Decarbonisation Technology - November 2021

Energy source

Exergy equation


Heat at a higher temperature has a higher value potentially Heat absorbed below ambient has work potential. Refrigeration is expensive Constant specific heat capacity Constant temperature Gibbs free energy at T 0 (relates to the efficiency of a battery)

Heat at temperature T, above ambient

∆ Ex = QH (1-T 0 /T)

Heat at temperature T, below ambient

∆ Ex = QC (T 0 /T-1) ∆ Ex = ∆ H - T 0 ∆ S ∆ Ex = ∆ H (1-T 0 /T LM1-2 ) ∆ Ex = ∆ H (1-T 0 /T) ∆ Ex = ∆ GT 0

Process stream

For a chemical reaction

For mixing and separation of components

∆ Ex = ∆ H - RT 0 Σ (n i ln(1/x i ))

Exergy Loss ( ∆ Ex) – a measure of the lost ability to do work

Table 1

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). The use of exergy loss as a measure of inefficiency is another established technique, albeit perhaps most often applied on an ad hoc basis. An amount of heat energy at say 200°C can be transferred and become the same quantity of heat energy at 50°C, through cooling of a process stream with cooling water for example. No energy is lost in this process, but exergy is lost. The heat at 200°C had

more exergy, more ability to do work/generate electricity, than the energy at 50°C, and was potentially more valuable, therefore. The heat could have been used to generate electricity in a heat engine, for example an organic Rankine cycle or a steam expander. Typical examples of exergy loss include reducing the temperature of a hot stream (above ambient), increasing the temperature of a cold stream (below ambient), mixing components and chemical reactions. The identification of exergy loss remains a valuable method to identify inefficiencies and opportunities for improvements within operations and is unaffected by the new focus on carbon. While the impacts of the prices of energy and the relative carbon emissions appropriate at an individual location, and the likely changes in these values, need to be properly taken into account, the analysis of process energy efficiency using pinch technology and exergy analysis remains valid, perhaps even more important in the era of decarbonisation, supporting the development of robust decarbonisation pathways based on rigorous analysis that match the ideal decarbonisation hierarchy.

David Hart


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