Sustainability 2022 , 14 , 4669
2of 18
glass, and plastics. A wide variety of constituents is present in DIS, which are short fibres, chemicals used as coatings, and fillers employed during paper manufacturing from virgin or recycled pulp to improve the paper quality, such as kaolin (Al 2 O 3 , SiO 2 ), talc (Mg 3 Si 4 O 10 (OH) 2 ), calcium carbonate (CaCO 3 ), clays, ink particles, extractive substances, and deinking additives applied for used paper recycling (e.g., Na 2 SiO 3 , NaOH, H 2 O 2 , CaCl 2 , fatty acid, and fatty acid soap) [10,11]. The lower heating value (LHV) of WP and DIS fluctuates from 15.0 to 26.61 MJ/kg as dry solid (DS) and 4.0 to 7.57 MJ/kg as DS, respectively [6,11]. In 2019, new paper and board production in the EU was 75.8 Mt of which 57.5 Mt were recycled generating 24.3 Mt of WP [12]. The quantity of DIS generated during the recycling of used paper is between 20% (for newsprint) and 40% (for tissue paper) of WP and is expected to reach between 48 and 86%, respectively, in the next 50 years [13,14]. In 2019, DIS production in the EU was in the range of 4.86 to 9.72 Mt as DS. The most common practices for disposal of the WP and DIS generated during used paper recycling are landfilling and incineration, which are responsible for the release of GHGs of CH 4 ,NO,N 2 O, CO 2 , CO, SO 2 , HCl, mercury, dioxins, furan, polychlorinated biphenyls, and polycyclic aromatic hydrocarbons to the environment [6,15–19]. However, many studies have considered energy recovery from municipal solid waste (MSW), whereas the potential benefits of WP and DIS appear to be underestimated [20] even though their valorisation could contribute to decreasing the energy consumption from fossil sources in the paper manufacturing sector. This, together with the avoided waste disposal, would reduce the GHGs emissions of such a sector. Thermal treatment appears to be very promising compared to the biological method for energy recovery from WP and DIS due to: • the higher conversion rate of carbonaceous content to energy product (more than 80% for thermal treatments whereas in the range of 30–60% for biological methods); • the lower processing time (thermal treatments require 30 to 70 min, whereas biological treatment needs between seven and 105 days) [21–23]. Considering the more common thermal treatments used for energy recovery from biomass, gasification is characterized by: • a high carbon conversion efficiency (CCE) (from 60 to 80%) and cold gas efficiency (CGE) (between 60 and 90%); • that migration of heavy or toxic metals from fed materials to the product phase is negligible; • the possibility to use the gaseous product as a fuel in internal combustion engines (ICE) or gas turbines or microgas turbines without any modification [17,24–30]. Gasification is a thermal treatment that converts the energy content of a biomass to a gaseous phase at a temperature higher than 700 ◦ C and atmospheric pressure in oxygen- deficient conditions. The gaseous product formed during biomass gasification consists ofH 2 , CH 4 , CO, CO 2 , and other lighter hydrocarbons with tar content and is commonly designated as syngas. Due to the easier availability and low cost, the air is frequently used as a gasifying agent to supply oxygen in the gasification process. However, based on the specification of syngas properties, other gasifying agents like pure O 2 , steam, CO 2 , a mixture of air–steam, O 2 –steam, and CO 2 –steam, may be also used. LHV of syngas varies between 3.0 and 9.73 MJ/Nm 3 depending on the properties of biomass used for the gasification and operating conditions [22,24,25,31,32]. Syngas composition and LHV as well as the performance of the conversion process (CCE and CGE) during air-gasification of biomass depends on the quality of fed materials and operating parameters, such as temperature and equivalence ratio (ER) [24,25]. For the sake of completeness, ER is the ratio between the actual air to biomass weight fed to the gasifier to the stoichiometric air to biomass weight required for complete combustion [33]. The available studies on the gasification of WP and DIS are limited [11,34]. Air gasifi- cation of WP and DIS blends (95% WP and 5% DIS by weight) in a pilot-scale circulating
Made with FlippingBook - Online magazine maker