Use of Additive to Mitigate Deposit and Corrosion Problems in Pulverized Biomass-Fired Boilers

Thesis from Manoj Paneru
University of Stuttgart, 2021

When replacing coal with biomass fuel in existing boilers which are originally designed for coal, ash related deposition and corrosion problems are of critical concern. The problem is inherent to the composition and properties of inorganic matter, namely ash, present in the biomass fuel original feedstock source. The interactive chemistry of ash inorganic species along the combustion process forms low melting ash components, like K-silicate, and corrosive ash components, like KCl. Such components have a direct implication to the boiler deposition and corrosion risk. Therefore, they are considered to be problematic ash species. One of mitigative option is to modify the ash formation chemistry to hinder the formation pathway of problematic ash species. Aluminosilicate minerals from the kaolin group, kaolinite and halloysite, are well known additives suitable to enforce such a mitigative chemistry.

In this work, the combustion behavior of various biomass fuels, woody and herbaceous, were investigated in relation to boiler deposition and corrosion severity associated with ash. The combustion tests include cases of biomass alone without an additive and biomass with an additive. The additive was supplied together with the fuel into the combustion chamber. The employed combustion test facility simulates a scenario of a pulverized fuel firing combustion system. The deposit and ash sampling location represents an environment, i.e. temperature and residence time, comparable to the super heater zone of a power plant boiler.

Online deposition sensors (ODS) are employed to quantitatively measure the outer deposit growth that is bulk ash accumulation. Further, the morphology, chemical composition, and fusion behavior of outer deposit ash bulk was characterized. Temperature controlled (cooled) deposit probe are implemented to investigate inner deposit growth and to evaluate the morphological and chemical composition of inner deposit ash and its implication to boiler tube material corrosion.

According to chemical composition the deposit ash is discussed as silicatic deposit and salt deposit. Silicatic deposit represents the ash particles dominated by silicon while the salt deposit represent these dominated by sulfur or chlorine.

The mitigative effect of additive was demonstrated by lower deposition propensity, improved morphology (less sintering), and overall reduction of molten ash components and salt species, especially KCl, in deposit ash. The absence of KCl apparently explains the lower corrosion activity in cases with additives compared to corresponding cases with biomass alone. The result shows that for both, silicatic and salt deposits, the potassium species are of uttermost concern. The mitigative chemistry is fundamentally related to potassium capture reactions driven by aluminosilicate (derive from additive mineral). In presence of aluminosilicate in the system, the potassium prefers to form refractory (high melting), stable (irreversible capture), and non-corrosive K-aluminosilicate instead of low melting K-silicate or corrosive KCl.

The amount of additives required for a certain biomass fuel is influenced by ash system chemistry of fired biomass fuel and the transformation level of reactive mineral, kaolinite or halloysite, in the given reaction system. The chemistry is essentially gas-solid capture reaction, K-species and aluminosilicate (kaolinite derive), respectively. The reaction basically proceeds outside the burning fuel/char particles. The potassium capture reactions are thermodynamically favored and not kinetically limited in high temperature combustion zone.

The biomass ash system chemistry governs the amount of gaseous K-species primarily released in the combustion zone. The transformation state of kaolinite, meta-kaolin or mullite, governs the effectiveness of additive particles to adsorb the gaseous species available in the system and later the chemical incorporation of potassium within the aluminosilicatic matrix of additive mineral. KCl prefers to remain as gas in the high temperature combustion zone. Therefore, chemically it is the last K-species to be consumed by the additive mineral. In the process, chlorine escapes to the flue gas as HCl.

This study investigates the applicability of HCl concentration in the flue gas measured within appropriate temperature boundaries as a suitable control parameter to evaluate and optimize the fuel specific additive amount. Further, the HCl concentration is a feasible parameter for a benchmark comparison across various commercially available aluminosilicate based mineral additives with regard to the effectiveness of capture chemistry in combustion application scenario. 

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