(569z) Process Design and Simulation for the Conversion of Coal to Methanol

Abundantly available Low Rank Coal (LRC) in Indonesia is one of promising resources to be converted to valuable intermediate chemicals such as Methanol which can be further converted to useful derivatives such as Gasoline, Olefin (Ethylene and Polypropylene), Ethanol, or Dimethyl-ether (DME). The conversion of Coal to Methanol (CtM) involves coal gasification to produce syngas (CO & H₂) and then followed by syngas conditioning and cleaning for methanol synthesis.  In an effort to meet energy and chemical demand in Indonesia, the coal based methanol (CtM) project is developed, aiming to produce 800.000 tones/year AA Grade Methanol from LRC with moisture content of 33% planned to be used as a feedstock to Ethanol Plant.

In order to provide suitable and better process design during detailed feasibility study, one of the challenges within this framework is to carry out rigorous and robust modelling for the coal based methanol plant which integrating the coal/solid processing with gaseous and liquid process modelling.

Aspen Plus^TM (Version 8.4) process simulation software was used for facilitating the process modelling and simulation since it can provide such an integrated solid and gas/liquid modelling approach.  A hierarchy model in Aspen Plus was applied for each process section to build an integrated coal based methanol Plant with process model involving solid, gas, and liquid processing. This approach provides opportunity to specify different suitable property packages. For instance, PR-BM (modified Peng Robinson equation for solid processing) property package was used in the Coal Pretreatment and Gasification and Peng Robinson property estimation method was applied in the Gas Conditioning and Cleaning Unit. As Aspen Plus process simulator facilitates process simulation for rigorous solid modelling, coal compositions and its properties (proximate and ultimate analyses including their heating value and moisture content) are possible to be used as basic input for the simulation. Furthermore, Aspen Plus^TM also allows for specifying coal particle size distribution (PSD) and grind-ability quality (HGI-Index) within process simulation.

Prior to gasification process, the process model is well established for drying of coal feedstock from moisture content of 33% to 5% in combining of coal grinding to form fine powder coal (average PSD of 90 μm) using available drying and grinding unit model in Aspen Plus. The power requirement for this process as a simulation result is comparable with vendor proposition. Furthermore, impact of particle size and coal quality (particularly grind-ability, HGI) on the grinding performance can be examined.  

As the heart of the process, the coal gasification process is well modelled to incorporate coal decomposition mechanism and syngas gas formation using an equilibrium/thermodynamic approach. The process conditions and requirement of the gasification (pressure of 50 bar, temperature of 1500 °C, O₂/Coal ratio about 0.5) are established accordingly to simulate syngas production (CO & H₂). Literatures data was used to verify the simulation results. Although the coal processing simulation is well established, a rigorous model involving detailed kinetics on the coal decomposition and syngas formation for coal gasification is still needed to be developed for the selected gasification technology (an entrained flow gasification technology). Combining with dynamic simulation approach and detailed computational fluid dynamics, this process model would not only suit for process performance evaluation but also provides insight on the reliability and safety aspect of the gasification process.  This simulation result is then used for challenging and justifying vendor proposal regarding to the coal consumption, utilities requirement, and waste estimation during the project development. 

After gasification process, further process model is developed to integrate this coal solid modelling with Gas Conditioning (Water Gas Shift Reaction), Gas Cleaning Unit (Acid Gas Removal Unit using Rectisol® Process and Sulfur Recovery Unit using Clauss Process) and Methanol Synthesis to produce Methanol and Syngas suitable for feedstock of Ethanol Plant. This approach resulted logic Heat & Material Balances, Utilities Requirement (water, steam, and power), and Equipment Sizing. Furthermore, capital expenditure of CtM Plant is well estimated using Aspen Economic Evaluation for further economic evaluation.