft fan is disposed within the flue gas feed means.

23. An apparatus according to any preceding claim, wherein the apparatus comprises a valve which is configured to control the flow of the flue gas being fed to the silo, preferably wherein the valve is disposed within the flue gas feed means.

24. An apparatus according to any preceding claim, wherein the apparatus comprises a supplementary (i.e. back-up) inert gas source for feeding inert gas to the storage silo to thereby create an inert atmosphere therein.

25. An apparatus according to claim 24, wherein the apparatus is configured to allow an operator to manually select the amount of the inert gas from the

supplementary inert gas source which is fed into the silo.

26. An apparatus according to either claim 24 or claim 25, wherein the apparatus comprises control means for automatically controlling the amount of the inert gas from the alternative source which is introduced into the silo depending on the concentration of oxygen in the flue gas.

27. An apparatus according to any one of claims 24-26, wherein the apparatus comprises an oxygen sensor which is configured to sense the concentration of oxygen in the flue gas, and means for varying the amount of the inert gas from the supplementary inert gas source which is introduced into the silo depending on the oxygen

concentration in the flue gas.

28. A flue gas treatment apparatus for preventing combustion of a flammable material contained within a storage silo, the apparatus comprising a flue gas conduit comprising a first end which is configured to be operably connected to a storage silo in which flammable material is stored, and a second end, which is configured to be operably connected to a flue gas outlet of a furnace in which the flammable material is combusted, and means for removing water vapour from the flue gas and/or means for extinguishing any flame or spark, which may be present in the flue gas.

29. A flue gas treatment apparatus according to claim 28, wherein the means for removing water vapour from the flue gas before it is fed to the silo comprises a condenser.

30. A flue gas treatment apparatus according to either claim 28 or claim 29, wherein the means for extinguishing any flame or spark, which may be present in the flue gas, comprises a flame trap.

31. A flue gas treatment apparatus according to any one of claims 28-30, wherein the apparatus comprises an induced draft fan which is configured to control the flow of the flue gas being fed to the silo.

32. A flue gas treatment apparatus according to any one of claims 28-31, wherein the apparatus comprises a valve which is configured to control the flow of the flue gas being fed to the silo.

33. A flue gas treatment apparatus according to any one of claims 28-32, wherein the apparatus comprises an oxygen sensor which is configured to sense the

concentration of oxygen in the flue gas.

34. Use of flue gas for preventing combustion of a flammable material contained in a storage silo.

35. Use according to claim 34, wherein the flue gas is treated prior to introduction into the silo, which treatment comprises removing, from the flue gas, water vapour and/or any flame or spark present therein.

36. A method of preventing combustion, in a storage silo, of a flammable material contained therein, the method comprising:

(a) combusting a flammable material in a furnace to thereby create a flue gas; and

(b) feeding the flue gas into a storage silo containing flammable material to thereby provide an inert atmosphere therein, and thereby prevent combustion of the flammable material.

37. A method according to claim 36, wherein wherein the method comprises removing water vapour from the flue gas before it is fed to the silo.

38. A method according to either claim 36 or claim 37, wherein the method comprises extinguishing any flame or spark, which may be present in the flue gas.

39. A method according to any one of claims 36-38, wherein the method comprises feeding the flammable material from the storage silo to the furnace where it is combusted.

40. A method according to any one of claims 36-39, wherein the method comprises pneumatically delivering or injecting the flammable material into the furnace.

41. A method according to any one of claims 36-40, wherein the method comprises delivering an excess of atmospheric gas into the furnace for combustion purposes.

42. A method according to any one of claims 36-41, wherein the method comprises feeding a first portion of the flue gas to the storage silo, and releasing a second portion of the flue gas to the atmosphere.

43. A method according to any one of claims 36-42, wherein the method comprises controlling the flow of the flue gas being fed to the silo using a first damper, a second damper, an induced draft fan and/or a valve.

44. A method according to any one of claims 36-43, wherein the method comprises feeding inert gas to the storage silo from a supplementary inert gas source.

45. A method according to claim 44, wherein the method comprises manually selecting the amount of the inert gas from the supplementary inert gas source which is fed into the silo.

46. A method according to either claim 44 or claim 45, wherein the method comprises automatically controlling the amount of the inert gas from supplementary inert gas source which is introduced into the silo depending on the concentration of oxygen in the flue gas.

47. A method according to any one of claims 44-46, wherein the method comprises sensing the concentration of oxygen in the flue gas, and varying the amount of the inert gas from the supplementary inert gas source which is introduced into the silo depending on the oxygen concentration in the flue gas.

48. A method according to 47, wherein the method comprises automatically feeding inert gas from the supplementary inert gas source into the silo when the concentration of oxygen exceeds a pre-determined set-point or amount.

49. A method according to any one of claims 44-47, wherein the method comprises calculating the amount of the inert gas from the supplementary inert gas source which is fed into the silo so as to maintain the concentration of oxygen in the inert atmosphere in the silo to an acceptable predetermined maximum.

50. A method according to claim 49, wherein the predetermined maximum of oxygen in the silo is a concentration of oxygen of 10% (v/v), more preferably 5% (v/v), and most preferably less than 3% (v/v), 2% (v/v) or 1% (v/v).