Modification of Biochar Formation during Slow Pyrolysis in the Presence of Alkali Metal Carbonate Additives

Abstract

In this work, slow pyrolysis of sawdust of Eucalyptus pilularis biomass and ternary molten carbonate eutectic [Li2CO3, 43.5%; Na2CO3, 31.5%; and K2CO3, 25% (mole percentage)] in thermogravimetric analysis at three different temperatures, 600, 750, and 900 °C, was studied. These salts affect the slow pyrolysis process, including changes in the volatile release mechanism and the morphology of remnant char material. The initial results show that, in the presence of molten carbonate, biomass particles make bubble-shaped larger particles, which result in less volatile emissions and more char residue. It is suggested that the ternary eutectic has a chemical diluent and catalytic role, particularly in the case of higher salt doping. Results from scanning electron microscopy images give strong evidence that molten carbonates capture volatiles inside swelling carbon particles, which causes the generation of various sizes of pores as well as char-making reactions, and at a higher temperature, the bubble-shaped particles will rupture. Swelling of this nature has previously only been observed clearly in coal precursors; however, this is the first observation in a biomass-based system. Also, at a temperature above 750 °C, decomposition of molten carbonate generates CO2 and carbon/carbonate gasification produces CO as well as a more “activated” biochar.

The interplay between ternary molten carbonate and biomaterials during pressurized slow pyrolysis

“…Eucalyptus pilularis (Blackbutt) sawdust and Sigma kraft lignin 19,41 were used as biomass samples with particle size range 75–150 μm. The ternary eutectic was blended in the mole percentage Li 2 CO 3 : 43.5%, Na 2 CO 3 : 31.5%, K 2 CO 3 : 25% from Sigma-Aldrich (melting point at ∼400 °C) and prepared in the same procedure of previous work.…”

“…Studies on three fundamental components of biomass structure (hemicellulose, lignin, cellulose) suggest a temperature range of 250 °C to 700 °C to accomplish complete devolatilization. 16,17 It is also reported that, at elevated temperatures, the morphology of remnant char improves by generation of a porous structure, 18,19 higher surface area, 20,21 and induced ordering of graphitic nanostructures. 22–24 Thus, not only the rate of heating is crucial in biochar preparation but also the final highest heating temperature has significant impacts on resultant char structure.…”

“…40 Hazard, cost and recyclability issues of additives limit selection of appropriate chemicals for practical materials generation. 8 Molten salts with high heat capacity and limited impact on the environment have also attracted attention in biomass treatment due to their processing advantages 19 specifically, lower melting point of binary or ternary carbonate is a promising incentive to decrease operational temperature during process. Application of molten salt in various industries such as solar thermal energy production is already commercialized.…”

“…8,42,43 Also, previous works of our group distinguished the ternary carbonate eutectic (Li 2 CO 3 /Na 2 CO 3 /K 2 CO 3 , with low melting point at ∼400 °C) influence on biomass slow pyrolysis under different operational conditions for multiple feedstock types, 41 gas atmospheres and HHT. 19,42,43 The ternary eutectic carbonate treatment increases porous char yield while promoting in situ tar reduction under nitrogen slow pyrolysis at temperatures below 600 °C. In cellulose undergoing treatment at 600 °C however molten carbonate has been seen to block generated pores during liquid–solid contacts between salt and cellulose, and is difficult to remove.…”

“…Evidence suggests the salt mixture evaporates to an extent, 43 decomposes under N 2 via eqn (1) 44 and in the presence of carbon particles will be consumed in a carbon/carbonate gasification pathway (eqn (2)): 45 M 2 CO 3(1) → M 2 O (s) + CO 2(g) M 2 CO 3(1) + C (s) → M 2 O (s) + 2CO (g) These observations suggest that during slow pyrolysis treatment of biomaterials with ternary eutectic carbonate, the impact of salt will vary within the temperature range of 350 °C to 900 °C depending on final temperature. 19,42,43 Although additive pyrolysis is heavily studied, the consequence of both additives and pressure on the slow pyrolysis process, and resultant biochar's morphology, is still vague. Certainly, it is beneficial to study the impact of two variable at the same time and compare with individual effects, particularly at high temperature when chemical reactions with gaseous products are taking place.…”