Cleaning up with Biofuels
Miles Davis, Consultant Scientist, WRc
Contaminated land is rarely out of the news. A study led by English Partnerships recently estimated that there are some 66000 ha of Brownfield sites in the England and Wales; a figure that is growing each year, with only a few thousand ha brought back into productive use. With the Landfill Directive (1999/31/EC) already prohibiting the co-disposal of non hazardous and hazardous material to landfill (ending the common practice of disposing of contaminated soils with other materials) and soon to require the pre-treatment of soils classified as hazardous prior to landfill disposal, the pressure has never been greater to find new contaminated site remediation alternatives.
Phytoremediation, or the use of plants to remediate land, potentially offers a solution to both problems. A recent study funded through the European Union Environment and Climate Programme (and with financial contributions from the Environment Agency), looked at the feasibility of using fast growing plant species to both remediate or stabilise contaminated soils and provide a renewable bioenergy, or energy crop source.
The 3 year project, undertaken by a European consortium and led by WRc, a leading research based environmental consultancy, studied three biofuel crops, Salix (willow), Phalaris and Miscanthus and involved extensive short and long term pot and field trials at study sites in the UK, Sweden and Spain. The key study objectives were to:
Are crops really up to the job?
The biofuels Phalaris, Salix and Miscanthus have all been shown to be able to be grown satisfactorily on heavy metal contaminated sites, with no apparent reduction in yield. These crops were found to decrease the solubility of heavy metals and therefore their probability of being leached and also decrease heavy metal soil concentrations, following crop harvesting. Consequently, with proper management of site conditions to enhance growth, industrially degraded land may be used to produce biofuels with environmental and economic benefits.
The ability of biofuels to remove significant amounts of metals from soil were found to be limited to Cd and Zn. Although Cu and Ni were also shown to be taken up, the extent was not considered to be enough to be considered phytoextraction. Although for these and the less plant available metals, the biofuel crops studied could be used for the stabilisation and productive use of contaminated land. Such phytostabilistation should not be seen as a poor alternative to phytoextraction, since the reduction of environmental risk is a key management objective.
The optimum phytoextraction method for a particular site, was shown to depend on a number of key factors, including:
To date no model has been developed to accurately predict an optimum combination of these factors. As a consequence of this, it is recommended that pot experiments are conducted prior to field experiments, in which the best combinations can be identified in a comparatively small time period. However, it should be noted that the potential to use phytoextraction to reduce the time needed to clean up soil to certain quality standards might be limited. On average doubled uptake rates (and therefore halved remediation time) can be expected without causing environmental damage.
If enhanced phytoextraction is to be achieved, a few major decisions can be made in advance:
A remediation end point, that is the reduction in the site-associated risk, can only be applied to phytostabilisation and not phytoextraction since too many variables will influence the rate of plant uptake and hence soil cleaning.
Heavy metals - up in smoke?
The presence of potentially toxic metals in biofuels is an important consideration with regard to potential aerial emissions of those metals as particulates or aerosols in flue gases and the disposal of contaminated ash. The concentration of metal in the unburnt fuel, the proportion of ash present in a fuel type, and the quality and efficiency of the conversion process will all influence the fate of the heavy metals present. Hence, fuel quality alone will not dictate the extent to which a particular fuel type or source may be compromised by its heavy metal content.
The extent to which the heavy metal content of a biofuel is a significant factor will vary with the use to which the fuel is put. Where wood is co-combusted with coal or other dirty fuels, the proportion of heavy metals contributed by the biomass fuel may be negligible, depending on the ratio of fossil fuel to wood, hence the presence of the metals in the biofuel is not important to its use in this context. Similarly, where biomass fuels are used as a base load fuel in a waste to energy plant to compensate for variation in the calorific value, the heavy metal quality of the biomass is likely to be unimportant. However, where a conversion facility is fuelled solely by biomass, the heavy metal quality of the fuel will be more important and the choice of fuel needs to account for the scale of conversion facility, the flue gas cleaning systems and the heavy metal content of the fuel.
Ash from coal and oil fired power plants is typically not recycled to land as a fertiliser. Flue gas cleaning in such facilities will be designed to accommodate a dirty fuel type and the adoption of biomass fuels is unlikely to result in additional gas cleaning. Bottom ash may be used as a building material however, air pollution control (APC) residues, ie filter fly ashes are high in a number of toxins and are disposed of in specialist containment facilities.
It would appear that in order of importance from high to low, technology type, biomass properties and heavy metal concentration dictate the extent to which a biofuel will contribute to increased heavy metal flows in the environment. Hence, it is unlikely that heavy metal concentration would be a criterion by which a type of biofuel crop would be selected over another.
The study concluded that growing biofuels on contaminated sites offered many environmental and economic advantages over conventional remediation techniques, and it identified three broad situations where biofuel production would be particularly beneficial, these were in: