BACKGROUND OF THE INVENTION

The conversion of carbonaceous fuels into lower carbon fuels and carbon dioxide, is well known. Just a few examples are the production of H2 from methane via steam methane reforming, the conversion of biomass into methane or ethanol via fermentation, and the reforming of coals to produce liquid fuels.

With the increasing concern around global CO2 emissions, low carbon fuels provide a mechanism for reducing emissions at their point of use. But since the production of low carbon fuels generates CO2 as a byproduct, this CO2 must be captured to achieve a systemic reduction in overall CO2 generation. Hydrogen can be substituted for carbonaceous fuels in most applications and generates no CO2 at its point of use. Thus a CO2 neutral hydrogen production would create a major step towards decarbonisation of the economy.

CO2 neutral hydrogen (‘green’ H2) is produced by electrolysis using renewable electrical power. But the existing green hydrogen industry is small, and the production cost of green hydrogen is high. If widely adopted, the additional renewable power demands for such CO2 neutral hydrogen would be substantial even to replace existing H2 production using fossil fuels.

The current route for bulk H2 production is through a reaction sequence termed steam methane reforming. A simplified overall equation is CH4 + 2 H2O -» CO2 + 4 H2

The steam methane reformer consists of two reactor stages operating at different temperatures, https://www.energv. ov/eere/fuelceils/hydrogen- production-natural-gas-reforming. The initial endothermic reaction between CH4 and water generates CO and H2. Depending on configuration of heat generation, some 10-40% of total CO2 emissions in the steam methane reforming process arise from the initial reaction stage. The CO2 content of the waste gas from this first stage is usually around 25%. The second reactor stage is a water gas shift reaction where the CO generated in the first reaction is reacted with additional water and converted to CO2 and additional hydrogen. This is followed by H2 separation, to produce an off-gas stream containing most of the overall CO2 emissions at high concentrations, typically with a composition greater than 95% CO2.

If the CO2 could be captured at the source of its production, the resultant carbon neutral hydrogen production via a steam methane reformer would be lower cost, and inherently more suited to large scale production, than electrolysis based on renewable power.

Depending on the source of energy feedstock, the quantum of CO2 produced per unit of steam methane reformed hydrogen will vary upwards from around 7 tonnes CO2 per tonne H2. As such, the conversion of fossil fuels to hydrogen, without capture of the resultant CO2, does little to reduce total CO2 emissions to the atmosphere.

It is equally possible to convert biomass into hydrogen, or lower carbon fuels such as methane or ethanol by anaerobic digestion. The CO2 content of the gas, once the low carbon fuel has been recovered, is predominantly CO2. The original biomass generation has absorbed CO2 from the atmosphere via photosynthesis. Thus, if the CO2 from this family of reactions were captured, the low carbon fuel would be net carbon negative on use. As such, CO2 capture would provide a pathway to large scale carbon-negative hydrogen or other carbon negative carbon fuels such as ethanol or methane.

For CO2 capture, depleted oil and gas reservoirs have been used to store large volumes of CO2. The CO2 must be concentrated at its production source and then pumped to the suitable storage locations. These locations are often large distances from the CO2 source. The cost of infrastructure for compressing and transporting the CO2 to the storage, are typically quoted in the range of US$50-US$150/tonne CO2. Questions also remain about the longevity of each storage location and whether leakage might ultimately occur.

In a separate field of endeavor, CO2 can also be sequestered permanently by chemical reaction with suitable ultramafic rocks. These may be naturally occurring minerals like serpentinites, or byproducts of other industrial processes such as various slags and fly ashes. The CO2 reacts chemically with the CaO or MgO fraction of the rock, to produce a carbonate. Mg(Ca)SiO3 + CO2 » Mg(Ca)CO3+ SiO2

The permanent storage of CO2 in nature occurs through this chemical sequestration. But the natural sequestration rate is slow. The naturally slow reaction is further limited by the limited surface area of ultramafic rock exposed to the CO2, and the inherently low concentration of CO2 (400ppm) in the atmosphere.

There have been many publications on this direct sequestration as a method of carbon storage, but no commercial application has been implemented. The direct sequestration can has followed broad themes, aimed at accelerating the reaction rate. Increasing the available surface area of rock, increasing the rock reactivity, and increasing the pressure of CO2.

To increase the surface area of rock, Walder (US 9,194,021 B2) teaches the use of heaps of suitable crushed rock, through which the combustion gas is passed. However, the reaction is still slow, and a large amount of CO2 is lost to the atmosphere in such a system.

In the second theme of enhancing rock reactivity, several authors such as O’Connor (O’Connor et. al. Research Gate 236576153 Continuing Studies on Direct Aqueous Mineral Carbonation for CO2 Sequestration, March 2002), Breuil (Breuil et. al Mineralogical Transformations of Heated Serpentine and Their Impact on Dissolution during Aqueous-Phase Mineral Carbonation Reaction in Flue Gas Conditions, Minerals 2019, 9(11 ), 680), and Benhelal (US 2021/0047197), have calcined serpentinite rocks to form a more reactive amorphous phase. When finely ground, this calcined rock sequesters CO2 more rapidly and more completely than the original mineral structure in the rock. The direct carbonation proceeds to 50 to 80% conversion of the available magnesium when fine rock is reacted at high pressure CO2 at elevated temperatures, over a duration of hours. But again, the cost of concentrating the CO2, including that from the calcination, and transporting the gas to the location of an expensive autoclave, has limited the practical application of the direct carbonation.

It is the object of the current invention to provide a process for the production of low carbon fuels, and efficiently capture the resultant CO2 at the point source of fuel generation. In so doing, the invention provides a pathway to large scale production of carbon neutral or carbon negative fuels, including hydrogen.

SUMMARY OF THE INVENTION

THIS invention relates to a process for the production of low carbon fuels, i.e the production of hydrogen or other low carbon fuels, including the steps of: reacting hydrocarbon/s with water to produce low carbon fuels product, and a CO2 by-product; providing porous wetted heaps or dams of particulate ultramafic rock, enclosed within a coating that is substantially impermeable to gas flow; and supplying the CO2 byproduct to a series of said porous heaps or dams, in which CO2 is sequestered with the particulate rock.

Preferably, the ultramafic rock is calcined.

Typically, the particulate ultramafic rock is: stacked in a heap with a free water content maintained at I Q- 20 by weight located on an impermeable pad enclosed in a coating that is substantially impermeable to gas flow; or contained in a dam enclosed in a coating that is substantially impermeable to gas flow.

Preferably, the CO2 gas is supplied into the enclosed heaps or dams to maintain a CO2 partial pressure inside the enclosure is greater than 0.1 , and typically greater than 0.2, preferably greater than 0.5 atmospheres, and up to 2 atmospheres within the heap.

Typically, the p80 of the particulate ultramafic rock is in the range 0.05- 10mm, and preferably between 0.1 and 1 mm, and the p10 is greater than 0.1 mm, and preferably around 0.2mm.

Water and CO2 are preferably circulated within the heaps or dams.

The CO2 may be bubbled into the base of the enclosed heaps or dams and allowed to rise through the enclosed particulate rock.

Low grade streams of CO2 containing less than 30% CO2 are typically reacted in a separate heap or dam to high grade CO2 streams containing more than 85% CO2.

The ultramafic rock may be a residue arising from prior processing of rock for the purposes of recovering other values, such as a residue arising from prior processing of rock for the purposes of recovering nickel or diamonds or asbestos, or byproducts or wastes from industrial processes such as fly ash or slag.

The heaps or dams are preferably arranged successively, and highgrade CO2 byproduct comprising more than 85%, typically more than 90% and up to 100% arising from the production of low carbon fuels, is assigned to the high-grade heaps or dams, and lower grade sources of CO2 byproduct comprising less than 30%, typically about 25% arising from the fuel production are assigned to the low grade heaps or dams for scrubbing CO2; and particulate rock in the enclosed heaps or dams are advanced counter- currently to the gas flows, such that the highest concentration of CO2 reacts with the partially reacted rock, and the lowest concentration of CO2 reacts with the freshest rock; such as to recover most of the CO2, and to utilise the sequestration capacity of the rock more fully.

Preferably, the heaps or sams are arranged such that the vent gas from the high-grade heaps or dams is assigned to the low grade scrubber heaps, such that each heap in series has a progressively lower CO2 concentration within the enclosed heap space.

The CO2 containing stream assigned to the high-grade heaps or dams may be the product of the water gas shift reaction after H2 has been separated.

The CO2 containing stream assigned to the high-grade heaps or dams may be the product of the processing of biomass to form a low carbon fuel.

Hydrogen used to calcine the rock or generate heat required in the production of a low carbon fuel, such as to reduce the amount of low- grade CO2 to be sequestered.

Gasses may be used to transfer heat between the rock preparation and the formation of low carbon fuels to maintain the heap or dam temperature between 25 ^e C and 80 ^e C, and preferably around 50 ^e C. Gasses may be used to redissolve the carbonated rock and the heat is used to disproportionate the resulting solutions to form magnesium carbonate.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a flow diagram of an embodiment of the invention with sequential enclosed CO2 sequestration heaps or dams in series;

Figure 2 is a graph showing CO2 absorption kinetics for Canadian Ultramafic Ni, Finish Ultramafic Ni, and Brazilian Ni slag; and

Figure 3 is a graph showing CO2 absorption of reactive rock ground to <75 microns in a vessel containing 100g of solids and 1 liter of CO2 gas.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is the integration of

• the production of lower carbon fuels from a carbonaceous energy source, to generate a CO2 as a byproduct gas,

• with the sequestration of the CO2 in a series of enclosed heaps or dams containing crushed ultramafic rock, where the resultant CO2 product stream from the fuel production forms an ideal feed for sequestration, and the resultant waste heat is utilised in feed preparation for sequestration.

In the embodiments in which the rock is calcined prior to sequestration, the integration can be further extended by the use of the low carbon fuel to calcine the rock used as the sequestration feed. And in a further set of embodiments, the sequestration can be used as an intermediate step in converting the CO2 generated from low carbon fuel production as a leachant for producing a valuable MgCO3 product.

The gas flows between the low carbon fuel production and sequestration processes are arranged to provide sufficient gaseous residence time for the absorption of most of the CO2, and sufficient solids residence time to utilise much of the inherent CO2 capacity of the rock.

Heat flows from the fuel production support one or more of rock preparation, sequestration rate, and product refining, depending on the preferred embodiment.

The primary off gas from the production of low carbon fuels is a source of concentrated CO2 containing close to 100% CO2. There may also be a secondary source of gas containing a lower grade of CO2, typically containing between 10 and 30% CO2.

The range of possible embodiments of the invention encompass different higher carbon fuel sources, different low carbon fuel products, different configurations of flows of the CO2 quantities and concentrations through heaps or dams containing the selected ultramafic rock, and either storage or further refining of the carbonated rock.

One such embodiment consists of several enclosed heaps for the sequestration of CO2 generated by hydrogen production generated from a methane source, is illustrated in Figure 1 .

Figure 1 shows a H2 production unit 10 that receives reactants 12 (CH4 and H2O) and releases H2 product 14 and two CO2 streams 16 and 18. One stream is high grade CO2 16 (with a CO2 content of more than 85%, typically more than 90% and up to 100%), and the other a lower grade stream 18 with a CO2 concentration less than 30%, typically about 25%. The H2 production unit 10 may be a steam methane reformer consisting of two stages of reactor which take place at different temperatures. reforming. Depending on the configuration of the hydrogen production, between 10-40% of total CO2 emissions arise in the low-grade CO2 stream, from this initial endothermic reaction. The second reaction is a water gas shift reaction where CO is converted to CO2 and additional hydrogen. H2 separation then produces a high-grade off-gas stream containing most of the CO2 emissions, typically at CO2 concentrations, greater than 95%.

The gas stream 16 is a high-grade CO2 stream is supplied to a series of CO2 sequestration heaps or dams (HG1 -3) containing crushed (particulate) rock and water.

The high-grade stream is fed to HG1 , with a gas overflow 20 to HG2 and ultimately a bleed 22 into one or more reactors HGn from which a stream 24 flows to a low-grade section LG1 to remove any ingress air. Also feeding this latter heaps or dams is the low-grade CO2 stream 18 with a bleed to a final scrubbing heap a vent 26 to atmosphere.

The freshest crushed rock 28 progresses from LG1 countercurrent to the gas flow to ultimately reach HG1 before being retired and stored 30, or further refined to recover values from the carbonated residue.

Heaps or dams are constructed from rock which has been crushed and/or agglomerated to create competent particles with sufficient pore space between the particles to enable bulk gas permeation or are saturated with water in a contained dam, into which the CO2 is injected.

The heaps or dams are sealed such that gasses can be recycled internally or vented in a controlled manner. A bleed point may provided at the top of the heap , to allow the enclosed gas to bleed from the top of the. The crushed rock is enclosed within a coating which is impermeable to gas flow and is applied around the external surfaces of the heap or dam. The coating may for example be a welded geotextile or similar polymer based sheet, or a spray layer composed of an organic material such as an epoxy, or an inorganic material like shotcrete or clay.

The high-grade carbon dioxide (CO2) gas stream 16 is supplied into the enclosed heap or dam HG, typically at the base of the heap or dam HG, such as to maintain a pressure above the heap or dam of around 1 atmosphere.

The internal partial pressure of CO2 in the HG heaps or dams is maintained at greater than 0.2 atmospheres, and preferably greater than 0.5 atmospheres (and up to 2 atmospheres), enabling CO2 18 to flow though the heap or dams HG prior to being vented to LG1 . As CO2 is consumed by the rock, additional CO2 is added the heap or dam. As the partial pressure of CO2 drops the gas is vented to LG1 .

Heaps or dams may be arranged such that gas flow is in series or in parallel, from HG1 to LG1 . The solids flow may be controlled by allocation of gas flow to each heap; and may be arranged such that the flow of solids is either in series or parallel.

In the case of the dam, the rock is saturated, and CO2 is bubbled into the base, and dissolves in the slurry as the bubbles rise. Alternatively, CO2 can be injected into a circulating stream of water and hence be carried by the recirculating water into the dam.

In the case of the heap, the heap located on an impermeable and the free water content of the heap is maintained at 10-20%. The CO2 is pumped into the base of the heap and where necessary collected within the seal and circulated through the heap.

The low-grade CO2 stream 18, is added to a separate sealed heaps or dams LG1 , and allowed to pass through the particulate rock thus reducing the CO2 concentration as the gas passes through the heap and the CO2 reacts with the crushed rock. When the CO2 partial pressure is sufficiently low, the gas is vented, enabling further low-grade CO2 to be added. The gas flow may be a single pass or may be recirculated within the seal. The vented gas stream may be processed to upgrade the CO2 content and recirculate this as a highgrade CO2 stream.

Sequestration rates are known to be proportional to the CO2 content in the gas from which CO2 is to be sequestered. Gaseous residence times available in a heap or dam, prior to needing to vent the gas to enable addition of further CO2 to be sequestered, is greatest when CO2 concentrations approach 100%. So the availability and use of high-grade CO2 from the production of hydrogen or low carbon fuels, aids both the sequestration rates and the extent of CO2 removal that can be achieved prior to requiring either scrubbing or upgrading.

The rate of rock reaction is also known to slow as sequestration proceeds. For this reason, the highest-grade CO2 source is directed to one or more of the heaps containing the slowest reacting rock. In this way the maximum conversion of the MgO or CaO content of the rock to the carbonate, can be achieved prior to the rock being discarded.

Examplel

Figure 2 shows the reactivity of CO2 in the presence of various forms of ultramafic rock containing 30% by weight water. The rocks have been ground to less 75 micron and are placed in the base of a sealed reactor at 70 ^e C with an overpressure of CO2.

Table 1 - Test Conditions Table 2 - Model Form CO@ Absorption= C1 *Days ^A C2

The Canadian ultramafic and Brazilian slag were tested in an atmosphere of 17% CO2, whilst the Finnish ultramafic test utilised 100% CO2.

Figure 2 illustrates the impact of reaction time on the uptake rate of CO2 into the solids for three different ore types. The reaction rate shows a logarithmic decay in reaction rate over time. The reaction rate of a fresh solid is higher than that which has been partially sequestered, presumably due to greater exposed unreacted surface area, and the availability of the most reactive mineral species.

Example 2

Figure 3 illustrates that under conditions where a thin film of water exists around the particles, and a reactive rock ground to <75 microns, the initial reaction rate with CO2 is fast. Most of the CO2 in the gas can be removed by the rock in less than 1 hour of contact between fresh rock and gas, in a vessel containing 100g of solids and 1 liter of CO2 gas. This supports the countercurrent flow of solids and CO2 containing gas, where the requisite residence time for CO2 is shortest when scrubbing low grade CO2 prior to venting. The residence time of both gas and solids in a high-grade heap or dam is dictated only by the need to vent impurities in the CO2 gas.

Table 3 - Test Conditions

Adding in additional moisture increased the kinetic rate at insertion of water which is primarily due to the uptake of CO2 into the water to form carbonic acid. Post this initial spike, the reactivity remained on the previous trajectory.

The ideal temperature for operating the sequestration heaps or dams is between 20 ^e C and 80 ^e C, and preferably between 40 ^e C to 50 ^e C. The sequestration reaction is exothermic, enabling the autothermal heating of the heaps when initially fed with 100% CO2. But under normal circumstances, the heat generation from a reaction of rock is insufficient to maintain this temperature for the full duration of the reaction. An initially rapid exothermic direct carbonation can be achieved by utilising the waste heat generated earlier in the integrated flowsheet, to warm the low grade heaps or dams where gas residence time is critical.

In another embodiment of the invention, a more reactive solid can be utilised for scrubbing the final CO2. Examples of such reactive species are rock which has been calcined, or fly ash from historical consumption of brown coal, or iron and steel making slags containing high levels of calcium.

The invention has a number of benefits, over and above the production of low carbon fuels with a net negative or very low carbon footprint.

Firstly, the integration can be achieved when the source and sink for CO2 are co-located, reducing the need for material transport between sites. For example, a biomass to ethanol plant could be established adjacent to a historical asbestos mine, thus rehabilitating the waste rock from the old mine.

In a similar case, where natural gas is piped near an ultramafic nickel mine, the gas could be utilised as the feedstock for hydrogen, while the residue of the nickel flotation could be used as the source of ultramafic rock.

The currently available network of pipelines, and the location of agricultural wastes, provide many opportunities to locate low carbon fuel production near suitable sources of rocks. Such co-location options open up the potential for rapid transition to a low carbon economy.

Secondly, in the case of hydrogen production from gas using the current invention, the presence of a gas pipeline from which the gas can be sourced also provides the infrastructure to connect hydrogen production with end users; either through blending the natural gas and the low carbon hydrogen, or by retrofitting the pipeline to make it compatible with hydrogen transport at higher concentrations, or simply by providing a corridor for a new pipeline to connect H2 production with end users.

Using the current invention, low carbon footprint production of low carbon fuels can be considered from a multiple set of carbonaceous energy sources including the sources such as coal and petroleum, without increasing total CO2 emissions.

The invention also provides for multiple opportunities to reduce the quantity of CO2 in the atmosphere.

As one example, the use of biomass as the energy feedstock enables fuel production with a large net carbon negative footprint. Plants naturally absorb CO2 from the atmosphere, and in a normal carbon cycle, a very large proportion of the resulting biomass is returned to the atmosphere by biodegradation. However, when the same biomass is converted to hydrogen or low carbon fuels with the CO2 being sequestered, the CO2 is totally removed from the carbon cycle. The invention can not only use nature to convert biomass into a fuel but achieving this in a strongly carbon negative method. The full value of the original photosynthesis is captured in the form of carbonated rock. As such, the invention enables much more efficient removal of CO2 from the atmosphere by agriculture or forestry.

As a second example, the use of the invention also allows the use of coal or petroleum products as feedstock for hydrogen production and use as fuel.

Such feedstocks can be converted to H2 and centrally sequestered with minimal net carbon footprint, rather than being used directly as fuel and creating many distributed CO2 sources to be addressed.

Fourthly, the availability of low carbon H2 provides a lower cost source of fuel or reduction gas than electrolytic hydrogen. Furthermore the hydrogen production process is inherently more scalable. This lower cost form of hydrogen creates an economic incentive for industries like steel and cement to convert to hydrogen, rather than incurring the energy and infrastructure associated with concentrating CO2 streams for transport to a remote location for CO2 storage.

Fifthly, the residue of direct carbonation represents a feedstock for generating magnesium carbonate and possibly other byproducts. Dissolution of the residue under a CO2 pressure can generate a solution of magnesium bicarbonate, which can be subsequently disproportionated under vacuum or at higher temperatures to produce a purified magnesium carbonate. The high-grade CO2 sources and heat sources generated from the production of low carbon fuels integrate neatly into this process.

In summary, the integration of production of low carbon fuels, with the sequestration of the high-grade CO2 sources in a series of enclosed heaps, enables much more efficient CO2 sequestration, by providing for ideal gas concentrations and temperatures to initiate and maintain high sequestration rates and high CO2 uptake by suitable rock.