Over fire air

The control of NO from stationary sources includes techniques of modification of the combustion stage (primary measures) and treatment of the effluent gases (secondary measures). The use oflow-temperature NO,.burners, over fire air (OFA), fiue gas recirculation, fuel reburning, staged combustion and water or steam injection are examples of primary measures they are preliminarily attempted, extensively applied and guarantee NO reduction levels of the order of 50% and more. However, they typically do not fit the most stringent emission standards so that secondary measures or flue gas treatment methods must also be applied. 

Model-based boiler optimization schemes have proved successful in many power plant and industrial boiler applications. Successful NOx reduction through this kind of optimization can avoid or postpone large capital expenditures for low NOx burners, over-fire air modifications, and selective catalytic reduction/selective noncatalytic reduction (SCR/SNCR). 

Above bed over-fire air is strategically mixed with the volatiles, which consist primarily of CO, COj, CH4, HjO, Nj and tars. Combustion of the volatiles proceeds, the tars are burned out, and heat is transferred to the boiler walls and convective tubes. Reactions above the bed occur first in a reducing environment, and then excess air is added gradually to the upper combustion zone. 

Combustion tests were conducted at the Bay Front Unit No. 3 of Northern States Power Co. in Ashland, WI, The coal-fired underfeed stoker with boiler rated at 45,400 kg/h steam at 5.1 MPa drum pressure and 455 C was modified to receive 4.6 m long whole tree sections from a charge chamber by means of a ram feeder. A higher pressure over-fire air system was added. The tests successfully demonstrated the feasibility of replacing coal with logs on a grate without a loss in boiler performance. 

It is shown that different over-fire secondary air supply leads to diffetent pollutant emissions at the outlet. Two different operating conditions have been considered to demonstrate this, (a) In Case I, 24% of the over-fire air was injected at zone 6, 19% at zones 7 and 8, and 32% at zone 9 (b) In Case 2, more over-fire air was injected at zone 6, (30%), while less air at zones 7 and 8, (15%). The under fire air and the third level or air jet (zone 9) was kept constant. 

The prediction of case 2. i.e. the effect of modifying the over-fire excess air is presented in Table 1. Although the air supply in the two cases provide roughly the same amount of air to the furnace, the CO emissions are fairly different, as found from the experiment. Air supply conditions in case 2 yields less effective combustion with higher emissions of CO. Modifying the mass flow of the over-fire air jets can significantly change the residence time of species in the ftimace. In case 2, the flow is accelerated by the first level of air Jets above the bed, while the intensity of the air curtain horn the second level of air jets has been decreased by 20%. 

The combustion process of wet wood chips and formation of pollutants in a biomass furnace have been investigated. Distributions of species CO, UHC, O2 where calculated numerically and compared to experimental data. It is shown that char, as flying particle, though in small amount has a significant influence on the CO emissions at the outlet. Numerical simulation indicates that half of the CO emission at the outlet is due to the combustion of flying char particles at the upper part of the furnace. Over-fire air staging has a significant influence on the residence time of panicles and gas species in the furnace, and thus the conversion of fuel and intermediate species to final products. 

From geological studies to aerospace engineering, physical modeling has been widely used in the industry to study complex fluid dynamics where engineering calculations or computational fluid dynamics are deemed either unreliable (the former) or uneconomical (the latter). In the field of combustion, physical modeling is employed in studying flow distribution involving combustion air, over-fire air (OFA), and flue gas recirculation (FGR) as well as isothermal flows in combustion chambers of furnaces, boilers, heat recovery and steam... 

Figure 2 Cost of NOx control for 500 MW base load oil and gas-fired utility boilers (BOOS Bumers-Out-Of-Service, OFA Over-Fired Air, LNB Low NOx Burners, FOR Flue Gas Recirculation, SCR Selective Catalytic Reduction) [adapted from ref. 7].

NOx emissions may be controlled by primary or secondary measures. Primary measures are aimed at reducing the formation of NOx. Examples of primary measures include fuel switching (e.g., moving from coal to oil or to gas) and in-combustion modifications. Examples of in-combustion modification such as Lx)w NOx Burners (LNB), Over-Fired Air (OFA), Bumers-Out-Of-Service (BOOS), Flue Gas Recirculation (FGR), etc. have been reviewed in the literature, e.g., see [10]. Secondary measures reduce NOx after it is formed. An example. Selective Non-Catalytic Reduction (SNCR), reduces NOx via ammonia injection at temperatures of between 1500 and 1700°F [11]. The use of reductants other than ammonia, such as urea [12] and cyanuric acid [13], has also been discussed. 

The environmental impact of nitrogen oxides has focused attention on emissions regulations in many countries in recent years. The NOx emission limits imposed by German law cannot be achieved by simply applying primary measures such as staged combustion, over-fire air, etc. this makes it necessary to apply secondary measures. Up to now, selective catalytic reduction (SCR) has dominated over other combustion control technologies. 

The conversion system is of batch type, over-fired, updraft air, and has a maximum capacity of 300 kWt. The methodology, described in Paper II, is based on the assumption of a steady-state conversion process, which is not the case for a batch reactor. Consequently, main assumption one above needs to be reconsidered and modified despite the fact that a batch conversion system is studied, which implies unsteady conditions, the process is assumed to be quasi-steady that is, the rate of change in the process variables in the range of interest is assumed to be slow compared with the response rate of the measurement system. 

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. 

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]. 

It is shown that different over-fire secondary air supply leads to different CO emissions at the outlet. The emissions of CO can be reduced thiough controlling the secondary air supply. Char formed in the bed is low in terms of its influence on the heat release, however it has significant influence on the CO distribution in the upper part of the furnace and at the outlet... 

This work is aimed at exploration of pollutant emissions (e.g. CO, UHC) in a 40 MW fixed-bed furnace burning wet wood chips, using both experimental measurements and theoretical simulations. The following issues are emphasized (1) the influence of turbulent flow motion, namely the over-fire secondary air supply on CO formations and (2) the influence of char burnout on CO formations. 

In order to decompose heat into fire air and the inflammable substance ( ), Scheele recalled that nitric acid has a great attraction for phlogiston, forming red fumes. He heated saltpetre in an 8 oz. glass retort with oil of vitriol (Fig. 20). At first the nitric acid went over red, then it became colourless, and finally... 

In 1779 Scheele published a supplement to his book on Air and Fire in which he records experiments made in 1778 on the action of a mixture of iron filings and sulphur supported on a stand in air confined over water (or, in very cold weather, brandy) in ajar graduated on the lower part by a varnished label. He concluded that our atmosphere must always contain, though sometimes with a little variation, the same quantity of fire air or pure air, viz. nine thirty-thirds . This (27 per cent.) is a more accurate result than the one-third found in 1777. 

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