Decarbonisation Technology - November 2021

to process changes consistent with reducing energy demands. How are pinch technology and similar approaches impacted by the need to decarbonise? Figure 4 shows the composite curves for a typical distillation process. The overlap between the hot and cold composite curves identifies the scope for heat recovery – in this case around 15 MW. Heat recovery is consistent with reducing energy costs, reducing overall carbon emissions and with the decarbonisation hierarchy. The grand composite curves show (see Figure 5 ), in this case, that the hot utility demand is generally at a relatively low temperature (120°C in this example) and the waste heat from the process is at a relatively high temperature (80°C). A heat pump is an appropriate solution, therefore, because of the relatively low temperature difference between the waste heat available and the heat required, easily identifiable by the shape of the grand composite curves around the pinch temperature. With a coefficient of performance of an estimated 4.5 in this case (the COP depends on the temperatures and the temperature lift), this heat pump in the UK would have a price per unit of heat which is lower than direct heat from gas or direct electricity, and will have a carbon emissions factor that is very much lower than alternatives. Pinch technology remains a valid, perhaps essential, method to assess opportunities to decarbonise. It has the added advantage that it requires the establishment of a comprehensive heat and mass balance for process operations – a sound basis for designing a decarbonisation plan. The first step towards decarbonisation should always be to reduce energy use – the process change and heat recovery options identified by a pinch analysis will always be valid actions where cost effective and practical. Similarly, optimum utilities will always involve delivering hot utility at as low a temperature as possible and cold utility at as high a temperature as possible. Heat pumps will always be most appropriate where the heat is transferred across the pinch and the temperature lift is relatively low. Direct heat transfer across the pinch will always be sub-optimal. Distillation columns should always ideally be all above or all below the pinch.

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Figure 5 Grand composite curves for distillation

marginal impacts on carbon emissions before they can develop pathways to decarbonisation. Marginal refers to the change in price or carbon emissions per unit increase or decrease in energy use. Forecasts of the future changes in these values are important, up to 10 years ahead for longer term investments, and the UK experience shows that these values can change significantly and quickly, driven especially by the actions of governments to tackle climate change. An ideal decarbonisation hierarchy for industrial sites is illustrated in the Figure 3 . Companies should ideally reduce their own energy use first (Scope 1 emissions) before turning to alternative utilities (Scope 2) or the procurement of green fuels or power and before considering opportunities in the wider community (Scope 3). In practice, it does often makes sense to develop opportunities in parallel. A powerful approach to the evaluation of energy performance and to the identification of optimum opportunities to reduce energy use is pinch technology. Developed in the 1970s and 1980s, this is an established thermodynamic approach to determining the minimum utility demands of a process (heating and cooling) and the temperature levels of the utilities required. It helps to design an optimum heat recovery network and then to identify the optimum utilities to deliver the required heat and power. It can show where heat pumps fit within the utilities strategy and can identify the opportunities and size of combined heat and power solutions. The approach can also point