PROCESS

Abstract

The present invention provides a process for obtaining solid recovered fuel and synthesis gas from a waste-based feedstock, comprising the steps of: I. converting the feedstock into a solid recovered fuel by means of a number of parameters pertaining to waste sorting, selection, comminution and/or screening; II. gasifying under suitable reaction conditions at least a portion of the solid recovered fuel to produce synthesis gas and by-product(s); and III. optionally cleaning at least a portion of the synthesis gas to produce clean synthesis gas and wastewater, wherein one or more of the solid recovered fuel, synthesis gas, and by-product(s) of the gasification are analysed during operation of the process, and wherein data from said analysis is used to control one or more parameters of step I) in order to influence reaction conditions in step II, and optionally step III).

Claims

1. A process for obtaining solid recovered fuel and synthesis gas from a waste-based feedstock, comprising the steps of: I. converting the feedstock into a solid recovered fuel by means of a number of parameters pertaining to waste sorting, selection, comminution and/or screening; II. gasifying under suitable reaction conditions at least a portion of the solid recovered fuel to produce synthesis gas and by-product(s); and III. optionally cleaning at least a portion of the synthesis gas to produce clean synthesis gas and wastewater, wherein one or more of the solid recovered fuel, synthesis gas, and by-product(s) of the gasification are analysed during operation of the process, and wherein data from said analysis is used to control one or more parameters of step I) in order to influence reaction conditions in step II, and optionally step III).

2. The process of claim 1 wherein said data includes information concerning the chemical composition, pressure and/or temperature of the synthesis gas during operation of the process.

3. The process according to claim 2 wherein the synthesis gas is analysed to determine one or more of H.sub.2:CO ratio, C.sub.14/C.sub.12 ratio, moisture content, wt. % of chlorides, and wt. % of inerts.

4. The process according to claim 1 wherein the solid recovered fuel is analysed to determine one or more of average particle size, average volume, moisture content, calorific value, wt. % of chlorides, wt. % of sulphur, biogenic content, chemical composition, grit content, glass content and inert content.

5. The process according to claim 1 wherein the by-product(s) of the gasification are analysed to determine tramp material mass flow.

6. The process according to claim 1 wherein the wastewater is analysed to determine wt. % of chlorides, and/or total flow of chlorides.

7. The process according to claim 1 wherein the parameters of step I) comprise: a) providing a feedstock which comprises a fine feed, a small feed, a main feed, and a coarse feed; b) shredding the feedstock to a first size; c) subjecting the feedstock to a first screening, which separates the fine feed, small feed and main feed from the coarse feed; d) subjecting the fine feed, small feed and main feed to a second screening, which separates the fine feed, the small feed, and the main feed; e) subjecting the coarse feed to a third screening, which separates the coarse feed into a light coarse feed, a medium coarse feed, and a heavy coarse feed; f) conveying one or more of the small feed, the main feed, the light coarse feed, and/or the medium coarse feed over one or more magnets to remove ferrous and/or non-ferrous metals from said one or more feeds; g) near-infrared scanning the medium coarse feed to identify and remove one or more plastics; h) subjecting the main feed to a density separation; i) shredding the small feed, the main feed, the light coarse feed, and the medium coarse feed to a second size; j) combining the small feed, the main feed, the light coarse feed, and the medium coarse feed into a final feed; and k) drying the final feed, optionally by using a belt dryer, to produce a solid recovered fuel.

8. The process according to claim 7 wherein the first screening is a trommel screen; and/or wherein the second screening is a flip-flop screen; and/or wherein the third screening is a wind sifter.

9. The process according to claim 7 wherein the first size is about 250 mm and/or wherein the second size is about 25 mm.

10. The process according to claim 7 wherein the plastics comprise one or more of polyvinyl chloride, a polyolefin, polystyrene, polyacrylonitrile, a polyacrylate, a polyurethane, a polyamide, a polyester, a polycarbonate, and an elastomer.

11. The process according to claim 7 wherein the density separation removes inerts, such as glass, stone, and grit, from the main feed.

12. The process according to claim 7 wherein one or more of the feedstock, the fine feed, the small feed, the main feed, the light coarse feed, the medium coarse feed, and/or the heavy coarse feed is analysed.

13. The process according to claim 7 wherein the parameters of step I) further comprise one or more of: a) selection of the feedstock; b) operation of the density separator; c) operation of the first, second and/or third screening; d) belt speed of the belt dryer; e) residence time in the belt dryer; f) amount of heat supplied in the belt dryer; g) flow rate of the feedstock through the process; h) type and quantity of the one or more plastics removed during the near-infrared scanning; i) addition of fine feed to final feed; j) rejection of one or more of the feed(s) to storage or disposal; and k) quantity of feedstock in each of the fine feed, the small feed, the main feed, the light coarse feed, the medium coarse feed, and the heavy coarse feed.

14. The process according to claim 1 wherein the analysis is performed continuously throughout the process.

15. The process according to claim 1 wherein the feedstock comprises one or more of household waste, commercial and industrial waste, and co-collected household and commercial waste.

16. The process according to claim 1 wherein at least about 95% of metals are removed from the feedstock and/or at least about 80% of inerts are removed from the feedstock.

17. The process according to claim 1 wherein the solid recovered fuel comprises one or more of: a) a particle size of less than about 25 mm in two dimensions; b) at least about 95% by weight of the solid recovered fuel having a volume of about 16,400 mm.sup.3 or less; c) no more than about 5% by weight of the solid recovered fuel being greater than about 75 mm in length; d) no more than about 15% by weight of the solid recovered fuel being smaller than about 840 μm; e) an average moisture content of from about 5% to about 15%, or about 10%; f) less than about 1% by weight of chloride; and g) a calorific value of from about 14 to about 22 MJ/kg.

18. Solid recovered fuel produced by step I) of a process according to claim 1.

19. Synthesis gas produced by a process according to claim 1.

20. A useful product manufactured by converting the synthesis gas according to claim 19.

21. The useful product of claim 20 being liquefied petroleum gas, naphtha, diesel or aviation fuel.

22. A control unit for monitoring a process according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0136] Preferred embodiments of the invention are described below by way of example only with reference to FIGS. 1 to 3 of the accompanying drawings, wherein;

[0137] FIG. 1 illustrates the process of obtaining solid recovered fuel from a feedstock in accordance with one embodiment of the invention.

[0138] FIG. 2 illustrates the feedback loops involved in steps I) and II) of the process in accordance with one embodiment of the invention.

[0139] FIG. 3 illustrates the feedback loops involved in steps I), II) and III) of the process in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

Production of Solid Recovered Fuel

[0140] FIG. 1 illustrates the process of obtaining solid recovered fuel from a feedstock in accordance with one embodiment of the invention. At the start of the process, a feedstock 101 is provided. The feedstock can be household, or co-collected house and commercial waste, which is also called municipal solid waste (MSW). Alternatively, the feedstock can be separately collected commercial and industrial waste (C&I).

[0141] The raw feedstock 101 is delivered to a feedstock reception area, where it is then loaded into a shredder 102. There may be a single shredder, or a plurality of shredders wherein the feedstock 101 is shared between said plurality of shredders. The feedstock is shredded to 250 mm.

[0142] The shredded material is then passed through a trommel screening process 103. This screening separates the material into a fraction with a size greater than 60 mm (the coarse feed) and into a fraction with a size less than 60 mm (the fine, small, and main feed). Depending on the screen size of the trommel (as the screen size may vary as a parameter which may be controlled in accordance with the invention and may not always be 60 mm), 2D materials pass through the trommel and 3D materials are screened off for separate processing.

[0143] The large fraction (termed the coarse feed) is then passed through a wind sifter 105, which uses a continuous jet of air to separate materials. The wind sifter 105 separates the coarse feed into heavy materials, light materials, and medium materials. The heavy materials, light materials and medium materials are also termed the heavy coarse feed, light coarse feed, and medium coarse feed respectively.

[0144] The heavy materials consist of inerts and glass and are rejected from the process because they cannot be used to form compliant solid recovered fuel. The light materials consist predominantly of paper and plastics. These materials are passed over a ferrous magnet 106 and a non-ferrous magnet 107 to maximise metal removal. The medium materials consist of heavier paper, card, and plastics, including polyvinyl chloride. These materials are passed over a ferrous magnet 106 and a non-ferrous magnet 107 to maximise metal removal. The medium materials are then passed through a near-infrared scanner 109 to identify and remove polyvinyl chloride based materials. Alternatively or additionally, the near-infrared scanner 109 may identify and remove other plastics. After these steps, the light materials and medium materials are delivered to the final shredder 110.

[0145] The smaller fraction separated at the trommel screening process 103 is subjected to different steps than the larger fraction. The smaller fraction (the fine feed, small feed and main feed) is passed through a double deck flip flop screen 104. This screening separates the material into a fraction with a size of from about 20 mm to about 60 mm (the main feed), into a fraction with a size of from about 6 mm to about 20 mm (the small feed), and into a fraction with a size less than about 6 mm (the fine feed). Both feeds are passed over a ferrous magnet 106 and a non-ferrous magnet 107 to maximise metal removal. The main feed is then subjected to a density separation 108 to remove any remaining inerts and glasses, which are to be rejected from the process. After these steps, the small feed and main feed are delivered to the final shredder 110.

[0146] All those feeds which are delivered to the final shredder 110 are shredded to a size of 25 mm, which conforms to the required specification of the solid recovered fuel. The shredded solid recovered fuel is then delivered to a belt dryer 111. Prior to delivery to the belt dryer 111, at least part of the fine feed, which was separated at the flip flop screen 104, may be added to the shredded solid recovered fuel.

[0147] All of the shredded solid recovered fuel may be delivered to a single belt dryer 111 or may be distributed across a plurality of belt dryers 111. The belt dryer 111 reduces the moisture content of the solid recovered fuel to less than, or equal to, about 10 wt. %.

[0148] The dried solid recovered fuel is sampled on leaving the belt dryer and analysed. The solid recovered fuel is analysed to determine one or more of the average particle size, the average volume, the moisture content, the calorific value, the wt. % of chlorides, the wt. % of sulphur, the biogenic content, the chemical composition, the grit content, the glass content and the inert content.

[0149] The remaining dried solid recovered fuel is delivered either to the gasifier feed systems for gasification 112, or to baling 113 for storage or export. The solid recovered fuel entering the gasification step 112 or sent for baling 113 needs to meet certain specifications, which are primarily determined by the requirements of the gasification step 112.

Feedback Loops

[0150] FIG. 2 illustrates the feedback loops involved in steps I) and II) of the process in accordance with one embodiment of the invention. In the illustrated process, feedstock 201 is converted into a solid recovered fuel 203 in a fuel conversion facility (FCF) 202. At least a portion of the solid recovered fuel 203 is delivered to a gasification step 204 where it is gasified to produce synthesis gas (syngas) 205.

[0151] Two of the steps which the feedstock may be subjected to in the fuel conversion facility 202 include a near-infrared scan 206, to remove plastic such as polyvinyl chloride, and a belt dryer 207, to vary the moisture content of the solid recovered fuel. The dashed arrows between the feedstock 201 and the near-infrared scan 206, and between the near-infrared scan 206 and the belt dryer 207, indicate that other steps may optionally also be present but not illustrated.

[0152] Exemplified in FIG. 2 are two feedback loops in accordance with the invention.

[0153] The first feedback loop is between the synthesis gas 205 and the near-infrared scanner 206. After gasification 204, the synthesis gas 205 is analysed to determine the H.sub.2:CO ratio. This ratio is important as a certain ratio is required for downstream reaction operations, such as Fischer-Tropsch synthesis. However, the H.sub.2:CO ratio of the synthesis gas 205 will be entirely dependent on the nature of the feedstock 201, because in a chemical process plant handling mixed feedstock streams derived from waste there is inherent and significant variability in the nature of the feedstock. As a result, downstream processing of the synthesis gas 205 is freighted with difficulty because of the variable nature of such gas arising from different feedstocks at different times in the production cycle. Wide variation in the synthesis gas H.sub.2:CO ratio creates problems in consistently and efficiently adjusting that ratio for suitability with the selected downstream reaction operation. This is particularly the case when the variability of feedstock is such as to give rise from time to time to H.sub.2:CO ratios which are above the preferred usage ratio of the downstream reaction.

[0154] Therefore, to provide a solution to this problem, data from the analysis of the synthesis gas 205 can be used to actively manage the amount of removal of high hydrogen contributing wastes, such as plastics, at the near-infrared scanner 206. The consequence of this is that the H.sub.2:CO ratio can be adjusted to account for the variations in the feedstock. This exemplifies how the present invention advantageously uses feedback loops which extend beyond solely within the fuel conversion facility 202, in that the products of downstream processes are analysed to control and influence upstream processes.

[0155] The second feedback loop is between the gasification step 204 and the belt dryer 207. The oxygen consumption and fuel gas consumption during the gasification step 204 is dependent on the moisture content of the solid recovered fuel 203. Therefore, the oxygen consumption and fuel gas consumption are analysed and used to control parameters of the belt dryer 207 in order to increase or decrease the amount of moisture removed from the solid recovered fuel during this step. Such parameters include the belt speed of the belt dryer 207, residence time in the belt dryer 207, and amount of heat supplied in the belt dryer 207.

[0156] FIG. 3 illustrates the feedback loops involved in steps I), II) and III) of the process in accordance with one embodiment of the invention. In the illustrated process, feedstock 301 is converted into a solid recovered fuel 303 in a fuel conversion facility (FCF) 302. At least a portion of the solid recovered fuel 303 is delivered to a gasification step 304 where it is gasified to produce synthesis gas (syngas) 305. At least a portion of the synthesis gas 305 is cleaned to produce clean synthesis gas 307 and wastewater 308. Other steps between the gasification 304 and gas clean up 306 may optionally also be present but not illustrated. The wastewater 308 is passed to a wastewater treatment unit 311 prior to disposal or reuse.

[0157] Two of the steps which the feedstock may be subjected to in the fuel conversion facility 302 include a near-infrared scan 309, to remove plastic such as polyvinyl chloride, and a belt dryer 310, to reduce the moisture content of the solid recovered fuel. The dashed arrows between the feedstock 301 and the near-infrared scan 309, and between the near-infrared scan 309 and the belt dryer 310, indicate that other steps may also be present but not illustrated.

[0158] Exemplified in FIG. 3 are three feedback loops in accordance with the invention.

[0159] The first feedback loop is between the synthesis gas 305 and the near-infrared scanner 309. After gasification 304, the synthesis gas 305 is analysed to determine the H.sub.2:CO ratio. This ratio is important as a certain ratio is required for downstream reaction operations, such as Fischer-Tropsch synthesis. However, the H.sub.2:CO ratio of the synthesis gas 305 will be entirely dependent on the nature of the feedstock 301. Therefore, depending on the results of the analysis of the synthesis gas 305, data from said analysis can be used to actively manage the amount of removal of high hydrogen contributing wastes, such as plastics, at the near-infrared scanner 309.

[0160] The second feedback loop is between the gasification step 304 and the belt dryer 310. The oxygen consumption and fuel gas consumption during the gasification step 304 is dependent on the moisture content of the solid recovered fuel 303. Therefore, the oxygen consumption and fuel gas consumption are analysed and used to control parameters of the belt dryer 310 in order to increase or decrease the amount of moisture removed from the solid recovered fuel during this step. Such parameters include the belt speed of the belt dryer 310, residence time in the belt dryer 310, and amount of heat supplied in the belt dryer 310.

[0161] The third feedback loop is between the wastewater 308 and the near-infrared scanner 309. Polymers such as polyvinyl chloride in the feedstock 301 contaminate the synthesis gas 305 with chlorides, which must be removed during the gas clean up 306. As a result, the clean synthesis gas 307 is substantially free of chlorides, and the wastewater 308 may contain chlorides which have been removed from the synthesis gas 305. This wastewater 308 is sent to a wastewater treatment unit 311 before disposal or possible reuse. The wastewater treatment unit 311 is required to remove chlorides from the wastewater 308 so that the water can be safely disposed or reused. The amount of treatment required depends on the amount of chloride present in the wastewater, which in turn is dependent on the amount of chloride-containing materials in the feedstock 301. Therefore, the wastewater 308 is analysed to determine the wt. % of chlorides, and the data from said analysis is used to actively manage and control the removal of high chloride contributing wastes (such as polyvinyl chloride) from the feedstock at the near-infrared scanner 309.

[0162] In some instances, there will be an interplay, and a necessary balance, between different feedback loops. For example, reducing the removal of high hydrogen contributing wastes, such as plastics, at the near-infrared scanner 309 will improve the H.sub.2:CO ratio and the synthesis gas 305 energy content. However, this consequently increases the need for caustic treatment of the wastewater 308. Therefore, there may be a dynamic optimum that exists for the relevant controlled parameters.

[0163] A further exemplary feedback loop in accordance with the invention is in controlling the calorific value of the solid recovered fuel by analysing and monitoring the heat input to the gasifier per unit mass of feedstock. A lower heating value may be corrected through an increased removal of moisture content at the belt dryer 310 and/or the addition of higher heating value materials such as plastic (in other words, less plastic is removed during the near-infrared scan 309). On the other hand, a higher than expected heating value may be corrected through a decreased removal of moisture content at the belt dryer 310 (such that the solid recovered fuel has a higher moisture content than before the correction) and/or the removal of higher heating value materials such as plastic (in other words, a greater amount of plastic is removed during the near-infrared scan 309).

[0164] A further exemplary feedback loop in accordance with the invention is in influencing the volume of caustic soda required in the caustic wash by analysing and monitoring the quantity of reactive halides in the solid recovered fuel.

[0165] A further exemplary feedback loop in accordance with the invention is in analysing the by-product(s) of the gasification to determine tramp material mass flow, and/or using the agglomeration detectors in the gasifier to analyse the formation of sticky materials. These can be used to control the ferrous and non-ferrous metal removal, and/or to control the density separation of the main feed. The density separation efficiency directly impacts the tramp removal rate. If the tramp removal rate increases, more CO.sub.2 is required to manage the removal process. This undesirably results in both a high CO.sub.2 demand, as well as introducing more CO.sub.2 to the synthesis gas, and thus influencing the reaction conditions of downstream processes. Therefore, analysing the tramp removal rate and controlling the density separation efficiency influences the quantity of CO.sub.2 in the synthesis gas.