Slagging Characteristics during Residential Combustion of Biomass Pellets

Abstract

Limited availability of sawdust and planer shavings and an increasing demand for biomass pellets in Europe are pushing the market toward other, more problematic raw materials with broader variation in total fuel ash content and composition of the ash forming elements as well as in their slagging tendencies. The main objective in the present work was therefore to determine the influence of fuel−ash composition on residual ash and slag behavior. Twelve different biomass pellets were used: reed canary grass (two different samples), hemp (two different samples), wheat straw, salix, logging residues (two different samples), stem wood (sawdust) as well as spruce, pine, and birch bark. The different pellet qualities were combusted in a commercial under fed pellet burner (20 kW) installed in a reference boiler. Continuous measurements of O2, CO, CO2, HCl, SO2, and total particle matter mass concentrations were determined in the exhaust gas directly after the boiler. The collected slag deposits, the corresponding deposited bottom ash in the boiler and the collected particle matter were characterized with X-ray diffraction (XRD) and scanning electron microscopy combined with energy dispersive X-ray analysis (SEM/EDS). For biomass fuel pellets rich in silicon (either inherent or contaminated with sand) and low content of alkaline earth metals the main part of the potassium reacted with the silicon rich ash-residual, forming sticky alkali−silicate particles, which were not entrained from the burner and thereby giving rise to/initiating slag formation. Silicon rich fuels, i.e. fuels were the ash characteristics were dominated by silicate−alkali chemistry, therefore generally showed relatively high slagging tendencies. Straw fuels have typically this ash composition but exceptions to these general trends exists (e.g., one of the hemp fuels used in this work). Wood derived fuels with a relatively low inherent silicon content therefore showed low or relatively moderate slagging tendencies. However, contamination of sand material to these fuels may greatly enhance the slagging tendencies.

Combustion and fuel characterisation of wheat distillers dried grain with solubles (DDGS) and possible combustion applications

“…fluidised beds and possibly grate combustors rather than powder combustors. Comparison of slag formation tendency in grate combustion for the pelletised fuels used in this study (dark) and some other biomass fuels combusted in similar conditions in previous studies (white) [37]. DDGS 800 *) not analyzed due to fast defluidization during the experiment.…”

“…bottom ash and slag) was quantified after each experiment and the products were collected for analysis, described in section 2.6. A previous study has shown that the variation in the slag formation between replicate combustion experiments is minimal [37].…”

Producción y uso de pellets de biomasa para la generación de energía térmica: una revisión a los modelos del proceso de gasificación

“…Para controlar esto Francia establece dos tipos de pellets de biomasas mixtas que poseen características específicas para su posible comercialización (Tabla III). Con base en los niveles de cloro y azufre estos productos sólo pueden ser utilizados para la producción de energía en grandes industrias que puedan financiar los sistemas de tratamiento de los gases de salida sin perjudicar en gran medida la rentabilidad del proceso, aunque la gran cantidad de residuos agrícolas que poseen muchos países [34] hace de esta industria un nuevo tema de investigación que propone un futuro interesante una vez se superen estos problemas de emisiones.…”

“…Con base en este proceso se lleva a cabo la mayor aplicación actual de los pellets, por la cual se genera energía térmi-ca para el calentamiento de espacios; y es sobre este tema que muchos científicos han llevado a cabo estudios dirigidos a evaluar la cantidad de emisiones generadas [35], formación de escorias que deriven en problemas de taponamiento de los quemadores residenciales [34] y la relación entre la ceniza producida con la composición de los pellets empleados [36].…”

Correlation between air flow rate and pollutant concentrations during two-stage oak log combustion in a 25 KW residential boiler

“…In case of agrobiomass pellets, however, it is quite challenging to obtain pollutant emission as low as in case of wood pellets due to the fact that at temperatures of approx. 750 °C slag is produced, hampering furnace operation (Gible et al, 2008 andJuszczak et al, 2012).…”