Kinetic modelling of the hydrothermal carbonization of compost derived from municipal solid waste
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abstract
Nowadays, municipal solid waste (MSW) management commonly includes a mechanical and biological
treatment process (MBT). The solid stream from the anaerobic digestion of the organic fraction can
be processed to obtain a compost, which can be used as fertilizer. However, compost production
is higher than the existing demand, and the expected developments on up-coming directives ruling
“End-of-waste” criteria are leading to barriers on the use of MSW-derived fertilizers [1]. Thus, the
development of new alternatives for the treatment of organic wastes and compost valorization are
necessary. This work deals with the valorization of compost from MBT, through the production of
catalysts by Hydrothermal Carbonization (HTC) [2]. HTC of the compost was carried out in a Teflon
vessel inserted in a stainless-steel body at different operating conditions (150-230 ºC,1-5 h, 1-4 g
of compost, 30 mL). A Doehlert Matrix was considered to plan the experiments. The carbon
balance and the kinetic equations were evaluated from experimental data reporting the carbon
content in the liquid (estimated by TOC analysis) and solid phases (estimated by elemental
analysis). A lumped kinetic model based on the elemental carbon content is proposed (Fig. 1),
anticipating that the compost (C) undergoes reactions that originate liquid intermediates (L),
reaction 1, and sequentially results in hydrochar (HC) and gases (G), reactions 2 and 3,
respectively. In addition, it was assumed that HC and G were also produced from the compost
directly, reactions 4 and 5, respectively. The highest kinetic constant at 190 ºC (8.3·10-4 min-1) was
found for the formation of the liquid soluble intermediates from compost (reaction 1), whereas the
production of hydrochar from the liquid intermediates (reaction 2) shows the lowest kinetic constant
(3.1·10-4 min-1). The lowest activation energy was estimated for reaction 1 (23 kJ·mol-1), while
reactions 2 and 4, related to the formation of hydrochar, resulted in the highest values (85 and 195
kJ·mol-1, respectively), meaning that the production of hydrochar strongly depends on the
temperature.
This work was financially supported by project “VALORCOMP - Valorización de compost y otros desechos procedentes de la fracción orgánica de los residuos municipales”, 0119_VALORCOMP_2_P, and project “AIProcMat@N2020 - Advanced Industrial Processes and Materials for a Sustainable Northern Region of Portugal 2020”, reference NORTE-01-0145-FEDER-000006, supported by NORTE 2020, under the Portugal 2020 Partnership Agreement, through FEDER; the Associate Laboratory LSRELCM
- UID/EQU/50020/2019 - funded by national funds through FCT/MCTES (PIDDAC); and CIMO UID/AGR/00690/2019
through FEDER under Program PT2020.
