The rapid decrease in the world reserves of hydrocarbons and natural gas and the considerable increase in their prices have stimulated the utilization of coal as the source of energy for the logical solution to the long-term energy problem. Meanwhile, coal has also been treated to produce hydrocarbons, pipeline gas and coal-derived liquid fuels, such as gasoline, methanol and olefins (e.g., by MTO process), for the chemical industry. As an example, a widely developed technique currently is coal gasification to syngas at the temperature of 1200-1500 C, which is the first step for coal chemistry to downstream chemical synthesis.

Different from these conventional routes for coal utilization, rapid pyrolysis of coal to gaseous products was ever paid much attention from 1960s to 1980s and seemed hindered for some reason in recent 20 years. When coal is heated to temperatures above 1500 C in milliseconds, a gaseous mixture is produced with the acetylene as the principal hydrocarbon constituent. Since the reaction environment is in hydrogen atmosphere, little CO and almost no CO2 are generated. In addition, the coal powders after the pyrolysis process can either serve as high-valued carbon materials or be burned to generate heat and electricity. Therefore, the coal pyrolysis to acetylene in hydrogen plasma is a rather clean process for coal utilization, which opens up a direct means for the conversion of coal to downstream chemical processes, e.g., to provide the feedstock to PVC industry. This novel process is an alternative to replace the conventional calcium carbide method with high energy cost and heavy pollution.

However, the high-severity operation of the coal pyrolysis process, i.e., high-temperature pyrolysis in a few milliseconds with a very rapid quenching rate, imposes great challenge to the commercialization in industry. This work is to introduce the latest development of a 2-MW plasma reactor for the purpose of coal pyrolysis to acetylene in China. The pyrolysis reactor consists of a V-shaped plasma generator, a downer reactor with unique coal injection nozzle design and a fast quenching operation. The volume concentration of acetylene in the product gas has reached 9.6% under the conditions of 2-MW power plasma and the feeding rate of 1 t/hr coal. The whole system has been scaled up to a 5-MW unit, which is ready for industry test.

To better understand the milliseconds reacting flow process inside the reactor, a CFD approach is established to model the coal pyrolysis in the hot hydrogen gas. The comprehensive model includes the sub-models for the turbulent gas flow, the multiphase interaction, the particle reactions, the gaseous reactions and the radiative energy transport. For gas phase, the Realizable k-£` model and a mixture fraction approach are chosen to model the turbulent reacting flow. For the solid phase, the discrete phase model (DPM) is employed. The chemical percolation devolatilization (CPD) model is used to describe the devolatilization behavior of rapidly heated coal powders, and a simplified surface reaction is introduced to consider the C-H reactions at extremely high temperatures (> 2000 K). All the numerical simulations are conducted in 3-D domains using FLUENT-6.3 software package.