Page 53 - New Trends in Eco efficient and Recycled Concrete

30 New Trends in Eco-efficient and Recycled Concrete

into the atmosphere (Maschio et al., 2011). BFA have a mainly inorganic fraction
and a minor organic fraction (unburned carbon).
The quantity and quality of ashes produced during biomass combustion are
strongly influenced by the characteristics of the biomass used (Masia ´ et al.,
2007) as well as the combustion technology and the combustion and operating
conditions of the process (Rajamma et al., 2009). Thus, the combustion of wood
generates fewer amounts of ashes to be managed because herbaceous biomass,
agriculture wastes and bark have a higher ash content compared to wood (Van
Loo and Koppejan, 2008; Masia ´ et al., 2007). On the other hand, the composition
of biomass varies not only according to vegetation type (Table 2.1)but also to
soil conditions and atmospheric dust particles deposited during cultivation or
storage, resulting in a very variable chemical composition of biomass ash
(Michalik and Wilczy´ nska-Michalik, 2012); for example the SiO 2 content is rela-
tively high in straw ash ( . 66 wt%) or beech bark (56 wt%), but very low in sun-
flower husk ash (,2.4 wt%) or corn bark (8.3 wt%); K 2 O content in ash varies
from 31.4 wt% in sunflower husks to ,4 wt% in hand beech bark ash. P 2 O 5
andCaO contentisveryhighin cornbarnash ( . 36 wt%) or beech bark ash
(17.7 wt%).
Table 2.1 summarises some examples of chemical composition depending on
the type of biomass. On the other hand, differences in operation temperatures
influence the amount of organic species (several salts and heavy metals) that vola-
tilise in the furnace, and consequently the relative composition of bottom and fly
ashes (Rajamma et al., 2009). Finally, BBA represent the higher percentage of the
total ashes produced in a grate furnace accounting for 60 90 wt% of the total ash
generated (Obernberger and Supancic, 2009). However, in bubbling fluidised bed
combustors (BFBC), the bottom bed ashes often represent the lower fraction vary-
ingbetween5and 17 wt%(Dahl et al., 2009; Latva-Somppi et al., 1998).
The foreseeable increase of large-scale utilisation of biomass during the next
years, according to the measures aimed at the implementation of renewable energy
sources that will reduce the problems derived from fossil fuels, will result in large
volumes of ash production. In this way, Directive 2009/28/CE to foster the use of
energy from renewable sources, includes the goal of using 20% renewable energy
by 2020; this goal will lead to the production of approximately 15.5 million tonnes
of biomass ash per year in the EU-27 (Obernberger and Supancic, 2009; James
et al., 2013).

2.1.6 Environmental and health aspects of biomass ashes
The environmental impacts of biomass ashes are related to their composition, which
depends on the origin of the biomass. In the case of agricultural and forest biomass
ashes’ composition was dominated by Si, Ca, K and P inorganic species; however,
industrial waste ashes were high in Si and Ca and to a lesser extent in Al and Mg
minerals (Vamvuka and Kakaras, 2011). In general terms, ashes from biomass
could be classified, according to the European Community’s legislation, as non-
hazardous industrial waste. Consequently, numerous possible applications of