Principal Research Results

Evaluation of Operating Condition of Air-blown Coal Gasifier Using CFD Tool
−Gasification Performance and Molten Slag Discharge under Low Air Ratio−
Background
Improvement of cold gas efficiency by low air ratio operation is important to achieve a highly efficient operation of an airblown coal gasifier. However, there is a possibility of increase of facility cost and difficulty of stable operation because of increase in
the amount of product char and difficulty of molten slag discharge due to decrease of temperature. There is an oxygen enriched air operation which can maintain the temperature and improve the per pass carbon conversion as one of the means to solve such a problem.
Then, it is important to clarify the influence of oxygen concentration in low air ratio on gasification performance and molten slag discharge.

Objectives
To perform numerical simulation to clarify influence of oxygen concentration in low air ratio on gasification performanceand molten slag discharge, and to establish evaluation method of operating conditions of air-blown coal gasifier.

Principal Results
Numerical experiment that changed the oxygen concentration of the gasifying agent in the extremely low air ratio ＊ 1, which
was difficult to confirm experimentally, was executed on the two stages air-blown coal gasifier (tested conditions : Fig. 1 and Table1), and the following findings were obtained.
(1) Effectiveness of an increase of the oxygen concentration of the gasifying agent to the per pass carbon conversion and the control of
the amount of product char was quantitatively clarified in extremely low air ratio conditions (Fig. 2). An important index of thecoal reactivity was obtained for the evaluation of the gasifier capacity.
(2) Slag temperature was predicted from the calculation results of the heat received by the slag flowing on the combustor inner walland the slag volume and residence time that were estimated by the gas-liquid two phase flow calculation ＊ 2. In addition, the influence of the air ratio and the oxygen concentration on the slag viscosity that is the most important factor of the slag discharge wereclarified by application of the semi-empirical slag viscosity model ＊ 3 (broken lines in Fig. 3).
(3) An index for the evaluation of the operating condition of the gasifier was obtained by the analysis of the tendency of the slag viscosity and the cold gas efficiency changing the air ratio and the oxygen concentration. The index can be used for the evaluation of
the operating conditions that can achieve high gasification efficiency and stable slag discharge ＊ 4 (Fig. 3).

Future Developments
Verification and improvement of the evaluation method presented in this paper will be done by the experimental study of themolten slag discharge characteristics using FRONTIA, newly set up coal gasifier in CRIEPI.
Main Researcher: Hiroaki Watanabe,
Research Scientist, Thermal Engineering, Energy Engineering Research Laboratory
Reference
Hiroaki Watanabe, 2004, “Prediction of Air-blown Entrained Flow Coal Gasifier Performance using Numerical Simulation
Technique”, CRIEPI Report, W03027＊1：According to the experience of the test of 2 t/d coal gasifier, it is difficult to discharge slag of Coal M less than air ratio 0.47.
＊ 2：Otaka et al., CRIEPI Report, W03021, (2004).
＊ 3：Browning, G. J. et. al., "An Empirical Method for the Prediction of Coal Ash Slag Viscosity," Energy & Fuels, 17,pp.731-737(2003).
＊4：It was assumed that the threshold viscosity that stable operation become to be difficult is the critical viscosity at which slagflow start to behave as a non-Newtonian fluid.

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6. Fossil Fuel Power Generation - Diversification and clean utilization of fossil fuels

It is possible to reduce air ratio to 0.425, keeping the amount of charequal to at air ratio 0.47, with 40% oxygen enriched operation.

(1) Coal : Australian M
Feeding rate : 100[kg/h](R/C = 60/40 [kg/kg])(2) Oxygen enriched air is
injected from combustor
and char burner.
(3) Burner velocity : Const.
(4) Performance is evaluatedincluding char recycling
system.

Fig.1 Calculated conditions
Table1 Calculated conditions

Air ratio−0.39, 0.41, 0.43, 0.47

Oxygenconcentration

vol%21, 30, 40

Fig.2 Trend of per pass carbon conversion and theamount of product char, changing air ratio andoxygenconcentration

The effects of air ratio and oxygen concentration on perpass carbon conversion and the amount of product charwere clarified. An important index of coal reactivity wasobtained for evaluation of gasifier capacity.
Influence of air ratio
LAir ratio

HH

Cold gasefficiency

LH

Slag viscosity

L

30% oxygen concentration
operationcan achieve stableslag dischargein air ratio
0.43 and improve 2%
ofcold gas efficiency
compared within air ratio
0.47

Influence of oxygen
L

Oxygenconcentration

HH

Cold gasefficiency

LH

Slag viscosity

L

Slag flow shows non-Newtonian fluid
behavior and difficulty of slag discharge
is expected when slag viscosity
becomes 150 Pa*s or more (red regionin Fig. 3). Operating shouldevade thisregion.

Fig.3 Trend of cold gas efficiency and slag viscosity
changing air ratio and oxygen concentration

The index that can be used for the evaluation of the operating conditions toachieve high gasification efficiency and stable slag discharge was obtained.

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