Biomass and MSW contain oxygen, nitrogen, sulfur, halogens, metals and other elements whose concentrations in the pyrolysis products must be controlled as per existing standards.

Sulfur and nitrogen have been shown to mostly cluster in the bio-oil phase [21]. Sulfur has been successfully avoided in pyrolysis oil from coal in the past [67] by the use of lime (CaO). Similarly, the same experiments showed that the oxygen weight fraction in oil could be as well significantly decreased. The main disadvantage of this technology is that the char will remain charged with calcium and that the gas phase will mostly receive this excess of sulfur [68]. However, the gas phase can be post-treated efficiently (scrubbing) and considering the high-level of inorganics and contaminants in the solid phase, it will not likely be used as bio-char or as activated carbon.

Adding CaO in the pyrolysis environment can also significantly inhibit the for­mation of liquid chlorinated organics in the bio-oil [69]. The co-feeding of CaO in MSW pyrolysis could eventually become widespread as this additive is relatively cheap and available.

Post-processing of bio-oil to remove oxygen, nitrogen and sulfur is also possible through hydrodeoxygenation using metal catalysts (and an H2 stream) [70]. For biofuels production, this post-treatment process is desirable, as the oxygen weight fraction can be reduced below 1 %. Furthermore, the effect of this post-treatment on the bio-oil chemical composition has not been investigated such that there is a possibility that it also generates higher value chemicals.

Most metals and metal oxides cluster in the solid phase due to the low tempera­ture associated with pyrolysis, which remains below metal sublimation or melting temperatures. Metals can also be absorbed by activated carbon produced from bio-char [56].

Nomenclature

Abbreviations

A = Pre-exponential factor [s-1] for a 1st order reaction d = Diameter

E = Activation energy [Jmol-1]

Fr = Froude number g = Standard gravity [m s-2] h = Heat transfer constant [Wm-2K-1] k = Thermal conductivity [Wm-1K-1] m = Dimensional & non-dimensional weight n = Order of reaction, non-dimensional Nu = Nusselt number Pr = Prandtl number

R = Universal gas constant OR Radius [Jmol-1K-1] OR m Re = Reynolds number

t = Time

T = Temperature

U = Velocity [m s-1]

Symbols

ц, = Viscosity [Pa s]

p = Density [kgm-3] rn = Angular velocity [s-1]

Subscripts

bed = Fluidized bed

bp = Bed of particles (in rotary drums)

g = Gas phase

p = Particle

t = Terminal (velocity)