Problem statement

Electrification of chemical industry

The chemical industry is responsible for a large fraction of global CO₂ equivalent emissions. Among the most polluting processes are naphtha cracking and hydrogen production for fertilizers. As significant contributors to global CO₂ emissions, they also present a large opportunity for reducing global carbon footprint. Naphtha cracking is an endo-thermic process that is heated partly by combusting the methane byproduct (15% by volume), resulting in a process that produces about 1 ton CO­₂ per ton of olefins. Upon electrification of naphtha crackers, the process is nominally emission free when green electricity is used. Hydrogen for fertilizer production is produced through steam cracking of methane, followed by a water-gas-shift step. This process produces approximately 9 ton CO­₂ per ton H₂ produced. Moreover, the hydrogen is first converted into ammonia, which is then oxidized to form nitric acid in the Ostwald process. In this process N­₂O byproduct, which has a Greenhouse Warming Potential (GWP) of 273x that of CO₂, is formed and released into the atmosphere in significant quantities. Electrification of these processes would replace the fossil feedstocks as source of energy and thus eliminate most of the associated CO₂ emissions, which is urgently needed to reach the climate goals of 2030 and 2050.

Plasma technology

Plasma technology provides a unique pathway to electrify processes by applying electrical power directly to the process gas itself. The resulting plasma phase can be thought of as an electrical flame that can reach temperatures of thousands of degrees in a matter of seconds. Because the power is deposited locally in the process gas, the reactor vessel does not need to heat up, resulting in ON/OFF switching times in the order of seconds – ideally suited to follow the intermittent supply of renewable energy sources. Because heat is applied extrinsically it can be used to drive chemistry in any stable, low enthalpy molecule, allowing for truly circular processes. Moreover, the plasma reactor’s electrodes can be made out of copper or steel and therefore do not inherently rely on any critical materials. The Brightsite consortium, a collaboration between knowledge institutes and industry (Maastricht University, TNO, Sitech Services, and the Chemelot Campus), works on scaling up plasma technology with the ultimate goal to scale up from lab scale to industrial scale. It is located on the Chemelot chemical site where it is surrounded by future end-users.

Methane plasma pyrolysis

Plasma pyrolysis of methane is the first plasma process being developed within the Brightsite consortium, which could have a significant benefit compared to more conventional methane valorization technologies. Upon electrification of steam crackers, a new purpose must be found for the methane by-product since burning it would nullify the emission reduction achieved in the electrification of the steam cracker. Plasma pyrolysis can be used to convert methane into acetylene or ethylene and hydrogen, opening up an avenue for valorization of the surplus methane. By producing polymer feedstocks as opposed to solid carbon, all of the co-products can be easily valorized, providing a significant economical advantage. This methane pyrolysis process is similar to the Huels process, which already employs plasma technology at industrial scale to convert methane to acetylene and hydrogen. The development goal of Brightsite is to improve the OPEX of the pyrolysis process by improving product yields and energy efficiency, as well as gaining a better fundamental understanding in the chemical pathways involved in the process. By focussing research on the direct production of ethylene, a significant improvement in energy efficiency can be attained by avoiding the exothermic hydrogenation of acetylene.

Further plasma processing opportunities

In addition to methane, plasma technology is developed for a range of other processes. Plasma processes can be competitive whenever stable feedstocks are involved or fast heating/cooling rates are required. One such application is the plasma fixation of nitrogen and oxygen to produce NO_x, a feedstock for fertilizer production. By producing NO_x directly, ammonia production can be omitted and the associated N₂O emissions eliminated, in addition to eliminating CO₂ emissions associated with the traditional steam reforming. By combining the pyrolysis of methane with nitrogen, a mixture of acetylene and HCN can be produced that, when catalytically converted to acrylonitrile, forms a pre-cursor for polyacrylonitrile. Additionally, conversion of CO₂ to CO and recycling of plastics is under consideration.

Methods

Fast quenching to control product selectivity

In the pyrolysis process methane can be converted to a range of hydrocarbon products. When considering product price and market size, acetylene and ethylene production is preferable over solid carbon, with ethylene the most desirable of the two. These hydrocarbons are formed through the Kassel mechanism, where hydrocarbons are subjected to successive dehydrogenation steps: 2CH₄ → C₂H₆ → C₂H₄ → C₂H₂ → 2C(s), balanced by production of molecular hydrogen. Due to the large spread in reaction rate constants of the different dehydrogenation steps, product selectivity can be tuned by quenching reactions to freeze kinetics once a certain product distribution is reached. Controlling residence times through appropriate application of the quench is thus a large focus in the research.

Scale-up strategy

Because a sequential scale-up takes considerable time and would not lead to maturity within the set timelines, a parallel scale-up strategy is pursued where different scales are developed simultaneously, each focusing on plasma technology of differing levels of maturity. Concretely, a 500kW pilot scale reactor will be constructed for investigation of the Huels process. Here the risk is minimized because the Huels process is already operating at industrial scale, demonstrating the process feasibility. A 50 kW “bench scale” reactor is being constructed for investigating opportunities to improve on the Huels process, still with a focus on ethylene production. The direct ethylene production, which is the significantly more challenging to achieve, is studied in lab scale reactors in the ~1kW range.

Experimental reactors

Most of our lab scale reactors are of the microwave type, where electricity is first converted in microwaves which are subsequently absorbed by the plasma to sustain it. The main advantage of this type of plasma is that it does not require electrodes and therefore has ample optical access. The setups are equipped with laser diagnostics such as Raman scattering for in-situ temperature and species measurements, and laser induced incandescence (LII) for investigations of soot growth. Downstream gas chromatography (GC) measurements measure the downstream composition. The 50 kW “bench scale” is a magnetically stabilized DC-arc, which better represents industrial scale DC-arc plasmas. It is equipped with a gas quench as well as a water quench to study the effect of different quenching strategies in a first scale-up step.

Results

Experimental

Downstream GC measurements recorded in the lab scale microwave reactors for a range of different parameters, provide insights into the ideal conditions for methane pyrolysis. In addition, it provides a means of tuning the conversion and selectivities by varying reactor conditions such as flow, pressure, and applied power. Raman scattering and LII were employed to gain a better understanding of the flow profiles in and around the plasma core, as well as the effect of radial temperature gradients.

Numerical modelling

In addition to the experiments, complementary numerical modelling was performed. 0D kinetic modelling was performed to understand the dominant chemical reaction mechanisms all the way to soot formation. These 0D models were complemented with computational fluid modelling (CFD) to estimate the typical flow and temperature profiles in the microwave reactor. Together the experiments, kinetic modelling, and CFD modelling paint a comprehensive picture of the pyrolysis process in the microwave reactor.

Conclusions

The chemical industry must rapidly be electrified in order to curb carbon emissions. Plasma technology is a promising candidate to electrify endothermic processes that require high temperatures, i.e. for driving chemistry in stable molecules to circularize product loops. Plasma technology is developed at Brightsite from lab to industrial scale, initially focusing on methane plasma pyrolysis. To scale up successfully, research is required over a large range of scales, with partners from knowledge institutes as well as well as industry. Following the successful scale up, these plasma processes pave the way for a green, circular chemical industry.