A CFD Simulation of Coal Syngas Oxy-Combustion in a High-Pressure Supercritical CO2 Environment

The Allam Cycle is an oxy-fuel supercritical CO2 power cycle that generates low-cost electricity from fossil fuels while producing near-zero air emissions. The turbine exhaust (sCO2) is then available for partial injection into underground storage while remainder is reused in the power cycle. Novel combustors required by this and other sCO2 cycles are critical to their commercialization. A conceptual design was developed for a coal syngas-fueled oxy-fuel combustor that meets the conditions of the Allam Cycle. The design of this combustor utilizes a 300MWe coal syngas-fired Allam Cycle thermodynamic analyses and ASPEN process models as inputs to the combustor. The primary inputs for design of the combustor included the fuel mixture compositions and respective flow rates for the constituent gases, pressures, and operating temperatures which were scaled to a 5MWth test article. The combustor was sized to accommodate the required pressures, heat release rate, flow rates, and residence times to produce well mixed turbine inlet flows with complete combustion. A preliminary design for a 5MWth test scale combustor was then developed, and a numerical study using Computational Fluid Dynamics (CFD) simulations was carried out to demonstrate the feasibility of that combustor. Steady-state RANS simulations were used to qualitatively examine the preliminary design of the 5MWth combustor and predict the fluid mechanics, heat transfer, and combustion. The purpose of the analysis was to verify the following criteria: 1) good mixing of the fuel and oxidizer in the primary zone, 2) uniform exhaust gas temperature and 3) efficient combustion with complete CO burnout. Additionally, the analysis investigated wall temperature and the impact of varying the fuel composition on combustion performance. The CFD model results were in good agreement with the equilibrium one-dimensional (1D) Aspen model results. The CFD predictions of the current conceptual design verified the identified key criteria for the combustor and demonstrated its feasibility.

Laminar Flame Speed Measurements of Synthetic Gas Blends With Hydrocarbon Impurities

New Laminar Flame Speed measurements have been taken for a wide range of syngas mixtures containing hydrocarbon impurities. These experiments began with two baseline syngas mixtures. The first of these baseline mixtures was a bio-syngas with a 50/50 H2/CO split, and the second baseline mixture was a coal syngas with a 40/60 H2/CO split. Experiments were conducted over a range of equivalence ratios from ϕ = 0.5 to 3 at initial conditions of 1 atm and 300 K. Upon completion of the baseline experiments, two different hydrocarbons were added to the fuel mixtures at levels ranging from 0.8 to 15% by volume, keeping the H2/CO ratio locked for the bio-syngas and coal syngas mixtures. The addition of these light hydrocarbons, namely CH4 and C2H6, had been shown in recent calculations by the authors to have significant impacts on the laminar flame speed, and the present experiments validated the suspected trends. For example, a 7% addition of methane to the coal-syngas blend decreased the peak flame speed by about 25% and shifted it from ϕ = 2.2 to a leaner value near ϕ = 1.5. Also, the addition of ethane at 1.7% reduced the mixture flame speed more than a similar addition of methane (1.6%). In general, the authors’ chemical kinetic model over predicted the laminar flame speed by about 10–20% for the mixtures containing the hydrocarbons. The decrease in laminar flame speed with the addition of the hydrocarbons can be explained by the increased importance of the inhibiting reaction CH3 + H (+M) ↔ CH4 (+M), which also explains the enhanced effect of C2H6 compared to CH4, where the former produces more CH3 radicals, particularly at fuel rich conditions.