Recycling of Biomass Bottom Ash from Fluidized Bed Combustion in Portland Cement Manufacturing: A Case Study Promoting the Circular Economy

Biomass bottom ash (BBA) constitutes about 20% of the by-products created during 100% biomass combustion in power and combined heat and power plants, with thus far limited utilization being reported. Our research is a case study from Poland examining the possibility of using BBA as a corrective additive to raw meal for the purpose of Portland clinker cement (PCC) production. BBA consists of mostly SiO2 (up to 86% mass). It has low alkalis content (on average 0.68% of Na2O and 2.68% of K2O) and low radioactivity indices, rendering it a safe material with no emission hazard. Morphological analysis confirmed that quartz grains in BBA, due to heating and cooling in a fluidized bed, underwent polymorphic transitioning twice, leading to multiple grain defects, which resulted in improved grindability and reactivity of BBA compared to natural quartz sand. A real-time industrial test was conducted for 5 weeks in one of Poland’s cement plants. BBA was fed to the rotary kiln (in proportion of 7% additive) as a substitution for natural sand (silica-bearing corrective additive) during the PCC manufacturing process, without any ash pretreatment or valorization being necessary. It was found that the addition of BBA to raw meal caused improvement of its sinterability (7% increase in clinkerization index). Results of this case study implement circular economy principles by permanently bounding a troublesome waste, i.e., BBA, into the structure of PCC without compromising the quality of the resultant cement (according to EN 197-1).