The Next Best Thing

Ruby Ray and Fabio Ruggeri describe an innovative process to produce substitute natural gas – a solution for low carbon energy

Substitute natural gas (SNG) from bio-based fuel provides a viable alternative way of decarbonising the gas grid, domestic heating supplies, and large-scale transportation, which should feature in the future energy mix as we transition away from fossil fuels. It is also a promising clean coal technology that provides a platform to achieve global emission targets for countries which are more dependent on coal-based energy.

Wood started looking at developing SNG in an energy efficient and cost-effective way as early as 2006. In 2012 Foster Wheeler (which was later acquired by Wood) patented an innovative catalytic methanation process, called VESTA, that can be adapted to any source of syngas.

SNG

 

SNG is neither a new terminology nor a new chemical; the Great Plains Synfuels Plant in North Dakota, US has been producing it from coal since the 1980s. It is essentially a high methane content gas that can be produced from diverse feedstocks such as coal, petcoke, biomass, waste and biogas, and has a combustion characteristic similar to natural gas. It requires minimal or no changes to be used as a replacement for natural gas. SNG production from solid feedstock involves gasification of the solid carbon source to syngas followed by catalytic conversion of syngas to SNG, as we’ll describe later. Gasification is an off-the-shelf technology and several pioneering catalyst companies have developed
SNG catalysts.

Why we need SNG in a decarbonised world

The combination of increasing energy demand, climate change and the need for energy security is creating long-term challenges in the global energy sector. Diversity of energy supply is necessary in order to meet these challenges and to achieve the target set out in the Paris Agreement. The IPCC Special Report on Global Warming of 1.5^oC emphasised that CO₂ emissions must reach net zero by 2050 in order to keep global warming below 1.5^oC.

Even though a diverse range of low-carbon generation technologies such as renewable and nuclear electricity have created a significant footprint in the global energy mix, fossil fuel-based energy still dominates the energy market. Natural gas is becoming the key player in the world’s energy supply in residential, commercial, and industrial applications due to its lower CO₂ emissions when compared to coal. The increasing demand for natural gas, its associated price, and limited supply in many parts of the world have led industries and researchers to pursue unconventional methods of natural gas production. The production of SNG is attractive where there is a huge demand, but a limited supply of natural gas.

The UK Government has moved from a previous target of 80% emission reduction to a new ambitious target to achieve net zero greenhouse gases by 2050. Much has been achieved since 1990 in the energy and power sector to reduce emissions in the UK but there are several sectors which are more difficult to decarbonise.

As reflected in Figure 1, the transportation sector is the biggest emitter followed by business and residential heating. The emission reductions from transportation and residential heating have only achieved 2% and 16% since 1990 in UK.

There is an increasing focus in the UK on the hydrogen economy as one of the sources for the net zero scenario to achieve decarbonisation of gas networks and also transport systems such as the hydrogen-powered train. However, this concept is localised to certain parts of the UK and comes with a number of financial and regulatory challenges. The approach to decarbonisation of the transport sector through electrification is upcoming; but this approach requires renewables to produce baseload electricity to achieve decarbonisation which is challenging with the fluctuating nature of certain renewable sources such as wind and solar. Also, electrification of the transport system requires huge infrastructure modification.

Renewable SNG production using bio-derived fuel is a clean and low-carbon alternative to conventional natural gas that can be transported and distributed using the existing natural gas grid infrastructure

Renewable SNG production using bio-derived fuel, when responsibly sourced, is a clean and low-carbon alternative to conventional natural gas that can be transported and distributed using the existing natural gas grid infrastructure. It provides one of the most flexible approaches to decarbonise widely-distributed end users such as industrial and residential heating and transportation systems by using compressed SNG. It can generate negative carbon emissions when coupled with CO₂ capture, as biomass is carbon neutral.

There are several countries in the world still heavily reliant on coal use, due to resource abundance and internal policies. The Paris Agreement forces many of these countries to look for clean coal technology to improve coal’s environmental performance and to produce power/energy with reduced emissions to make better use of this plentiful resource. Using coal for SNG production is an exciting and promising development. If a carbon capture integrated SNG process such as VESTA is utilised, it can reduce dependency on natural gas by injecting SNG into the gas grid and also provides a platform to achieve global emissions target with coal use. 

Vesta technology

Catalytic synthesis of methane from syngas involves the following equilibrium reactions:

CO +  3H₂ ↔ CH₄  +  H₂O (1)

CO₂  +  4H₂  ↔  CH₄  +  2H₂O (2)

Both of these reactions are strongly exothermic, although CO₂ methanation is less exothermic than CO methanation. In order to achieve high methane yields, low temperatures and high pressures are required. Standard methanation catalysts generally have to work in a reaction temperature range of 250-600°C, while properly-stabilised catalysts can tolerate temperatures up to a maximum of 700°C. Due to the very large amount of heat released during the methanation reaction, the criticalities to be faced in the design of a methanation process are the control of the reactors’ outlet temperature² and the capability to recover the reaction heat. In order to moderate the exothermic methanation reaction temperature and avoid a reaction runaway, several techniques can be employed, such as recycling of products, diluting process gas with inert gas or steam, or installing isothermal reactors.

The most common choice of standard technologies to moderate the exothermic heat of reaction is recycling reacted gas which has, as the main drawback, the need for a recycle compressor. Figure 2 shows a typical conceptual scheme for the standard methanation process based on recycling converted gases. The main sections shown are gasification, sour gas shift, acid gas removal, and methanation. The recycle compressor complicates the scheme and adds to the investment cost and the overall power consumption. Moreover, in order to obtain the desired SNG quality, a complex adjustment of the syngas composition has to be performed across the sour gas shift block, as well as a compositional control on a continuous basis in order to achieve SNG of the correct quality without downstream functionality issues.