Page 171 - New Trends In Coal Conversion

134 New Trends in Coal Conversion

can be used in cofiring systems with little or no modification (Dai et al., 2008), but the
catalysts of the SCR system are susceptible to poisoning from volatile inorganic com-
pounds, such as alkalis and phosphates, which are expected to be present in increased
amounts in the flue gases from cofiring plants. The issue of the cofiring impact on SCR
operation still requires further research (Karampinis et al., 2014).

5.6.3 Ash
Concrete admixtures represent an important market for some coal combustion ash by-
products. One of the issues associated with biomass cofiring is how to deal with the ash
left over after the combustion of both fuels in the combustor. The cofiring technology
employed determines the nature of the ash left at the end of the combustion process.
The ash contents of different biomass and coal feedstocks differ significantly in
composition, and it is still not appropriate to use fly ash from cofiring wood with
coal. Literature suggests that herbaceous biomass fuels which contain alkali and
chorine may compromise several important concrete properties, but there is not
enough evidence to preclude the fly ash from biomass energy source for the supply
of concrete additive (Bhuiyan et al., 2018).
Fly ash from the gasification of biomass in fluidized beds can be reused as fuel for
power generation because it may have high energy content due to unburned carbon. On
the other hand, potentially, the ash originating from combustion or gasification of
biomass can be used as natural fertilizer or in fertilizer production, as it is rich in
Mg and Ca (Tumuluru et al., 2012).

5.7 Oxy-fuel cofiring

Oxy-fuel combustion is a technology for the GHG abatement. In this technology, fuel
is combusted with the aid of a mixture of oxidizer and recycled flue gas, which pro-
vides a rich stream of CO 2 . This technology has mainly been widely used for coal com-
bustion (Kanniche et al., 2010; Nakod et al., 2013; Rebola and Azevedo, 2015; Riaza
et al., 2011; Singh et al., 2013), but the literature is scarce for biomass cofiring.
An experimental study evaluated the ignition and burnout performance of coal and
biomass mixtures in oxy-fuel conditions using an entrained flow reactor (EFR) (Arias
et al., 2008b). The results showed that the ignition temperature had strong dependence
on the combustion environment. A delay in the ignition was observed for the burning
with a mixture of 79%CO 2 e21%O 2 compared with air firing. This significantly
affected the flame temperatures. When the O 2 fraction in the CO 2 /O 2 mixture was
higher than 30%, early ignition took place at comparatively lower temperatures. It
was concluded that the use of biomass blend has a low impact during air combustion,
but a significant improvement in the burnout of the coal/biomass blends was found un-
der oxy-fuel cocombustion conditions.