Effect of boron-based additives on char agglomeration and boron doped carbon microspheres structure from lignin pyrolysis

• Boron additives were used to improve the properties of lignin pyrolytic char. • The glass transition of boron modified lignin was inhibited. • The crosslinking of B-anions with O–H reduced the relative slip of lignin chains. • Boron additives inhibited agglomeration and promoted the carbonization of lignin. • B-doped spherical char was obtained after pyrolysis of lignin-boron hybrids. To prevent the thermal softening of lignin during the pyrolysis process, the influence of boron-based additives (boric acid, sodium borate and potassium borate) on lignin softening, pyrolysis process were investigated with metal bases (Ca(OH) 2 and Mg(OH) 2 ) as comparison, and the influence mechanism of boron based additives was explored in depth. It is found that the boron additives can effectively inhibit the agglomeration of lignin particles. For pyrolysis products, boron additive promoted the carbon-forming reaction of lignin with carbon sequestration more than 70 wt%, and inhibited the release of volatiles, especially for phenolic compounds. It also inhibited the generation of CO 2 and CH 4 , while promoted the release of CO. The boron element in lignin char mainly exists in the form of BCO 2 and B 2 O 3 with more hydroxyl groups on the surface. Furthermore, boron-modified lignin chars are spherical with more uniform and smaller particle size compared with metal bases modified lignin chars.