Book - School of Science and Technology

Miscellaneous fuels 315burn natural gas alone is attractive in the sense of capital expenditure. Properly equippedand sited near to a demand for heat supply (see Chapter 24), a case just might be made forsuch usage. Otherwise, the profligate consumption of such a refined source of energy (andone of limited reserves) is incomprehensible to the authors.Miscellaneous fuelsIn addition to the more usual fuels and sources of energy, others are in use for a variety ofreasons. To give but two examples:Timber, as logs or off-cutsIf burnt in a properly designed stove with an integral boiler, timber may in somecircumstances provide primary or back-up facilities at domestic level for heating tapwater or a few radiators. When sold as fuel, timber may be either waste off-cuts or logspurpose-cut from stick which is either diseased or otherwise unsuitable for use as a rawmaterial in manufacture. It may originate from a wide range of growth, imported or homeproduced, and be green, air dried, seasoned or oven dried.The principal characteristics of timber are set out in Table 11.13 together with, forcomparison, those for a medium volatile coal. Moisture content, as would be expected,has a very significant effect upon the calorific value, of the order of plus or minus 30%,and any manufacturer's performance data are likely to be rather higher than resultsachieved in practice due to the firing problems which arise from the high volatile content.Waste productsThese, from a very wide variety of sources in manufacturing industries and from municipalcollection, may be burnt at a relatively low efficiency in large scale specialist furnacesTable 11.13 Properties of typical timberDescriptionMediumvolatilecoal(30 lb)AveragehardwoodTimberAveragesoftwoodTypical ultimate analysis (%)carbon 81.0 45 44hydrogen 4.5 6 5nitrogen 1.4 1 1oxygen 3.8 38 37sulphur 1.3 ± traceash 5.0 2 1moisture a 3.0 8 12Calorific value (MJ/kg)gross 32.9 19 17net 31.8 17 16Bulk density b (kg/m 3 )green ± 600 450air dried 750 300 250a Moisture quoted for air dried timber.b Based on half value of solid specific mass.