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Biomass fly ash and biomass bottom ash 29

Gasification. Gasification is a thermochemical process in which biomass is trans-
formed into a combustible gas known as syngas or synthesis gas. The operating tem-
perature ranges between 700 and 1400 C, depending on the type of technology used

and the conditions of the process, and a gasifying agent (water vapour, oxygen, air or a
mixture of these) is used. The gas generated is more adaptable than the original solid
biomass and it is characterised by its low or medium calorific value. It can be
exploited in various ways through combustion processes to produce electricity and/or
thermal energy or as synthesis gas, transforming it into products with higher added
value.
Biochemical processes. The biochemical processes of transformation of biomass into
gaseous or liquid fuels, such as biogas or bioethanol, are supported by different types of
microorganisms. The microorganisms, whether contained in the original biomass or
added for the process, produce the degradation of the complex molecules of the biomass
to more simple compounds characterised by their high-energy density. These procedures
are usually applied in the case of natural or residual biomass with a high moisture con-
tent. The most-used biochemical technologies are anaerobic digestion and alcoholic
fermentation:
Anaerobic digestion. This is basically a fermentation process. The biomass is trans-
formed through bacterial action without oxygen producing a gas compound of methane
and CO 2 that can be used to produce electricity through gas turbines or in heat and
steam processes.
Alcoholic fermentation. A variety of biofuels can be produced from waste resources
including liquid fuels, such as ethanol or methanol, that can replace significant quanti-
ties of fossil fuels in many transport applications. Biofuels are produced through the
chemical reactions transesterification and esterification.

2.1.5 Ash production from biomass combustion

The process of energy production by combustion of biomass is considered to have
important environmental advantages; however, it also has the disadvantage that it
generates large amounts of ashes that affect the conversion process reducing the
efficiency of combustion systems, causing extra cost for boiler cleaning and mainte-
nance, and hinder further utilisation of biomass materials as combustion fuels
(Frandsen, 2005; Wang et al., 2012; Werther et al., 2000). Ash production can
become an environmental problem if they are not properly managed.
Two types of waste are generated in the combustion of biomass (Picco, 2010):
bottom and fly ashes. Biomass bottom ash (BBA) includes the coarse fraction and
is formed by the total or partially burnt material. Bottom ashes are produced in the
combustion chamber (James et al., 2013) and are composed of sand particles,
mainly quartz, often mixed with mineral impurities contained in the biomass
(Modolo et al., 2013); these impurities can be minerals, and they are usually
responsible for slag production due to the melting point decreasing; they are also
responsible of the presence of sintered ash particles in the bottom ashes. Biomass
fly ashes (BFA) are the particles separated from the stream of gases outside the
combustion chamber, so they are the finest fraction of the ashes. They are separated
from the combustion gases by specially designed systems to avoid their emission