Primary air rates

Rogers experimental work [2] deals with the combustion of a fuel bed in a batch reactor (pot furnace). Rogers primary objective was to study the effect of the primary air rates (underfire air) on the burning and ignition rates of wood fuel. 

The heat accumulation in the bed surface layer causes the ignition of the char combustion process. The heat is supplied from the over-fire process (see Figure 58C). When the char combustion process commenced, the macroscopic ignition front sustains itself with heat from the exothemic oxidation reactions. Large amounts of the heat released by the char combustion zone are also conducted and radiated away both upwards and downwards in the bed. The downward propagation rate of the macroscopic ignition front is controlled by several factors, such as biofuel moisture content, primary air rate and air temperature [33]. The temperature of the macroscopic propagating char combustion zone is around 1000-1200°C in batch-bed combustion of solid biofuels [38,41]. 

Low primary air rate of 6%-12% according to kiln and fuel requirements... 

With a favourably designed kiln burner the following primary air rates will be required ... 

Most of the commercial gas—air premixed burners are basically laminar-dow Bunsen burners and operate at atmospheric pressure. This means that the primary air is induced from the atmosphere by the fuel dow with which it mixes in the burner passage leading to the burner ports, where the mixture is ignited and the dame stabilized. The induced air dow is determined by the fuel dow through momentum exchange and by the position of a shutter or throtde at the air inlet. Hence, the air dow is a function of the fuel velocity as it issues from the orifice or nozzle, or of the fuel supply pressure at the orifice. With a fixed fuel dow rate, the equivalence ratio is adjusted by the shutter, and the resulting induced air dow also determines the total mixture dow rate. 

The Socony Vacuum design consisted of separate vessels for reaction and regeneration. Units constructed in the late 1940s employed a pneumatic lift design which allowed for high catalyst circulation rates. A typical design is shown in Figure 20, which allowed for a primary air stream to convey the catalyst. A... 

Air Flow - The capacity of a multijet flare to induce air flow must be calculated, to make sure that it is adequate to meet the maximum air flow requirement for smokeless combustion. (W, of Equation 4 below must be > W, of Equation 5). The term air flow capacity refers to the primary air flow rate which will be induced around each jet, and may be estimated from the following equation ... 

The primary air flow rate per jet necessary for smokeless combustion depends on the molecular weight and degree of unsaturation of the flare gas. Experience indicates that it varies linearly with percent unsaturates, from a minimum of 20 % excess air for a flare gas containing 0 % unsaturates to 35 % excess air for a gas containing 67 mol % unsaturates. Based on this relationship and a gas flow rate of 72.2 mVh per jet, the required primary air flow rate can be computed directly from the gas composition, or approximated conservatively from the following equation ... 

The following example provides some indication of the effects of induction. Primary and secondary air are mixed respectively at the rates of 0.5 m s , the primary air velocity at outlet being 5.0 m s , with the secondary air velocity assumed to be zero. 

Constant flow An assembly within which the primary airflow rate is modulated and mixed with air induced from the surroundings by means of a fan (also known as series type). 

Induction supply ATD An air terminal device in which the primary air from the duct induces secondary airflow from the treated space in such a way that a high rate of mixing between the air from these two sources takes place within the device. 

Induction terminal unit An air terminal assembly which by virtue of the configuration of the primary air inlet(s) within the unit can induce secondary air from the surrounding atmosphere before being discharged to the treated space. The tlow rate of the primary air may or may not be variable. The inlet aperture(s) for the secondary air may be fixed or adjustable by means of manual remote control. 

Primary airflow rate The mass or volume of air entering a supply air terminal device in unit time from an upstream duct or a plenum box. Or the air leaving through an opening and entering a space. 

Example 28.4 Primary air and chilled water Eor the same application, primary air reaches induction units at the rate of 0.4 kg/s and at conditions of 13°C dry hulh and 72% saturation. Chilled water enters the coils at 12°C and leaves at 16°C. What will he the room condition and how much water will he used ... 

Pulverized fuel coal burners (typically turbulent air burners, vertical burners, or nozzle burners) receive hot primary air containing the PF and introduce the mixture to secondary air in such a way that it provides a stable flame. The flow rates of both primary and secondary air are controlled by dampers. An ignitor is required to initiate combustion, and the flame front is maintained close to the burner, with the heat of combustion used to ignite incoming PF. A flame safety device electronically scans the flame and initiates corrective action if required. 

The transport of heat to the conversion zone controls the conversion rate in regime n. Regime II exists in the mid range of the volume flux of primary air and is characterised by a conversion zone without extension and an off-gas with relatively high contents of combustibles. Regime II is a consequence of macroscopic conversion front rates being equal to the overall conversion rate. Consequently, the conversion zone has no thickness and no distinct bed process structure. 

Figure 18 displays mass flux curves plotted against time. This particular selection of curves shows the difference in conversion gas rates with respect to wood fuel. Wood chips are significantly easier to convert than 6 mm wood pellets, which in turn have higher mass flux of conversion gas than fuel wood for a given volume flux of primary air through the conversion system. 

Figure 22 displays the time-integrated mean of mass flux as function of standard volume flux of primary air for all the three wood fuels, respectively. As indicated by Figure 22, the time-integrated mean of mass flux of conversion gas exhibits a hyperbolic relationship with the volume flux of primary air. In the low range of volume fluxes the conversion gas rate increases up to a maximum. After the maximum point is passed, the mass flux of conversion gas decreases due to convective cooling of the conversion reaction. 

As been demonstrated by the graphs above, the fuel wood is significantly more difficult to thermochemically convert than wood pellets and wood chips, at a given volume flux of primary air and for the conversion concept studied. The relatively low conversion gas rates should be one of the most crucial factors in the explanation of... 

The main reason why fuel wood is more difficult to combust than wood pellets or wood chips is that fuel wood displays significantly lower conversion gas rates than wood pellets and wood chips for a given conversion system and range of volume flux of primary air. The conversion rate is positively coupled to the... 

Thermocouples were placed inside the fuel bed, to be able to follow the temperature history, which contained a great deal of information about the thermochemical processes occurring in the bed. The sample ports were positioned at 45, 145 and 245 mm above the grate. These ports could also be used for gas sample probes. The primary air could be preheated up to 200°C and the air flow rate could be varied between 150-1500 1/min. 

The underfired combustion mode corresponded to the operation of an updraft gasifier. The tests carried out on wet wood chips and peat lumps (35-60%) showed that the combustion rates are many times higher than the same conditions for the overfired mode. Koistinen et al were able to gasify wood chips with a moisture content of 58% d.b. However, the off-gas was so humid that it was not ignitable until the end of each run when the drying had ceased. Koistinen et al concluded that the combustion rate increased linearly with an increased primary air flow rate. 

The primary air flow rate and moisture content of the fuel was given before each combustion run. Flue gas samples were taken continuously and the bed temperature was monitored on-line. The bed weight and mass loss rate were also continuously measured. 

The primary objective of Gort s experimental work [10] was to study the effect of volume flow of primary air on ignition front rate and combustion rate. Another aim was to study the dependency of ignition front rate and combustion rates on moisture content, volatile content, and particle size. 

However, it is possible to directly or indirectly measure the mass flux (mass flow) of conversion gas. Several authors have measured the mass loss of the fuel bed as function of primary air velocities and biofuel [12,33,38,53] by means of a balance. Most of them have used the over-fired batch conversion concept. They utilise the relationship illustrated by Eq. 16 (formulised in amounts instead of flows) above and the assumption that no ash is entrained. As a consequence, the mass loss of the batch bed with time equals the conversion gas. In the simple three-step model [3], an assumption of steady state is made, which is not relevant for batch studies. If it is practically possible, the method of using a balance to measure the conversion gas rate is especially appropriate for transient processes, that is, batch processes. 

Regime I prevails at low volume fluxes of primary air. The overall conversion rate is controlled by the diffusion rate of oxygen to the oxidation of the solid char. Regime I is characterized by the fact that the overall conversion rate is slower than the macroscopic ignition rate, which implies that solid fuel convertibles are accumulated behind the macroscopic ignition front. In other words, the conversion zone is growing thicker. [9,33,40]... 

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