Maximising the valorisation of organic waste locally available via carbon-to-nitrogen ratio Supply Composite Curve shifting

Abstract

Valorisation of organic waste can lead to cleaner production in the energy sector. One factor affecting the decision for organic waste valorisation to value-added products through biological processes is the carbon-to-nitrogen (C/N) ratio. All biological processes have preferential C/N ratios for optimum performance, while organic waste comes with a wide range of C/N ratios. The mismatch of the C/N ratio between the supply stream (organic waste) and the demand stream (biological process) can lead to suboptimal process performance and affect resource allocation. In this study, a new graphical C/N ratio Pinch Analysis approach was proposed by plotting cumulative carbon mass flowrate versus the cumulative nitrogen mass flowrate as x- and y-axes for the supply and demand sides. A series of graphical Supply Composite Curve (SCC) shifting, namely SCC right-shifting, SCC end-shifting, SCC detaching, and SCC down-shifting, were developed explicitly tackling different supply stream conditions in the hypothetical case studies while satisfying the demand streams with the aid of external supply. The external supply was determined by filling the gaps formed after the SCC was shifted to the right of the Demand Composite Curve (DCC). Specific heuristics were established to assess the range of C/N ratio for the external supply that is eligible and preferred to satisfy the demand streams. Stepwise procedures for mass flowrate allocation to mix the supply and match the demand were introduced. In this study, the demands for Case Study 1 were satisfied by 59.15% OWLA with 40.83% ES 3. For Case Study 2, the demands were satisfied by 76.19% OWLA with 23.81% ES 1. For Case Study 3, the demands were satisfied by 91.54% OWLA with 8.46% ES 1. The integration of the C/N ratio element in the Pinch-based Analysis of SCC shifting and exploring new optimisation scope can act as an advising tool for any individual, party, or organisation to optimally valorise the organic waste found within a local region.