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Description
To address the climate change and energy crisis caused by traditional fossil fuels, and to promote the achievement of "carbon neutrality" and "carbon peak" goals, the development and application of biomass energy has garnered widespread attention. Among the challenges faced by biomass energy, the alkali and alkaline earth metals (AAEMs) present in biomass ash can lead to boiler corrosion and slagging. Ash removal treatments such as water leaching and acid leaching have been proven effective in mitigating these issues. However, the water consumption associated with ash removal treatment was too high to limit the application of deashing process. This study proposed a cycling leaching process to reduce the water consumption of eucalyptus bark deashing.
Initially, the study examined the impact of leaching time, acetic acid concentration, and water consumption on the ash removal from eucalyptus bark through acid leaching (AL), step leaching (SL), and circle leaching (CL) via single-factor experiments. The results indicated that ash removal rate increases rapidly with leaching time and acetic acid concentration before reaching a plateau, while water consumption had a minimal impact. Response surface methodology revealed the maximum ash removal efficiency for AL, SL, and CL were 77.37 wt.%, 79.35 wt.%, and 80.55 wt.%, respectively. The circle leaching eucalyptus bark (CL-EB) exhibits the highest ash fusion temperature and the lowest slagging tendency.
Furthermore, the study investigated the leaching behavior and kinetic of AAEMs from eucalyptus bark in different leaching processes. The experimental results showed that when the water consumption was 500+300 mL, the leaching time was 120 min, and the acetic acid concentration was 2 mol/L, the leaching efficiency of K+, Ca2+, Na+ and Mg2+ under different treatment processes were in the order of AL<SL<CL. Kinetic studies indicated that SL and CL enhanced the leaching equilibrium concentration (Cs), leaching rate constant (k) and initial leaching rate (h) values for AAEMs, effectively promoting ion leaching.
Calorific value and thermogravimetric analysis were also conducted for eucalyptus bark (Raw-EB), and bark treated under optimal ash removal conditions with AL (AL-EB), SL (SL-EB), and CL (CL-EB). The calorific values were found to be in the order: Raw-EB<SL-EB<AL-EB\<CL-EB. The combustion process of these samples could be divided into three stages, and the activation energies calculated using the Flynn-Wall-Ozawa method were presented for each sample.
Finally, the NSGA-II algorithm was used to optimize the objectives of treatment cost and ash removal rate. The results suggested that at a 78 wt.%% ash removal rate, the treatment cost of AL was 1.9 and 3.3 times higher than that of SL and CL, respectively, with the cost order being: AL > SL > CL.
In conclusion, the ash and AAEMs content of EB could be effectively removed by CL, the slagging tendency could be reduced, the calorific value of fuel could be obviously increased, and the combustion activity can be reduced. Compared with the other two processes, CL’s processing cost was the lowest. This study provides the basic information for the ash removal of eucalyptus bark.
