WO2008015473A2

WO2008015473A2 - Process for creating carbon sinks

Info

Publication number: WO2008015473A2
Application number: PCT/GB2007/050444
Authority: WO
Inventors: Paul Lawrence Younger
Original assignee: University Of Newcastle-Upon Tyne
Priority date: 2006-07-29
Filing date: 2007-07-26
Publication date: 2008-02-07
Also published as: WO2008015473A3; GB0615182D0; PE20080641A1

Abstract

A process for creating a carbon deposit in a pit comprises the steps of: a. depositing a growing medium in the base of the pit; b. allowing or causing water ingress to the pit; c. growing wetland plants in the growing medium to directly sequester carbon dioxide from the atmosphere; d. allowing wetland plants to die back to deliver biomass below the water line; and e. controlling the level of water in the pit such that at any time the surface of the water is a small distance above the growing medium surface.

Description

Process for Creating Carbon Sinks

Field of the Invention

The present invention relates to a process for creating carbon sinks and in particular to a process for creating long term carbon sinks.

Background of the Invention

It is generally acknowledged within the scientific community that one of the causes of global warming is the production of greenhouse gases resulting from the burning of fossil fuels. Political efforts to reduce emissions of greenhouse gases have resulted in the creation of a market in what are known as "carbon credits". Producers of greenhouse gases can offset their production by purchasing "carbon credits". Carbon credits can be generated by producers of renewable energy, and by organisations which actively sequester carbon dioxide from the atmosphere, subject to certain requirements, those being: that the displacement of carbon emissions which its activities yield are in line with the principle of 'additionally', which means that:

(i) The activity replaces carbon emissions which would otherwise take place or else actively removes CO2 from the atmosphere.

(II) The activity is not in itself a response to a regulatory obligation.

(III) The activity is not sufficiently economically attractive that it would be likely to proceed spontaneously with private sector investment.

Abandoned surface mine voids present environmental problems. Historically, surface mine voids have been used for land fill, however, in many cases this is not a suitable use, either because environmental regulations would be breached by such use (in Europe environmental legislation largely prevents disposal of waste below the water table), or because the abandoned mine is in such a geographically remote location that transportation of waste is uneconomical. Many abandoned surface mine voids are allowed to flood to form pit lakes, which have unnaturally great ratios of depth / surface area and become 'meromictic' in limnological terms. Many pit lakes pose their own environmental problems. Around 25% of them contain water that is highly acidic and/or metalliferous, due to leaching of contaminants from weathered rocks in and near the flooded void. In particular, acidic conditions often make the metals Fe, Al, Mn (and less commonly also one or more of Pb, Cu, Cd, Ni, Zn and Se) more mobile and bio-available, resulting in an environment inhospitable for wildlife. Birds landing on contaminated pit lakes may themselves be vulnerable to poisoning, and may also serve to transmit toxic metals further up the trophic chain far beyond the limits of the former mine site in the event that they become subject to predation. Even where they contain good quality water, pit lakes tend to lose much water to the atmosphere by evaporation, consuming natural groundwater (and to a lesser extent surface water) resources which are often in short supply in the semi-arid and and areas where many large modern surface mines are found.

It would be desirable to provide a method of creating carbon sinks.

It would also be desirable to provide an alternative use for abandoned surface mine voids.

It would also be desirable to provide an alternative use for abandoned surface mine voids which does not pose the environmental problems associated with pit lakes.

In the present specification the words "void" and "pit" are used interchangeably and refer to any enclosed space in the ground, whether man-made or naturally occurring. Whilst performance of the process of the invention is particularly suited to pit lakes resulting from abandoned surface mines, the process of the invention may also be performed in other voids, such as a void resulting from the quarrying of industrial minerals such as limestone or shale for example. Summary of the Invention

According to one aspect of the invention there is provided a method of creating a carbon sink as specified in Claim 1.

According to another aspect of the invention there is provided a carbon deposit as specified in Claim 17.

The present invention provides a convenient process for sequestering atmospheric carbon. The sequestration of carbon may give rise to a right to trade carbon credits, subject to the requirements set out above being met. Furthermore, environmental damage associated with abandoned pit lakes may be reduced.

Brief Description of the Drawings

In the drawings, which illustrate a method of creating a carbon deposit according to the invention, and is by way of example:

Figure 1 is a schematic representation of a pit lake and illustrates the elements of an embodiment of the invention, and

Figure 2 is a cross-sectional elevation of the embodiment illustrated in Figure 1.

Detailed Description of the Preferred Embodiments

Referring now to Figures 1 and 2, a disused surface mine site 1 includes an open pit 2, surrounding land 3 and a surface drainage feature 12. This surface drainage feature may be a natural dry channel, formerly dry land, a natural stream or river, a natural lake and/or wetland, a man-made canal and/or impoundment, an estuary or man-made harbour, or open sea. The natural water table 4 within the surrounding land 3 rises to within a height x of the surface. The elevation of the water table and therefore the height x may vary seasonally.

As is clear from Figure 2, the bottom 5 of the pit 2 is below the height of the water table 4. This means that throughout the duration of operation of the pit as a productive mine, water must be pumped out of the pit to allow working to continue. In the absence of pumping, the pit 2 would simply flood. The prior art method of dealing with spent pit mines is simply to cease pumping thereby allowing the pit 2 to flood, forming a pit lake.

The method of the invention requires inter alia the controlled flooding of the pit 2 in order to provide conditions in which biomass may grow. As such a pump 9 is provided which is connected to a pipe 10, and which maintains the depth of water 6 in the pit within an optimal range.

In order to commence the process of biomass production and hence the creation of a carbon sink, a wetland must be created in the pit 2. This requires that the pit 2 is first substantially cleared of water. A body 7 of growing medium, which may comprise soil from the surface of the surrounding land 3 is deposited on the bottom 5 of the pit and seeded with a suitable plant. Suitable plants include (but are by no means restricted to) Typha sp., Phragmites %p., Iris pseudacora, and J uncus sp.. These plants may be seeded into the soil placed in the bottom of the pit, or the soil may be left to become seeded naturally through deposition of seeds carried in the atmosphere. Another method of seeding involves depositing a growing medium containing seeds, rhizomes or emergent plants in the bottom of the pit. For example the surface drainage feature 12 may well contain such plants, which might be readily transplanted together with sediment dredged from the same.

Once the growth medium and seeds are in the bottom of the pit, a controlled amount of water is allowed into the lower part of the pit 2. The level 6 of water in the pit 2 is controlled by a pump 9, which draws water out of the pit 2 through pipe 10 and transfers the water to the surface drainage feature 12 via pipe 11. The level of water 6 is controlled such that in the growing season there is a standing depth of water of between 0.15m and 0.5m above the level of sediment in the pit 2. Whilst a wetland regime can be maintained with less than 0.15m standing depth of water, and can even withstand periods in which the surface of the growing medium is dry, the result is that terrestrial plant species colonise the growth medium, those species being poorer producers of biomass. In the season during which plants are dormant the water level may be increased in order to stimulate collapse of plant biomass growth which occurred in the growing season preceding the said dormant season. The level of water may be increased by anything up to six metres above the surface of the growing medium, but would more typically be increased to about two metres above the surface of the growing medium. Increasing the depth of water above the surface of the growing medium reduces the requirement for pumping water from the pit at least temporarily.

As plants 8 grow, so does the surface of growing medium 7, which in addition to the original material introduced to the base 5 of the pit now includes plant litter and other sediment (which may have been carried down to the floor of the pit by running water or the wind). As mentioned above, no further growing medium is intentionally added into the pit, but as the plants 8 go through their growth and decay cycle sediment is created which advances up the pit 2. In effect there is additional growing medium in the form of sediment. The ma_jority of plant roots are present in an upper layer of the growing medium of about one metre in depth. Whilst some roots do penetrate the growing medium to depths much greater than one metre, the growing medium behind the upper layer becomes a store of carbon rich biomass.

As the level of growing medium advances up the pit 2 the level of water 6 is increased until the surface of the sediment reaches some maximum level. Normally the maximum level will correspond to the natural water table level 4. However, where the rock mass above the water table is of relatively low permeability, the process may be continued by adding water from outside the pit until the surface of the sediment reaches the top of the pit 2 (or an even higher level if the pit is enclosed within an impoundment).

The carbon sink consists of the organic matter deposited as the plants go through their growth cycle, which involves growth through photosynthesis, which is based on fixing of CO2 from the atmosphere, and dying back in the winter, during which time carbon fixed in the plant materials is stored below the water level, where the bulk of it is preserved in solid form. For the types of plant used, there is no requirement to add further soil beyond that which is initially necessary to form the plant growth medium. As the volume of biomass advances up the pit, new plants grow in the organic matter residues left by dying plants.

In order to speed up the process of creating a deposit of carbon, additional organic matter may be introduced into the pit, with the water level being controlled such that the introduced organic matter is located below the water line. Such organic matter may be green waste compost for example.

Pits left behind by surface mining are suited to this type of carbon sinking because of their depth, which is typically from tens of metres to some hundreds of metres. This provides for the build up of significant volumes of biomass before the biomass front reaches the maximum level.

An advantage of applying the carbon sink formation process of the invention to abandoned surface mines is that the generated biomass is isolated from the external atmosphere, and hence a supply of oxygen, by the walls of the pit on all sides and a body of water on the its upper surface. Aerobic degradation of organic matter releases significant quantities of CO2 to the atmosphere. Isolating the generated biomass from the atmosphere ensures that degradation thereof is primarily by an anaerobic process, rather than an aerobic process, which would be the case if the biomass were exposed to the atmosphere. Anaerobic degradation occurs much more slowly than aerobic degradation emitting significantly less carbon to the atmosphere. Furthermore, as the volume of biomass grows, the biomass towards the bottom of the pit is compressed by the weight of the overlying, younger biomass and the water, increasing the density of the deposit as a whole and thus the total mass of carbon per unit volume remaining in long-term storage. The action of compression also removes oxygen from the deposit, thereby enhancing the anaerobic condition within the pit.

Two gases are likely to be produced by the stored biomass: carbon dioxide and methane, both of which are greenhouse gases. As the quantities produced are typically very small in comparison to the amount of carbon remaining below the sediment surface, the net loss of carbon from storage can be quantified (by means of periodic atmospheric monitoring above the pit surface) and discounted from the total amount of carbon claimed as having been placed in long-term storage. In economically favourable circumstances, some or all of the methane could be captured using pipes buried in the sediment during accumulation and / or using a transparent cover 13 emplaced above the growing plants. Captured methane can be used to generate energy supplying part of the power for the pump 9, delivered using conduit 14.

Another advantage of the process of the invention is that the type of wetland being used to create and deposit biomass is capable of remediating environmental and pollution problems associated with abandoned surface mine voids.

Whilst in the example illustrated the level of water in the pit is controlled by pumping, other suitable means of controlling the water level in the pit may be used. For example, in some pit environments it may be possible to control the water level by using wells to intercept groundwater before it can enter the pit, and / or by intercepting surface water inflows. Alternatively, for a pit on a hill or mountain, a tunnel or borehole directed horizontally into the pit from the side of the hill or mountain may be used to drain water and control the water level in the pit using a valve.

Finally, in certain geological settings a void may exist (or may be created) which does not intersect the water table. In low permeability rock, or with a suitable low -permeability liner (such as compacted clay, geotextile or other synthetic material) it would be possible to undertake the creation of a carbon sink by diverting other water into the void as needed to maintain the desired depth range above the sediment surface.

Claims

1. A process for creating a carbon deposit by creating a wetland in a pit, the wetland being created by following the steps of: i) depositing a growing medium in the base of the pit; ii) allowing or causing water ingress to the pit ; iii) growing wetland plants in the growing medium through multiple growth and collapse cycles in the growing and dormant seasons respectively; iv) controlling the level of water in the pit such that at any time the surface of the water in relation to the growing medium surface is capable of maintaining the wetland;

and wherein repeated growing cycles cause the growing medium surface to rise up the pit and a body of carbon rich biomass to build up in the pit beneath the growing medium surface.

2. A process for creating a carbon deposit according to Claim 1, wherein the level of water in the pit is controlled such that at any time the surface of the water is a small distance above the growing medium surface.

3. A process for creating a carbon deposit according to Claim 1 or 2, wherein the growth medium deposited in the pit includes wetland plant seeds.

4. A process for creating a carbon deposit according to any preceding claim, wherein the growth medium deposited in the pit includes wetland plant seedlings.

5. A process for creating a carbon deposit according to any preceding claim, wherein the growth medium deposited in the pit includes mature wetland plants.

6. A process for creating a carbon deposit according to any preceding claim, wherein the growing medium is seeded by air borne seeds.

7. A process for creating a carbon deposit according to any preceding claim, wherein the plant species are selected from the group comprising: Typha sp., Phragmites fφ., Iris pseudacora, and J uncus sp.

8. A process for creating a carbon deposit according to any preceding claim, wherein the pit is a surface mine void, a quarry void, or a natural void.

9. A process for creating a carbon deposit according to any preceding claim, wherein the water level is controlled by pumping water from the pit.

10. A process for creating a carbon deposit according to any preceding claim, wherein the water level is controlled by intercepting water ingress into the pit and controlling its subsequent flow into the pit.

11. A process for creating a carbon deposit according to any preceding claim, wherein the water level is controlled by gravitational drainage of the pit via an inclined borehole or tunnel which debouches at some point on the ground surface beyond the pit boundaries at an elevation lower than the minimum water level in the pit.

12. A process for creating a carbon deposit according to any preceding claim, wherein the pit lies above the water table, and wherein said pit is lined with a low permeability material to retain water within the said pit.

13. A process for creating a carbon deposit according to any preceding claim, wherein the water level in the pit is controlled such that at any time the depth of water above the growing medium surface is less than or equal to six metres.

14. A process according to any preceding claim, wherein the water level in the pit is controlled such that at any time during the growing season the surface of the water is between 0.15m and 0.5m above the growing medium surface.

15. A process according to any of Claims 1 to 14, wherein in the dormant season the distance between the surface of the water and the growing medium is between 0.5m and 6m.

16. A process according to Claim 15, wherein in the dormant season the distance between the surface of the water and the growing medium is between 0.5m and 2m.

17. A process according to any preceding claim, wherein additional organic matter is introduced to the pit and wherein the water level is controlled such that the added organic matter is located below the surface of the water.

18. A process for creating a carbon deposit according to any preceding claim, wherein methane gas released from the wetland in the pit is collected.

19. A process for creating a carbon deposit according to Claim 17, wherein the collected methane gas is used as a fuel source for a device used in controlling the level of water in the pit.

20. A carbon deposit created in a pit by application of the process as claimed in any of Claims 1 to 18.

21. A process for creating a carbon deposit substantially as described with reference to and as illustrated in the drawings.