2013 1(12)

LOW-EMISSION BURNING OF PREMIXED GAS-AIR MIXTURES IN THE CHAMBER BY COMBUSTION PRODUCTS RECIRCULATION

Soroka B.S.¹, Zgurskiy V.A.¹, Khinkis M.²

¹ The Gas Institute NAS of Ukraine, Kiev, Ukraine
² Gas Technology Institute (GTI), Des Plaines, USA

Two layouts of burning process and the relevant designs of combustion chamber have been tested under CFD modeling and 2D numerical simulation:

• cylindrical combustion chamber CC (without Recirculation Sleeve RS) equipped with conical nozzle at the burner axis to deliver the total flow rate of combustible mixture under one-stage combustion of natural gas (methane) – air mixture;

• the same equipped with RS arranged inside the CC coaxially to the chamber’s walls.

Some types of chemical kinetic mechanisms and turbulence models have been tested to make option the most suitable ones.

The following statements have been found for the case of methane-air mixture burning in simplified design of water-cooled ΡΡ without RS:

• The results obtained are testifying of strong influence of the combustion mechanism and turbulence model being chosen on computed environmental characteristics (NO and CO concentrations in flue gases) while the energy (heat transfer) indices are insignificantly dependent on the chosen constituents of complex computation algorithm;

• CO concentration is strongly dependent on air excess factor λ, especially in the range of 1.0 ≤ λ ≤ 1.15, the [CO] (λ) correlation is sharply lowering by λ value increase within 1.0 ≤ λ ≤ 1.05. Thus in essence the principally known [CO] (λ) regularities for industrial burners have been confirmed.

The most attractive way of minimization of NO issue by combustion makes combination of separate techniques of reduction the NO formation. Particularly two methods of harmful nitrogen oxides lowering have been considered in this paper: arrangement the RS inside the CC coaxially the walls to organize combustion products recirculation and a weak combustion air humidification. Due the first technique [NO]_ex  at the exit of CC could be reduced approximately 2.6 times by the CC walls temperature 1200K while the [NO]_ex is decreased by 4.3 – 4.5 times even in case of small vapor content (20.4g/kg dry air) by simultaneous effect of two mentioned ways of NO reduction.

This vapor content in oxidant (d_a = 20.4 g/kg dry air) causes reduction of [NO]_ex formation by 1.725 – 2.0 times in comparison with the case of dry air-oxidant application. As the positive fact must be mentioned [CO]_ex decrease simultaneously with [NO]_ex reduction by RS use and by intensive combustion products recirculation – if the last process makes (by10 – 12 %) the single impact factor and by  (17.5 – 24 %). - in case of an air humidifican. By two considered factors joint impact [CO] _ex could be reduced by 25 – 30%

Unfortunately the power efficiency of CC calculated by account of Q_w by these conditions for the case of humid air flow is reduced for 3.15 – 3.45% in comparison with dry air oxidant. Arrangement CC with RS provides some increase of Q_w (5.35 – 5.75 %) and corresponding small decrease of T_fl,ex. By lowering temperature of the combustion chamber walls from 1200 to 600 K significant reduction of [NO]_ex concentration takes place both under dry and humidified air application as an oxidizer: by 7.04 times for dry air and by 8.16 times – for air of d_a = 20.4 g/kg dry air vapor content.

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