Moisture content, fuel

The objectives of Koistinen et al s [7] research was to study the differences and similarities in the effects of the moisture content, fuel bed type and the particle size of different biofuels on the combustion rate, released grate effect and the combustion behaviour in general. 

Fluidized-bed combustion systems use a heated bed of sandlike material suspended (fluidized) within a rising column of turbulent air to burn many types and classes of waste fuels. The vendor claims that this technique results in improved combustion efficiency of high moisture content fuels and is adaptable to a variety of waste -type fuels. The scrubbing action of the bed material on the fuel particle is said to enhance the combustion process by stripping away the carbon dioxide and char layers that normally form around the fuel particle. This allows oxygen to reach the combustible material much more readily and increases the rate and efficiency of the combustion process. 

The fuel characteristics are provided in Table 2. There are significant variations in ash content and net calorific value due to the fuel nature and moisture content. Fuel chemical compositions are in the typical ranges for softwood Pinus radiata) and hardwood ( . nitens) (12). 

Based on a similar approach, a statistical analysis was conducted to determine whether correlations of CO and CH4 emissions could be established with a number of variables including fuel moisture content, fuel particle size distribution, fuel feed rate and excess air ratio. With the limited number of observations, the best model for CO was derived as a function of fuel moisture content (Fig, 1) for the combustor used for these studies. There was no evidence for correlation of the other variables. The measured CH4 contents also had an increasing trend with higher moisture contents of the fuels, but could not to be fitted in any correlation equation. 

Waste Wood Moisture Content Fuel Consumption... 

Fuel Specific gravity Bulk density, kg/m Moisture content, wt %... 

It is useful to examine the combustion process appHed to soHd wastes as fuels and sources of energy. AH soHd wastes are quite variable in composition, moisture content, and heating value. Consequendy, they typically are burned in systems such as grate-fired furnaces or duidized-bed boilers where significant fuel variabiUty can be tolerated. 

Where T)is flame temperature in K MC is moisture content of the waste, expressed on a total weight basis SR is defined as stoichiometric ratio or moles O2 avadable/moles O2 required for complete oxidation of the carbon, hydrogen, and sulfur in the fuel, ie, 1/SR = equivalence ratio and is temperature of the combustion air, expressed in K. In Fnglish units, this equation is as follows ... 

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%. 

In theory, whole-tree-energy plants have the potential to be more efficient than existing wood-fired generators, which are fueled by chipped wood with a relatively high moisture content (45%). The dried whole trees have a moisture content below 25%, and whole-tree plants potentially can be built to have a greater capacity and to employ high pressure, high temperature steam. 

The moisture content of peat or brown coal that is briquetted for fuel must be reduced to about 15% for satisfactory briquetting. Mechanical or natural means are used because of the cost of thermal drying. Moisture is sometimes desirable. About 8% is necessary for prevention of combustible loss from a chain-grate stoker. 

A direct diyer consumes more fuel per pound of water evaporated, the lower the final moisture content. 

An incinerator will usually have a fuel of varying chemical composition and physical properties, as well as varying moisture content and heating value. Also, the fuel fired in one locality may be vastly different from that fired by an incinerator of similar size and design in another locality. Refuse production in the United States has been estimated to average 2.5 kg per person per day in 1970, increasing to 10 kg per person per day by the year 2000. 

Proximate analysis - a relatively low-cost analysis in which moisture content, volatile combustible matter, fixed carbon, and ash are determined. The fuel value of the sludge is calculated as the weighted average of the fuel values of its individual components. 

When an incinerator is designed, the waste to be burned must be characterized for properties such as heat content (fuel value), percent moisture, chlorine content, metals content, and physical characteristics. The size and physical layout of the incinerator should be based on those waste properties. 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

Note The LHV to HHV ratio of 90% for methane (21,526/23,881 = 90 %) is typical of that for natural gas, while this ratio is roughly 94% for fuel oils. Common coals typically have a LHV to HHV ratio of 92 to 96% depending upon the hydrogen and moisture content. Typically, gas turbine based cycles are presented on an LHV basis. Conventional power plants, such as coal-, oil-, and gas-fired steam generator/steam turbine cycles are presented on an HHV basis within the U.S, and on an LHV basis in the rest of the world. 

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