Evaluation of tar from the microwave co-pyrolysis of low-rank coal and corncob using orthogonal-test-based grey relational analysis (GRA)

Abstract

Tar is a potential alternative to crude oil and can be produced through the pyrolysis technology of low-rank coal. However, the low H/C ratio of coal leads to unsatisfactory tar yield and quality. The use of corncob as a hydrogen donor to assist the microwave pyrolysis of low-rank coal through orthogonal-test-based grey relational analysis was investigated. The yield, structure, and component of tar under the optimum conditions were discussed. Results showed that pyrolysis time had the most obvious effect on tar yield, followed by corncob particle size, microwave power, and corncob additive amount, but the difference was small. The maximum tar yield of 8.85% was achieved under the following conditions: microwave power of 700 W, pyrolysis time of 30 min, corncob particle size of 0.420–0.841 mm, and corncob additive amount of 30%. The positive synergistic effect resulted in increased tar yield and decreased long-chain carbon structure and heteroatom organics in tar, thereby producing “lighter” and more “valuable” tar. This positive synergistic effect was due to the H free radicals and reactive oxygen-containing groups produced from corncob cracking.