Page 56 - New Trends in Eco efficient and Recycled Concrete

32 New Trends in Eco-efficient and Recycled Concrete

biomass ash have been widely studied, including, for example (Vassilev et al.,
2013a), soil amendment and fertilisation, bulk utilisation or in construction materi-
als. However, logistical problems and variation in the quality of the ash, the lack of
legislation and regulations in many countries, among other factors, lead most coun-
tries to deposit the ash in landfills increasing the economic and environmental
impact (Vamvuka and Kakaras, 2011). The management of biomass is becoming an
increasing economic and environmental burden.
The increasing amount of BBA and BFA all over the world, as a consequence of
the use of biomass to produce energy, suggests necessary recycling to reduce land-
fill disposal costs as well as negative environmental impacts, but also as a conse-
quence of the ‘zero-waste’ objective including in circular economy concept
(Maschio et al., 2011). Consequently, it is necessary to study new potential uses for
biomass ashes and the construction sector could be a good option.

2.2 Overview of biomass ash characteristics

2.2.1 Classification of biomass ashes
Different typologies of biomass are usually applied in biomass thermal power plants
(BTPP) for the production of electric energy. These typologies are mainly related to
the vegetation of the region where they are located, and the nearby areas. Mixtures
of different types of biomass are usually applied to produce electricity, such as bio-
mass waste from mass pruning processes, from the agricultural industry as forestry
waste, from urban areas as gardens or from inter-urban areas such as roads, etc.
BTPP are usually close to the areas where huge quantities of biomass are
obtained. There are mainly two technologies in BTPP to burn biomass: fluidised
bed combustors (BFBCs) and circulating fluidised bed combustors (Modolo et al.,
2013). Fig. 2.5 shows a thermal plant with a fluidised bed combustor (Hinojosa
et al., 2014). These technologies present differences in the pattern of gas solid
hydrodynamics in the reactor, the size of the bed particles, the heat and mass trans-
fer rates in the reactor, and the temperature and flue gas composition profile along
the reactor (Van Loo and Koppejan, 2008). These variables influence the character-
istics of the ashes produced during biomass combustion.
In every BTPP, two types of ash are obtained, BBA and BFA. BBA is the por-
tion of non-combustible residue found in the furnace or incinerator, whereas BFA is
the portion of ash that escapes through the chimney and is retained to prevent it
from being released into the atmosphere (Cabrera et al., 2014).
On the one hand, BBA are composed of sand particles purged from the original
bed, inorganic components, as soil and sand, and the unburnt biomass fraction.
BBA represent the higher percentage of the total ashes produced, around 55%
65% (Modolo et al., 2013). These ashes are obtained through discharges that are
required to renew and to avoid agglomeration and defluidisation (Van Loo and
Koppejan, 2008).