Calorific values, gaseous fuels

Hoppesteyn P.D.J, (1999) Application of Low Calorific Value Gaseous Fuels in Gas Turbine Combustors. DPhil thesis, Technical University Delft. 

In the early experiments that were made with water-gas mixtures in attempts to synthesize valuable organic liquids, methane apparently was the only product obtained until the effect of pressure was understood, after which the progress toward methanol synthesis was rapid. Aside from the work that had to do quite largely with the production of methane for the enrichment of water-gas or other low calorific value gaseous mixtures in countries without cheap oil, considerable attention has been paid to a study of the effects of various catalysts on the synthesis from the viewpoint of perfecting the methanol and higher alcohol syntheses. Processes have, at the same time, been proposed for the synthesis of liquid fuels from water-gas mixtures in an attempt to produce motor fuels from cheap coal. 

The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight (or volume in the case of a sample of gaseous fuels) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 °K. Net calorific value assumes the water leaves with the combustion products without fully being condensed. Fuels should be compared based on the net calorific value (Table 2.55). The calorific value of coal varies considerably depending on the ash, moisture content and the type of coal, while calorific values of fuel oils are much more consistent. Table 2.55 shows that oil and gas have about 30% less specific CO2 emission than coal, based either on carbon content or energy output the only reason for this difference is the water content of the fuel and the energy loss due to water evaporation. Water condensation and heat recovery seems a way to increase the net efficiency and hence reduce emissions. 

All these fuel gases contain more than 50 % hydrogen and 10-30% methane, the other main components being CO, higher hydrocarbons, CO2 and Nj. In many parts of the world natural gas of calorific value of approximately 38MJ/m has become the widely-used gaseous fuel. 

Owing to the differing combustion characteristics and calorific values of the gaseous fuels which are commonly available [natural gas, liquefied petroleum (bottled) gas], slight variations in dimensions, including jet size and aeration controls, are necessary for maximum efficiency it is essential that, unless the burner is of the All Gases type which can be adjusted, the burner should be the one intended for the available gas supply. 

The bomb calorimeter provides the most suitable and accurate apparatus for determination of the calorific values of solid and liquid fuels. Since the combustion takes place in a closed system, heat transfer from the calorimeter to the water is complete, and since the reaction is one between the fuel and gaseous oxygen, no corrections are necessary for the heat absorbed during the reduction of the oxidizing agent. In addition, the losses due to radiation can be reduced to comparatively small quantities, and more important, can be determined with a considerable degree of accuracy. Corrections due to the heat evolved in the formation of nitric and sulfuric acids under the conditions existing in the bomb can be determined accurately. 

Gross calorific value (gross heat of combustion at constant volume) heat produced by combustion of a unit quantity of a solid or liquid fuel when burned at constant volume in an oxygen bomb calorimeter under specified conditions, with the resulting water condensed to a liquid not applied to gaseous fuels and applies to a volatile liquid fuel only if it is suitably contained during the measurement closely related to the internal energy of combustion for the same reaction at constant standard temperature and pressure. 

The heat flow rate (Q) of a gaseous fuel is calculated as the product of its volumetric flow rate at standard conditions (V0) and its calorific value (CV). The Wobbe index (WI) measures the ratio between the net CV and the square root of specific gravity (SG). With orifice-type flow sensors, the advantage of detecting the WI is that it eliminates the need to separately measure the specific gravity this is because the product of the WI and orifice pressure drop results in a constant times the heat flow rate (KxQ), without requiring a separate measurement of SG. 

Particular attention should be paid to the fact that standards contain various formulas for computations and calculations, conversion factors, and correction factors. For example, a method of test for calorific value of gaseous fuels by the water-flow calorimeter (ASTM D 900-46T) has tables showing corrections for reduction of observed barometric heights to standard conditions. These corrections include those of temperature, latitude, and altitude. A standard method of testing and tolerances for woolen yarns (ASTM 403-44) has formulas for computing yarn numbers and a yarn number conversion table. 

Determinations of the calorific value (C.V.) of many gaseous fuels, and of all solid fuels, are performed in constant volume calorimeters. That is to say, values of qp= AV are determined. When the fuel is burnt, however, at one atmosphere pressure, additional energy of expansion (+ve or -ve) against the atmosphere is involved, and the value of q actually realized, that is qp=AH, may be significantly different. We start with equation 2.7 ... 

The volume of gaseous fuels used can be measured accurately with turbine meters. However, to measure their mass, it is essential to apply temperature and pressure corrections to the measured volume. Changes have been observed of over 2 % in the calorific value of natural gas — sufficient to cause a significant change in the degree of burning. As a result, many operators have installed calorific value meters to detect any changes as they occur. 

American Society for Testing and Materials, Calculating Calorific Value and Specific Gravity (Relative Density) of Gaseous Fuels, D 3588-81 (1981). 

Methods of testing cement. Physical tests. Test for heat of hydration Specification for calorimeter bombs Guide for determination of calorific values of solid, liquid and gaseous fuels (including definitions)... 

Hv(Tb) and dy both can be estimated at combustion reference conditions using DIN 51857 1997-03 Gaseous fuels and other gases - Calculation of calorific value, density, relative density and Wobbe index of pure gases and gas mixtures [11], It is normal to give a Wobbe Index without units avoid any possible confusion with the volumetric heating value of the gas [12]. 

Analysis of gaseous fuels is necessary for quality control. The most important quality criteria are the calorific value (CV) and the sulfur (S) content. The CV is related to the quantity of heat energy, which is liberated when the fuels are bmrned the sulfur... 

There has been growing interest lately in alternative raw materials containing methane which is an essential component of gaseous fuels as natural gas. They have not been utilised yet because of low content of combustible components, mainly methane. The main condition of their utilisation as fuels would be the enrichment of these gases in methane up to high concentrations in methane, which would results in the calorific value corresponding to that of natural gas. 

For gaseous fuels the calorific value can be calculated from the volumetric percentages of the constituents (reckoned for standard conditions, i. e., at 0° C and 1013 mbar) ... 

Equilibrium is characterized by constancy of some properties but it is not the only requirement. When some of the properties of a system are constant but equilibrium does not exist, the system is said to be in a steady state. In the experimental determination of calorific value of a gaseous fuel by Boy s Gas Calorimeter, the following conditions characterise the steady state ... 

A fuel gas is a fuel In the gaseous stale. Ii contains some of the cumbustible gases like H, CO, CH, or other hydrocarbons, Small amounts of non-cumbiistiblc gases like CO), and O may also be present. Calorific value may vary from about 1000 k c d/in to 12000 k cal/m. ... 

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