GB2615165A

GB2615165A - Carbon sequestration process, storage repository and plant

Info

Publication number: GB2615165A
Application number: GB2217564.0A
Authority: GB
Inventors: Brown James; Booth John
Original assignee: Lapwing Energy Ltd
Current assignee: Lapwing Energy Ltd
Priority date: 2022-11-24
Filing date: 2022-11-24
Publication date: 2023-08-02
Also published as: GB202217564D0

Abstract

Biomass is converted to biochar via pyrolysis, and water added to produce a slurry. The slurry is pumped into porous geotextile bags 7, 8 located in underground storage pits 1 wherein a proportion of the water can percolate from the bags while the biochar remains contained within. The filled bags are covered with backfill to form a storage repository. Conditions are maintained in the repository such that the bags remain flooded with water, holding the biochar under anaerobic conditions. Flooding may be maintained naturally by placing the bags below the water table 5. Pyrolysis by-products may be used for power generation.

Description

CARBON SEQUESTRATION PROCESS,

STORAGE REPOSITORY AND PLANT

TECHNICAL FIELD OF THE INVENTION

This invention relates to the capture and storage of atmospheric carbon.

BACKGROUND

The global agri-food sector is responsible for circa 30% of greenhouse gas (GHG) emissions and 60% of lost natural environments around the world. To hit Net Zero, carbon emissions from every sector of the economy need to be abated and carbon sequestration implemented to remove both past and difficult-to-remove emissions whilst protecting the natural environment.

Atmospheric carbon dioxide is a greenhouse gas which causes gradual warming of the biosphere and climate change. Carbon sequestration is a strategy which focuses on removing carbon dioxide from the air, capturing the carbon and then storing it under conditions which prevent, or at least greatly slow down, its -2 -return to the atmosphere.

Photosynthesis is a natural process in which chlorophyll in green plants uses light energy to create carbohydrates and oxygen from carbon dioxide and water. The carbon is retained in the biomass while oxygen is released back into the air. Whilst vast amounts of carbon is captured in this way the majority is, sooner or later, returned to the air, either through natural decay brought about by microbial decomposition, use of the biomass as food or fertilizer, or combustion through use as a source of fuel. It has been proposed to store biomass underground in disused and re-sealed oil or gas wells, mines or natural caverns, but even then, natural processes occur which, over time, returns carbon to the atmosphere.

Biochar is a material which is produced from wood by a process of pyrolysis, which involves heating the dried material to a high temperature under anaerobic conditions (lack of oxygen). Biochar is high in carbon and is commonly used as a fertiliser to improve soil fertility.

The present invention was born out of a necessity for an agricultural operator to develop a transition plan for peat land to enable food production to be improved and continued into the future without having an irreversible, negative impact on the environment. -3 -

SUMMARY OF THE INVENTION

The present invention proposes a carbon sequestration process which utilises photosynthesis to remove CO2 from the atmosphere via production of a suitable biomass. In a preferred embodiment the biomass is short rotation coppice willow (SRCW) grown on rewetted peatland which simultaneously abates landscape soil emissions from agriculturally drained lowland peat. Such emissions account for about 3% of total UK GHG emissions. The biomass is harvested as a crop and subjected to high temperature pyrolysis which produces biochar, a solid material of approximately 86% carbon. Long term stable carbon sequestration is achieved by burying the biochar in a contained, waterlogged condition, providing a highly concentrated and easily verifiable carbon mass-storage solution.

Energy from the pyrolysis may be utilised in Controlled Environment Agriculture (CEA) to enhance food production. In this way, the invention addresses the inherent dilemma of bioenergy crops, namely the loss of land from food production.

The invention includes a carbon storage repository produced by the process and a carbon storage plant which incorporates the storage repository.

BRIEF DESCRIPTION OF THE DRAWINGS -4 -

The following description and the accompanying drawings referred to therein are included by way of non-limiting example in order to illustrate how the invention may be put into practice. In the drawings: Figure 1 is a diagrammatic cross section through a proposed carbon storage repository; Fiaure 2 is a general layout of a carbon storage plant; Figure 3 is a plan view of a carbon storage repository included in the plant; Fiaure 4 is a longitudinal section through the repository, taken at the position IV-IV indicated in Fig. 3; Figure 5 is a transverse cross section through the repository, taken at the position V-V indicated in Fig. 3; Figure 6 is a detailed vertical cross section through one of the long-term monitoring points shown in Fig.s 3 to 5.

DETAILED DESCRIPTION OF THE DRAWINGS

Current agriculture practices on reclaimed lowland peat lands emit vast amounts of GHG emissions. By rewetting lowland peat, these emissions can be abated. Short Rotation Willow Coppice (SRCW) -5 -is a fast growing woody bioenergy crop that can be grown quickly on rewetted peat and efficiently captures carbon dioxide from the atmosphere through photosynthesis. SRCW was selected as the most suitable bioenergy crop for the following reasons: * SRCW is one of the most used perennial bioenergy crops in Europe and the UK and can be coppiced every 3-4 years.

* Willow roots can tolerate periods of waterlogging and can regrow naturally in rewetted fen peatlands following coppicing.

* It is grown commercially on wet peatlands for a range of traditional uses such as basket making. This means that the techniques, infrastructure and machinery needed are already proven and available.

* SRCW can also tolerate nutrient-rich and heavy metal contaminated conditions for phytoremediation.

* SRCW has the ability to generate high yields of woody material per annum.

* Experimental analysis shows SRCW to have a high energy generation potential which can be utilised for other purposes.

Initial ground preparation for SRCW production involves clearance of existing foliage or previous crop residue. Planting of willow bulbs or whips is followed by a period of growth with monitoring and minimal weeding where necessary. SRCW has a three year cycle time, taking approximately three years to grow and be dried after initial coppicing. A conventional forage harvester can be used for harvesting, fitted with stronger cutting blades. When the SRCW is ready to be harvested it is chipped and stored under drying conditions, reducing its moisture content so that decay -6 -does not occur. Transportation of the harvested crop is easily carried out using conventional farm tractors and trailers. Storage, mixing and drying can be carried out in existing grain barns. Transfer of the dried material can be by conventional farm loader.

After drying the SRCW is subjected to a pyrolysis process in a very high temperature kiln with little or no oxygen so that it does not combust or release CO2. The chemical compounds (i.e. cellulose, hemicellulose and lignin) thermally decompose into a combustible gaseous product syngas, liquid bio-oil and biochar. This biochar has a high carbon content of around 86%. Bio-oil and syngas are potential biofuels which can be used for power generation. For present purposes the kiln is configured to control the pyrolysis conditions (temperature, residency time, reaction gas) to optimise the quantity of carbon in the biochar, power generation capability, and biochar stability. It has been found that low temperature pyrolysis produces a large quantity of bio-oil which, whilst practical for storage, is very poor quality and would require substantial refining for energy use. Experimental studies have shown the optimum temperature for biochar and syngas production to be in the region of 760°C +/-10%.

Biochar is highly flammable, and there is a spontaneous combustion risk when handling, transporting and storing dry biochar. The biochar is therefore sprayed with water within the pyrolysis plant as part of the cooling and stabilisation process, achieving a moisture content of about 10%. This also increases its mass, which greatly increases the energy needed to raise its -7 -temperature to the point of ignition.

Damp biochar may be discharged from the pyrolysis plant into storage containers and suspended in water for transportation to a long-term storage repository. However, the risks and CO2 emissions associated with vehicular transport can be reduced by pumping biochar to a local final storage repository while suspended in water. An example of a suitable carbon sequestration and storage plant is described below.

Effective carbon storage requires the biochar to be stored in a verifiable, stable and contained state for an indefinite period. To achieve this objective the biochar is buried in a permanent storage repository that is flooded to stabilise the biochar and prevent CO2 emissions. Referring to Fig. 1, the site chosen for the repository is excavated to a suitable depth below the natural water table to form a pit 1 with a flat bottom 2 and sloping sides 3 and 4. By way of example, if the water table 5 is around 1.6m below the surface 6 the bottom of the pit may be approximately 3.6m below the surface. The biochar is placed into porous bags, e.g. two bags 7, 8 stacked one above the other and each filled to a depth of approximately 1m. The pit 1 is back-filled with the previously excavated soils 9 up to the original surface level 6, providing a soil depth of approximately 1.6m above the filled bags.

The carbon storage repository is particularly suitable for poor quality reclaimed land such as that found on the Humberhead levels in England. Since the land was drained 400 years ago, -8 -significant erosion has led to land loss of typically 4m from the original ground level. The present proposal involves rebuilding such ground by reversing the lowering that has resulted from peat drainage. By removing the remaining peat and some subsoil, a layer of biochar approx. 3m deep can be placed and then covered with the subsoil and then the topsoil reinstated. This offers the capacity for substantial levels of solid carbon storage exceeding all known on-land storage capacity and is far higher than the total carbon stock of any UK natural ecosystem (including the original deep peat).

The storage conditions for biochar are key to ensuring carbon sequestration, avoiding the risk of oxidation and subsequent CO2 re-emission. Recent studies have found the mean survival time for biochar to vary greatly from just 8 years to almost 4,000 years. Appropriate design of a secure and auditable biochar storage facility is therefore very important. It is known that higher temperature production (650°C and above) produces biochar which is more stable than low temperature production (300°C) when stored in dry mineral and peat soils. Nevertheless, it has been found that the short-term stability of biochar in peat is not significantly influenced by pyrolysis temperature when stored under wet anaerobic conditions, and all biochar appears to be stable in waterlogged soils. Furthermore, biochar in long-term waterlogged storage is protected from both combustion and decomposition, and therefore offers a high level of long term stability. -9 -

Fig. 2 is an example of a carbon sequestration and storage plant which processes SRCW to produce biochar which is prevented from interaction with atmospheric oxygen through aqueous submersion. The plant includes a pyrolysis shed 10 in which dried and seasoned willow chips are processed in a pyrolysis furnace 11 under anaerobic conditions to produce biochar. Outside the shed 10 the biochar is discharged into a reception hopper 12 and mixed with water from a water tank 13 to form a slurry. A pump 14 transfers the biochar/water mix along a transfer conduit system 15 to an adjacent long-term storage repository flanked by earth embankments 16 and 17. The storage repository is large enough to hold four storage cells, each containing two stacked 7m x lm storage bags. As shown, one pit contains a double cell 18 which is already filled and covered with back-fill, together with a second cell 19 containing two further stacked storage bags which is still being filled. The adjacent area 20 for the remaining two cells is reserved for future excavation. The biochar is deposited within the excavated pits in bags made from porous geotextile which retains the biochar and allows water to pass through. Once the bags are filled, the contents are left to settle and the next bag is brought on line upstream of the first. Water leaving the second bag maintains the saturation of the first. Once a cell is filled, sufficient overburden soils are placed over the bags to counter the uplift caused by water pressure, so the whole bay of bags can be submerged below the prevailing water table. Calculations of water displacement show a drained restoration soil of 1700kg/m3 at 0.42m depth countering the uplift from a 0.8m deep store of biochar. The geotextile bag separates the biochar from the -10 -confining soils preventing reduction of the restoring force. Initial settlement to the repository is to be expected due to the consolidation of the biochar from 180kg/m3 to 225 kg/m3. Such settlement would occur over the first 10 years, leaving a stable earthworks structure able to retain the biochar. Thus, it is reasonable to assume that these structures could continue to perform in perpetuity unless disturbed through physical intervention. The biochar sludge residue will further drain over time, but the residual biochar material retains a sufficiently high moisture content to mitigate the risk of combustion.

Fig.s 3 to 5 show one cell of a carbon storage repository in sectional detail. The cell is excavated to a suitable depth below the natural water table 30 to form a pit 31 with a flat bottom 32 and sloping sides 33 -36. The biochar is pumped via tansfer conduit system 15 into porous bags 37 of porous geotextile material which are normally used in mining to retain particles having a greater density than water. One bag per cell is shown in this example. The biochar typically includes particles which have a density less than water so rather than filling from the bottom upwards and draining through the top and sides the biochar fills the bag from the top down. To ensure that the filling process proceeds without hindrance retaining straps 38, e.g. of webbing, netting, mesh etc. are installed over the top of the bag and fixed to the sides of the pit using ground anchors 39. When the bag is filled to capacity the pit 31 is back-filled with the previously excavated soils 40, reinstating the ground up to the original level 41.

For reliable and verifiable long-term carbon sequestration the repository is subject to an annual inspection for signs of movement, with sampling checks between the bottom of the carbon store and the upper ground water level to maintain a record of the saturation. To achieve this a perforated ground water sampling pipe 42 is installed longitudinally through the middle of the bag. Each end of the pipe 42 connects with a riser 43, 44 leading up to surface level 41. Upright gas sampling ducts 45 are also inserted into the bag commencing slightly above the bottom of the pit, allowing gas samples to be taken periodically. The upper ends of the risers 43, 44 and the ducts 45 provide long-term monitoring points by which ground water and gases can be periodically monitored. The top portion of the monitoring points is shown in greater detail in Fig. 6. Tubular sliders 50 received over the upper ends of the upright pipes 51 are provided with resilient gas seals 52 which, when the sliders are pushed down over the pipes 51, seal the concentric gap and prevent the escape of gases from the store into the atmosphere. The upper ends of the sliders are provided with removable caps 53 which prevent entry of dirt and debris. The sliders can be moved up and down the pipes 50 so that the sampling points can remain at ground level with settlement of the backfill or a rise in ground level due to future above-ground operations.

Further long-term validation can be carried out at any point in the future by employing a small diameter borehole rig to core-sample the repository. Small target pads of concrete 46, Fig. 4, are cast -12 -beneath the bag prior to filling, enabling any future validation to determine the lower level of the biochar repository. The core drilling areas are indicated by the hatched areas 47 in Fig. 3.

Carbon storage repositories which involve land-raising would have an embankment on each side of the biochar, a flow control mechanism to control the rate of filling, and restoration soils. Repositories can also be located in suitable areas such as quarries to combine carbon storage with land reclamation.

The present sub-soil storage approach is capable of storing 1 Mt CO2e in 45ha for land-raise repositories, and 22ha for quarry-based repositories, under highly stable conditions. Compared to field dispersal of biochar, this contained solution is more stable, more secure, involves lower transportation costs and emissions, and is much easier and cheaper to monitor and verify. The present solution also mitigates against collateral environmental impact.

The present storage process has other potential environmental advantages. Sequestered biochar can act as a carbon water filter improving water quality and cleaning out nitrates or phosphates from a watercourse. Biochar has also been suggested as suitable for drinking water treatment processes and can avoid the production of carcinogenic byproducts from chlorination. Application of wood-based biochar prepared using fast pyrolysis was successful in removing pollutants like arsenic, cadmium, fluoride, lead and chromium during the water purification process.

-13 -Increasing pyrolysis temperature enhances the surface area and nnicroporosity of biochar which increases absorption and capacity to remove contaminants.

Through careful management of water levels using a decant structure, biochar storage areas can become adjustable subterranean reservoirs. This can then become part of the system of flood risk mitigation for local towns and villages. In reverse, reservoir capacity can be used to reduce the abstraction requirements of a farm.

The land-raise and quarry options both involve removing topsoil and overburden, burying the biochar, and then replacing the overburden and topsoil. Apart from the environmental buffer zones above these dam structures themselves, the raised land can be returned to a wide range of agricultural uses -including biomass production if desired. Land immediately above the dam structures can be covered with suitable short rootstock planting to both stabilise and contain the surface and also provide additional habitats for wildlife.

The energy by-products from the plant can be utilised in a CEA system, producing higher value foods, replacing the change in land use and subsequent displacement of food production.

The pyrolysis process may generate enough syngas and heat to operate the plant itself, and in addition, there may be sufficient excess to export electricity and heat for secondary uses. This can -14 -be used to power CEA facilities that replace the food production capacity that would otherwise be lost through change of land use from food production to biomass. Once operational, a small pyrolysis plant can typically produce enough energy as a by-product to generate 0.2 MW (Net) of electricity and 0.4 MW of recoverable heat per hour. This energy can be used for powering existing cold stores and pack house, all of which are covered under agricultural permits. As the fresh SRCW feedstock is not a waste product, no waste permit for incineration is required.

In summary, the proposed carbon storage process sequesters and abates significant quantities of carbon, and also produces food with measurable positive environmental and social impact. The process is highly scalable with the total area of degraded UK lowland peat able to support over 10 of the commercial-scale plants. Both the growing of willow and biochar storage can be easily adapted to match production in different locations. In principle the process can utilise biomass produced from any land area, and store biochar in any quarry, deep mine or area of lowered ground surface (e.g. opencast mining) where permanent waterlogging can be implemented. It is believed that the on-land biochar storage potential provided by the present invention could deliver a large proportion of the UK's CCS needs at low cost, and without taking that land area permanently out of productive use.

Whilst the above description places emphasis on the areas which are believed to be new and addresses specific problems which have been identified, it is intended that the features disclosed -15 -herein may be used in any combination which is capable of providing a new and useful advance in the art.

Claims

-16 -CLAIMS1. A carbon sequestration process: - harvesting a biomass derived from photosynthetic plant material containing captured atmospheric carbon; - converting the biomass to biochar using pyrolysis under anaerobic conditions; - adding water to the biochar to produce a pumpable biochar slurry; - placing porous geotextile bags into underground storage pits; - pumping the slurry into the geotextile bags wherein a proportion of the added water can percolate out of the bags while the biochar remains contained within the bags; - covering the bags with backfill to form a permanent storage repository; - maintaining long-term conditions within the storage repository such that the bags remain flooded with water holding the biochar under anaerobic conditions.
2. A carbon sequestration process according to claim 1 wherein the biomass is obtained from coppiced willow.
3. A carbon sequestration process according to claim 2 wherein the coppiced willow is harvested with a three year cycle.
4. A carbon sequestration process according to any of claims 1 to 3 wherein the harvested biomass is chipped and stored -17 -under drying conditions.
5. A carbon sequestration process according to any preceding claim wherein the pyrolysis is carried out under conditions which produce a porous granular material with a lower density than water.
6. A carbon sequestration process according to claim 5 wherein the pyrolysis is carried out at a temperature above 650°C.
7. A carbon sequestration process according to claim 5 wherein the pyrolysis is carried out at a temperature of 760°C +110%.
8. A carbon sequestration process according to any preceding claim wherein the biochar is sprayed with water following pyrolysis as part of a cooling and stabilisation process.
9. A carbon sequestration process according to any preceding claim wherein the biochar is mixed with sufficient water at the pyrolysis facility to form the slurry and the slurry is pumped from the pyrolysis facility into the porous geotextile bags.
10. A carbon sequestration process according to any preceding claim wherein the porous geotextile bags are filled sequentially such that water percolating out of a bag which is being filled is used to maintain the saturation of bags which have -18 -already been filled.
11. A carbon sequestration process according to any preceding claim wherein the underground storage pits are excavated below the natural water table.
12. A carbon sequestration process according to any preceding claim wherein the underground storage pits have a substantially flat bottom and sloping sides.
13. A carbon sequestration process according to any preceding claim wherein retaining straps are installed over the top of the bags to hold them at the bottom of the pits during filling.
14. A carbon sequestration process according to claim 13 wherein the ends of the retaining straps are fixed to the sides of the pit using ground anchors.
15. A carbon sequestration process according to claim wherein a perforated ground water sampling pipe is installed substantially horizontally through the bags.
16. A carbon sequestration process according to claim 15 wherein the water sampling pipe is connected to a riser pipe leading to surface level.
17. A carbon sequestration process according to any preceding claim wherein gas sampling pipes are inserted into the -19 -bags commencing near the bottom of the pit and leading to surface level.
18. A carbon sequestration process according to any of claims 15 to 17 wherein upper ends of the pipes leading to surface level are provided with tubular sliders with removable caps.
19. A carbon sequestration process according to any preceding claim wherein target pads of concrete are cast beneath the bag prior to filling.
20. A carbon storage repository: - underground storage pits; - porous geotextile bags containing biochar installed in the underground storage pits; - backfill covering the biochar filled bags; wherein the biochar filled bags are flooded with water to maintain the biochar under anaerobic conditions.
21. A carbon storage repository according to claim 20 wherein retaining straps are installed over the top of the bags to hold them at the bottom of the pits during filling and the ends of the retaining straps are fixed to the sides of the pit.
22. A carbon storage repository according to claim 20 or 21 wherein ground water sampling pipes are inserted into the bags leading to surface level.
-20 - 23. A carbon storage repository according to claim 20 or 21 wherein gas sampling pipes are inserted into the bags leading to surface level.
24. A carbon storage repository according to claim 22 or 23 wherein upper ends of the pipes leading to surface level are provided with tubular sliders with removable caps.
25. A carbon storage plant: - a carbon storage repository according to any of claims 20 to 24; - a pyrolysis facility which converts a biomass derived from photosynthetic plant material into biochar under anaerobic conditions; - a mixing arrangement associated with the pyrolysis facility for adding water to the biochar to produce a biochar slurry; - a conduit to conduct the biochar slurry from the pyrolysis facility to the carbon storage repository.