Combining petroleum coke and natural gas for efficient liquid fuels production
Introduction
Despite the current trend towards a shift to green and sustainable energy sources with reduced greenhouse gas (GHG) emissions, the use of fossil energy continues to dominate the world's energy mix. According to the 2017 BP statistical review of world energy, a total primary energy of 13.3 billion tonnes of oil equivalent was consumed in 2016 [1] of which fossil energy contributed 86% of the total.
Over 90% of Canada's oil deposit is made up of heavy oil [2] which leaves between 15% and 40% of the oil as residues in the refinery distillation unit [3,4]. To maximize profit and also meet demands for transportation fuels, refiners further crack these residues in coker units producing unwanted solid carbon material by-product known as petroleum coke (petcoke) [5]. The type of petcoke produced depends on the coker unit used [5]. Fluid and delayed coke are produced in Canada by Syncrude and Suncor respectively [6] with delayed coke getting more industrial attention [7]. Generally, petcoke is classified as either fuel grade or graphite depending on their sulfur content [8] and could be used for wide range of applications such as anode, fuels in kilns, etc. Reports on petcoke availability have ranged between 56 and 150 million tonnes per year (Mt/yr) [4,8]. Recently, stockpiled petcoke produced in Canada by Syncrude, Suncor, and CNRL near Fort McMurray, Alberta is estimated to be about 100 (Mt/yr) [9] with reasons being due to lack of transportation and carbon sequestration [4]. Such a large amount of solid fuel is sufficient to create dedicated power and chemical production facilities solely running on petcoke.
Therefore, just like coal, petcoke can be used for power generation in supercritical pulverized coal (SCPC) or integrated gasification combined cycle (IGCC) plants since it has a competitive heating value of 34.7 MJ/kg [10] and can be obtained at very low cost since it is a waste product. On the downside, petcoke is a dirty fuel with 5–10% higher CO2 emissions per unit energy produced [4]. Hence, replacing petcoke with coal is obviously not environmentally advisable. However, the conversion of petcoke to make liquid transportation fuels could potentially reduce greenhouse gas emissions by offsetting some petroleum usage. Thus, this study explores petcoke conversion strategies that could be both economically competitive and environmental friendly.
Some studies have examined different commercial plants which use petcoke either alone or in combination with coal for power and chemical productions [11,12]. However, research on the techno-economic analysis of petcoke conversion to power, chemicals, and fuels are limited. Orhan et al. [13] carried out a techno-economic analysis of producing power and hydrogen from petcoke. For a petcoke feed rate of 4200 t/day, 437 MW of electricity and 12.87 tonne/hr of hydrogen (also equivalent to 437 MW of energy content) were produced resulting in a thermal efficiency of 25.6% on a lower heating value basis. A preliminary economic analysis was carried out by Jacob Consulting on a 4 Mt/yr petcoke plant that produces either methanol or hydrogen in which the net present value (NPV) was estimated as a function of oil and gas prices [9]. The study showed that there is a business case for using petcoke as a feedstock for fuel production. Similarly, the National Energy Technology Laboratory in the United States Department of Energy (NETL) performed a techno-economic study of converting petcoke to hydrogen, industrial grade steam, fuel gas, power, and liquid fuels [14]. They also concluded that there is a business case for petcoke conversion to fuels under the right market conditions. However, neither study looked at the potential environmental benefits of using waste petcoke as a resource, nor did they examine any potential synergies that could be exploited by combining petcoke with natural gas as feedstock.
Recently, Salkuyeh et al. [15], carried out an optimization study on a petcoke and natural gas polygeneration plant for the production of dimethyl ether (DME), methanol, olefins, and Fischer-Tropsch (FT) liquids. The result of the study showed that market prices strongly determined which of the possible products were the most economically optimal to produce. In addition, it was found that depending on market prices, the economically optimal polygeneration plant used either 20% petcoke (and 80% gas), or no petcoke at all, with the gas-only configuration strongly preferred with the market prices at the time of the study. However, the work only looked at economics and did not consider any potential environmental benefit by using waste petcoke as a resource instead of stockpiling. In addition, the study did not look at petcoke-only designs.
Therefore, this work focuses on the environmental benefits of using waste petcoke for FT liquids production. In this study, different petcoke conversion strategies are proposed as pathways by which petcoke can be converted to FT liquids. The design configurations evaluated are as follows: petcoke standalone gasification (PSG) in which petcoke is the only feedstock; petcoke integrated natural gas reforming (PG-INGR) which integrates petcoke gasification and natural gas reforming in the tubes of the gasifier's radiant syngas cooler; and petcoke external natural gas reforming (PG-ENGR), which uses petcoke gasification and natural gas reforming as separate units but blends their syngas products together. These designs were adopted from the work of Adams et al. [16].
The objective of this paper is to ascertain in detail the economic and environmental performance of the three proposed petcoke conversion strategies. A process flow diagram showing the different conversion strategies is shown in Fig. 1. A petcoke slurry is gasified in the presence of O2 (99.5%) to produce syngas with low H2/CO ratio. Depending on the configuration, either boiler feed water (BFW) or a natural gas/steam mixture is fed through the gasifier's radiant syngas cooler tubes (for natural gas, the tubes are packed with catalyst) providing the cooling duty required to cool the hot gasifier syngas and simultaneously either producing steam or syngas respectively in the process. This distinguishes the PSG and PG-ENGR configurations from the PG-INGR design as the hot gasifier syngas heat is used to drive the endothermic steam natural gas reforming process in PG-INGR instead of making high pressure steam (HPS) in the PSG and PG-ENGR designs.
The gasifier syngas is quenched and flashed to remove entrained impurities and contaminants. Water gas shift and carbonyl sulfide (COS) hydrolysis are employed to raise the H2/CO ratio and convert COS to H2S respectively. Natural gas derived syngas from the PG-INGR or PG-ENGR design is mixed with the shifted gasifier syngas and sent to the methyl di-ethanolamine (MDEA) based acid gas removal unit for H2S and CO2 removal. The absorbed H2S is sent to the Claus unit for sulfur production while aqueous MDEA enriched with piperazine is used to capture CO2 which is sent to the CO2 compression unit for sequestration. A small amount of H2 is extracted from the syngas via a pressure swing adsorber (PSA) unit for use in a downstream hydrocracker. The syngas is then sent to the FT reactor where it is converted to FT syncrude. The syncrude is distilled and hydrocracked to produce gasoline and diesel fractions. To promote the formation of liquids, 90% of the unconverted hydrocarbons are recycled back to the FT reactor while the remaining is combusted for HPS generation. For comparison, variants of each of these three designs were considered with no CO2 capture and sequestration (CCS), resulting to a total of six designs considered in this analysis.
Section snippets
Process simulation/assumptions
This paper considers the use of delayed coke produced in Alberta, Canada and conventional pipeline natural gas as the feedstocks that is to be converted to liquid fuels. Based on the NETL petcoke study [14], the petcoke feed rate is 219 t/hr (dry basis) with the natural gas feed rate sized according to the amount of radiant heat recovered in the gasifier that is available to drive the endothermic natural gas reforming reaction. The properties of the feedstock are shown in Table 1.
The simulation
Process simulation
The converged Aspen Plus simulation files for all six cases are available for download on the digital archive at PSEcommunity.org.1 The results of the 6 petcoke conversion strategies (with CCS and w/o CCS) are briefly summarized in Table 5 detailing the feed rates, total electricity consumption, fuels production rates, by-product rate, and cradle to plant exit life cycle GHG emissions. Also, feed carbon, fuel, and
Conclusions
The results showed that converting petcoke into liquids with CCS through a traditional gasification route is not profitable nor were the environmental benefits worth the cost. However, by tightly integrating natural gas reforming with petcoke gasification (by performing the gas reforming in the tubes of the radiant syngas cooler), the results were much more promising. This integration strategy uses the excess heat from the gasifier to drive the endothermic reforming process of gas reforming,
Acknowledgements
The authors received funding from an NSERC Discovery grant (RGPIN-2016-06310) for this research.
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