Combustion, optimum

Using many small burners to utilize the whole wall area is a way to achieve good temperature uniformity. (See figs. 3.4 and 3.5, and sec. 7.4.) There are large burners that can hold the burner wall as hot as the point of conventional maximum heat release. These adjustable thermal profile burners (fig. 6.1) can automatically hold a desired temperature profile by controlling the spin of the products of combustion. Optimum use of furnace space and overall refractory wall radiation usually favors the hottest possible burner wall (maximum flame spin, minimum flame length). In... 

For optimum combustion, the fuel should vaporize rapidly and mix intimately with the air. Even though the design of the injection system and combustion chamber play a very important role, properties such as volatility, surface tension, and fuel viscosity also affect the quality of atomization and penetration of the fuel. These considerations justify setting specifications for the density (between 0.775 and 0.840 kg/1), the distillation curve (greater than 10% distilled at 204°C, end point less than 288°C) and the kinematic viscosity (less than 8 mm /s at -20°C). 

Control Devices. Control devices have advanced from manual control to sophisticated computet-assisted operation. Radiation pyrometers in conjunction with thermocouples monitor furnace temperatures at several locations (see Temperature measurement). Batch tilting is usually automatically controlled. Combustion air and fuel are metered and controlled for optimum efficiency. For regeneration-type units, furnace reversal also operates on a timed program. Data acquisition and digital display of operating parameters are part of a supervisory control system. The grouping of display information at the control center is typical of modem furnaces. 

Carbon. Most of the Earth s supply of carbon is stored in carbonate rocks in the Hthosphere. Normally the circulation rate for Hthospheric carbon is slow compared with that of carbon between the atmosphere and biosphere. The carbon cycle has received much attention in recent years as a result of research into the possible relation between increased atmospheric carbon dioxide concentration, most of which is produced by combustion of fossil fuel, and the "greenhouse effect," or global warming. Extensive research has been done on the rate at which carbon dioxide might be converted to cellulose and other photosyntheticaHy produced organic compounds by various forms of natural and cultivated plants. Estimates also have been made of the rate at which carbon dioxide is released to soil under optimum conditions by various kinds of plant cover, such as temperature-zone deciduous forests, cultivated farm crops, prairie grassland, and desert vegetation. 

In general, the following steps are used to ensure optimum regeneration and catalyst performance recovery alow temperature first pass to remove low boiling point hydrocarbons and other volatile matter, an initial combustion step to remove a portion of the sulfur and carbon, and a final combustion step to remove the remaining carbon to the target level. 

Metals in the platinum family are recognized for their ability to promote combustion at lowtemperatures. Other catalysts include various oxides of copper, chromium, vanadium, nickel, and cobalt. These catalysts are subject to poisoning, particularly from halogens, halogen and sulfur compounds, zinc, arsenic, lead, mercury, and particulates. It is therefore important that catalyst surfaces be clean and active to ensure optimum performance. 

A high-nickel alloy is used for increased strength at elevated temperature, and a chromium content in excess of 20% is desired for corrosion resistance. An optimum composition to satisfy the interaction of stress, temperature, and corrosion has not been developed. The rate of corrosion is directly related to alloy composition, stress level, and environment. The corrosive atmosphere contains chloride salts, vanadium, sulfides, and particulate matter. Other combustion products, such as NO, CO, CO2, also contribute to the corrosion mechanism. The atmosphere changes with the type of fuel used. Fuels, such as natural gas, diesel 2, naphtha, butane, propane, methane, and fossil fuels, will produce different combustion products that affect the corrosion mechanism in different ways. 

It has been emphasised in the earlier chapters that the thermal efficiency of the gas turbine increases with its maximum nominal temperature, which was denoted as T. Within limits this statement is true for all gas turbine-based cycles and can be sustained, although not indefinitely, as long as the optimum pressure ratio is selected for any value of Ty, further the specific power increases with T. However, in practice higher maximum temperature requires improved combustion technology, particularly if an increase in harmful emissions such as NO is to be avoided. 

Fig. 5.2 shows that for the single-step cooled CBT plant at a given combustion temperature, the overall efficiency of the cooled gas turbine efficiency increases with pressure ratio initially but, compared with an uncooled cycle, reaches a maximum at a lower optimum pressure ratio. Fig. 5.3 shows that for a given pressure ratio the efficiency generally increases with the combustion temperature even though the required cooling fraction increases. 

Several techniques are available in the literature for evaluation of the flame temperature, exit temperature, equilibrium composition of combustion products, and performance parameters of energetic composites [11-13]. The optimum combination of the composite ingredients is determined by thermodynamic means, so as to arrive at a composition having maximum performance... 

The life and necessary maintenance of a gas turbine are heavily dependent upon both the operating regime and the fuel quality. Continuous firing on natural gas provides the optimum availability, which will be progressively eroded if the plant is subject to frequent inter-mptions (i.e. stops and starts) from both cold and hot conditions. With a maximum interval between inspections of some 8000 hours, it may be anticipated that the combustion section will require most attention. Every 16,000 hours (or less) the turbine section will need inspection. While a major inspection of the entire unit will be necessary every 31,500 hours. Under optimal conditions, the... 

Combustion equipment can be set to give optimum efficiency at the time of commissioning but this condition will not be maintained. Wear and tear on control valves, partial blockage of filters, sooting of surfaces, etc. will all cause a fall in efficiency. To counter this, regular maintenance is desirable, and must include routine flue analysis and burner adjustment. 

Reference to Figure 19.1 shows how efficiency can be adversely affected by deviation from the optimum air/gas ratio. By maintaining combustion close to stoichiometric, efficiency will be improved, but the practical limitations of burners discussed above must be noted. 

Carbon monoxide is usually sampled as the second parameter in conjunction with carbon dioxide or oxygen. In theory, as the optimum is usually to have near-stoichiometric combustion without CO breakthrough it is the most reliable gas to sample. A problem is that although small quantities of CO usually indicate the need for additional air, they can also be caused by flame chilling and careful interpretation of results is needed. 

Most combustion equipment is not controlled by means of a feedback from flue gas analysis but is preset at the time of commissioning and preferably checked and reset at intervals as part of a planned maintenance schedule. It is difficult to set the burner for optimum efficiency at all firing rates and some compromise is necessary, depending on the control valves used and the control mode (e.g. on/off, fully modulating, etc.). 

Combustion equipment, when first commissioned, can be set to operate at its optimum efficiency. With time, however, there will be a deterioration due to blockage of air filters and breather holes, wear in valve linkages, etc. Such changes may have safety implications if gas-rich firing is a consequence. 

Combustion controls such as oxygen trim help to maintain optimum operating conditions, especially on gaseous fuels. Instrumentation can give continuous visual and recorded information of selected boiler and plant functions. To be effective, it must be maintained and the data assessed and any required action taken before the information is stored. 

Boiler plants are a major user of energy. The combustion efficiency of a boiler plant can easily be set at the optimum, and Table 30.2 suggests the parameters for this for various fossil fuels ... 

Andreev Chulko (Ref 7) claim that the pressure above which PETN combustion becomes unsteady and accelerates decreases with increasing PETN particle size. Other studies suggest that there is an optimum particle size for DDT (see Sect VII of Propellants, Solid in this Vol). In a recent study, Bernecker et al (Ref 16) found that at a given degree of compaction, 20-micron Tetryl had a longer run-up to detonation, 12, than 470-micron Tetryl. This is shown in Fig 3... 

The detector is based on the combustion of sulfur-containing compounds in a hydrogen rich air fleuie of a FID to form sulfur monoxide. The hydrogen/air flow rate ratio is the most critical parameter controlling the production of sulfur monoxide. Under optimum conditions sulfur monoxide may account for up to 20% of the sulfur species in the flame. Sulfur monoxide is a free radical and a very reactive species that is short lived however, it can be stabilized in a vacuum, and a ceramic probe under reduced pressure can be used to sample it in the flame and transfer it to... 

Thus, the technique can become counterproductive. A typical arrangement for selective non-catalytic reduction is shown in Figure 25.30. Aqueous ammonia is vaporized and mixed with a carrier gas (low-pressure steam or compressed air) and injected into nozzles located in the combustion device for optimum temperature and residence time10. NO, reduction of up to 75% can be achieved. However, slippage of excess ammonia must be controlled carefully. 

It will be clear from the above that the optimum types of oxidising materials are those of highest density and dense forms of ammonium nitrate are always used. The combustibles can be dense also, although it is sometimes necessary to add at least a proportion of the combustible in an absorbent form to ensure adequate sensitiveness. Wheat flour may be regarded as typical of a dense combustible woodmeal is a useful and cheap combustible of intermediate properties. 

The next major improvement was the development of combustion modified PU foam. The original CMHR polyurethane foam was developed in the USA (26) and contained hydrated alumina and halogenated flame retardants but was made in a single operation. It was used in institutions, public buildings, hotels etc. but its high density and less than optimum physical properties... 

Another possibility to adjust the combustion air flow to the supplied fuel gas flow is to maintain a constant level of C02-content in the flue gas. Thus, a stoichiometric value can be adjusted for any type of fuel gas, which varies only slightly from the optimum value. 

As indicated in Fig. 3.21, the ionization current shows a maximum near the stoichiometric level of combustion. By introducing a set point for the ionization current the excess air can be adjusted in order to achieve optimum combustion conditions. 

---