Burner Characteristics

The pyrolysis chamber is generally heated with a high velocity gas burner. In order to avoid hot-spots, an impingement plate is used such that there is no flame impingement on the vessel itself. In newer designs however the chamber is heated indirectly with a hot air burner so that hot spots and flame impingement problems are eliminated. 

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Figure 2.3 shows the composition of the flue gas as a function of the amount of air present. The combustion process is usually operated so that enough air is provided to convert all the fuel into carbon dioxide (C02), but not much more. This percentage of excess 02 is not a constant. It varies with boiler design, burner characteristics, fuel type, air infiltration rates, ambient conditions, and load. 

When a customer agrees to purchase gas, product quality is specified in terms of the calorific value of the gas, measured by the Wobbe index (calorific value divided by density), the hydrocarbon dew point and the water dew point, and the fraction of other gases such as Nj, COj, HjS. The Wobbe index specification ensures that the gas the customer receives has a predictable calorific value and hence predictable burning characteristics. If the gas becomes lean, less energy is released, and if the gas becomes too rich there is a risk that the gas burners flame out . Water and hydrocarbon dew points (the pressure and temperature at which liquids start to drop out of the gas) are specified to ensure that over the range of temperature and pressure at which the gas is handled by the customer, no liquids will drop out (these could cause possible corrosion and/or hydrate formation). 

Whereas there is no universally accepted specification for marketed natural gas, standards addressed in the United States are Hsted in Table 6 (8). In addition to these specifications, the combustion behavior of natural gases is frequently characteri2ed by several parameters that aid in assessing the influence of compositional variations on the performance of a gas burner or burner configuration. The parameters of flash-back and blow-off limits help to define the operational limits of a burner with respect to flow rates. The yeUow-tip index helps to define the conditions under which components of the natural gas do not undergo complete combustion, and the characteristic blue flame of natural gas burners begins to show yellow at the flame tip. These... 

Natural gas is attractive as a fuel ia many appHcatioas because of its relatively clean burning characteristics and low air pollution (qv) potential compared to other fossil fuels. Combustion of natural gas iavolves mixing with air or oxygen and igniting the mixture. The overall combustion process does not iavolve particulate combustion or the vaporization of Hquid droplets. With proper burner design and operation, the combustion of natural gas is essentially complete. No unbumed hydrocarbon or carbon monoxide is present ia the products of combustioa. 

With this type of burner, a wide variety of raw materials, ranging from propane to naphtha, and heavier hydrocarbons containing 10—15 carbon atoms, can be used. In addition, the pecuhar characteristics of the different raw materials that can be used enable the simultaneous production of acetylene and ethylene (and heavier olefins) ia proportioas which can be varied within wide limits without requiring basic modifications of the burner. 

Consider the case of the simple Bunsen burner. As the tube diameter decreases, at a critical flow velocity and at a Reynolds number of about 2000, flame height no longer depends on the jet diameter and the relationship between flame height and volumetric flow ceases to exist (2). Some of the characteristics of diffusion flames are illustrated in Eigure 5. 

Unless material characteristics hmit the gas temperature, the inlet temperature is usually fixed by the heating medium employed i.e., 400 to 450 K for steam or 800 to 1100 K for gas- and oil-fired burners. The proper exit-gas temperature is largely an economic function. Its value may be determined as follows ... 

A refractory quarl is usually an integral part of forced-draft burners. Suitable design of burner and quarl can determine the flame characteristics. Long, short, pencil or even flat flames are possible. 

Figure 19.2 shows how the production of CO can vary with excess air for two typical burners. It is seen that to limit CO to, say, 50 ppm with burner B, 3 per cent oxygen in the flue is needed, and with burner A, which exhibits better mixing characteristics, only 0.75 per cent excess oxygen is required. It is also seen that the heel in the curve is more pronounced with burner A such that... 

Fuel gas to fuel gas purging The purge may be carried out by burning the gas under supervision on an appliance or appliances. If the two fuel gases have different burning characteristics and Wobbe numbers, correct burner selection is important. 

Characteristics of various atomizers are given in Table 24.1. Primary considerations are selecting the best principle for the type of fuel, the size of the burner/boiler and the type of application. Other important characteristics are ability to operate with the minimum of excess air, turndown ratio and questions of durability and maintenance. 

Oil burners Type Size range Dual-fuel capability Fuel type Atomizing Atomizing pressure viscosity (bar) (cS) Turndown ratio Flame characteristics Main applications... 

Owing to the differing combustion characteristics and calorific values of the gaseous fuels which are commonly available [natural gas, liquefied petroleum (bottled) gas], slight variations in dimensions, including jet size and aeration controls, are necessary for maximum efficiency it is essential that, unless the burner is of the All Gases type which can be adjusted, the burner should be the one intended for the available gas supply. 

A cook recognizes the tell-tale signs of an overflowing pot of boiling water—the characteristic hiss as the water hits the hot gas burner and evaporates and the familiar accompanying yellow glow. Chemistry on the atomic level is responsible for the brilliant yellow illumination. 

As can be seen in Fig. 5.44 as well, flames in gas heaters have a similar emission spectrum. Besides the UV surveillance ionization electrodes are often used in gas burners. The method is cheap and secure but it disturbs the combustion process since the electrode has to be placed close to the flames. New developments in gas heaters focus on catalytic combustion on a metal mesh. There, an ionization electrode would fail due to the lack of a flame. However, the characteristic UV emission is still present... 

Fig. 5.44 Emission spectra of oil and gas flames in burners. The characteristic peak at 1=310 nm determines the signal of a UV flame-monitor.

The characteristics of the lifted flame are worthy of note as well. Indeed, there are limits similar to those of the seated flame [1], When a flame is in the lifted position, a dropback takes place when the gas velocity is reduced, and the flame takes up its normal position on the burner rim. When the gas velocity is increased instead, the flame will blow out. The instability requirements of both the seated and lifted flames are shown in Fig. 4.37. 

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