Andrew Coe and James Paterson explain developments on part of the solution to the dual energy challenge using an almost century-old reaction
GLOBAL energy demands are increasing and so too is the need for more renewable and sustainable sources of energy to help transition us to a net zero GHG emissions world, the dual energy challenge. Last year, the EU increased its renewable energy target to 32% for 20301, with many countries planning to ban fossil fuel powered cars by 2040. However, the transportation industry is one of the most challenging sectors to adapt to using low-carbon fuels. Transportation modes such as aircraft, heavy-duty and marine vehicles demand high power and energy capacity that are largely unmet by renewable technologies.
What if a solution to help solve the dual energy challenge was found using an almost century-old reaction? Johnson Matthey and BP have been collaborating for the past two decades to develop an efficient reactor system and catalyst for the Fischer-Tropsch process. This will offer a cost-effective method of converting any carbon source into liquid hydrocarbon fuels.
The Fischer-Tropsch process was originally developed by Franz Fischer and Hans Tropsch in 1925. It is a way of converting any sustainable carbon source into liquid hydrocarbon, effectively creating synthetic fuel:
[2H2 + CO]n + H2 --> CH3(CH2)n-2CH3 + nH2O
Syngas, a mixture of gaseous hydrogen and carbon monoxide, is used as a source of carbon. Syngas can be generated from various carbon sources, including coal, natural gas, municipal solid waste and biomass. The process mainly produces linear long-chain hydrocarbons. These require a further hydrocracking procedure to break and isomerise them into alkanes with the correct properties for fuel applications. In the quest for sustainable fuel solutions, synthetic fuels promise a cleaner way to power cars, heavy-duty vehicles and planes. The latest developments in Fischer-Tropsch technology mean that the production of fuel from sustainable carbon sources is now closer to being commercially viable at all industrial scales.
Catalysts are required for the Fischer-Tropsch process to increase the rate of reaction and make the process industrially viable. The most commonly-used catalyst is cobalt. Prior investigations found cobalt to be a more active and stable catalyst that afforded lower water-gas-shift activity and longer lifetimes than iron catalysts.2 Commercial synthesis of hydrocarbons occurs at moderate temperature (200-240°C) and pressure (20-40 barg). During the process, hydrogen and carbon monoxide are converted into long-chain paraffins or waxes over the supported catalyst (see Figure 1). Diffusion and mass transfer therefore play a key role in Fischer-Tropsch selectivity due to the higher mobility of H2 over CO within the catalyst pores.
Over the nine decades since its inception, many different Fischer-Tropsch process reactors have been designed. The reactions are all highly exothermic, making efficient removal of heat essential for any design. Industry favours fixed-bed tubular reactors as they are a proven technology with manufacturers at scale. They work by holding the catalyst in place via a static bed. This has the advantage of preventing catalyst loss through product contamination. The reactors have a modular design, which makes increasing capacity as simple as adding tubes. However, fixed-bed reactors are limited in terms of catalyst productivity and selectivity. Construction is also costly, requiring balance between tube diameter and catalyst particle size to achieve optimal conditions. An alternative synthesis route is through slurry reactors where diffusion constraints are minimised. This type of reactor is more efficient at heat removal and small catalyst particles are used to reduce mass transfer resistance. However, they can suffer from catalyst attrition, which leads to catalyst loss and product purity issues. Slurry reactors are also less straightforward to scale up compared to fixed-bed alternatives.
Since 1996, JM and BP have been collaborating to bring Fischer-Tropsch synthesis to the industrial scale. Our first major joint venture was to build the Nikiski demonstration plant in Alaska in 2002, based on our first generation (Gen1) conventional tubular reactor technology. Nikiski was the largest Fischer-Tropsch facility ever built in the US, producing 300 bbl/d of synthetic crude product from pipeline natural gas feedstock. By the time the Nikiski plant was decommissioned in 2009, the plant had exceeded all its performance goals related to: catalyst productivity, hydrocarbon selectivity, carbon monoxide conversion, methane selectivity, and catalyst lifetime. The catalyst selectivity and productivity targets were achieved by optimising the commercial catalyst activation procedure to match the laboratory-scale performance. A single charge of catalyst ran for just over 7,000 hours enabling us to predict an expected three-year lifetime without any regeneration. Other Fischer-Tropsch technologies can reach similar catalyst lifetimes but require periodic regeneration cycles.
The integrated plant combined three processes for testing Fischer-Tropsch technology: a proprietary compact reformer for syngas generation, fixed bed Fischer-Tropsch reactor, and mild hydrocracking of Fischer-Tropsch waxes to produce synthetic crude. The original fixed bed tubular reactor technology was developed as a method of monetising stranded natural gas in remote locations. However, it was only competitive at large scale (>30,000 bbl/d) in areas with low natural gas prices and high oil prices.
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