Using Pyrolysis Tar to meet Fuel Specifications in Coal-to-Liquids Plants

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1 Using Pyrolysis Tar to meet Fuel Specifications in Coal-to-Liquids Plants Jaco Schieke, Principal Process Engineer, Foster Wheeler Business Solutions Group, Reading, UK Abstract: Indirect Coal-to-Liquids (CTL) facilities (i.e. liquids production via gasification and Fischer-Tropsch synthesis) are often sited adjacent to the coal mine feeding the facility, but these coal deposits are often found in remote locations; far from coastal refinery hubs where synergies with conventional refining operations can be exploited. The ability to produce a final saleable product as opposed to a blendstock is therefore an important consideration when designing a CTL facility. This opens up marketing opportunities in the immediate area of the facility and reduces the logistical cost of shipping the CTL product to a blending facility. There are several challenges to meeting final product specifications from a low temperature Fischer-Tropsch-based CTL facility. Hydrocracked Fischer-Tropsch diesel, although attractive from a cetane and sulphur perspective, has a lower density than conventional diesel. Fischer-Tropsch naphtha on the other hand is a very good chemicals feedstock, but typically has a lower octane than required. Foster Wheeler recently performed a number of feasibility studies on projects where there was a requirement to produce saleable product from the CTL facility. This article describes some of the key challenges and potential solutions to meeting this constraint. Through the integration of the refining needs with the coal and gasification technology selection, these challenges were addressed. By selecting the right combination of gasifier and coal, combined with the recovery of tar and oil by-products from the gasification process, the blendstock requirement to meet product specifications was substantially reduced. Coal conversion into transportation liquids, commonly referred to as Coal-to-Liquids (CTL), through the conversion of syngas generated from coal gasification, is considered in a number of countries as a potential solution for coal-rich nations to reduce dependence on imported oil and refined products. This paper examines one of the key technical challenges in the development of a CTL facility; namely producing fuels meeting final market specifications, rather than producing blendstocks. Although a seemingly minor issue in the development of a CTL project, the inability to meet final market product specifications, in domestic or export markets, poses a substantial business risk to the viability of a CTL project. If a CTLderived product is unable to meet the final market specification, the viability of the project will be entirely dependent on availability of blendstock, which may increase project risk and logistical costs. Fuel specifications are generally determined by state or national governments. Although significant variation is still encountered in fuel specifications around the world, the drive for cleaner fuels, and pressure by engine manufacturers are leading to a convergence in specifications. The key global specifications are those used in the United States and the European Union.

2 This paper compares typical low temperature Fischer-Tropsch (LTFT) derived CTL diesel against the Euro IV and Euro V diesel fuel specifications, and considers ways in which coal and gasification technology selection can play a role in addressing the shortfalls of CTL diesel. The paper is based on feasibility studies that Foster Wheeler recently completed on potential large-scale CTL facilities, in South Africa, China and the United States. In all cases, these facilities were minemouth facilities at inland locations. All sites were remote, away from conventional refineries which would typically be the buyers of a blendstock product. Regional markets however existed, where a final product could be sold directly to the enduser. The table below presents CTL fuel properties typical of straight-run (SR) CTL diesel. This diesel is produced from LTFT synthesis, followed by hydrocracking of the synthetic waxes and hydrotreating of the lighter products. The Euro IV and Euro V fuel specifications under discussion are compared against this typical CTL product. Property Unit SR CTL Euro IV Euro V Density kg/m 3 (min/max) / /845 1 Cetane Number > 51 >51 Sulphur ppm <5 <50 <10 Notes 1) or Arctic diesel specification of 800/845 kg/m 3 It can be seen that CTL-derived diesel is an excellent diesel product and exceeds Euro IV and Euro V standards in all respects but density. The cetane number and sculpture specifications were well within the market specification, which makes LTFT diesel an ideal high cetane, low sulphur blendstock, particularly for conventional refiners that are unable to blend excess light cycle oil into their diesel pool. The low density of CTL-derived diesel does however pose a problem when targeting a final product. The lower density of CTL-derived diesel would not be a problem, if it were not for the consumer issues associated with the lower density. Due to the lower density, the volumetric energy content of CTL diesel is less than that of conventional diesel. Because fuel is sold on a volumetric basis, the fuel is therefore not compatible with the market specification. The consumer perceives this as reduced volumetric fuel efficiency when using CTL-derived fuels. Therefore, in order to produce a compatible diesel product, a CTL facility requires additional measures in order to meet market density specifications. This can take one of two forms: (i) blending with an external blendstock prior to the end user sale, or (ii) additional processing steps within the CTL facility. It is however not possible to increase straight-run CTL diesel density using current technology without incurring a significant cost and yield penalty. It should be noted that meeting the less stringent Arctic diesel specification of 800kg/m 3 may be feasible, but this severely limits the potential market for CTL diesel. Figure 1 shows a typical indirect CTL facility, consisting of gasification, followed by acid gas removal and Fischer-Tropsch synthesis. The Fischer-Tropsch products then undergo mild hydrocracking followed by the hydrotreatment of olefins. This straight-run diesel can be used as a diesel blendstock, while the straight-run naphtha from the process is either upgraded to improve the octane through isomerisation or reforming or sold directly as a cracker feed to a chemical complex.

3 O 2 Gasification Coal Isomerisation Gasoline Acid Gas Removal LTFT Synthesis Raw syngas Clean syngas LTFT Wax SR CTL Naphtha Hydrocracking/ treating CCR Reforming SR CTL Diesel Diesel Blendstock Figure 1: Typical indirect Coal-to-Liquids Flow scheme The use of low-temperature gasification presents an alternative route to final product densification. Low-temperature gasification and, specifically, fixed-bed technologies, produces a substantial amount of tar and pyrolysis oil as part of the gasification process. This occurs due to the counter-current flow of synthesis gas and coal in these gasifiers, where the hot synthesis gas liberates volatile pyrolysis tar and oils from the coal without cracking. This happens in the upper part of the gasifier, where temperatures are sufficient to devolatilise the coal, but low enough to limit cracking. Where LTFT-derived diesel is a low density, high cetane material, pyrolysis products are high density, low cetane blendstocks. Density of pyrolysis products ranges upwards of 865kg/m 3 and therefore makes an ideal blendstock to increase the density of the LTFT diesel. The amount of pyrolysis product produced is heavily dependent on the coal feedstock rank and grade. Although the proximate analysis gives an indication of the volatile matter in the coal, the most reliable estimate of pyrolysis oil can be obtained from a Fischer assay of the feedstock. The Fischer assay measures the condensable liquid produced when heating a sample to 500 C for one hour in the absence of oxygen, a process very similar to conditions inside the gasifier. This analysis is useful when assessing different potential coal feedstocks and should be considered when targeting maximum pyrolysis oil production. In a typical CTL flow scheme using fixed-bed gasification, the pyrolysis product is recovered in two stages: coal tar is knocked out from the raw synthesis gas during initial quench, and gasification naphtha is recovered during the cooling of synthesis gas to the acid gas absorber feed temperature. The coal tar typically contains some of the coal dust entrained in the raw syngas after gasification. This has to be taken into consideration when selecting the refining route, as filtration and solids removal is difficult and adds another degree of complexity to the flowscheme. There are two main routes for upgrading the pyrolysis products: carbon rejection (e.g. coking) or hydrotreatment. Foster Wheeler s work focused predominantly on the hydrotreating route, as this yields the maximum product from refining the tar. The flowscheme considered, shown in Figure 2, consisted of filtration, followed by fractionation and separate hydrotreating of the resulting tar distillate and naphtha streams. The residue was recycled back to the gasifiers, although coking was considered an attractive option to increase product yield. The flowscheme used conventional fixed-bed hydrotreating technologies, but the opportunity exists to hydrotreat the unfiltered feed in an ebullated-bed reactor. Hydrotreated distillate was blended with the straight-run CTL diesel, whilst naphtha was combined with the conventional LTFT naphtha and sent to a reformer. Other refinery options are discussed in more detail in Lamprecht (2010).

4 O 2 Coal Gasification Acid Gas Removal LTFT Synthesis Raw syngas Clean syngas Gasification Naphtha LTFT Wax Filtration & Distillation Hydrotreating Hydrocracking/ treating Isomerisation CCR Reforming SR CTL Diesel Gasoline (and Reformate) Diesel (and Blendstock) Figure 2: Indirect Coal-to-Liquids Flow scheme incorporating Pyrolysis Tar and Oil The results from Foster Wheeler s studies are presented in Figure 3 below. The final product slate from an indirect CTL facility is shown for the two cases considered: CTL with tar blending, and CTL without tar blending. The benefit of utilising the gasification pyrolysis products in the final fuel blend is evident. On-specification diesel production increases by 22 percentage points from 5% to 27% of the overall product. This equates to a 33% increase of saleable diesel in the diesel pool and a commensurate reduction in the blendstock requirement. The remainder of the diesel product will require further blending to meet the density specification, but the amount of blendstock required is reduced by one third. The study by Lamprecht (2010) has shown that all diesel could be produced to meet the specification if sufficient pyrolysis products are available in the facility. The quantity required is more than would be produced in a conventional CTL facility and other options would have to be considered to increase the pyrolysis tar yield. Without Tar Blending With Tar Blending 10% 9% 19% 39% 21% RON95 Gasoline Reformate 63% 3% 5% 4% Diesel Diesel Blendstock 27% Figure 3: CTL Facility Product Slate without and with Gasification Tar (and Oil) Blending

5 An LTFT-based CTL facility typically aims to maximise diesel production, but it can be seen from the results that the balance of the products comprises about a third of the total products. The gasoline products indicated in Figure 3 (RON95 gasoline and reformate) arises from the further processing of the LTFT naphtha into gasoline. Although LTFT naphtha makes an excellent reforming feedstock for chemicals production, an isolated facility may again need to target onspecification gasoline production rather than naphtha export. is produced in the facility as needed to meet the specification for light ends in gasoline. The remainder of the paper considers the gasoline value chain and the ways in which pyrolysis products aid the production of gasoline. The straight chain, paraffinic LTFT-derived naphtha has a very low octane content and further refining is required to increase the octane of this stream. This is typically achieved through isomerisation of the C5/C6 fraction, and continuous catalyst regeneration (CCR) reforming of the heavier components. The severity of the CCR process is determined by the N2A (naphtha + 2 x aromatics) of the feed. The straight-run LTFT naphtha has a very low N2A, leading to a very high severity and low yields in the CCR reformer. The high aromaticity of the hydrotreated tar oil fraction makes an ideal blendstock to the CCR reformer feed, increasing the feed N2A and reducing the severity requirement. Additionally, the aromaticity of the pyrolysis products improves the octane of the straight-run LTFT naphtha. In the Foster Wheeler studies there was an excess of reformate from the CCR reformer, because the gasoline pool was already constrained by the aromatics limit. The gasoline pool typically consists of a combination of straight-run naphtha, isomerate and reformate. When blending only these components to meet a final gasoline specification, the facility is typically constrained by the benzene and/or aromatics specifications of the gasoline when targeting a Euro IV or Euro V specification. Although the pyrolysis products therefore help in increasing the octane, an octane booster may still be required in order to meet final product specifications. This could take the form of MTBE, TAME, ETBE or ethanol, depending on local regulations. Care must be taken, in selecting the octane booster, as the trade-off between vapour pressure (RVP), aromatics and octane remains a constraint in a refinery blending only reformate and isomerate. In conclusion, the Foster Wheeler studies demonstrated that the use of gasification pyrolysis products can aid in meeting final diesel specifications from an LTFT-based CTL facility, by providing a high density blendstock for the final diesel pool. In addition, the gasoline pool will also benefit from including pyrolysis products due to the increased octane and lower severity of the CCR reactor. The selection of a low temperature gasification technology, combined with the use of the resultant pyrolysis tars within the refinery, has the potential to greatly improve the fuel pool produced within a standalone CTL facility. References 1. Lamprecht D, Nel R, Leckel D, Production of On-Specification Fuels in Coal-to-Liquid (CTL) Fischer-Tropsch Plants Based on Fixed-Bed Dry Bottom Coal Gasification, Energy Fuels, 2010, 24 (3), pp Foster Wheeler All rights reserved.