Abstract
Hydrothermal carbonization (HTC) has been seen as a potentially beneficial process for converting wet biomass into value-added products. It is, however, necessary to overcome the challenges associated with handling the powdered form of hydrochar—a solid product of the HTC process—by controlling the formation of dust and facilitating smoother transportation and distribution in a potentially wide marketplace. In this paper, the authors investigate the environmental consequences of different alternatives for using hydrochar pellets produced from mixed sludges from pulp and paper mills in Sweden, using the environmental life cycle assessment (E-LCA). Two scenarios for possible end-uses of hydrochar in combined heat and power (CHP) plants as a source of energy (heat and electricity) were assessed. In these scenarios, hydrochar pellets were assumed to be combusted in CHP plants, thereby avoiding the use of combustible solid wastes (Scenario A) and coal (Scenario B), respectively, to recover energy in the form of electricity and heat. The environmental damages to Human Health, Ecosystem Quality, Climate Change, and Resources are evaluated based on 1 tonne of dry sludge as the functional unit. The results from this analysis illustrate that Scenario B, in which hydrochar replaces coal, offers the greatest reduction in all the environmental damage characterizations, except the Resources category. The displacement of energy-based coal due to hydrochar combustion contributed most significantly to the environmental damages wrought by the system—ranging from 52% in Resources to 93% in Ecosystem Quality. Overall, the results highlight that the application of hydrochar pellets for energy recovery to offset waste- and coal-based energy sources has great environmental benefits. The favorability of sludge hydrochar over solid wastes as fuel for CHP plants may be counter-intuitive at first, since HTC is an energy-intensive process, but when accounting for the necessity of dependence on imports of wastes for instance, the hydrochar pellet may well emerge as a good option for CHPs in Sweden.
Highlights
Incineration plants for energy recovery may have the supply of heat as their sole reason for existence, in which case, they are referred to as heat-only boilers
Because of a lack of data associated with the energy required for hydrochar combustion, and the distribution of heavy metal elements during this process, hydrochar combustion is assumed to behave in the same way as sludge incineration does; the incineration module of mixed sludges [30] was employed for both scenarios A and B to model hydrochar combustion in combined heat and power (CHP) plants including emissions and the flue gas treatment process
The authors have studied the application of hydrochar pellets from pulp and paper mill sludges in CHP plants to substitute different fuel materials with the aim of identifying opportunities for improving its environmental performance
Summary
Incineration plants for energy recovery may have the supply of heat as their sole reason for existence, in which case, they are referred to as heat-only boilers. In another work [19], the energy use impact and associated climate change effects of producing hydrochar briquettes co-formed with coal fines for use as a fuel in power plants has been investigated using E-LCA as the tool. The use of E-LCA to comparatively analyze the environmental impacts and damage consequences associated with the utilization of hydrochar pellets in CHPs as a source of energy for heat and electricity generation, has enabled the authors to contribute to some of the said purposes of the tool referred to above. Hydrochar pellets are produced from pulp and paper mill sludge and are assumed to substitute coal and solid wastes in CHPs. This study is part of the continuum to earlier ones by the authors, in which alternate approaches to handling mixed sludges from pulp and paper mills were studied and analyzed for their environmental footprints
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