Abstract
This is the first in a series of three papers describing combustion of biosolids in a 5-kW bubbling fluidized bed, the ash chemistry, and possible application of the ash produced as a fertilizing agent. This part of the study aims to clarify whether the distribution of main ash forming elements from biosolids can be changed by modifying the fuel matrix, the crystalline compounds of which can be identified in the raw materials and what role the total composition may play for which compounds are formed during combustion. The biosolids were subjected to low-temperature ashing to investigate which crystalline compounds that were present in the raw materials. Combustion experiments of two different types of biosolids were conducted in a 5-kW benchscale bubbling fluidized bed at two different bed temperatures and with two different additives. The additives were chosen to investigate whether the addition of alkali (K2CO3) and alkaline-earth metal (CaCO3) would affect the speciation of phosphorus, so the molar ratios targeted in modified fuels were P:K = 1:1 and P:K:Ca = 1:1:1, respectively. After combustion the ash fractions were collected, the ash distribution was determined and the ash fractions were analyzed with regards to elemental composition (ICP-AES and SEM-EDS) and part of the bed ash was also analyzed qualitatively using XRD. There was no evidence of zeolites in the unmodified fuels, based on low-temperature ashing. During combustion, the biosolid pellets formed large bed ash particles, ash pellets, which contained most of the total ash content (54%–95% (w/w)). This ash fraction contained most of the phosphorus found in the ash and the only phosphate that was identified was a whitlockite, Ca9(K,Mg,Fe)(PO4)7, for all fuels and fuel mixtures. With the addition of potassium, cristobalite (SiO2) could no longer be identified via X-ray diffraction (XRD) in the bed ash particles and leucite (KAlSi2O6) was formed. Most of the alkaline-earth metals calcium and magnesium were also found in the bed ash. Both the formation of aluminum-containing alkali silicates and inclusion of calcium and magnesium in bed ash could assist in preventing bed agglomeration during co-combustion of biosolids with other renewable fuels in a full-scale bubbling fluidized bed.
Highlights
As the world’s population grows larger, it becomes increasingly important to manage and recycle nutrients from biomass back to soil for a more sustainable food and energy production
Several studies have shown that biosolids can improve the combustion properties of biomass by reducing the risk of corrosion and fouling, reducing bed agglomeration tendencies and possibly enabling a higher process temperature for some problematic fuels.[11−15] Even if the high ash content in biosolids may pose a challenge for monocombustion it means that relatively small amounts of biosolids in co-combustion scenarios will have a large impact on the overall ash chemistry.[11,14−16]
If zeolites are present in Energy & Fuels biosolids, they are likely to be associated with cations, such as sodium or calcium, so when biosolids are used in cocombustion, the importance of zeolites as an alkali adsorbent is probably smaller than has been suggested in previous publications.[11,37]
Summary
As the world’s population grows larger, it becomes increasingly important to manage and recycle nutrients from biomass back to soil for a more sustainable food and energy production. A resource that is related to biomass production in agriculture is municipal sewage sludge, known as biosolids after further treatment such as digestion This waste stream resource contains a lot of energy, even after biogas production through digestion, and the high content of macronutrients in biosolids makes it interesting both as a sustainable nutrient resource for new crops and as a co-combustion fuel where the ash produced could be used as a nutrient resource.[1−5] Incineration of biosolids has the benefits of energy recovery, destruction of both pathogens and anthropogenic chemicals (e.g., pharmaceutical agents and persistent organic pollutants)[6−8] with the possibility of creating an ash with macronutrients available for plants.[5,9,10]. Such an approach may decrease the dependency of mineral-bound
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
