Comprehensive environmental impact assessment of a combined petroleum coke and natural gas to Fischer-Tropsch diesel process

Highlights

• Life cycle assessment of petcoke and natural gas conversion to diesel was evaluated.

• Different design configurations operating with and without CCS were explored.

• The impacts of electricity grid emissions on the overall life cycle were analyzed.

• GHG emissions and Cost of CO₂ avoided of 281 gCO₂ eq/km and $144/tCO₂ eq was estimated.

Abstract

In this study, a well-to-wheels life cycle assessment was conducted to determine the environmental impacts from disposing of petroleum coke by converting it into liquid fuel. Specifically, this work is an extension of the life cycle assessment of the petcoke standalone gasification to diesel study which also includes two new pathways for converting petroleum coke and natural gas to Fischer Tropsch diesel operated with and without carbon capture and sequestration (CCS). Impact categories were calculated using the EPA’s TRACI 2.1 US-Canada 2008 midpoint method in SimaPro software. In addition, the impact of grid emissions on the overall process was assessed using two representative Canadian locations with high (Alberta) and low (Ontario) grid emissions. The results of each impact category were compared among the designs and against conventional petroleum and oil sands derived diesel. Key findings showed that the proposed designs when operated with CCS in the low-emissions-grid location had life cycle GHG emissions between 281–291 gCO₂-eq/km compared to the conventional petroleum and oil sands derived diesel with GHG emissions of 305 and 348 gCO₂-eq/km respectively. Nevertheless, the various tradeoffs between processes indicated that there was no clearly superior design among the candidates. However, the design which uses a natural gas reformer that is integrated directly into the radiant syngas cooler of a petcoke gasification unit has the lowest cost of CO₂ avoided ($144/tCO₂-eq), and so is likely the best choice for reducing environmental impacts.

Introduction

Petcoke is the unwanted solid waste that is produced when petroleum residues or oil sand bitumen is upgraded to lighter fractions. Despite the high market value of these liquids, petcoke is generally of limited use. For example, unprocessed petcoke from a coker unit is usually high in sulfur and considered low grade (fuel grade); as such, it is most commonly used as an inexpensive fuel source for boilers, cement kilns, etc. However, government restrictions on petcoke combustion have prohibited its use as fuel, at least in parts of North America (Stockman, 2013). Some petcoke can also be further processed (calcination operation) to produce high-grade (anode grade) petcoke, which is used in the steel and aluminum industries. The remaining produced petcoke (generally unprocessed petcoke) is stockpiled indefinitely, which poses a number of problems. Besides occupying valuable land at storage locations, this stockpiled coke is responsible for a variety of environmental impacts, such as air and water pollution, as well as having potentially detrimental effects on the respiratory health of nearby populations (Andrews and Lattanzio, 2020). These concerns highlight the need for the development of a benign petcoke “end of life” strategy.

Wabash River Energy Ltd.’s 262 MWe power plant offers one potential solution for disposing of low-grade petcoke, as its GE 7FA turbine is driven by syngas generated by petcoke gasification (Amick, 2000). Additionally, Wabash River Energy has also shown that petcoke can be used in a coal gasifier without any operational issues. The Tampa Electric Polk Power Station provides another possible solution for petcoke disposal, as its integrated gasification combined cycle (IGCC) utilizes a combination of petcoke and coal to produce electricity (DOE/NETL, 2004). Besides power generation, petcoke has been used for fuel and chemical production. For instance, the Coffeyville syngas plant uses petcoke from the Coffeyville refinery as a zero-value waste fuel to produce 1300 tons of ammonia per day (Brown, 2013). This clearly illustrates how petcoke stockpiles can be used to produce value-added products. Similarly, polygeneration studies conducted by the US Department of Energy’s National Energy Technology Laboratory (NETL) have found that the conversion of petcoke to hydrogen, fuel gas, industrial-grade steam, and Fischer-Tropsch (FT) liquid fuels is an economically attractive option when petcoke is zero cost (Kramer, 2003). A different study examined the optimization of petcoke-natural gas polygeneration for the production of olefins, methanol, dimethyl ether (DME), and FT liquids (Salkuyeh and Adams, 2015). The results of this study showed that petcoke-natural gas polygeneration is not economical when petcoke is used above a certain ratio (petcoke/natural gas). Petcoke has also been proposed for use in power-to-gas technology in order to produce a range of products, including fuels and chemicals (Ranisau et al., 2017). That study was the first to employ a mixed-inter linear programming to assess the economics of using electrolysis and surplus grid electricity for petcoke disposal in a polygeneration system. Recently, we presented six strategies for converting petcoke to FT diesel and gasoline that focused on design efficiency, NPV, and GHG emissions (Okeke and Adams, 2018). Our results indicated that this approach is economically promising when petcoke gasification is tightly integrated with a natural gas reforming process. Although the conversion of petcoke to liquids has tremendous technical and economic potentials, detailed studies of this process’ environmental impacts do not exist. Therefore, a life cycle assessment—which is a systematic approach to evaluating the environmental impacts of a particular process to ascertain its overall effects on the eco-system—was conducted in this work.

Alternatives liquid fuels, such as those made from coal, gas, and biomass, can potentially have lower life cycle impacts and/or be competitive with conventional fuels in terms of actual cost or cost of CO₂ avoided (Hoseinzade and Adams, 2019; Okeke et al., 2019). In many cases, though, alternative fuels have none of these advantages. For example, Jaramillo et al. (2009) conducted a life cycle assessment that compared GHG emissions from vehicles (fuel economy of 19.1 km per liter) operated with coal-to-liquid (CTL) fuels against coal-based electricity powered plug-in hybrid electric vehicles (PHEV) and coal-based hydrogen-fuel-cell vehicles (FCV) using a functional unit of gCO₂-eq per km traveled. Their results showed that PHEVs produced less GHG emissions than the CTL vehicle and FCVs, and that the CTL vehicle and FCVs also produced more GHG emissions than petroleum-powered vehicles. Similarly, the NETL conducted a cradle-to-grave (CTG) life cycle assessment comparing the relative GHG emissions of FT diesel (FTD) made from Illinois #6 coal and petroleum-based diesel for sport utility vehicles (SUV) (Marano and Ciferno, 2001). Their results showed that the coal-derived diesel had a GHG life cycle of 583.5 gCO₂-eq/km, while the Wyoming sweet crude oil had a GHG life cycle of 290.8 gCO₂-eq/km. For the gas-to-liquid (GTL) process, Forman et al. (2011) used a substitution allocation method to perform a well-to-wheels (WTW) GHG life cycle assessment with the results of their assessment showing a GHG emissions of 292.9 gCO₂-eq/km using a vehicle fuel economy of 8.6 litres per 100 km. Biomass-to-liquid (BTL) fuel production has also received considerable research attention due to its so-called “carbon neutrality” and reduced environmental impact. For example, Kreutz et al. (2008) performed a BTL LCA and found that, when the gasifier char carbon is assumed to be sequestered, BTL has a negative GHG emissions life cycle, even without CO₂ capture and sequestration.

Although these examples demonstrate that other alternative fuel pathways are promising from an environmental standpoint, studies that address the environmental aspects of petcoke-to-liquids are limited. Recently, we presented the WTW LCA of petcoke standalone gasification (PSG) which showed to have a reduced GHG emission compared to conventional and oil sands diesel (Okeke and Adams, 2019). In this work, we have extended the LCA of the PSG system by recomputing the GHG emissions using the 5th IPCC assessment report factors and also estimated the cost of CO₂ avoided. In addition, two new designs that combines petcoke and natural gas to FT diesel were also presented in order to ascertain the environmental benefits of such synergy. The new design configurations studied are petcoke gasification and external natural gas reforming (PG-ENGR) and petcoke gasification integrated with natural gas steam reforming (PG-INGR) both with and without CCS. Our prior work (Okeke and Adams, 2018) on these configurations consisted of techno-economic analyses that used Aspen Plus v10 process-simulation software to compute process stream flows, energy consumption and production, equipment sizes and costs, and profitability. Therefore, this study aims to examine the environmental impacts of a petcoke-to-liquids process known as the petcoke-derived-diesel (PDD) process. Altogether, the environmental performance of six configurations were presented and compared amongst each other and against the conventional and oil sands diesel.