Fuel switching is not straightforward

From coated wires to glass, high temperature processes are used in the manufacture of a wide variety of products. With its plentiful supply and low cost over the last 50 years or more, the majority of these processes rely on natural gas.

As COP26 convenes to set global targets for emissions reductions, the incentives for decarbonisation of industrial processes are set to increase. But with deeply engrained process knowledge to manufacture the perfect product, fuel switching is not necessarily straightforward and numerous technical and practical considerations must be navigated. With deadlines accelerating, now is the time to understand your processes to avoid product quality degradation at switchover.

High temperature manufacturing processes convert constituent ingredients physically and chemically, relying on the thermal input as the driving energy for the reactions. Often, the rate of the core reactions, as well as other transformations enabled by the created atmosphere, are critical to ensuring the process occurs as intended, and that the desired quality of product is manufactured.

The energetic content of natural gas, the balancing of radiative and convective heating (and occasionally conductive) and the moisture formed on combustion have long been used to design complex thermal and psychrometric process, using the equipment line length and run speed to also control timing. Importantly, over time many processes have been tweaked away from their design points, encapsulating operator knowledge on how the process and equipment responds to the real-world variation of a myriad of conditions.

An initial start-point towards replicating the process with a lower carbon intensity is therefore often to formally characterise processes, codifying often intangible and tribal operator knowledge. Often, it is not the characterisation at the design point that matters here, but how the process is actually managed when there are variations around this point, whether they be environmental or product based. A numerical model of the theoretical process, incorporating real-world experience enables determination of the likely determinant process parameters and manifest output product quality aspects.

Characterisation of these to populate the model and link the process to the output may be possible using existing equipment and process instrumentation or require a bespoke measurement system. Likewise, normal operation and variation may provide adequate date, or more value may be derived from running a DOE (design of experiment) matrix – the maturity of the process and the model into which the data is to be incorporated will determine the most suitable characterisation path. Once the current process is characterised, the challenge of replicating those exact characteristics can begin.

Due to the different thermal profiles of different heating technologies, a perfect replication from a different heating source can often be hard to achieve, let alone numerous options to be able to pick the most palatable new heating technology. Substitution of natural gas therefore is rarely a direct swap-out. The use of low life-cycle carbon methane, such as biogas, is a near direct substitution, but even then can vary in total composition enough to warrant that other process changes are required.

The production, availability and cost of biogas are of course also important considerations. The further the gas moves from the composition of natural gas, including for example through blending hydrogen, the greater the difference in combustion temperature and moisture, and the greater the scale of modification required. Carbon capture at the exhaust is often cited, but technology maturity, cost, carbon disposal and on-going (“pure”) natural gas availability are all important factors to determine the viability of these routes.

Design of a new carbon-free process often allows for a fresh, multi-technology approach, using different heating and other thermal control technologies for different parts of the process. In addition, other process control such as humidification may require separate equipment, but offers the advantage of independent parameter control. Fuel switching is also often not a direct input Joule for Joule swap as new equipment is likely to be more efficient, with better thermal targeting to where the energy is required for the process. With low carbon energy usually more expensive than incumbents the impact of the cost of transition on the final product is therefore somewhat mitigated.

Ultimately, what is sought is the lowest total cost of decarbonisation which incorporates many factors - not just the cost of the energy itself (and how that may change over time), but also the capital cost of the new process machinery including ancillary equipment which may be required by a change in the core process, and costs associated with new energy infrastructure and connection and with any required fuel treatment.

Another class of costs are those borne from the operational impact during the change of production equipment including a potential initial higher scrap rate as the new process beds in, and any up-front R&D, pilot testing and equipment development costs. The certainty of supply of the new energy vector is also an important factor, especially where there is likely to be competing demand from numerous other customers making similar switches, and where there may be a lag in the construction and scale up of supply, transmission and distribution infrastructure.

Situations may well arise where direct process replication just isn’t possible. In this case, revisiting the current product formulation and/or design may open up avenues to achieve the same output quality with a low carbon process. Ingredient or product substitution may yield the same product but require a more achievable temperature or humidity profile with a substitute fuel. Interestingly, this may also enable product improvement or other sustainability gains to be made, as those inputs with the largest waste, control or other overheads may be (partially) substituted first. The customer marketing value of reducing the environmental footprint of the final product also comes into play here.

With both product and process parameters being modified, scale-up and satisfying overall product demand become the next challenges, and consideration of these from the outset can prevent bottle necks and revenue throttles later down the line. Pilot plants, parallel lines and integration with remaining process equipment such as feeders and packaging should be thought through. In this phase of implementation the risk-mitigation is around mirroring the original production rate, although opportunities for increasing the rate, or reducing scrap, should not be overlooked.

Likewise, the specification and procurement of new equipment often affords the opportunity to add functionality to benefit process control and monitoring, enabling long term opex reductions through digitalisation, pro-active maintenance and real-time process optimisation.

With the rhetoric about now being the time for action, never more is this true than in process decarbonisation, where the up-front lead times are long, and the implementation times even longer. Though with a deep understanding of the science of the process and good planning the task need not be as daunting. Risks and costs can be mitigated and product output maintained.

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