Gasification Systems | Niksa Energy Associates

Overview

Single-stage entrained-flow gasifiers often operate at temperatures where syngas compositions are equilibrated, so detailed reaction kinetics are unnecessary. But in cooler second stages and, especially, in fluidized bed gasifiers, conversion rates are usually limited by the process chemistry, so that accurate kinetics are required to predict cold gas efficiencies and detailed syngas compositions. NEA’s expertise in solid fuel gasification focuses on three of the most formidable technical challenges: fuel flexibility, tar in syngas, and the levels of minor species. All of NEA’s chemical reaction mechanisms have already been validated for any coal, pet coke, or biomass sample, and have already been deployed for literally hundreds of different solid fuel samples. NEA’s detailed reaction mechanism for tar conversion is the only one based on legitimate depolymerization kinetics. At moderate temperatures it describes the elimination of heteroatoms in tar into oils and additional noncondensables. For progressively hotter temperatures, the mechanism automatically shifts toward a channel for soot production, accompanied by the reduction of all gaseous hydrocarbons into methane and acetylene. In our most comprehensive gasifier simulations, we use a 551-step reforming mechanism to accurately describe the complete syngas composition, including trace levels of hydrocarbon gases.

NEA's Gasification Kinetics Cover the Fuels and Conditions in Commercial Applications

A Japanese government R&D sponsor hired NEA to formulate all the chemistry submodels in a CFD simulator for a 150 tpd entrained coal gasifier. NEA’s mechanism for devolatilization at elevated pressures was validated against a database representing 99 coals at pressures to 16.7 MPa. Our char oxidation mechanism was validated for applications to 3 MPa. The char conversion kinetics were also expanded for simultaneous gasification by steam, CO₂, and H₂ and validated against test data on over 2 dozen coals. Learn More. Learn More.

The National Energy Technology Laboratory of the U. S. DoE purchased NEA’s complete suite of chemistry submodels for coal gasification at moderate temperatures, including devolatilization, char oxidation, char gasification, tar conversion, and gas reforming. These mechanisms accurately predicted the product gas compositions from several tests in Southern Company Services’ Power System Development Facility (PSDF) for diverse operating conditions with subbituminous and hv bituminous coals. Since then, NETL developed an interface between NEA’s PC Coal Lab^® and its own comprehensive kinetics package called Carbonaceous Chemistry for Computational Modeling (C3M).

A leading CFD company hired NEA to formulate improved fuel reaction submodels for pressurized applications. This project coupled NEA’s PC Coal Lab^{^®} software into both the CFD package and into ASPEN Plus, for detailed analyses of advanced coal utilization technologies, and delivered a new NO_X submodel for the CFD simulations. Learn More.

A Japanese utility OEM routinely uses NEA’s PC Coal Lab^® to develop small, distributed gasifiers for biomass and low-rank coals. These calculations are used to manage the different product gas compositions with various forms of biomass and coal, and also tar-related problems, such as sticky deposits and diminished fuel conversion efficiencies.

Anticipate Problems Caused by Residual Tar in Syngas

PC Coal Lab^® contains two separate approaches to describe tar destruction. One assumes rapid conversion of tar into soot, which is appropriate for high-temperature (> 1000°C) applications. The second is based on an expansion of the FLASHCHAIN^{^®} reaction mechanism to describe the secondary pyrolysis of tar at temperatures to 1000°C. At moderate temperatures, tars decompose into oils, additional noncondensables, especially hydrocarbons, CO, and H₂, and polynuclear aromatic hydrocarbons (PAH). The new mechanism predicts the oils and additional noncondensables and the yields and compositions of PAH for any thermal history. It is suitable for simulations of volatiles transformations in fixed- and fluidized bed gasifiers, and also in multi-stage systems, where volatiles may be processed at conditions that are much different from those in the primary stage.

NEA is currently expanding its framework for tar destruction to cover catalytic tar destruction agents, as a means to interpret syngas compositions from a diverse assortment of biomass forms from a low-temperature, fluidized bed gasifier. A reaction mechanism that combines separate channels for catalytic and pyrolytic tar conversion has been formulated, and is now being validated with literature datasets on nickel-based catalysts as a preliminary to interpretations of the commercial process.

Predicting Minor Species Often Requires the Most Extensive Reaction Mechanisms

A major American OEM for solid fuel gasification technology hired NEA to develop simulations to predict the levels of residual CH₄ in the syngas from their moderate temperature gasifier for a variety of solid fuels. By adopting ChemNet™ CFD Post-Processing as the simulator framework, NEA combined its best-in-class reaction mechanisms for chemistry in the solid phase with a 551-step reforming mechanism among the gaseous species and NEA’s state-of-the-art mechanism for tar destruction at moderate temperatures. Whereas virtually all the leading simulation teams in the USA were given opportunities to accurately interpret the reported syngas compositions, NEA’s simulations were the only ones that accurately predicted the residual CH₄ levels over a broad domain of gasifier conditions, scales, and fuel quality.