British Chemists Examine The Fate of Heavy Metals in Wood Bark Burning.

[NNadir]

Introduction

The controlled combustion of wastes of agricultural, industrial, and municipal origin promises to be an increasingly viablealternative to direct disposal to landfill. It allows useful energy to be recovered during thermal conversion, and the residual combustion ashes occupy only a small fraction of the initial waste volume. Furthermore, the European Union Directive 1999/ 31/EC specifically precludes the landfilling of combustible wastes, defined by maximum limits of 10% loss on ignition or
6% total organic carbon. The heat produced in the cocombustion process can be harnessed to generate electricity or provide district heating in the same way as heat from coal or natural gas-fired power stations. This practice will thus replace some fossil fuel-derived energy, thereby reducing dependence on expensive and often imported fossil fuels. Substitution of biomass-derived wastes for fossil fuels will also reduce the netcarbon dioxide emissions to the atmosphere...

The present, fifth, article now focuses on the co-combustion of spruce wood-bark with waste fuels, viz. plastic and agricultural wastes and pulp sludge. The potential use of wood-bark as the base fuel, rather than coal, has two major benefits. First, it is effectively CO2-neutral, being of biomass origin, and second, the concentrations of most trace elements in wood-bark are significantly lower than their counterparts in coal, manganese being a notable exception. On the negative side, the energy density of bark is less than coal. Although the availability and quality of bark may be more variable than coal, this is not likely to be a problem for large paper mills and timber processors. Seven of the trace elements selected for this study, viz. cadmium, chromium, lead, mercury, nickel, selenium, and zinc, have been chosen on the basis of their perceived toxicity...

I am surprised to see that mercury contamination is a significant business in these studies. I should not be surprised to learn that the source of the mercury found in biomass originally was coal based. In any case, burning the wood bark with plastics seems to retain the mercury in the ash:

Mercury. Here, almost complete (90-99%) volatilization has been observed from wood-bark, from agricultural waste, and from their blends (Figure 9). This is consistent with thermodynamic equilibrium model predictions of complete volatilization of mercury as elemental Hg vapor (>98%) and gaseous HgO (<2%). The presence of chlorine in the waste fuels is predicted to generate small quantities of HgCl2, which is also in the vapor phase at these temperatures. The influence of chlorine on mercury emissions has been noted by other investigators, such as Furimsky.13
However, blends that included pulp sludge or plastic waste retained mercury to a significantly higher degree, between 20 and 30% in the case of bark plus plastic waste at 800 °C. Furthermore, there is clear evidence of mercury enrichment on sinter ashes of some wood-bark/waste blends for all three waste fuels (Figure 10). The sinter ashes spent a considerable time at elevated temperature in an oxidizing environment, and all samples were found to contain less than 0.1 wt % carbon.

The article is from Energy and Fuels. The abstract of the article can be found here: http://pubs.acs.org/cgi-bin/abstract.cgi/enfuem/asap/abs/ef058013r.html

[eppur_se_muova]

1. Interesting. So many combinations to try. nt