Numerical study of coal gasification in a dual-CFB plant based on the generalized drag model QC-EMMS

Coal gasification involves the conversion of coal into syngas. The process typically consists of a series of reactions and stages aimed at achieving high carbon conversion and syngas production rates. Presently, circulating fluidized beds are widely used in coal gasification processes due to their wide coal adaptability and high gasification intensity. Currently, dual-circulating fluidized bed plants are being developed for large-scale coal gasification. However, the complex gas-solid flow and reaction characteristics during field tests are difficult to be revealed due to several challenges attributed to the limitations of the measurement and the coupling between the chemical reactions, heat transfer, and fluidized gas-solid flow. Some of the key operating parameters such as the solid mass circulation rate still cannot be effectively measured.

Lately, full-loop numerical simulations have been proposed to guide the selection and measurements of the operating parameters and model the internal flows and reactions in the newly developed large-scale coal gasification plants. However, considering the remarkable effects of particle clusters during the gas-solid fluidization process, accurate simulations require modeling of the gas-solid drag, which have proved difficult due to the heterogeneous nature of the gas-solid flows and the complex dual-circulating fluidized bed structures. A generalized drag model QC-EMMS developed based on the Energy Minimization in Multi-Scale (EMMS) theory has been used to address most of the above challenges. Comprising of different sub-models, the QC-EMMS model is capable of accurately predicting the drag over a wide range of flow conditions. Despite its promising results, it has usually been tested in the simulation of cold-flows in circulating fluidized bed risers, thus the need to extend it to other simulations involved in the coal gasification.

To this note, a team of Tsinghua University researchers: Mr. Yang Liu and led by Professor Haiying Qi in collaboration with Director Pengju Huo and Chief Engineer Xiaohong Li from Shaanxi Yanchang Petroleum Company Limited developed the generalized QC-EMMS model, applied in the Eulerian-Eulerian approach, to conduct a full-loop numerical simulation of the coal gasification process in the newly-designed dual-circulating fluidized bed plants. Specifically, a coal gasification process of a 60 m high pilot plant with an average daily capacity of 100 tons of coal was modeled. The main aim was to predict the internal flow structures and reactions as well as the unmeasured parameters to achieve appropriate design modifications. To validate the feasibility of the QC-EMMS model in simulating the complicated circulating fluidized bed plants, the authors compared the obtained results to those predicted by the conventional Gidaspow drag model. The work is currently published in the journal, Fuel Processing Technology.

The author accurately predicted the parameters such as the solid mass circulation rates as well as the pressure, temperatures, and syngas compositions. Most importantly, these predictions were observed to agree well with those of field test data compared to the Gidaspow model results, thus confirming the QC-EMMS model’s applicability for simulating large circulating fluidized bed plants. Moreover, the simulation revealed adverse effects of the previously high loose gas flow rate that is prevalent in the first stage of the circulating fluidized bed, that providing support for appropriate adjustments.

In a nutshell, the authors reported an accurate simulation of the coal gasification process by a QC-EMMS drag model based on the Eulerian-Eulerian approach. The study revealed more insights into the gas-solid fluidization and reactions in the newly developed large-scale coal gasification plants. This would enable appropriate prediction and adjustments of coal gasifier operating parameters to optimize the flow pattern and enhance the reaction efficiency. The accuracy and applicability of the model were successfully validated by comparing the predicted results to those from the field data. In a statement to Advances in Engineering, Professor Haiying Qi said their designed model is a promising numerical tool for enhancing the efficiency and operation of large-scale industrial circulating fluidized bed processes.