This invention relates to apparatus, in the form of a furnace, for the carbon neutral processing of limestone to produce carbon dioxide and calcium oxide, and also to a method for producing carbon dioxide and calcium oxide, in a carbon neutral way using the furnace. The invention finds particular use in the production of carbon dioxide on an industrial scale for the subsequent manufacture of a carbon neutral synthetic fuel.

The phrase“carbon neutral” is used herein to mean that the amount of carbon dioxide produced during the process is counter-balanced by the subsequent absorption or capture of an equivalent or even greater quantity of carbon dioxide by the calcium oxide also produced by the apparatus.

Fossil fuels are non-renewable energy resources which are rapidly depleting. The combustion of fuel manufactured from fossil fuels creates large quantities of greenhouse gases. With increasing concerns of climate change due to greenhouse gases, there is a need to reduce the amount of air pollution caused by the combustion of fuels and by industrial manufacturing processes. Due to the limited number of oil reserves, it is necessary to transport large quantities of oil from the oil reserves to the consuming areas, often over great distances. The transportation of oil in this way inevitably causes more pollution, additional to that from the burning of the oil being transported.

Scientists devoting their lives to, and specialising in, climate science warn us that climate change is the largest of all threats that could bring about the extinction of humankind. Of all living scientists, most now agree that climate change is one of the very largest risks to our continued existence as a species. It is incumbent upon all, including innovators and all those involved in the protection of new ideas to try to do all that we can to encourage solutions that will help in the fight against climate change for the sake of all humanity.

In an attempt to reduce fossil fuel use and eliminate pollution caused by the burning of such fuels, there is an increasing need for environmentally sustainable energy sources. Processes for producing synthetic fuels using carbon dioxide and hydrogen are well established. However, obtaining carbon dioxide directly from the atmosphere is not only expensive but is also problematic in that the extraction process can create yet even more pollution.

Furthermore, the carbon neutral processing of limestone on an industrial scale (for example over 10 tonnes per hour) brings about yet more challenges. Such large-scale quantities are required to compete with existing polluting processes, such as industrial fossil fuel kilns, and thus to meet the demand for competitively priced carbon neutral fuel, particularly since economies of scale are pertinent to such processes. In any solution high efficiency is paramount. The extremely high ratio of expensive heat to large quantities of relatively inexpensive feedstock mean that it is essential that heat loss is kept to an absolute minimum; if any part of the process is inefficient then the process can become commercially unviable at an industrial level and therefore faulty in concept.

The above problems are compounded by the fact that the temperature for calcining limestone has to be specific. If it is greater than 1000°C, the limestone can be burnt, which is not only inefficient but can pollute the process. However, the corollary, calcining at variable temperatures that are allowed to drop below approximately 800°C, causes incomplete and inefficient calcining which adversely affects commercial viability.

Furthermore, it has been discovered through prototype testing that there are difficulties meeting the structural requirements of a carbon-neutral industrial furnace; such difficulties could not easily have been foreseen. A furnace for the carbon neutral calcining of limestone needs to be big enough to produce an industrial size output of carbon dioxide and quicklime. Conventional internal void rotary kiln designs are good up to a point but, when larger industrial sizes are required, such designs begin to be problematic. There are no size restrictions on carbon polluting kilns as they can be placed outside the containment of factory walls. As such they can be extremely large in order to produce an industrial size output of quicklime; indeed, they require a substantial supply of oxygen which is taken from the surrounding air to fan the combustion needed. While this is inefficient in terms of wasted heat energy, it is optimal for efficient combustion, and the inefficiency is compensated for by the relatively low cost of the fossil fuel used. This luxury (of cheap fossil fuels) is not an option for a commercial furnace that is designed to produce "carbon neutral carbon dioxide".

It is a principal aim of the present invention to address the environmental damage caused by the combustion of fossil fuels and to provide a carbon neutral furnace and a method for producing carbon dioxide from limestone, on an industrial scale, which can be used for the subsequent manufacture of a synthetic and environmentally sustainable fuel. The invention aims to reduce energy consumption and the production of harmful emissions by the manufacture of synthetic fuels, so as to have a positive impact on the environment and climate change.

According to this invention, there is provided apparatus for the processing of limestone, comprising: a furnace having an annular outer vessel defined by concentric inner and outer cylindrical walls, and an inner chamber defined by the inner cylindrical wall, a reaction chamber being defined between the inner and outer cylindrical walls, the outer vessel having an inlet for the introduction of limestone to the reaction chamber and outlet means for the release of carbon dioxide and calcium oxide from the reaction chamber; heating means within the inner chamber of the furnace arranged to be supplied with heat and/or electricity from an energy source; and a curved blade within the reaction chamber, extending between the inner and outer cylindrical walls and arranged to channel limestone introduced into the reaction chamber toward and along the inner cylindrical wall, whereby the temperature of the heating means is raised to transfer heat to limestone contained within the reaction chamber to an extent sufficient to release carbon dioxide from limestone.

According to a second but closely related aspect of the present invention, there is provided a method for processing limestone comprising the steps of: a) operating heating means disposed within an inner chamber of a furnace, to raise the temperature within the furnace, using heat and/or electricity derived from an energy source;

b) introducing limestone into a reaction chamber, defined by inner and outer cylindrical walls of an outer vessel of the furnace, through an inlet thereto, to be heated by the heating means;

c) operating the furnace to channel limestone from the inlet through the reaction chamber; and

d) collecting carbon dioxide and calcium oxide released by heating the limestone, through outlet means in the outer vessel, whereby the heat transferred from the heating means to the limestone and the channelling of the limestone along the reaction chamber causes calcining of the limestone to produce carbon dioxide and calcium oxide.

To maximise heating of limestone along the reaction chamber, it is preferable that the inlet in the outer chamber is located at, or towards, one end of the outer vessel. The limestone inlet end of the outer vessel will be referred to hereinafter as the first end, and the other end of the outer vessel, remote from the first end, will hereinafter be referred to as the second end.

Preferably the heating means comprises a non-combustion based heat source. Calcining of limestone by heating releases carbon dioxide and produces calcium oxide (quicklime). The heating of limestone in conventional rotary kilns is carried out by burning fossil fuels, which inevitably gives rise to pollution and is environmentally unsustainable. The apparatus of this invention preferably addresses this problem by using an indirect heat transfer process which does not require combustion.

The apparatus may further comprise an energy source supplying heat and/or electricity to the heating means. The energy source may be directly or indirectly connected to the heating means and may be provided within close proximity to the furnace or remote thereto. The efficient release of carbon dioxide from limestone requires a temperature within the furnace optimally in the region of 900-1000°C. The heating means and energy source should be arranged such that the specific temperature of this order can be achieved within the furnace. Carbon dioxide can be released at lower temperatures but, as discussed hereinbefore, this can lead to incomplete and inefficient calcining, thereby resulting in a commercially nonviable system.

The energy source may be a nuclear and/or a renewable energy source, configured to produce heat and/or electricity, and thereafter to cause the temperature of the heating means to rise.

Where the energy source is nuclear, the apparatus may comprise a nuclear reactor. There are many types of nuclear reactor including, but not limited to: a water cooled reactor, a liquid metal cooled reactor, a gas cooled reactor (GCR), a molten salt reactor, a generation IV reactor, a boiling water reactor (BWR), a pressurised water reactor (PWR), a breeder reactor, a high temperature gas cooled reactor, a vodo-vodyanoi energetichesky reactor (PWR-WER), a Canada deuterium uranium reactor (CANDU reactor), a D20 PWR, an advanced gas-cooled reactor (AGR), a high temperature helium cooled reactor, a traveling-wave reactor (TWR), a nuclear fusion reactor, a light- water-cooled graphite-moderated reactor (LWGR), or a thorium-fuel reactor and/or a thorium dual-fuel reactor.

Where the energy source is renewable, it will be derived from natural resources such as sunlight, wind, rain, tides and geothermal heat, which are renewable in the sense that the energy is naturally replenished. In addition to these sources, renewable energy may also be derived from biomass and biofuel plants, where the fuel source for those plants is itself renewable.

By employing any of the energy source arrangements described above, or in other ways, the heating means employed in this invention may be supplied with energy to cause the temperature within the furnace to be raised sufficiently for the calcining of limestone and so the production of carbon dioxide.

Regardless of the type of energy source, the heating means of the apparatus is ideally configured to facilitate a uniform distribution of heat transfer to the limestone within the furnace. The heating means is preferably positioned directly adjacent to at least part of the inner cylindrical wall. The heating means may be provided in an annular form, within the inner chamber, around the inner cylindrical wall. In practice however, depending on how the apparatus is used, limestone may tend to locate and be transferred along only the lower half of the reaction chamber. As such, it may be more efficient for the heating means to be located only around the lower half of the inner cylindrical wall so as to transfer heat specifically to limestone within the reaction chamber. The heating means may be arranged in a semi-circular configuration in the shape of half an annulus. The heating means preferably extends along the length of the outer vessel (i.e. between the first and second ends thereof) to maximise the supply of heat to limestone as it is conveyed therealong. Furthermore, it may be advantageous to locate the heating means as close to the inner cylindrical wall as is possible to ensure efficient heat transfer to limestone contained within the reaction chamber.

The heating means is preferably stationary and may comprise a radiant heater, such as a radiant tube. Materials such as Inconel tube or APMT alloy may be utilised for this purpose; ceramics can also be used, but the strength and radiance properties are less preferable. The heating means being provided adjacent the inner cylindrical wall serves to maximise the heated surface area for limestone within the reaction chamber. Additionally, in this configuration the size of the reaction chamber and the limestone flow path, defined by the blade, can be minimised. The reaction chamber therefore defines a restricted channel with the heating means transferring heat directly thereto to facilitate the most effective, efficient heat transfer. This is in contrast to traditional kilns which have an internal void heat arrangement and inefficient distance between the heat means and the area within which the limestone is located. In the prior art radiation heat and convection heat has a limited effect. The arrangement of the heat means, the reaction chamber and the blade in the present invention serve to address such inefficiencies.

The heating means may comprise a heat transfer arrangement for transferring heat from the energy source to the interior of the furnace, the heat transfer arrangement including feed and return conduits arranged to allow a heat transfer fluid contained therein to extract heat from the energy source for transfer to the interior of the furnace thereby to heat the limestone to a temperature sufficient for the release of carbon dioxide. There are many types of heat transfer fluid which are well known to those skilled in the art and the term“heat transfer fluid” is used herein to mean any fluid capable of facilitating effective heat transfer.

Alternatively, and in a preferred arrangement, the heating means may be one or more electrical resistance heating element arranged to be supplied with electricity derived from the energy source. In this arrangement, the electrical resistance heating element disposed within the furnace is electrically powered and the energy source generates electricity which may be supplied through a suitable control unit to the heating element, to raise the temperature within the furnace. Advantageously, the energy source may generate electricity directly utilising the thermoelectric effect and so typically may comprise thermocouples, thermopiles, thermionic converters or similar apparatus. In the alternative, the energy source may be arranged to generate electricity indirectly, for example by heating water to produce steam and using the steam to power a turbine driving an electricity generator.

The curved blade may extend partly or entirely between the inner and outer cylindrical walls of the outer vessel. Preferably the apparatus is configured to facilitate rotation of the blade about the axis of the inner chamber. The blade may be freely disposed within the reaction chamber so that it may rotate therearound with respect to the inner and outer cylindrical walls. Alternatively, the blade may be integrally mounted to the inner or the outer cylindrical wall of the outer vessel. In this arrangement, the blade may protrude radially from the inner or the outer cylindrical wall, having a free edge remote from the mounting edge. In a particularly preferred arrangement, the blade extends entirely between and is mounted to both the inner and outer cylindrical walls. In any configuration where the blade is mounted to the outer vessel, the annular outer vessel is ideally configured to rotate about the axis thereof. Roller and motor mechanisms may be provided to effect rotation of the outer vessel. As an alternative arrangement, instead of rollers, the apparatus may comprise a shaft mechanism, as generally known in the art.

A primary function of the curved blade is to ensure that limestone is transferred across the heated reaction chamber at a uniform, totally metered rate. To achieve this, the blade preferably extends from the first end to the second end of the outer vessel, having a free end at each distal extremity. The outer vessel may include a distal protrusion for the introduction of limestone into the reaction chamber and the blade preferably extends along the distal protrusion. The blade is preferably curved to define a spiral shape at the periphery of the inner chamber around the inner cylindrical wall; this arrangement provides a defined path for the limestone. The presence of the blade ensures that limestone is conveyed along the path in measured amounts. The processing of the limestone is thereby carried out in an even fashion facilitating full and stable processing. The blade may be curved transversely to define a concave surface for the channelling of limestone. This shape may assist to direct the limestone towards the heated inner cylindrical wall. The blade may comprise a plurality of radial vanes interconnected to form a single axial blade. The spacing between adjacent turns in the spiral blade is ideally uniform as too is the pitch of the blade.

The apparatus may advantageously comprise two like blades mounted to the cylindrical walls of the outer vessel in an entwined, synchronised manner, taking inspiration from nature by resembling part of the configuration of the molecular shape of a double-stranded DNA molecule. Such a configuration may increase the efficiency of the apparatus.

Preferably, the furnace comprises a rotary annular outer vessel which defines an inner stationary chamber. The outer vessel and the inner chamber are generally cylindrical, with the inner and outer cylindrical walls of the outer vessel being configured such that the outer vessel may rotate about a horizontal axis. In an alternative arrangement, the outer vessel may be arranged to rotate about an axis inclined at a small angle to the horizontal. An incline may allow the force of gravity to assist with energy input for the flow of limestone into the interior of the furnace.

The provision of the curved blade and heating means and their particular configurations, as defined hereinbefore, facilitates progressive heating of limestone as it is transferred along the reaction chamber. In this way, there is no requirement for a pre-heater at the inlet of the furnace, although this will not preclude an integral pre-heating facility should this add further efficiency; the temperature of limestone at the first end of the outer vessel will be relatively low and calcining will not occur at this stage of the process; instead this stage will serve as a preheating phase. As limestone is conveyed, by the curved blade, along the reaction chamber towards the second end of the outer vessel, the temperature of the limestone will progressively increase. As the limestone passes through the middle section of the furnace, decomposition of the limestone into carbon dioxide and calcium oxide (quicklime) will begin; this stage is the calcining phase.

The outlet means may comprise one or more outlets in the outer cylindrical wall of the outer vessel for the release of carbon dioxide. In a preferred arrangement the outlet means may further comprise a fan system creating a pressure depression within the furnace. This result should be reasonably easy to achieve by a person skilled in the art using a conventional fan system. Valves can be complex and prone to failure, whereas a fan is easily maintained and could be repaired, if necessary, without cooling the furnace down. Nevertheless, as an alternative, or additionally, the one or more outlets may comprise gas permeable membranes and/or gas valves. Preferably, the apparatus further comprises a stationary casing which houses the furnace and which may facilitate temporary containment of the carbon dioxide. As heated gas tends to rise, the casing may be configured to receive carbon dioxide at the upper end thereof. Carbon dioxide may exit the outer vessel, by way of the one or more outlets, and pass into the upper end of the casing. In an alternative arrangement, the outlet means may comprise outlets through the curved blade. Such outlets may be in the form of small vent openings through the blade which allow carbon dioxide gas to flow through the reaction chamber. The casing is preferably arranged to conduct the carbon dioxide away from the furnace. One or more channels may be provided to direct the carbon dioxide from the reaction chamber. This arrangement serves a dual purpose; firstly, it facilitates the structured capture of carbon dioxide from the process; and secondly, it releases pressure from within the furnace. The need for the apparatus to sustain a particular level of pressure within the reaction chamber has been found, by prototype testing, to be a very important factor to achieve maximum efficiency; if the pressure within the furnace is not released, processing of limestone within the reaction chamber will be inhibited. Ideally the pressure within the reaction chamber should be maintained at atmospheric level. The provision of a fan system or gas valves, or similar control-flow devices, would allow the pressure within the reaction chamber to be regulated. Advantageously, in each of the embodiments, the casing may further comprise a fan directed to assist the drawing away of carbon dioxide from the vessel.

Preferably, a plurality of outlets is provided. Where a plurality of outlets is provided, these may be equidistantly spaced around the outer cylindrical wall of the outer vessel and/or densely dispersed around the outer wall in the calcining phase stage of the furnace. The number and location of the outlets may be determined to sustain atmospheric pressure within the reaction chamber.

Calcining of limestone within the reaction chamber will produce carbon dioxide and calcium oxide (quicklime). The cooled quicklime released from the furnace will absorb carbon dioxide from the atmosphere. The quicklime could be used in vehicle exhaust filters or along motorways, to absorb carbon dioxide from vehicle exhaust gases, or could be made into mortar-like slabs which could be utilised in sea defences, new quays and the like. Quicklime is particularly good at absorbing carbon dioxide when placed in water, and this characteristic could be most beneficial in projects around the coastline. Thus, the process may become carbon neutral. Indeed, when quicklime is placed in the ocean it can absorb up to twice the amount of carbon dioxide; in this way the process may in fact be carbon negative. So that quicklime produced by the calcining of limestone in the apparatus may be readily used for other purposes, the apparatus may further comprise a quicklime outlet. The quicklime outlet may be disposed towards or at the second end of the outer vessel. The configuration of the furnace, and particularly the curved blade, ensures that the flow of limestone is controlled; this arrangement encourages calcium oxide to be expelled from the quicklime outlet in measured thrusts.

The inner chamber of the furnace (defined by the inner cylindrical wall) defines a cavity (central void). The void extends in the centre of the inner chamber preferably throughout all or a substantial part of the length of it. As a result, the channelling and moving of the limestone throughout the reaction chamber as well as the resulting carbon neutral calcination only take place at the outer periphery of the outer vessel. The apparatus of the present invention in fact allows limestone to be calcined evenly at a constant optimum temperature throughout the length of the furnace at the periphery of the outer vessel thereby providing the efficiency necessary to convect, conduct and radiate heat in a manner that is viable for an industrial scale carbon neutral process, to enable the overcoming of the inefficiencies of existing electrical kills which have been shown not to be viable for the purpose of calcining industrial amounts of limestone. In contrast, existing kilns tend to have combustion-based heating means disposed within the core of the kiln. In such arrangements the limestone is tumbled around the heated core and there is inevitably a lot of space between the limestone and the walls of the kiln. Such space is beneficial in these types of kiln as air is required to allow combustion. In contrast, the present invention does not require combustion and thus does not need vast amounts of air to operate. Yet, the invention serves to optimise the effects of radiation and convection of heat to make the most of the efficiencies needed for industrial scale production. Preferably, the central void can be utilised to collect and transfer calcium oxide from the second end of the outer vessel to the first end thereof. This can be highly advantageous in minimising the space required for housing the furnace; by providing all inlets and outlets at one end (preferably the first end), access space is not required at the other end (e.g. the second end) which can be located against or close to a wall or other surface of the room containing the furnace. Of course, such transfer could take place externally of the apparatus, e.g. beneath the casing, but this would add inefficiencies by minimising the working area for housing the apparatus, which would not serve to maximise feedstock capacity.

To facilitate such transfer the apparatus may further comprise a cylindrical or annular tube feed arrangement. This feed arrangement may simply urge calcium oxide along a defined path from the second end to the first end of the furnace. In practice limestone, transforming into calcium oxide, will travel along the bottom half of the outer vessel, as it is rotated. It will be advantageous to move the resulting calcium oxide at the second end of the furnace up to the highest point of the outer vessel to facilitate its despatch back to the beginning of the furnace through the internal void. Whilst it is appreciated that energy is required in order to transfer calcium oxide, this will need to be done at some stage anyway in order to be utilised. It should also be borne in mind that expelled calcium oxide will weigh less than the original weight of the limestone. If the furnace is inclined with the limestone inlet at the first end, then the relaying of calcium oxide back to the beginning of the furnace at the limestone inlet end can be carried out along an optimum trajectory by collecting the calcium oxide from an upper point of the second end of the furnace and transferring it to the lower point of the first end of the furnace. In this way the transfer will require only minimal energy.

One way to achieve this may be to have a capped section at the second end of the outer vessel, essentially forming a bucket, to collect calcium oxide at the blade end. As limestone is conveyed along the reaction chamber it is likely to collate towards the lower half of the outer vessel but as the limestone reaches the final turn of the blade it will become encapsulated in the capped section and, upon further rotation of the blade, it will be carried to the top of the second end of the outer vessel and thereafter expelled into a stationary cylindrical shoot which extends from the top of the second end down towards the bottom of the first end for collection. If inadvertently any limestone misses the shoot it will fall back into the blade path. Preferably, in this arrangement, the capacity of the capped section is large and the width thereof greater than the pitch of the blade. The height of the capped section may however be relatively short to allow the limestone to“tip-out” into the shoot as the blade rotates.

In an alternative embodiment, the feed arrangement may comprise a reverse curved blade arranged to channel calcium oxide expelled from the reaction chamber along the furnace to the limestone inlet end thereof. The reverse blade may operate in the same way as the curved blade utilised in the reaction chamber but will transfer calcium oxide in the opposite direction to limestone. The same features of the curved blade, as discussed herein, with regards shape and configuration may equally apply in respect of the reverse curve blade. The reverse curved blade may be arranged within the void but to avoid interference with the heating means and associated efficiency issues, the reverse blade is preferably disposed around the outer vessel. In this arrangement, the reverse curve may be disposed within an auxiliary annular vessel located around the outer vessel. The auxiliary vessel may be defined by secondary inner and outer cylindrical walls. In a particularly preferred arrangement, the auxiliary vessel and the outer vessel share a common wall, i.e. the secondary inner cylindrical wall of the auxiliary vessel may in fact be the outer cylindrical wall of the outer vessel. In this arrangement the same motor and rotation mechanism may be used for both blades and these will be configured in a manner which is well known to persons skilled in the art. Preferably at the second end of the apparatus, the secondary outer cylindrical wall will extend along the apparatus further than the common wall and the inner wall of the outer vessel, to allow calcium oxide at the second end to fall out of the outer vessel and into the auxiliary vessel, automatically upon rotation. In this embodiment, the secondary outer cylindrical wall maybe shorter at the first end of the apparatus to facilitate the introduction of limestone into the reaction chamber.

The central void may instead or additionally be filled with insulative material. Preferably, insulation is provided between the casing and the vessel to maximise heat transfer to the limestone.

The apparatus may further comprise a mechanical feed system for the introduction of limestone. The mechanical feed system may comprise a standard shoot system. Preferably, the system includes two plates including an introduction plate designed to constrict limestone into the inlet of the furnace. The inlet of the furnace ideally comprises an opening into the reaction chamber. In a preferred arrangement, the opening exposes part of the blade of the apparatus. The introduction plate may be arranged to constrict limestone into the opening with sufficient thrust so as to facilitate rotation of the inner chamber in combination with the electric motor. The feed system may further comprise a valve plate arrangement located above the introduction plate which serves to regulate the release of limestone into the inlet. This arrangement allows limestone to be consistently measured and compacted into the reaction chamber. This facilitates the compacting of the limestone between the blades in the initial exposed blade part of the reaction chamber so as to take advantage of convection of heat, in addition to heat radiation. The mechanical feed system actively pushes measured amounts of limestone through defined areas where specific heat is applied. The curved blade collects and effectively pulls the limestone to the heated reaction chamber.

Preferably at the first end of the outer vessel an end cap is located and arranged to prevent limestone falling out of the reaction chamber past the free end of the blade with the initial thrust. This end cap may be placed at the first end around the free end of the blade. The end cap may be substantially“U” shaped in cross section with an end plate arranged to close off the free end of the blade and having an exposed region through the outer cylindrical wall configured to allow limestone through the inlet to the exposed blade. The end plate preferably includes one or more openings for access to the core of the furnace; such openings may be provided for the location of insulated pipes to the core of the furnace.

The mechanical feed system may further comprise a starting cylinder, attached to a hopper and having an opening in the top. In this way limestone need only be unceremoniously dumped into the starting cylinder, with the curved blade taking care of conveying measured amounts of limestone to be treated. Such treatment is in conjunction with measured amounts of heat so that the whole process is very efficient.

The apparatus may further comprise a self-regulating feedback control system configured to monitor operational status of the furnace and to vary the rotation speed and limestone feed rate in response thereto. The system may be configured to monitor the carbon dioxide emitted from the furnace and the limestone introduced thereto.

A carbon neutral furnace would optimally be placed within the walls of an insulated building to minimise heat loss and optimise usable heat recovery. However, such an arrangement causes two major challenges to arise: -

The first is a size challenge: the cost of housing a typical industrial size internal cavity furnace rotary kiln is a problem commercially. Such a kiln would have to be as big as a very large aircraft hangar, and this would cause significant difficulties with regard to the building and maintenance thereof as well as costs associated with attempting to minimise heat loss.

The second is an engineering challenge: contrary to carbon polluting open rotary kilns, an important aspect of the carbon neutral furnace of the present invention is that it is preferably sealed to prevent carbon dioxide being expelled into the immediate environment. There is a commercial need to seal the kiln, not only to avoid heat loss and efficiently to apply expensive heat to where it is needed, but also to capture carbon dioxide, the latter not being concern in the case of a carbon polluting kiln. Prototype testing has shown that the requirement for very large industrial scale, will put an unacceptable strain on essential parts of a carbon neutral kiln where there is a conventional central void heat pit, such as strain on cantilevered heat resistant elements especially at temperatures of up to 1000°C. Also, sealing the unit increases the pressure which inhibits the reaction, and puts strain on valves, exits and instruments. The apparatus of the present invention serves to overcome these two challenges. Preferably, therefore the casing of the apparatus of the present invention is sealed. Such a casing may serve to provide containment for the off-gas emitted as the by-product of the process. It is appreciated that this arrangement may marginally increase maintenance complexity but the frequency of required maintenance should be reduced and the downsides should be offset by the improved reliability of the apparatus in combination with the lowering of downtime and running costs. This has direct benefits for an industrial size furnace. The sealing of a furnace ensures maximum heat efficiency but does bring about two challenges that have the effect of inhibiting the calcining reaction. Firstly, as indicated above, this arrangement increases the pressure within the reaction chamber and secondly, it increases the retention of carbon dioxide therein. The configuration of the present invention serves to address these problems by providing appropriate outlets for the release of carbon dioxide and reduction of pressure. It is crucial for limestone to be at a certain temperature at a particular point in the process. This is vital in an industrial scale carbon neutral furnace in order to work efficiently and effectively in order to compete with fossil fuel-based processes. Preferably, the blade and outer vessel, where applicable, is configured to rotate at variable speeds. The rate of operation of the apparatus may be varied by controlling the rotation speed of the blade (either directly or indirectly via the outer vessel, depending on the arrangement) and/or the limestone feed rate. This allows for adjustment of the process parameters to allow for variations in feedstock properties.

The carbon dioxide produced using the apparatus of the present invention may be provided to other industries; for example it could be transferred to an oil corporation facility to be forced into a spent oil well in order to release remaining crude oil, while the carbon dioxide is sequestered in the process. In this way, the carbon dioxide is used in the production of carbon neutral fuel; the newly liberated crude oil will be burned in an internal combustion engine but the carbon dioxide then produced in that combustion, will be offset by the calcium oxide produced in the process of the apparatus.

Alternatively, by connecting the apparatus to a hydrogen plant and to a synthetic fuel production plant, the apparatus of this invention may be used directly to convert synthesis gas to fuel such as methanol and butanol. Butanol may be used as a gasoline substitute without requiring any further processing. The high temperatures and pressures produced by the apparatus during the process may be used within the synthetic fuel plant to facilitate the conversion.

The design of the furnace allows it to be sufficiently small to be housed in a relatively modest sized building, whilst still having the output of an extremely large traditionally designed kiln.

The apparatus as hereinbefore defined could be used for the production of “carbon neutral” carbon dioxide for use in sequestration and/or carbon neutral fuel production. As in the previous example, carbon dioxide obtained using the furnace of the present invention, may be injected into spent or partially spent oil wells to release residues of crude oil or hydrocarbon fuel for use in engines or domestic or industrial heaters. This carbon dioxide can also be used for injection into hydraulic fracturing strata operations to release hydrocarbon gases. The said carbon dioxide can remain sequestered in such wells or strata and the calcium oxide produced by the furnace may be utilised to neutralise the effect of the carbon dioxide produced when the liberated hydrocarbon fuel is combusted.

In a second aspect of the present invention, there is provided a carbon neutral fuel production plant incorporating apparatus as hereinbefore defined.

The furnace of the present invention is designed for the carbon neutral processing of limestone to produce“carbon neutral” carbon dioxide which could be used in making carbon neutral, greenhouse gas neutral, or low carbon, fuels, gasses, chemicals and cement and in facilitating the production of carbon neutral, greenhouse gas neutral, or low carbon, fuels, gasses, chemicals and cement.

The furnace of the present invention facilitates continuous calcining which means it can process a constant supply of limestone with no downtime. The furnace is not limited to processing limestone in batches as with a carbon neutral traditional designed internal void kiln. The present invention thus overcomes the problems of the prior art associated with inefficiencies, heat losses and disruption in temperature variations. So that the invention may be better understood, an embodiment will now be described in detail, but by way of example only, with reference to the following drawings in which:

Figure 1 shows a side cross sectional view through apparatus of the present invention;

Figure 2 is an end cross sectional view through part of the apparatus;

Figure 3 is an end cross sectional view through another part of the apparatus;

Figure 4 is a perspective end view of part of the apparatus of Figure 1 ;

Figure 5 is a simplified cross-sectional view of the blade and calcium oxide collection mechanism;

Figure 6a and Figure 6b are views of the end section of the blade illustrating the calcium oxide collection mechanism;

Figure 7 is an alternative embodiment of outlet means and blade of the apparatus of the present invention; and

Figure 8 is an end cross sectional view through part of apparatus showing an alternative embodiment of blade configuration incorporating a calcium oxide transfer arrangement.

In general conventional rotary kilns are designed specifically for use in combusting calcium carbonate (limestone) to cause the calcium carbonate to decompose into carbon dioxide and calcium oxide (quicklime). These kilns are fired with natural gas or oil, in a manner which is well known in the art, to cause the limestone to decompose.

Unlike conventional kilns, the apparatus of the present invention does not require combustion in order to operate. Referring to Figures 1 and 2, the apparatus of this invention comprises a generally cylindrical stationary casing 10 having a furnace 1 1 housed therein. The furnace 1 1 includes a rotatable annular outer vessel 12 formed by coaxial inner and outer cylindrical walls 13, 14. A reaction chamber 15 is defined between the inner cylindrical wall 13 and the outer cylindrical wall 14 and is arranged to receive limestone 16 to be calcined. The casing 10 includes an annular gas containment area 17 and insulation 18 is provided between the containment area 17 and the furnace 1 1.

The casing 10 and outer vessel 12 may be constructed of a conventional refractory material, reinforced as necessary, in a manner well known to those skilled in the art, but further use may be made of the specialist materials, as hereinbefore discussed (or other materials) if particular longevity is required for specific furnaces. The furnace 1 1 has, at one end (the first end 21 ), an inlet 22 for the introduction of limestone 16, that inlet 22 being provided with a mechanical feed system 23, which will be explained in more detail below. The outer vessel 12 of the furnace 1 1 is horizontally positioned such that the other end of the furnace (the second end 24) is at the same height as the first end 21 and includes a distal protrusion 20 for the introduction of limestone into the reaction chamber 15. The inner cylindrical wall 13 defines an inner chamber 25. Heating means is disposed within the inner chamber 25. The heating means is an electrical resistance heating element 26 arranged to be supplied with electricity derived from an energy source 27. The heating element 26 extends in a half annular configuration and is located against the bottom half 28 of the furnace, against the inner cylindrical wall 13. In practice, limestone will gather in the lower half 28 of the outer vessel 12 as it is conveyed along the reaction chamber 15. The heating element 26 extends along the length of the outer vessel 12 in order to maximise heat transfer. In this arrangement, the electrical resistance heating element 26 is electrically powered and the energy source 27 generates electricity which may be supplied through a suitable control unit (not shown) to the heating element 26, to raise the temperature within the furnace 1 1 . Electricity supply cables 29 are connected to that element 26 and are provided with electrical, thermal and mechanical insulation to allow the supply of electricity to the element 26 to an external control unit (also not shown). In turn, the energy source 27, which could be any of the carbon neutral, nuclear or renewable energy sources as hereinbefore described, is connected to the control unit and the heating element 26 is powered from the nuclear energy source 27, to raise the temperature within the furnace 1 1 sufficiently to cause calcining of the limestone.

The apparatus includes a curved blade 32 within the reaction chamber 15 arranged to channel limestone introduced into the reaction chamber 15 toward and along the inner cylindrical wall 13. The curved blade 32 is mounted to both the inner and outer cylindrical walls 13, 14. The blade 32 extends the full axial length of the outer vessel 12, including along the distal protrusion 20 thereof and is curved to define a spiral shape along the length of the reaction chamber 15; this arrangement provides a defined path for the limestone. The presence of the blade 32 ensures that limestone is conveyed along the path in measured amounts. The processing of the limestone is thereby carried out in a uniform fashion facilitating full and even processing. The blade 32 is also curved transversely to define a concave surface for the channelling of limestone towards the heating element 26. The spacing between adjacent turns in the spiral blade 32 is uniform as too is the pitch of the blade.

A plurality of outlets, in the form of gas valves 33, are provided, equidistantly spaced around the outer cylindrical wall 14 of the outer vessel 12, for the release of carbon dioxide. Alternatively or additionally, these could be gas permeable membranes. The gas valves 33 are defined in the outer cylindrical wall 14 of the outer vessel and openings 34 are provided in the insulation 18 to allow carbon dioxide to escape from the reaction chamber 15 to the containment area 17 of the casing 10. A fan 35 is provided adjacent the containment area 17 of the casing 10 to assist the drawing away of carbon dioxide from the vessel 12. The gas valves 33 are designed to regulate the release of carbon dioxide from the furnace 1 1 so that it can be conveniently captured whilst maintaining an optimum level of pressure within the reaction chamber 15.

An alternative embodiment is illustrated in Figures 7 and 8. As seen in Figure 7, the curved blade 32 may comprise a series of outlets in the form of small vents 47 which allow carbon dioxide to pass more easily between blade turns. In this arrangement channels 52 are provided to direct carbon dioxide from the reaction chamber 15 to the containment area 17. The fan 35 assists the drawing of carbon dioxide from the reaction chamber 15 and from the containment area 17.

As the curved blade 32 is mounted to the inner and outer cylindrical walls 13, 14 of the outer vessel 12, the outer vessel 12 is configured to rotate thereby facilitating rotation of the integral blade 32. As best illustrated in Figure 3, the apparatus includes a rotation mechanism in the shape of two or more rings 36 (only one visible) which extend around the outer vessel 12, between the gas containment area 17 of the casing 10 and the vessel 12, and which are connected to roller drives 37 to effect rotation of the outer vessel 17. As an alternative arrangement, instead of rollers 36, the apparatus may comprise a shaft mechanism, as generally known in the art; if the blade 32 were freely disposed within the reaction chamber 15 (instead of the mounted configuration illustrated) so that it may rotate therearound with respect to the inner and outer cylindrical walls 13, 14, then a cantilever drive shaft rather than a roller mechanism may be more preferable.

The inner chamber 25 defines a cavity (central void). The central void is utilised to collect and transfer calcium oxide 38 from the second end 24 to the first end 21 of the outer vessel 12. In the embodiments illustrated in Figures 1 to 6, within the reaction chamber 15, at the second end 24 of the outer vessel 12, the blade 32 leads to a capped section 39 which defines a container 40 arranged to collect calcium oxide 38, created in the calcining of limestone in the reaction chamber 15, as it reaches the end of the outer vessel 12. As limestone is conveyed along the reaction chamber 15 it will collect towards the lower half 28 of the outer vessel 12 but as the calcium oxide 38 transformed from the limestone reaches the final turn of the blade 32 it will become encapsulated in the container 40 and, upon further rotation of the blade 32, it will be carried to the top of the second end 24 of the outer vessel 12 and thereafter expelled into a stationary cylindrical shoot 41 which extends within the central void 25 from the top of the second end 24 down towards the bottom of the first end 21 for collection. This can be highly advantageous as this allows for movement of machinery and personnel to be at a single end as well as minimising the space required for housing the furnace 1 1 and it also allowing all inputs and outputs to be at a single location.

In an alternative embodiment, as illustrated in Figure 8, an auxiliary annular vessel 58 is provided around the outer vessel 12. The auxiliary vessel 58 is defined by a secondary outer cylindrical wall 59 and the outer cylindrical wall 14 of the outer vessel 12. In this way the outer vessel and the auxiliary vessel share a common wall 14. A reverse curved blade 57 is arranged within the auxiliary vessel 58 to channel calcium oxide, expelled from the outer vessel, along the furnace to the limestone inlet end thereof 21 . The reverse blade 57 operates in the same way as the curved blade 32 utilised in the reaction chamber 15 but is configured in an opposite direction so as to transfer calcium oxide towards the first end 21 of the apparatus. As with the curved blade 32, the reverse curve blade 57 is mounted to both the outer cylindrical wall 14 and the secondary outer cylindrical wall 59. The same motor and rotation mechanism may be used for both blades and these will be configured in a manner which is well known to persons skilled in the art. Although not visible in the Figures, at the second end 24 of the apparatus, the secondary outer cylindrical wall 59 will extend along the apparatus further than the common wall 14 and the inner wall 13 of the outer vessel 12, to allow calcium oxide at the second end 24 to fall out of the outer vessel 12 and into the auxiliary vessel 58, automatically upon rotation. Again, although not visible in the Figures, in this embodiment, the secondary outer cylindrical wall 59 is shorter at the first end 21 of the apparatus to facilitate the introduction of limestone into the reaction chamber 15. In this particular embodiment, the curved blade 32 will include the vents 47, as hereinbefore described and as shown in Figure 7, to allow the carbon dioxide to be directed out of the outer vessel 12.

A frame 42 is provided to support the rotatable annular outer vessel 12, inner chamber 25, the roller rings 36, the roller drives 37 and the drive motor 46. Although not illustrated, the frame may support the apparatus suspended at a height.

The mechanical feed system 23 is provided at the inlet 22 for the introduction of limestone. The mechanical feed system 23 comprises an introduction plate 43 to constrict limestone into the inlet 22 of the furnace 1 1 . The inlet 22 of the furnace 1 1 includes an opening 44 in the outer cylindrical wall 14 into the reaction chamber 15, which exposes part of the blade 32 of the apparatus. The introduction plate 43 may be arranged to convey limestone into the opening 44 with sufficient thrust so as to facilitate rotation of the outer vessel 12 in combination with the electric motor. The feed system 23 also includes a valve plate 45 arrangement located above the introduction plate 43 which serves to regulate the release of limestone into the inlet 22. This arrangement allows limestone to be consistently measured and compacted into the reaction chamber 15. This facilitates the compacting of the limestone between the blade turns in the initial exposed blade part of the reaction chamber 15 so as to take advantage of convection and radiation of heat. The mechanical feed system 23 actively pushes measured amounts of limestone through defined areas where specific heat is applied. The curved blade 32 collects and effectively pulls the limestone to the heated reaction chamber 15.

As illustrated in Figure 3, at the first end 21 of the furnace 1 1 , an end cap 48 is located and arranged to prevent limestone falling from the free end 49 of the blade 32 out of the end of the furnace 1 1 with the initial thrust. This end cap 48 is located adjacent the outer vessel 12 and around part of the blade 32, solely at the distal protrusion 20 inlet portion of the furnace 1 1 . The end cap 48 is substantially“U” shaped with the open end 50 arranged to allow limestone through the opening 44 to the exposed blade 32. The end cap 48 also includes a rear wall 51 defining an exit for internal parts of the apparatus.

The mechanical feed system 23 also comprises a starting cylinder 54, attached to a hopper 55 and having an opening 56 in the top. In this way limestone need only be unceremoniously dumped into the starting cylinder 54, and the curved blade 32 takes care of conveying measured amounts of limestone to be processed. This treatment will be in conjunction with measured amounts of heat so that the whole process is highly efficient.

As discussed above, the heating element 26 may be formed from relatively thin walled Inconel for hot strength, corrosion and creep resistance. The outer vessel 12 could also be formed from Inconel for the same reasons, if cost allows. The blade 32 could be formed from a more cost-effective material, such as stainless steel, which allows manipulation into a suitable shape. The blade 32 may be fabricated from a series of identical shorter sections. As will be appreciated by the skilled person, the welding of Inconel to stainless steel should not pose a problem. The casing 10 serves predominantly a containment function, although it will also serve to support the furnace 1 1 . The casing 10 will preferably be formed from a material being sufficiently heavy to resist corrosion, such as mild steel, to ensure a very long service life. Preferably the apparatus includes insulation 18, which may be a low thermal mass ceramic fibre; such material has a high performance and is easy to install at a low cost.

The configuration of the blade 32 within the rotary outer vessel 12 and the arrangement of the heating element 26 in the inner chamber 25 ensures that the apparatus of the present invention can operate efficiently in a continuous way, processing a constant supply of limestone with no downtime. The furnace 1 1 is not limited to processing limestone in batches as with traditional internal void kilns. The present invention thus overcomes the problems of the prior art associated with inefficiencies, heat losses and disruption in temperature variations.