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And its greenhouse implications
The title of this paper indicates that two issues more important in the early 2000s than even a decade or so previously are going to be addressed. One is the ‘fuel use of waste’. The finiteness of reserves of conventional fuels, in particular oil, is becoming clearer and clearer and for part of 2006 the price of crude oil, adjusted for erosion in the value of a dollar over the period under consideration, was higher than it had been since around 1870 (yes, 1870: in 1970 a sum equivalent to $20 at the 2006 value of the dollar would buy a barrel of oil). The world consumes 80 to 85 million barrels per day of crude oil. World activity is critically dependent on the availability and price of crude oil and political ramifications of oil supply and demand are of course enormous.
‘Alternative fuels’ are therefore being extensively investigated in order to achieve reductions in usage of petroleum fuels, and these include wastes of various kinds. Amongst the waste materials to have found application as fuels are household waste and agricultural waste. Nowadays the acceptability of a fuel for use depends on the contribution if any which on burning it makes to the carbon dioxide content of the atmosphere. Hence the word ‘greenhouse’ in the title of the paper provides an indication of the second issue which, as it relates to the first, the paper will address.
Brief comments on the relevant fuel technology
The units in SI for calorific value are MJ kg-1, and such values are given below for a selection of conventional fuels and of ‘waste fuels’.
In concluding this section we anticipate two facts from our later discussion of MSW. First, some low-rank coals in the bed-moist state have calorific values the same as that for raw MSW. For example, the brown coals in Victoria, Australia, are of calorific value 7 to 8 MJ kg-1, yet they are used to make electricity on a grand scale. Drying by evaporation and during milling prior to burning brings the calorific value to within the range given for coals in the previous table at the stage. Secondly, there is what is sometimes termed ‘solid waste/ crude oil equivalence’. This simply means that a barrel (159 litre) of crude oil and a tonne of MSW are capable of releasing on combustion the same amount of heat, actually about 7 GJ. These facts considered it is clear that the potential for the fuel use of MSW, once described as an ‘energy ore’, is considerable.
Background on the greenhouse effect and global warming
When greenhouse gases are present in the atmosphere they cause it to be transparent to solar radiation received by the earth but opaque to the radiation of much longer wavelength which the earth itself emits, and a rise in temperature is the result. The most important greenhouse gas is carbon dioxide. Currently its level in the atmosphere is about 370 p.p.m. In the early nineteenth century, when industrialisation was just beginning, it was about 280 p.p.m. In New Testament times it was about 260 p.p.m. These data are known from analysis of ‘ancient air’ trapped in bubbles of known age in the Antarctic ice layer. Nitrous oxide (N2O), one source of which in the atmosphere is forest fires, and methane (CH4) are both more powerful greenhouse gases than carbon dioxide.
Valuation of fuels The point was made previously that a tonne of MSW and a barrel of crude are equivalent in that on burning they release the same amount of heat. However, it would be insane to offer for a tonne of MSW the current price of a barrel of crude. The reasons are obvious: the crude can be straightforwardly refined to make a range of fuels for immediate use. The MSW, besides being unhygienic and unpleasant to work with, would first require removal of non-combustibles such as bottles and cans. It would then need to be dried, shredded and perhaps pelletised. Even when it was at this advanced stage of processing it would be suitable only for certain types of combustion plant and would probably leave a significant amount of corrosive ash.
One cannot therefore interchange dollars and megajoules in assessing the value of a fuel. It is quite possible for the energy return on energy invested (EROEI) for a particular fuel to be less than unity, meaning that more energy is needed to obtain it and prepare it for use than would be obtained from such use. That is why in the US, in particular in Colorado and in Utah, vast amounts of oil in the form of shale, well exceeding the known conventional oil reserves of the entire Middle East, have not been brought into use and perhaps never will be.
A waste substance for fuel use might have at its origin a ‘negative financial value’, meaning that unless used as a fuel it would have to be disposed of professionally for which a charge would be made. Spent cooking oil from fast food chains is a case in point. If a fuel like that was used in a way which did not break even financially such use could be validated if losses were lower than the cost of professional disposal, meaning in effect that the magnitude of the ‘negative financial value’ was reduced.
Wood and other cellulosic wastes as fuels
Wood has of course been used as a fuel throughout history and it was only in the late nineteenth century that coal ‘overtook’ wood as the primary solid fuel in the US. There has been much revival of fuel use of wood in recent years because of its carbon neutrality (see next page). One has to distinguish between wood grown with fuel use as its intended purpose and wood waste diverted to fuel use. It is with wood waste only that this article is concerned, e.g., sawdust, pelletised wood and wood shavings
The carbon neutrality of wood fuel
The carbon content of any carbon dioxide molecule released in the burning of wood was in the recent past a carbon dioxide molecule in the atmosphere ‘fixed’ by the tree. When wood fuel is burnt therefore carbon dioxide is simply returned to where it came from and the effect on the carbon dioxide content of the atmosphere is nil, and wood fuels are said to be carbon neutral. A fully grown tree absorbs about 25 kg per year from the atmosphere which is returned there when the dead wood is burnt. By contrast, when coal or petroleum-derived fuel are burnt newly created carbon dioxide is released and this adds to the carbon dioxide content of the atmosphere.
The firing of wood with or instead of coal
It is often not realised that partially or totally to replace coal with wood, for example in the generation of electricity, leads to the formation of more carbon dioxide per unit heat released, not less. The calculations in the shaded area below, taken from another recent publication by the author, show this.
It is clear then that the carbon neutral fuel produces significantly more carbon dioxide than the conventional fuel per unit heat produced. The previous calculation uses arbitrary though typical values for the quantities involved, and that wood fuel produces more carbon dioxide than the coal other things being equal is in fact a general result. Why then is the wood fuel to be preferred on carbon dioxide emission terms? Simply for the reason given in the previous section, that unlike the carbon in coal the carbon in wood fuel was in the recent past carbon dioxide in the atmosphere, so when the wood is burnt it is simply being put back where it came from. Biomass and coal might be of course be co-fired. Imagine that in our last example where the total heat release rate is 10 MW the coal derived CO2 has to be reduced by 5% to comply with local requirements. The calculation is set out below.
There are two further reasons why in greenhouse terms fuel use of wood, on its own or with coal, is advantageous. One is that if it was not so used it might simply be incinerated in which case it will release CO2 into the atmosphere with no ‘return’ at all from the heat. Another is that if not destroyed by burning at all it might be taken to a landfill where, over time, it would decompose and release into the atmosphere methane which is in fact a much more powerful greenhouse gas than carbon dioxide.
Some current scenes of coal-biomass co-firing
The following have been selected from the numerous examples worldwide of coal-biomass co-firing for electricity generation. Before considering them we should note that in some countries including the Netherlands there is insufficient available biomass for target reductions in carbon dioxide to be attained by this means therefore some import of biomass might be necessary. Though not lacking biomass, the UK does import some in the form of Illipe meal from Malaysia. Illipe seeds on crushing yield a semi-sold material which is widely used in cosmetics, as the basis of skin creams and so on. The solid residue (‘meal’) remaining retains some of this material and in subsequent combustion this makes for ease of ignition. Similarly Shea meal, containing like Illipe meal residual extractables, is exported from Ghana to the UK.
At Fiddler’s Ferry, Cheshire UK coal is co-fired with biomass in the raising of electricity at 2000 MW level. At Tilbury in the Thames estuary a power facility generating at up to 1020 MW operates with 5% of the coal replaced by sawdust. At Liddell Power Station in New South Wales, Australia, there is co-firing of sawdust with local bituminous coal. Australia produces 50 million tonne of biomass waste annually.
In Mauritius, a small island nation in the Indian Ocean, coal and sugar cane residue (‘bagasse’) are co-fired to generate electricity at about 70 MW and also heat, in a Combined Heat and Power cycle. The installation has flexibility to respond to seasonal variations in the availability of bagasse. At some times of year coal will be the dominant fuel whilst at other times the primary fuel will be bagasse. Over a year about a third of the power will be from bagasse combustion, reducing the coal import requirement very significantly. Cotton waste, where it is available, is also suitable for co-firing with coal. This takes place in countries including Paraguay. The table below gives examples of biomass fuels.
Municipal solid waste (MSW) as fuel
We saw in an earlier part of this article that this has a calorific value of typically 7 to 8 MJ kg-1 and that there are significant disadvantages in its use. Even so, there has been extensive use of MSW as a fuel since 1898. In that year heat recovered from the burning of household waste at the New York City Incinerator in East 17th St. was put to use and this was the first time that any return on heat from burning such a substance had been obtained. In the USA at the present time 15 to 20% of the total MSW generated finds its way to power generation, one of the largest such operations being in southern California where 360 tonne of MSW per day are burnt generating 10 MW of electricity for sale. ‘Success stories’ in MSW usage in the UK include South East London Combined Heat and Power (SELCHP) and Sheffield Heat and Power further details of each are easily obtainable from the web.
Raw MSW always has a negative financial value. It is not straightforward to judge the acceptability of MSW in carbon dioxide emission terms. Some components of MSW will be carbon neutral, e.g., cardboard. Others will not, e.g., plastics, the starting material for which is petroleum based. Some legislatures make the approximation that for carbon accounting purposes MSW is treated as being 50% carbon neutral.
Concluding remarks
This article has dealt with biomass fuels and with MSW and has treated suitability for fuel use alongside greenhouse issues. We can expect a great deal more by way of biomass combustion and coal/ biomass co-firing as a way of reducing the carbon dioxide content of the atmosphere as the 21st Century takes its course. The author will be pleased to receive comments from readers.
J.C. Jones DSc FIChemE FRSC University of Aberdeen Clifford Jones is a Reader in the Department of Engineering at the University of Aberdeen. He has held academic posts in Australia and the UK and is the author of six university-level textbooks with a seventh in press. He has lectured on his work in countries including the US, Thailand, South Africa, Sweden, Spain and India and also has significant broadcasting experience. [email protected] Examples of miscellaneous biomass fuels for use alone or with coal. All of the fuels in the table are carbon neutral wer generation at Ely, Engl ndCit us peelFlor da US Can b e blende
Published: 10th Jan 2006 in AWE International
