Adsorptive desluphursation of diesel fuel on unprocessed amarula (sclerocarya birrea) wastes or synthesized activated carbons from biomass wastes

Abstract

Diesel fuel has been found to contain highly concentrated organo-sulphur compounds which have bad impact economically, environmentally, and health-wise. Adsorptive desulphurization (ADS) is one of the promising processes which are carried out under atmospheric conditions. Amarula (Sclerocarya birrea) waste biomass from the production plant of Amarula liqueur, was utilized as a low-cost adsorbent and as a source of synthesized activated carbon for reducing sulphur content in diesel fuel. The performance of gasification char (waste from syngas production) was also compared with Amarula shells waste biochar. The Amarula wastes Biomass: fruit, seed, and shell waste were used as adsorbents to reduce the content of sulphur in dibenzothiophene model diesel fuel. The results showed that raw Amarula Shells (AmSh) waste had the highest adsorption efficiency as compared to Amarula seeds (AmSe) and fruit (AmWa) wastes. The effect of adsorption temperature revealed that the sorption is more favourable at room temperatures. The selected Amarula shell wastes showed that as the adsorbent quantity increases, the sorption efficiency also increases. The three Amarula wastes biomass were processed to synthesise activated carbons (ACs) using pyrolysis, and then steam activation at 800 °C for 45 min. The adsorption efficiency of DBT in model diesel fuel was found to improve on Amarula wastes ACs with the order of AmShAC-ST > AmSeAC-ST > AmWaAC-ST. The effect of steam residence time on selected Amarula shell wastes biomass revealed that the desulphurization efficiency of DBT increased outstandingly with increased steam residence time. The AmShAC-ST produced at 90 min steam resident time reduced the highest content of DBT within 30 min. The gasification chars from a down draft gasifier (DG) and a plasma gasifier (PG) were utilized for adsorption of DBT in model diesel fuel. The Algae biochar (ALGC-DGBC) was from gasification of Algae binder mixed with coal- fines in a down-draft gasifier. The Wood-DGBC was a waste product from wood pellets gasifier in a down-draft gassier, the Wood-PGBC was a by-product from Plasma gasification of wood pellets. These chars were able to reduce sulphur content in DBT model diesel fuel with Amarula shells biochar (AmShBC) being the best performer as compared to other chars. The KOH and steam were used as activating agents on biochar to improve their performance. The results showed that the gasification chars treated with the steam agent had a higher desulphurization efficiency of DBT than the ones treated with KOH agent. The desulphurisation efficiency trend: AmShBC-ACST > Wood-PGBC-ACST > Wood-DGBC-ACST > ALGC-DGBC-ACST was achieved for steam activation. In the contrary, for KOH/BC the trend was in the order of Wood-DGBC-KOH > Wood- PGBC-KOH > AmShBC-KOH > ALGC-DGBC-KOH, but with lower ADS efficiency values as compared to steam/BC. Pseudo-second-order was found to be the best fit kinetic model on experimental data. While Langmuir isotherm was found to be a better fit as compared to Freundlich isotherm for both processed and unprocessed Amarula wastes biomass. Thermodynamic studies were carried out to determine the spontaneity of the adsorption of DBT on Amarula wastes adsorbents. Qualitative analysis was carried out by techniques such as TGA, XRD, FESEM, TEM, BET and FTIR. The raw Amarula waste biomass had more O-functional groups as compared to their biochar and activated carbons. After thermal/steam activation of biomass, the micropores and mesopores were increased. This was concluded as the reason for improved desulphurisation efficiency of diesel fuel. The selected AmShAC-ST produced at 90 min steam residence time had increased mesopores and less micropores with a BET surface area of 1 194 m2/g and the highest pore volume of 0.98 cm3/g. On the other side, the gasification chars were found to have more micropores than mesopores. The ACs from KOH activation had less mesopores than the ACs from steam activation. Therefore, the above desulphurization’s data was found to contribute to the design of a small-scale adsorptive desulphurization plant for diesel fuel.