Methane calorific value

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. 

A low calorific value gas, which includes nitrogen from air, could be produced for boiler or turbine use in electric power production, or an intermediate calorific value gas containing no nitrogen for an industrial fuel gas, or synthesis gas for chemical and methane production could be provided. This approach which has been studied in Russia, Europe, Japan, and the United States, is stiU noncommercial in part because it is not economically competitive. 

For SNG manufacture, it is necessary to reduce the residual hydrogen to a minimum in order to achieve a high calorific value. This is best realized if the synthesis gas, instead of having a stoichiometric composition, contains a surplus of C02 which can be utilized to reduce the H2 content by the C02 methanation reaction to less than 1% according to equilibrium conditions. The surplus C02 must be removed at the end of the process sequence. It is, of course, also possible to operate a methanation plant with synthesis gas of stoichiometric composition then there is no need for a final C02 removal system. The residual H2 content will be higher, and therefore the heating value will be lower (cf. the two long term runs in Table II). 

Worked Example 3.11 The wood mentioned in our title question is a complicated mixture of organic chemicals so, for simplicity, we update the scene. Rather than prehistoric men sitting around a fire, we consider the calorific value of methane in a modem central-heating system. Calculate the value of A Hc for methane at 25 °C using molar enthalpies of formation AH. ... 

If the subsequent stage is at a lower temperature, carbon oxides and hydrogen recombine to methane, increasing the calorific value. Following C02 removal, very little enrichment is required lo achieve a product fully interchangeable with natural gas. 

If the rich gas from the CRG reactor is passed over another bed of high-nickel catalyst at a lower temperature, the equilibrium of the five components is reestablished. Carbon oxides react with hydrogen to form methane and the calorific value of the gas is increased. It should be noted that this methanation step differs from that encountered in ammonia synthesis gas production because of the high steam content the temperature rise is reduced and there is no possibility of temperature runaway as the... 

Tabic 3 shows the effect of the second methanation stage on product calorific value. While consumption of LI G is minimized, the capital cost is increased and Ihe overall thermal efficiency is slightly reduced. 

The calorific value of the product from hydrogasification is lower than that from single methanation, particularly with high carbon/hydrogen feedstocks because of the additional steam required. However, by adding a final metlianator. the calorific value can be increased to that obtained from double methanation, again with increased capital cost and reduced efficiency. This process (Fig. 2) is used in the first operational SNG plant in the United States at Harrison, N.J. Typical gas analyses are given in Table 4. 

Of particular interest are the curves in Fig. 5. This form corresponds to the most common case when the temperature T0 is so low that, even when very little heat transfer is present, self-ignition is completely out of the question. For all ordinary explosive mixtures of hydrogen, carbon monoxide, methane and other industrial combustible gases with air, at initial room temperature, self-ignition is completely impossible. However, for sufficient fuel concentration and sufficient calorific value of the mixture, the temperature which develops in combustion may be large enough to ensure a very rapid chemical reaction or to ensure the possibility of steady combustion (branch AB of the curve). 

The value obtained for the lower limit of inflammation of methane when mixed with air is 5-6. The calorific value of methane is 189T. Substituting m the above equation a value of 1059 is obtained for k, and the relative values for L for other gases can then be calculated. 

As can be seen the pyrolysis gas contains mainly hydrogen, methane and other hydrocarbons. In our case the yield was 167 kg for each ton of used tyres. The average calorific value was determined to be approx. 34.500 kJ/m3. The odour was specific and intense. 

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