Introduction

The present invention relates to a process for ash remediation and in particular to the remediation of waste ash, for example Air Pollution Controlled Residue (APCR).

Background to the Invention

Fly ash, also known as pulverised fuel ash (PFA) is the ash resulting from the burning of pulverised coal in coal-fired electricity power stations. PFA has been used for many years as an alternative to sand and cement grouts due to the technical, rheological, durability and economic advantages offered. PFA is typically combined with Portland cement in the range 60%-80% Portland cement and 40%-20% PFA depending on the application.

During the period 1999 to 2014, production of PFA in the UK was between 4 million and 7 million tonnes per annum. Four million tonnes of PFA was used in construction, predominantly as grout for land stabilisation.

Burning coal is a major source of greenhouse gases. The 2016 Paris Agreement on climate change aims to hold the increase in the global average temperature to less than 2 °C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1 .5 °C above pre-industrial levels. Whilst the UK’s reliance on coal for power has reduced from 70% in 1990 to 3% in 2020, as part of its response to the Paris Agreement, the UK and many other countries have committed to reducing or eliminating their use of coal for power.

One part of the response to global warming has been for countries to legislate to reduce the environmental impact of the waste that is created. For example, the 2014 Waste (Scotland) Regulations provided for the prohibition of the landfilling of biodegradable municipal waste from January 2025. This legislation along with the closure of coal fired power stations has helped hasten the transition to low carbon power and strengthened the Energy from Waste and biomass sectors.

Energy from Waste (EFW) facilities such as waste wood biomass, biomass in general, gasification, fluidised bed and moving grate technologies all produce a waste ash known as Air Pollution Controlled Residues, (APCR) as a product of flue gas clean up. APCR is a hazardous waste, consequently, the number of locations at which raw APCR may be disposed is very limited. There is very limited capacity for the treatment or landfilling of APCR in Scotland therefore practically all APCR produced in Scotland is transported to England for disposal, principally in hazardous waste landfill sites at significant financial and environmental cost. The definition of APCRs may include some types of boiler ash especially wood ash and waste incinerator ash.

UK Patent Application GB2532860 describes a process for remediation of an ash byproduct from a Combined Heat and Power Plant by combining the ash by-product containing metal contaminants with a liquid mobilisation agent to create a mobilised fly ash. The mobilised ash is heated to form a chloride salt precipitate and a de-salinated ash by-product which is combined with a liquid mobilisation agent. The metal is bound in a complex to prevent metal contaminants from being chemically active in the mobilised de-salinated fly ash and chemicals are added to decontaminate the fly ash by separating the metal contaminants from the fly ash which is then dried.

Summary of the Invention

It is an object of the present invention is to create a process for ash remediation and in particular to the remediation of Air Pollution Control Residue created at an energy from waste facility. It is another object of the present invention to create a decontaminated grout product which is suitable for use as a grout material and/or a material suitable for use as a secondary constituent of building materials including Portland cements e.g. CEM-II.

In accordance with a first aspect of the invention there is provided a method for treating an air pollution control residue (APCR), the method comprising the steps of: creating a wet APCR mixture; promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product; adding an aqueous liquid to the APCR reaction product to form a diluted APCR reaction product; separating contaminants from the APCR reaction product in the diluted APCR reaction product by creating an aqueous solution of contaminants in the diluted APCR reaction product and a decontaminated APCR in suspension in the aqueous liquid; and forming a solid decontaminated APCR product.

Preferably, the step of creating a wet APCR mixture comprises adding water to the APCR.

Preferably, the step of creating a wet APCR mixture comprises adding between 5% and 25% by weight of water to the APCR.

Preferably, the step of creating a wet APCR mixture comprises adding between 10% and 20% by weight of water to the APCR.

Preferably, the step of creating a wet APCR mixture comprises adding up to 40% by weight of water to the APCR

Preferably, the step of creating a wet APCR mixture comprises adding 35% by weight of water to the APCR Preferably, the step of creating a wet APCR mixture comprises mixing the APCR as a batch in a vessel or container

Preferably, the step of creating a wet APCR mixture comprises mixing the APCR as a a continuous batch process.

Preferably, the step of promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product comprises exposing the wet APCR mixture to atmospheric conditions.

Preferably, the step of promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product comprises periodically mixing or turning the wet APCR mixture.

Preferably, the step of promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product comprises adding moisture to the wet APCR during the exothermic reaction.

Preferably, the step of adding moisture comprises maintaining the moisture content of the wet APCR mixture at an or near an initial level of wetness at which the wet APCR was created.

Preferably, the step of adding moisture maintains a moisture content at between 10% and 40% water by weight of the APCR.

Preferably, the step of adding moisture comprises the controlled addition of water from natural precipitation and/or manual or automated application of potable water rain water, ground water or process water including treated effluents. Preferably, the step of promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product comprises allowing the reaction to progress substantially to completion.

More preferably, the reaction progresses to a point where the temperature of the APCR reaction product is within approximately 4°C to 2°C of the ambient temperature.

Preferably the APCR reaction product has a water content of between 5% and 45%. by weight.

Preferably the APCR reaction product has a water content of between 5% and 40% by weight.

Preferably the APCR reaction product has a water content of between 10% and 25% by weight.

Preferably, the step of adding an aqueous liquid to the APCR reaction product to form a diluted APCR reaction product mobilizes at least some of the contaminants.

Preferably, the aqueous liquid washes the APCR reaction product.

Optionally, the water may be potable water, rain water, ground water, process water including treated effluents or borehole water.

Preferably, the ratio of aqueous liquid to APCR reaction product by volume is approximately 3:1.

Optionally, the ratio of aqueous liquid to APCR reaction product by volume is approximately between 2:1 and 5:1.

Preferably, the aqueous liquid is at ambient temperature. Preferably, the ambient temperature is between 5°C and 25°C.

Preferably, the step of separating contaminants from the diluted APCR reaction product comprises agitating the diluted APCR reaction product.

Preferably, the step of separating contaminants from the diluted APCR reaction product comprises heating the diluted APCR reaction product.

Optionally, the diluted APCR reaction product is heated to a temperature of up to a temperature above ambient temperature.

Optionally, the diluted APCR reaction product is heated to a temperature of up to 70°C.

Optionally, the diluted APCR reaction product is heated to a temperature of up to 60°C.

Preferably, the step of agitating the diluted APCR reaction product is conducted in a ball mill.

Optionally, the step of agitating the diluted APCR reaction product is conducted in a cone tank.

Optionally, the step of agitating the diluted APCR reaction product is conducted in a vertical tank

Optionally, the step of agitating the diluted APCR reaction product is conducted in a horizontal tank.

Optionally, the step of agitating the diluted APCR reaction product is conducted in an agitated tank. Optionally, the step of agitating the diluted APCR reaction product is conducted in a hammer mill.

Optionally, the step of agitating the diluted APCR reaction product is conducted in a pendulum mill.

Preferably, the aqueous solution of contaminants in the diluted APCR reaction product is transferred to a tank where it is agitated to prevent settlement of the decontaminated APCR.

Preferably, the step of forming a solid decontaminated APCR comprises filtering the aqueous solution of contaminants in the diluted APCR reaction product.

Preferably, the aqueous solution of contaminants in the diluted APCR reaction product is filtered using a filter press.

Optionally, the filter press is a cross flow filter press.

Preferably, the step of forming a solid decontaminated APCR comprises applying pressure to the aqueous solution or suspension of contaminants in the diluted APCR reaction product to expel at least some of the aqueous solution or suspension.

Preferably, the step of forming a solid decontaminated APCR comprises applying a pressure of between 6 and 12 bar.

Preferably, the step of forming a solid decontaminated APCR product comprises applying a pressure of 8.5 bar.

Preferably, the step of forming a solid decontaminated APCR comprises applying a pressure for between 10 and 20 minutes. Preferably, the solid decontaminated APCR product has a wetness of between 50% and 60% dry solid content.

Preferably, the solid decontaminated APCR product is dried after removal of the aqueous solution of contaminants.

Preferably, the APCR product is dried to a moisture content of less than 10%.

Preferably, the APCR product is dried to a moisture content of less than 5%.

Preferably, the APCR product is dried using a fluid bed dryer.

Preferably, the APCR product is dried using a superheated steam dryer.

Preferably, the APCR product is dried using turbulent hot air to remove moisture from the surface of particles.

Preferably, the APCR product is dried using passive solar drying.

Preferably, the APCR product is dried using heat from solar concentrators.

Preferably, the step of forming a solid decontaminated APCR product further comprises dry milling the APCR Product.

Preferably, the step of dry milling is undertaken with APCR product which has a moisture content of less than 10%.

Preferably, the step of dry milling is undertaken with APCR product which has a moisture content of less than 5%.

Preferably, the step of dry milling is undertaken using a ball mill. Optionally, the step of agitating the diluted APCR reaction product is conducted in a pendulum mill.

Optionally, the step of agitating the diluted APCR reaction product is conducted in a hammer mill.

In accordance with a second aspect of the present invention there is provided a decontaminated APCR product suitable for use as a construction material.

In accordance with a third aspect of the invention there is provided decontaminated APCR product suitable for use as a construction material and made using the process of the first aspect of the invention.

Brief Description of the Drawinqs

The present invention will now be described with reference to the accompanying drawings in which: figure 1 is a schematic diagram of an example of a process in accordance with the present invention; figure 2 is a schematic diagram of an example of a container within which contaminated APCR is collected after its removal from an EFW facility; figure 3 is a schematic diagram which shows an example of a vessel a wet APCR mixture is created; figure 4 is a schematic diagram which shows an example of an open container or area where the exothermic reaction occurs; figure 5 is a schematic diagram which shows an example of a vessel in which the diluted APCR reaction product is agitated; and figure 6 is a schematic diagram which shows an example of a filtration device which separates the decontaminated ash from a contaminated aqueous solution; figure 7 is a graph which shows chloride concentration in the wash water after washing the conditioned APCR for a range of wash times and temperatures; figure 8 is a graph which shows electrical conductivity in the wash water after washing the conditioned APCR for a range of wash times and temperatures; figure 9 is a graph which shows mass loss percentage for a range of wash times and wash temperatures; and figure 10 is a schematic diagram which shows an example of a vessel in which the decontaminated APCR is dry milled.

Detailed Description of the Drawings

The present invention relates to a process for decontaminating APCR and for creating a product which is suitable for use as a construction material, for example a grout used in land stabilisation.

Figure 1 is a flow diagram 1 which shows an example of a process in accordance with the present invention.

In a first stage of the process, contaminated APCR is removed from an EFW facility and transferred into a container. Water is added and mixed in with the contaminated APCR to create a wet APCR mixture 3. The next stage in the decontamination process 5 is promoting an exothermic reaction in the wet contaminated APCR mixture to create an APCR reaction product, after which a diluted APCR rection product is formed 7. The contaminants are separated from the APCR in a diluted APCR reaction product 9 after which a solid APCR product is formed 11 which may be formed into a powder.

Figures 2 to 10 show a more detailed example of the process steps which may be used to implement the process of the present invention.

Figure 2 is a schematic illustration of a container within which contaminated APCR is collected after its removal from an EFW facility. Figure 2 shows a container 10 which comprises an inlet 12 for receiving contaminated APCR, a chamber 18 where the contaminated APCR is stored an outlet 14 through which the contaminated APCR exits the chamber 18.

In use, contaminated APCR with a bulk density of 800 to 1300kg/m ³ is received from a powder tanker. The tanker blows the ash into the receiving chamber 18. The container is fitted with pneumatic pads 16 through which air is blown to agitate the ash to prevent the ash from sticking to the sides of the chamber 18. To stop condensation forming in the compressed air, a dryer is fitted to the compressed air line. The contaminated APCR then exits the chamber 18 and drops onto twin augers (not shown) and is screwed along a series of augers until it enters a vessel where the contaminated APCR is combined with water.

Figure 3 is a schematic diagram 13 which shows an example of a vessel 15 in which a wet contaminated APCR mixture is created. The vessel 15 has a square cross section and is shaped as an elongated channel. It comprises an inlet 22 through which it receives dry contaminated APCR, paddle screws 17 are arranged linearly along the bottom of the vessel and a row of sprinklers 20.

In use, the contaminated APCR is transferred to the vessel 15 via the augers, the twin paddle screws 17 mix the contaminated APCR and push it forward whilst the rows of sprinklers 20 above spray water onto the ash. Moisture content in manipulated by adjusting the flow of water from the sprinklers 20. A moisture content of between 10wt% and 20wt% produces a mixture which has a consistency of a viscous paste or sludge.

As shown in figure 1, the next stage in the decontamination process 5 is promoting an exothermic reaction in the wet APCR mixture to create an APCR reaction product.

Figure 4 is a schematic diagram 21 shows an example of an open container where the exothermic reaction occurs. The wet APCR 25 is transferred to vessel 23 where an exothermic reaction occurs in air. The wet APCR 25 may be mixed or turned over to increase its exposure to air in order to speed up the reaction, which is allowed to proceed substantially to completion. The point at which the reaction is deemed to be substantially complete is when the temperature of the wet APCR 25 matches the ambient temperature to within approximately 2°C. In the conditions where the wet APCR is exposed to atmospheric conditions and ambient temperatures of approximately between 4 °C and 15°C, the reaction takes between 3 and 10 weeks, the reduction in time to the lower end of this range occurs when the wet APCR 25 is turned regularly.

Given that the contaminated APCR is created from the burning of biomass, the precise make up of different batches of contaminated APCR will vary depending upon the biomass which is being burned. A complex range of mineral forms such as oxides, hydroxides and sulphates may be present in the wet APCR and these will react with water producing an exothermic reaction for example calcium oxide (free lime) may react with water to produce calcium hydroxide that will also react with water producing an alkaline solution. In addition, the minerals may react with atmospheric CO2 in carbonation reactions.

During the conditioning phase over a period of weeks, the temperature of the wet APCR may rise to between 80-90 degrees Celsius. The wet APCR will also react with atmospheric carbon dioxide from the air bringing about a reduction of approximately 1-2 pH units from initial values of approximately pH 11-12. Changes in chemical speciation of heavy metals during conditioning and a reduced pH will reduce leachability an important benefit in the environmental acceptability of the final product e.g., a constituent of grout material or other building material.

It is beneficial for such reactions to take place slowly and be completed or near to completion during a conditioning phase and not during subsequent washing to avoid these reactions having a deleterious effect to the strength of a grout material or building material containing the treated APCR. Other reagents including those from waste streams e.g. ferric sludge from water treatment may be added during conditioning or washing to assist with the immobilisation of heavy metals such as lead.

In this example of the present invention, the step of adding an aqueous liquid to the APCR reaction product to form a diluted APCR reaction product 7 of figure 1 and the step of separating contaminants from the APCR in the diluted APCR reaction product by creating an aqueous solution or suspension of contaminants in the diluted APCR reaction product 9 occur in a reaction vessel in which the diluted APCR reaction product is agitated.

In another example of the present invention, the step of promoting an exothermic reaction as undergone in a container such as that shown in figure 4 is enhanced by the addition of moisture to the wet APCR mixture. The purpose is to maintain the moisture content of the wet APCR mixture at an or near the initial level of wetness at which the wet APCR was created. The moisture level is typically between 10% and 40% water by weight of the APCR. The moisture may be added by a controlled addition of water during the period over which the exothermic reaction occurs and the water may be from natural precipitation and/or manual or automated application of potable water rain water, ground water or process water including treated effluents. Figure 5 is a schematic diagram which shows a reaction vessel 31 in which the diluted APCR reaction product 39 is agitated. In this example, the reaction vessel comprises a ball mill. A ball mill is a type of grinder used to grind or blend materials and is used in mineral dressing processes, paints, pyrotechnics, ceramics and selective laser sintering. Balls of different sizes, which are made of a hard abrasive material, move freely around the inside of the cylinder and this movement against and through a material in the cylinder to agitate and grind against the material. The inner surface of the cylinder is also often covered in an abrasive material. The size of the material particles is reduced by impact as the balls move around. Typically, ball mills are used to grind down mix together solid materials and to decrease their overall particle size of the matter.

As shown in figure 5, the ball mill 31 will generally comprise a cylindrical drum 33 which is coupled to a drive mechanism 35. In the example shown, the drive mechanism comprises rollers which are in contact with the outer surface of the cylinder. Drive shafts inside the cylinder and belt drives are also typically used. Abrasive balls 37 of different sizes are shown along with the diluted APCR reaction product 39. In this example of the present invention, an aqueous liquid, typically water is added to the APCR reaction product to form a diluted APCR reaction product 39 and this is introduced into the cylindrical drum 33 via hatch 41.

In this example, the diluted APCR reaction product comprises approximately 3 parts water to one part APCR reaction product by volume. The APCR reaction product has 39% moisture content and this ratio was preferred for the removal of chlorides whilst also allowing for the removal of other contaminants. The APCR reaction product is washed as a diluted APCR reaction product in the ball mill cylinder 33 for approximately 15 minutes where it is agitated by the abrasive balls 37 in the ball mill cylinder 33.

One effect of washing in the ball mill is to remove the contaminants from the APCR reaction product and to place them in solution with the water and the decontaminated APCR in suspension in the aqueous liquid. Suitably, the ball mill may have external dimensions of approximately 2m diameter x 3m length with a high-density alumina lining. The gearbox and magnetic brake are mounted on the drive end with a support structure and base plate. Non-drive end bearings are mounted on base plates. The Ball Mill may be mounted on concrete foundations with steel support structures to fit under both drive and non-drive ends of the Ball Mill with a final height off the bottom of the mill to the floor of 350mm.

In other embodiments of the present invention, higher ratios of water to APCR reaction product are possible and the water may be heated. These parameters are more efficient for mobilizing the chlorides into the wash water at a cost of higher energy input.

In other embodiments of the present invention, separation of the contaminants from the APCR reaction product may be achieved using cone tanks, vertical and horizontal tanks and agitated tanks. Advantageously, ball mills are mechanically simple and efficient and easy to operate.

The process of washing the conditioned APCR was undertaken for different durations and at different temperatures. The effect of these changes in condition on the chloride content, electrical conductivity and the mass loss are shown in figures 7 to 9.

Figure 7 is a graph 81 which plots chloride concentration (ppm) on the Y-axis 83 for wash samples under different conditions, on the x-axis 85. The conditions are 20°C for 15 minutes 97, 20°C for 30 minutes 95, 45°C for 15 minutes 93, 20°C for 60 minutes 89 and 45°C for 60 minutes. A combination of longer washing time and increasing temperature was determined to result in higher removal of chloride. Values for pH (not shown) were broadly similar for all treatments ranging from 10.6 to 11.0.

Figure 8 is a graph 101 which shows electrical conductivity on the Y-axis 103 for wash samples under different conditions, as plotted along the x-axis 105. The conditions are 20°C for 15 minutes 117, 20°C for 30 minutes 115, 45°C for 15 minutes 113, 20°C for 60 minutes 109 and 45°C for 60 minutes 107. Conductivity of the wash water generally increased which indicates a higher concentration of ions in the wash water which has been agitated for longer and at higher temperatures, most notably for the sample at 45°C for 60 minutes 107.

Figure 9 is a graph 121 which shows mass loss percentage on the Y-axis 123 for wash samples under different conditions as plotted on the x-axis 125. The conditions are 20°C for 15 minutes 139, 20°C for 30 minutes 137, 45°C for 15 minutes 135, 20°C for 60 minutes 133 and 45°C for 60 minutes 131. Mass loss was broadly consistent during the washing process (stirred solution) at between 13-14%.The comparatively higher value for 20°C and 60 minutes should not be taken to represent a significant difference and is likely explained by experiment error. Previous 1 and 2 represent prior results from gentle shaking (10.8%) and vigorous shaking (11.9%) respectively, both at 20°C, 20 minutes.

Once the contaminants have been placed in solution in an aqueous liquid and the decontaminated APCR in suspension in the aqueous liquid, a solid decontaminated APCR product 11 figure 1 is produced. Prior to this step, the decontaminated APCR in suspension in the aqueous liquid may be stored in a tank where agitators or stirrers are used to prevent the decontaminated APCR from settling out from the suspension.

Figure 6 is a schematic diagram which shows an example of a filtration device which separates the decontaminated ash from a contaminated aqueous solution.

In this example, the solid decontaminated APCR product is formed in a filter press 51. A filter press is a tool used in separation processes to separate solids and liquids. In this example, the filter press comprises a frame 53, 55 which holds in position a stack of horizontally arranged filters 57 each of which has a corresponding collection chamber 59. In use, the decontaminated APCR in suspension in the aqueous liquid which contains contaminants is fed in to the filter press 51 via inlet 61 under a pressure of approximately 8.5 bar. A solid cake of decontaminated APCR is collected in the first collection chamber and as the aqueous liquid continues to be fed in, the decontaminated APCR is collected in the subsequently arranged chambers in the stack which are linked by fluid channels 65. Outlet 63 is connected to a tank (not shown) where the contaminated solution is collected. Once a batch of solution/APCR suspension has been filtered, the pressure is removed and an APCR cake is removed 67 from the chamber 59. In this example, the filter press is filed in around 15 seconds and pressure applied for around 15 minutes. The cake has a moisture content of about 60% dry solid content.

The example of a filter press shown in figure 6 is intended to illustrate the principle of operation of the press. Other types of filtration may be used, for example cross flow filtration.

Further drying of the cake may be achieved using a fluid bed dryer. The decontaminated APCR cake is broken up into a powder and fed into a fluid-bed system wherein the “wet” powder is conveyed over a perforated bed. Hot drying air is blown through the holes of a perforated plate. The wet solids are lifted from the bottom which causes the solids to behave as a fluid. The air velocity is adjusted to keep the moving layer of material fluidized. Fluidized bed technology in dryers increases efficiency by allowing for the entire surface of the drying material to be suspended and therefore exposed to the air.

In one example, the filter cake is dried using a superheated steam dryer.

In a conventional air dryer, each cubic metre of air brought into a dryer needs to be heated to a control temperature of up to 100°C and is discharged to atmosphere after use because of excess moisture contained in the air after it has dried the product. Advantageously, superheated steam may be is continually recirculated.

Dry superheated steam at atmospheric pressure has a temperature of above 100°C. It has a heat transfer coefficient which is approximately double that of air, it has better penetration of the heap debris because steam has higher viscosity than hot air and a lower surface tension which means that it penetrates quicker into the cake and has an increased drying effect.

During the process, the temperature is controlled, and additional heating further penetrates into the product and evaporates the remaining water, while the steam generated continues to be vented from the chamber. Heating continues until the product is dry at which stage ambient air is introduced into the dryer to cool both chamber and products, via automatically controlled dampers (fresh air and exhaust).

In this example, the dryer may process up to 4,000 kg/hr of wet residue materials containing a moisture content of approximately 35%. The dried materials would exit the dryer at 1% residual moisture.

The estimated throughput drying time to dry the filter cake material from a maximum 35% to 1% would be approximately 60 minutes, subject to in feed material condition. The dryers vented exhaust steam source will be contained and condensed into hot liquid and its latent energy recovered if possible.

Further drying of the cake may also be achieved using systems based on turbulent hot air to increase drying efficiency.

In suitable climates, further drying of the cake may also be achieved using passive solar drying outdoors or make use of solar thermal energy from solar concentrators to provide some or all of the heat for drying.

Once dried, the powder may be further which may be reduced by dry milling to reduce and homogenise the particle size of the APCR powder. Figure 10 shows a ball mill similar to that shown in figure 5 and suitable for dry milling the decontaminated APCR powder. The ball mill 31 will generally comprise a cylindrical drum 33 which is coupled to a drive mechanism 35. In the example shown, the drive mechanism comprises rollers which are in contact with the outer surface of the cylinder. Drive shafts inside the cylinder and belt drives are also typically used. Abrasive balls 37 of different sizes are shown along with the APCR powder 50. In this example of the present invention, the dry decontaminated APCR powder is introduced into the cylindrical drum 33 via hatch 41. After dry milling, the APCR powder is ready for use in a land fill grout.

In other examples, the dried powder may be processed using a pendulum mill or a hammer mill of known type.

One feature of the present invention is that it takes the ash by-product from energy to waste plants and decontaminates it to allow its use as a building product such as a grout material and/or a material suitable for use as a constituent of building materials.

In line with the objectives of the Paris Agreement on climate change, analysis was undertaken to determine the environmental impact of onsite treatment of APCR to produce a PFA replacement media in comparison with offsite disposal of APCR in a hazardous landfill.

Carbon footprint is characterised by the release of greenhouse gas to the atmosphere. The unit used to describe this is kilograms of carbon dioxide equivalent release (kgC02e). The six greenhouse gases can be compared on a like for like basis relative to one unit of CO2. C02e is calculated by multiplying the emissions of each of the six Kyoto Protocol greenhouse gases by their 100-year global warming potential (GWP). These are Carbon Dioxide (CO2), Methane (CPU), Nitrous Oxide (N2O), Hydrofluorocarbons (HFCs), Perfluorocarbons (PFCs) and Sulphur Hexafluoride (SF6) (Carbontrust.com, 2019).

In order to carry out the carbon footprint analysis, a functional unit (F.U) must be determined for calculation. This is deemed to be one tonne of APCR in its untreated and raw state, all carbon footprints are calculated in reference to this unit. The onsite treatment of APCR involves collection from the ash silo at the onsite EFW plant and transport to the ash conditioning plant. In this example, the APCR is processed as follows. The following analysis assumes the following:

EFW plant produce 20,000 tonnes of APCR per year;

The APCR source is close to the conditioning plant, less than 25km;

Landfill of the contaminated APCR is around 300km from APCR source; and Transport of materials by diesel powered trucks.

Stage 1 - Collection of APCR from onsite EFW

A powder tanker is driven to the site, a distance of 16km, performs 4 round trips of 1km between the APCR ash silo at the EWF plant and the ash conditioning operation. This amounts to a 36km total journey made per tanker operation day.

Table 1 - Carbon Footprint of Stage 1

Diesel Fuel 10.584 115.370 0.313 0.3010

Stage 2 - Ash Conditioning In this stage, 15% moisture is added to the APCR in the conditioning plant. It is then moved to the conditioning pad, where it is exposed to atmospheric conditions in order to allow the conditioning process to proceed. The water added to the APCR evaporates due to the exothermic reaction and so is not present within the material which is sent to stage 3. In other examples, between 5% and 40% moisture may be added.

The Carbon Footprint due to energy usage in stage 2 is shown below in Table 2. Table 2 - Carbon Footprint of Stage 2

Diesel Fuel 0.7978 1 8.6962 0.2810 2.4436

Electricity 3.5890 kWh 4.8500 0.6250 3.0313

Total - 13.5462 - 5.4749

The water used in conditioning is termed as Process Water from Ground Water source in the CCALC2 database (Ccalc.org.uk, 2019). The carbon footprint due to the use of water in APCR conditioning is shown below in Table 3.

Table 3 - Carbon Footprint due to Water Usage

0.150 6.520 0.978

Stage 3 - Washing Process In this stage of the process, the APCR goes through a washing process in order to create the PFA replacement media.

Table 4 - Carbon Footprint of energy used in Stage 3

Electricity 19.531 kWh 0 625 12.207

During the treatment process, there is a portion of material removed which must be sent to landfill. This material is 0.5% of the original mass of APCR. Hence 0.005 tonnes per tonne of APCR. This material is highly concentrated in heavy metals and so is characterised in the CCALC2 database as having the highest possible carbon footprint for landfill material. This is 0.015 kgCC e per kg of material. The carbon footprint of landfilling this material is outlined below in table 5.

Table 5 - Carbon Footprint due to landfill of contaminant

0.005 15.000 0.075 Alternative/Current Disposal Route - Landfill APCR

The alternative to utilising APCR as a PFA replacement media is to landfill the material. This involves transport to a hazardous landfill site, machinery movements and landfill capping to be carried out. This material will produce leachate which must be treated and so has a high carbon footprint for landfilling. Few such sites exist, the nearest being a round trip of approximately 570 km.

7.53 litres of fuel are consumed per tonne of APCR transported. The carbon footprint of APCR haulage is highlighted below in Table 6.

Table 6 - Carbon footprint due to APCR Haulage

Diesel Fuel 7.530 82.077 0.313 25.690

The carbon footprint of landfilling the APCR is displayed below in Table 7. The material is taken to be landfill of ferrous metal due to the high concentration of heavy metals present in the APCR. This is seen to be the most representative material available in the CCALC2 database.

Table 7 - Carbon Footprint due to Landfill of APCR

APCR 0015 14500

Results

The carbon footprint of proposed onsite treatment of APCR and current offsite disposal by landfill are displayed below in Table 8.

Table 8 - Carbon Footprint Totals

Stage Carbon Footprint (kgC02e/F.U)

Onsite Treatment 19.0359 Offsite Disposal 40.1900 In this example, the carbon footprint of onsite treatment of APCR is significantly less than the carbon footprint of solely transporting the APCR to the offsite treatment facility, without considering any carbon footprint arising from its actual disposal. This clearly displays the environmental benefit of the present invention as exemplified above.

Improvements and modifications may be incorporated herein without deviating from the scope of the invention.