Find opportunities to decarbonise with pinch technology

Pinch technology and exergy analysis can steer energy efficiency and process decarbonisation decisions.

David Hart
Energy Intelligent Solutions

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Article Summary

The need to decarbonise is clear: to satisfy stakeholder demands associated with climate change. For many organisations this translates into significant disruption to established operations, higher operating costs and significant capital expenditure. The speed with which organisations need to act varies depending on local regulations and demands, and on the local prices and carbon intensities of energy sources, but in all cases there is a need to quickly and robustly develop clear pathways for the future. Many companies have stretching targets for carbon reduction which they have committed to achieving.

Pinch technology and conventional thermodynamic and energy performance assessments need to be adjusted to cover situations where, for example, electrification is more appropriate than using fuels to generate heat; in other words, where work (or electricity) is less valuable than heat. This is counter to solutions generally applied in the past where carbon reduction was not a consideration.

Figure 1 shows trends in energy prices in the UK for the last decade. The UK is characterised by high cost electricity and lower cost gas, pushing companies to use gas in favour of power, to generate power from gas using gas engines/turbines with heat recovery and to avoid increased electricity use, for example avoiding using heat pumps. A higher relative price for electricity, or work, is consistent with the conventional view of thermodynamics that work has a higher value than heat and that there needs to be a focus on avoiding exergy losses.

The trends in UK carbon emissions intensities for typical utilities are shown in Figure 2. The decarbonisation of electricity is pushing organisations towards electrification rather than using fossil fuels, and away from combined heat and power and towards the use of heat pumps — the exact opposite of the actions to reduce operating costs. This is driven by the increased percentage of power derived from low carbon sources, resulting in lower carbon emissions from grid electricity. Reducing carbon emissions is inconsistent with reducing costs and with the traditional thermodynamic approach.

It is interesting to note the changes in the carbon intensities over the last five years and the associated changes in acceptable technologies: combined heat and power (CHP) has switched from a low carbon solution to a carbon increasing solution in just a few years, but remains one of the few opportunities to significantly reduce utility operating costs (power costs) in the UK. Heat pumps have become the first choice for low temperature heating duties but rely on cheap power to be cost effective. Electricity can be a lower carbon option than steam from gas fired boilers, especially where the efficiency of steam generation and distribution is relatively low, but operating costs will increase considerably.

The signals from energy prices and carbon intensities in the UK are not mirrored everywhere. The actual and relative prices of gas and power are highly variable across the world. Similarly, the relative carbon intensities depend on the sources of grid power which can range from almost zero carbon hydro and nuclear power to high carbon power from diesel engines and coal fired power plant. Organisations need to establish the marginal costs of power and heat and the 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 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.

The choice of the residual required hot and cold utility will depend on the local emissions factors and prices — in the UK CHP will save money but increase carbon emissions, in other locations this may well be reversed. In some cases, the residual hot utility demand will be best served using low carbon electricity.

Where electricity costs and/or emissions are high, then alternative refinery fuels may be a better choice. The use of hydrogen as a fuel for industrial utilities is uncertain, however — hydrogen availability is limited in the near term and price is uncertain and may be driven by demand and the type of use. The technologies associated with hydrogen (power generation and hydrogen generation) are in development or currently involve high capital costs. The use of hydrogen in the longer term may be limited to applications where decarbonisation using other means is not easy, for example heavy transport and high temperature processes (and the price for hydrogen may reflect this).

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