Combustion efficiencies

Products of Incomplete Combustion Emission Limits. Products of iacomplete combustion typically are not directly measured duting the trial bum. Instead, levels of carbon monoxide (qv) emissions are used as an iadication of combustion efficiency. High combustion efficiencies are assumed to result ia acceptable levels of products of incomplete combustion. If carbon monoxide emissions are measured at less than 100 ppmv dry basis, the standard is met. However, if emissions are greater than 100 ppmv, no more than 20 ppmv of total hydrocarbons (qv) are allowed at the iaciaerator stack duting the trial bum. 

Because of the wide variation in composition and properties of brown coal (see Table 3), efficient combustion of these fuels caimot be accomphshed by a single system. The moisture content limits combustion efficiency because some chemical energy is required to convert Hquid water to steam in the flue gases. The steam then increases the dew point of the gases, requiring higher temperatures to avoid condensation in the stack. For fuels up to 25% moisture content, 80% efficiency can be achieved. As the moisture content increases to 60%, the efficiency decreases to 70% and efficiency continues to decline about another 1% for each additional 1% moisture to 70%. 

Another furnace that does not require fuel preparation is the stoker boiler, which was used by New York State Electric Gas Corporation (NYSEG) in its TDE tests. At NYSEG, the stoker boiler, which has a 1649°C (3000°E) flame temperature (as does the cyclone boiler), has routinely blended low quaUty coal, and more recently, wood chips with its standard coal to reduce fuel costs and improve combustion efficiency. In the tire-chip tests, NYSEG burned approximately 1100 t of tire chips (smaller than 5x5 cm) mixed with coal and monitored the emissions. The company determined that the emissions were similar to those from burning coal alone. In a second test-bum of 1900 t of TDE, magnetic separation equipment removed metal from the resulting ash, so that it could be recycled as a winter traction agent for roadways. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

PressurizedFIuidized-Bed Combustors. By 1983 the pressurized fluidized-bed combustor (PFBC) had been demonstrated to have capacities up to 80 MWt (49). PFBCs operate at pressures of up to 1500 kPa (220 psi) and fluidization velocities of 1—2 m/s. Compared to an AFBC of the same capacity, a PFBC is smaller, exhibits higher combustion efficiencies with less elutfiation of fine particles, and utilizes dolomite, CaCO MgCO, rather than limestone to capture SO2. 

Fluidized combustion of coal entails the burning of coal particles in a hot fluidized bed of noncombustible particles, usually a mixture of ash and limestone. Once the coal is fed into the bed it is rapidly dispersed throughout the bed as it bums. The bed temperature is controUed by means of heat exchanger tubes. Elutriation is responsible for the removal of the smallest soHd particles and the larger soHd particles are removed through bed drain pipes. To increase combustion efficiency the particles elutriated from the bed are coUected in a cyclone and are either re-injected into the main bed or burned in a separate bed operated at lower fluidizing velocity and higher temperature. 

Soot. Emitted smoke from clean (ash-free) fuels consists of unoxidized and aggregated particles of soot, sometimes referred to as carbon though it is actually a hydrocarbon. Typically, the particles are of submicrometer size and are initially formed by pyrolysis or partial oxidation of hydrocarbons in very rich but hot regions of hydrocarbon flames conditions that cause smoke will usually also tend to produce unbumed hydrocarbons with thek potential contribution to smog formation. Both maybe objectionable, though for different reasons, at concentrations equivalent to only 0.01—0.1% of the initial fuel. Although thek effect on combustion efficiency would be negligible at these levels, it is nevertheless important to reduce such emissions. 

In AFBC units, heat is removed from the flue gas by a convection-pass tube bank. The particulates leaving the boiler with the flue gas consist of unreacted and spent sorbent, unburned carbon, and ash. Multiclones after the convection pass remove much of the particulate matter and recvcle it to the combustor, increasing the in-furnace residence time an improving combustion efficiency and sulfur retention performance. Bubbling PFBC units do not have convection-pass tube banks and do not recycle solids to the boiler. 

Give an example of how opacity monitoring of a coal-fired boiler could be used to improve combustion efficiency. 

Measures such as improved process design, operation, maintenance, housekeeping, and other management practices can reduce emissions. By improving combustion efficiency, the amount of products of incomplete combustion (PlCs), a component of particulate matter, can be significantly reduced. Proper fuel-firing practices and... 

When primary fume capture is performed by the enclosure, furnace off-gas combustion efficiency is lower than experienced by furnace direct evacuation control. The off-gas, rich in carbon monoxide (CO), rises from furnace roof openings and partially burns and cools with enclosure air. Significant levels of CO have resulted in the enclosures and exhaust ducting from this type of combination. These levels are not explosive but present a potential hazard to personnel working in the enclosure or in downstream fume cleaning equipment. 

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 ... 

The modem FB boiler is a compact economical design (typically more compact than SM boilers) and provides minimum combustion efficiencies of 80% for gas-fired and 83% for oil-fired models. They usually are small units, seldom exceeding 150 hp, and operate on a variety of fuels at typically 4 to 5 sq ft of surface area per hp. 

Checks to the air and flue gas system include visually inspecting the furnace and periodically monitoring all fans, levels of draft, furnace pressure, excess air demands, and combustion efficiency. 

Typically, online cleaning must be carried out fairly slowly and carefully, so that the programs may take 3 to 6 months, perhaps longer, before particularly satisfactory results are achieved. If the boiler is particularly dirty before an online clean is considered necessary, then the combustion efficiency will be lower than desirable. It consequently will take some time before this reduced efficiency improves significantly, and therefore there is an additional fuel cost that must be considered, as well as the cost of the online cleaning program. 

Fuel treatments have been used for very many years as an aid to improving the combustion efficiency process. Old formulations often used saw dust, wood flour, common salt, zinc sludge, ground oyster shell, and similar crude ingredients, but could still provide a dramatic effect when thrown into a fire. The metallic salts present (sodium in salt, zinc in sludge, and calcium in shell) acted as catalysts that dramatically lowered the ignition temperature of soot deposits from around 1100 °F/590 °C to only 600 °C/315 °C the fire burned vigorously and the soot disappeared. 

Furnace area and superheater slagging may occur at low furnace or superheater temperatures (below 450-500 °C) due to high vanadium content in fuel oil. These high levels of vanadium in the fuel reduce the eutectic temperature of the noncombustibles, creating a molten deposit that holds unbumed carbon and contributes to a thickening of the slag. The trapped carbon is unavailable for combustion, and this process consequently reduces the overall fuel combustion efficiency. 

NOTE Where the primary problem is to improve combustion efficiency, the product typically is added to the fuel handling system via an automatic feeder and used continuously at a rate of 0.5 and 1.5 lb per ton of fuel. 

One of several different types of flue-gas analysis equipment (such as electronic, Fyrite, or Or sat types). They are used to determine boiler fuel combustion efficiency. 

In the complicated reaction networks involved in fuel decomposition and oxidation, intermediate species indicate the presence of different pathways that may be important under specific combustion conditions. While the final products of hydrocarbon/air or oxygenate/air combustion, commonly water and carbon dioxide, are of increasing importance with respect to combustion efficiency—with the perception of carbon dioxide as a... 

Santos V, Morao A, Pacheco Ml, Cirfaco L, Lopes A (2008) Electrochemical degradation of azo dyes on BDD effect of chemical structure and operating conditions on the combustion efficiency. J Environ Eng Manage 18(3) 193-204... 

The result C02 emission from combustion process is shown in Table 11, assuming that average combustion efficiency in all cases is 97.5% [Hu(raw) > 10 MJ/kg RDF],... 

Hydrogen is highly flammable over a wide range of temperature and concentration. Although its combustion efficiency is truly outstanding and welcomed as a fuel of the choice for the future, it inevitably renders several nontrivial technological challenges, such as... 

---