Lower heating values

The volatiles contents of product chars decreased from ca 25—16% with temperature. Char (lower) heating values, on the other hand, increased from ca 26.75 MJ /kg (11,500 Btu/lb) to 29.5 MJ /kg (12,700 Btu/lb) with temperature. Chars in this range of heating values are suitable for boiler fuel apphcation and the low sulfur content (about equal to that of the starting coal) permits direct combustion. These char products, however, are pyrophoric and require special handling in storage and transportation systems. 

Seam thicknesses and depths vary tremendously. The most favorable deposits have shallow overburdens and thick seams that cover large areas. Acceptable stripping ratios, ie, overburden thickness to coal thickness, depend on the quaHty of the fuel. Ratios up to 10 1 have been used for bituminous coals, but lower ones are required for lignitic coals because of the lower heating value per unit weight. 

The energy consumption (lower heating value of the feedstock plus fuel) of the low pressure process has successively improved from over 38.3 GJ/1 when it was first introduced to 29.0—30.3 GJ/thy the mid-1990s. Natural gas-based reforming plants have advanced to the point where the scope for stiU further gains in efficiency is small and the gains costiy to obtain. 

The proximate analysis of charcoal is 20-25% volatile matter, 70—75% fixed carbon, and 5% ash. Charcoal briquets have lower heating values than charcoal lumps, because of additives in the briquets. The higher heating value of lump charcoal is - 28 kJ/kg (12,000 Btu/lb). The higher heating value of briquets is 23 to 25 kj/kg ( -- 9,900-10,800 Btu/lb). 

If the substitute fuel is of the same general type, eg, propane for methane, the problem reduces to control of the primary equivalence ratio. For nonaspiring burners, ie, those in which the air and fuel suppHes are essentially independent, it is further reduced to control of the fuel dow, since the air dow usually constitutes most of the mass dow and this is fixed. For a given fuel supply pressure and fixed dow resistance of the feed system, the volume dow rate of the fuel is inversely proportional to. ypJ. The same total heat input rate or enthalpy dow to the dame simply requires satisfactory reproduction of the product of the lower heating value of the fuel and its dow rate, so that WI = l- / remains the same. WI is the Wobbe Index of the fuel gas, and... 

Figure 27-11 gives the theoretical air requirements for a variety of combustible materials on the basis of fuel higher heating value (HHV). If only the fuel lower heating value is known, the HHVean be calculated from Eq. (27-6). If the ultimate analysis is known, Eq. (27-7) can be used to determine HHV. 

Lower Heating Value. The lower heating value of the gas is one in whieh the H2O in the produets has not eondensed. The lower heating value is equal to the higher heating value minus the latent heat of the eondensed water vapor. 

Fuel lower heating value. In many plants, on-line fuel analyzers have been introdueed to not only monitor the turbine performanee but to also ealeulate the fuel payments, whieh are usually based on the energy eontent of the fuel. 

Selective solubilization of materials of lower heating value in the thermal conditioning process. 

Height of flame center above flare tip, m h = Height of flare tip above grade, m F = Fraction of heat release radiated from the flame m = Mass flaring rate, kg/s H = Lower heating value of the flare gas, MJ/kg r = Relative humidity, percent The following are the calculation steps ... 

Many preliminary analyses of gas turbines are based on the assumption of a closed air standard cyclic plant, and for such analyses the use of tj as a thermal efficiency is entirely correct (as discussed in the early part of Chapter 3 of this book). But most practical gas turbines are of the open type and the rational efficiency should strictly be used, or at least its approximate form, the arbitrary overall efficiency tjq. We have followed this practice in the latter part of Chapter 3 and subsequent chapters even though some engineers consider this differentiation to be a somewhat pedantic point and many authors refer to tjo as a thermal efficiency (or sometimes the lower heating value thermal efficiency ). 

In electrical power stations a new measure of the performance is the amount of CO2 produced per unit of electricity generated, i.e. A = kg(C02)/kWh this quantity can be non-dimensionalised by writing A = A( 16/44)(LCV) where (16/44) is the mass ratio of fuel to CO2 for methane and (LCV) in its lower heating value. However, presenting the plant s green performance in terms of A directly allows the cost of any tax on the carbon dioxide to be added to the untaxed cost of electricity production most easily. 

Another objective of gas processing is to lower the Btu content of the gas by extracting heavier components to meet a maximum allowable heating limit set by a gas sales contract. If the gas is too rich in heavier components, the gas will not work properly in burners that are designed for lower heating values. A common maximum limit is 1100 Btu per SCF. Thus, if the gas is rich in propane and heavier components it may have to be processed to lower the heating value, even in cases where it may not be economical to do so. 

Enthalpy of reaction and standard entlialpy of reaction are not always employed in engineering reaction/combustioii calculations. The two other terms tliat hai C been used are tlie gross (or liighcr) heating value and tlie net (or lower) heating value. These arc discussed later in this Section. 

Heat Value of Fuel Lower heating value... 

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