Review of the use of additives to mitigate operational problems associated with the combustion of biomass with high content in ash-forming species

The impact of additives addition on the combustion behavior of biomass with high content in ash-forming species is evaluated in this review. Their influence on both emissions (particulate matter and gaseous emissions) and deposits formation (fouling and slagging) are here considered. The uncertainty in the availability, under current sustainability criteria, of good-quality woody biomass (i.e. woody biomass with low content in ash-forming species, mainly derived from stem wood), along with the growing demand for biomass fuels, has caused the pellet industry to attempt to diversify the sources of raw material. Other types of biomass such as bark, non-woody biomass (cereals and herbaceous materials), and residues from the agricultural industry are also potentially useful as raw materials due to the large volumes available. These fuels present however some challenges. They vary strongly in composition, impacting significantly their combustion behaviour. The high content in ash-forming species, such as alkali and alkaline-earth species, chlorine, phosphorous, nitrogen, and silicon can lead to an increase in gaseous (e.g. sulfur oxides and nitrogen oxides) and particulate matter (PM) emissions. They can also lead to operational problems, such as fouling, corrosion, slagging, and agglomeration during the combustion process. There are several routes to mitigate these ash-related problems, whose applicability depends on the technology and scale. In the present review, the use of additives to reduce emissions and deposits formation, as well as further operational problems these may trigger, is evaluated. However, the high heterogeneity in biomass composition and varying nature of the aforementioned combustion issues hinders the possibility to use a “one-size-fits-all” additive, resulting in the need for developing further understanding on the impact of different additives according to biomass composition and combustion conditions. For example, aluminosicates have proven to be effective to reduce fine particulate matter emissions, but they increase the gaseous emissions of HCl and SOx. The impact of Ca-based additives on PM and alkali-induced slagging is inconclusive, although they can capture gaseous emissions such as HCl and SOx. The additive application method and combustion conditions play on top a very significant role. For all this, reaching conclusions on the type of additive, amount, and application method is very challenging. With this work the authors aim at providing an overview of the state-of-art on the use of additives in biomass combustion, as well as to offer a framework to advance in the regulation on the use of additives.