Morphology and phosphate distribution in bottom ash particles from fixed-bed co-combustion of sewage sludge and two agricultural residues.

The purpose of this study was to provide detailed knowledge of the morphological properties of ash particles, including the volumetric fractions and 3D distributions of phosphates that lay within them. The ash particles came from digested sewage sludge co-combusted with K- and Si-rich wheat straw or K-rich sunflower husks. X-ray micro-tomography were combined with elemental composition and crystalline phase information to analyse the ash particles in 3D. Analyses of differences in the X-ray attenuation enabled calculation of 3D phosphate distributions that showed high heterogeneity in the slag particles. This is underscored by a distinct absence of phosphates in iron-rich and silicon-rich parts. The slag from silicate-based wheat straw mixtures had lower average attenuation than that from sunflower husks mixtures, which contained more calcium. Calculated shares of phosphates between 7 and 17 vol% were obtained, where the highest value for a single assigned phosphate was observed in hard slag from wheat straw with 10 % sewage sludge. The porosity was notably higher for particles from pure wheat straw combustion (62 vol%), compared to the other samples (15-35 vol%). A high open pore volume fraction (60-97 vol%) indicates that a large part of the pores can be accessed by the surroundings. For all samples, more than 60 % of the discrete (closed) pores had an equivalent diameter < 30 µm, while the largest volume fraction consisted of pores with an equivalent diameter > 75 µm. Slag from sunflower husk mixtures had larger pore volumes and a greater relative number of discrete pores >75 µm compared to wheat straw mixtures.

Ash Transformation during Fixed-Bed Co-combustion of Sewage Sludge and Agricultural Residues with a Focus on Phosphorus.

This work investigates the ash transformation during fixed-bed co-combustion of sewage sludge mixtures with the agricultural residues wheat straw and sunflower husks, focusing on the fate of phosphorus (P) in the resulting ash fractions. The study aims to determine suitable process parameters for fixed-bed combustion of fuels previously investigated in single-pellet experiments. The pure fuels and fuel mixtures were combusted in a 20 kWth residential pellet burner while monitoring the flue gas composition, temperature, and particulate matter formation. Subsequently, the different ash fractions were collected and characterized by CHN, SEM/EDS, and XRD analysis. The results showed that co-combustion of sewage sludge and agricultural residues reduced the formation of particulate matter as well as the formation of slag. Co-combustion of sewage sludge with either agricultural residue resulted in a change in phosphate speciation, displaying higher shares of Ca and lower shares of Fe and Al in the formed orthophosphates as well as amorphous phases containing higher shares of K. The formation of K-bearing phosphates was hindered by the spatial association of P with Ca and Fe in the sewage sludge, the incorporation of available K in K-Al silicates, and the depletion of K in the P-rich melt phase. Compared to mono-combustion, co-combustion experiments showed the potential for improving the combustion performance and reducing the risk of slag formation. The outcome suggests that co-combustion is a feasible path to integrate waste streams in fixed-bed energy conversion with simultaneous formation of phosphates enabling P recovery.

Preparation and Characterization of Pulp and Paper Mill Sludge-Activated Biochars Using Alkaline Activation: A Box-Behnken Design Approach.

This study utilized pulp and paper mill sludge as a carbon source to produce activated biochar adsorbents. The response surface methodology (RSM) application for predicting and optimizing the activated biochar preparation conditions was investigated. Biochars were prepared based on a Box-Behnken design (BBD) approach with three independent factors (i.e., pyrolysis temperature, holding time, and KOH:biomass ratio), and the responses evaluated were specific surface area (SSA), micropore area (S micro), and mesopore area (S meso). According to the RSM and BBD analysis, a pyrolysis temperature of 800 °C for 3 h of holding and an impregnation ratio of 1:1 (biomass:KOH) are the optimum conditions for obtaining the highest SSA (885 m2 g-1). Maximized S micro was reached at 800 °C, 1 h and the ratio of 1:1, and for maximizing S meso (569.16 m2 g-1), 800 °C, 2 h and ratio 1:1.5 (445-473 m2 g-1) were employed. The biochars presented different micro- and mesoporosity characteristics depending on pyrolysis conditions. Elemental analysis showed that biochars exhibited high carbon and oxygen content. Raman analysis indicated that all biochars had disordered carbon structures with structural defects, which can boost their properties, e.g., by improving their adsorption performances. The hydrophobicity-hydrophilicity experiments showed very hydrophobic biochar surfaces. The biochars were used as adsorbents for diclofenac and amoxicillin. They presented very high adsorption performances, which could be explained by the pore filling, hydrophobic surface, and π-π electron-donor-acceptor interactions between aromatic rings of both adsorbent and adsorbate. The biochar with the highest surface area (and highest uptake performance) was subjected to regeneration tests, showing that it can be reused multiple times.

Phosphorus release from hydrothermally carbonized digested sewage sludge using organic acids.

Hydrothermal carbonization (HTC) is a treatment technique with great potential for sanitizing digested sewage sludge (SS) and converting it into valuable products. In particular, phosphorus (P) recovery from hydrothermally carbonized SS has attracted special attention in recent years. This work aims to examine the leaching efficiency of P and the consequent release of metals and heavy metals from SS hydrochars (at 180, 215 and 250 °C) using organic acids (oxalate and citrate) over a range of pH values (0-4) and extraction times (5 min-24 h). Both organic acids triggered P extraction efficiencies exceeding 75 % at the lowest pH, but only oxalate reached a nearly complete P release from hydrochars at pH > 0 and for all carbonization temperatures. Low HTC temperature (180 °C) and short extraction time (5 min) were the optimal conditions treatment for P recovery when reacted in oxalate solutions of maximal pH buffering capacity (pH = 1.4). However, oxalate leaching also transferred metals/heavy metals into the P-leachate, with the exception of Ca being retained in the solid residue from HTC as Ca-oxalate precipitate. Different characterization methods confirmed the presence of this precipitate, and provided information about the surface and morphological changes of the SS hydrochars following acid treatment. The results suggest that HTC not only a promising technique to sanitize and reduce the volume of SS, but also an efficient means for P recovery using oxalic acid, thus contributing to the circular economy of P.

Morphological characterisation of ash particles from co-combustion of sewage sludge and wheat straw with X-ray microtomography.

Combustion of phosphorus-rich residual streams can produce nutrient-rich ashes and these can be used either in further processing or as materials for direct nutrient recycling. The latter requires knowledge on morphological parameters of such ash particles that may impact plant growth, nutrient availability, and soil physical properties. The present work aims to determine the porosity, pore size, and specific surface area of ash particles, and discuss these properties in light of literature concerning interaction with soil water and plant roots. Bottom ash particles from combustion of sewage sludge and wheat straw and their co-combustion were analysed with X-ray microtomography. Image analysis provided information on morphology, specific surface area, porosity, and pore structure on a micrometre scale resolution. Co-combusting sewage sludge with wheat straw resulted in differences in ash particles' porosity and pore structure compared to combustion of pure fuels. Pure wheat straw ash displayed 62 vol% porosity while there was no apparent difference between 10 wt% or 30 wt% mixtures of sewage sludge, with a porosity of 29-31 vol%. Open pore volume comprise the largest part of the porosity (72-99 vol%) enabling interaction between surrounding pore water and nutrients. Overall, the ash particles display large open volume fractions and thin particle walls which may lead to rapid weathering and extensive interaction with soil water. The particles generally contained pore openings over 200 µm towards the surroundings, which provide opportunities for interaction with microbes and roots from a variety of plant species in addition to nutrient transport by soil water.

Design and validation of an advanced entrained flow reactor system for studies of rapid solid biomass fuel particle conversion and ash formation reactions.

The design and validation of a newly commissioned entrained flow reactor is described in the present paper. The reactor was designed for advanced studies of fuel conversion and ash formation in powder flames, and the capabilities of the reactor were experimentally validated using two different solid biomass fuels. The drop tube geometry was equipped with a flat flame burner to heat and support the powder flame, optical access ports, a particle image velocimetry (PIV) system for in situ conversion monitoring, and probes for extraction of gases and particulate matter. A detailed description of the system is provided based on simulations and measurements, establishing the detailed temperature distribution and gas flow profiles. Mass balance closures of approximately 98% were achieved by combining gas analysis and particle extraction. Biomass fuel particles were successfully tracked using shadow imaging PIV, and the resulting data were used to determine the size, shape, velocity, and residence time of converting particles. Successful extractive sampling of coarse and fine particles during combustion while retaining their morphology was demonstrated, and it opens up for detailed time resolved studies of rapid ash transformation reactions; in the validation experiments, clear and systematic fractionation trends for K, Cl, S, and Si were observed for the two fuels tested. The combination of in situ access, accurate residence time estimations, and precise particle sampling for subsequent chemical analysis allows for a wide range of future studies, with implications and possibilities discussed in the paper.

Fate of metals and emissions of organic pollutants from torrefaction of waste wood, MSW, and RDF.

Torrefaction of municipal solid waste (MSW), refuse-derived fuel (RDF), and demolition and construction wood (DC) was performed at 220°C and a residence time of 90min in a bench-scale reactor. The levels of toxic polychlorinated dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) contained in emission from the torrefaction process were evaluated. In addition, main ash-forming elements and trace metals in the raw feedstock and char were determined. The use of MSW in fuel blends with DC resulted in lower PCDD and PCDF emissions after torrefaction, compared with the RDF blends. The migration of chlorine from the feedstock to the gas phase reduces the chlorine content of the char which may reduce the risk of alkali chloride-corrosion in char combustion. However, trace metals catalytically active in the formation of PCDD and PCDF remain in the char, thereby may promote PCDD and PCDF formation during subsequent char combustion for energy recovery; this formation is less extensive than when the feedstock is used.

Phosphorus and cadmium availability in soil fertilized with biosolids and ashes.

The recycling of hygienized municipal sewage sludge (biosolids) to soil as the source of phosphorus (P) is generally encouraged. The use of biosolids, however, has some concerns, such as the presence of elevated concentrations of potentially toxic trace elements, and the possible presence of pathogens, hormones and antibiotics. Organic substances are destroyed during combustion whereas trace elements could partly be separated from P in different ash fractions. Biomass combustion waste (ash) can instead be considered as an alternative P source. This study evaluates and compares the impact of biosolids and their combustion residues (ashes), when used as fertilizers, on P and Cd solubility in soil, plant growth and plant uptake of these elements. Biosolids were also amended with K and Ca to improve the composition and properties of P in ashes, and incinerated at either 800 °C or 950 °C. Combustion of biosolids improved the Cd/P ratio in ashes by 2-5 times, compared with the initial biosolids. The low Cd content in ashes (4-9 mg Cd (kg P)(-1)) makes this material a particularly attractive alternative to mineral fertilizers. Significantly higher pore water P (as well as total N) was measured in soils containing biosolids, but plants produced a higher biomass in soil fertilized with ashes. The K and Ca amendments prior to biosolids combustion generally decreased the total Cd in ash, but had little effect on P and Cd uptake and biomass growth. Similarly, the combustion temperature had negligible effect on these factors as well.

Combustion of Biosolids in a Bubbling Fluidized Bed, Part 1: Main Ash-Forming Elements and Ash Distribution with a Focus on Phosphorus.

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.

Effect of residue combinations on plant uptake of nutrients and potentially toxic elements.

The aim of the plant pot experiment was to evaluate potential environmental impacts of combined industrial residues to be used as soil fertilisers by analysing i) element availability in fly ash and biosolids mixed with soil both individual and in combination, ii) changes in element phytoavailability in soil fertilised with these materials and iii) impact of the fertilisers on plant growth and element uptake. Plant pot experiments were carried out, using soil to which fresh residue mixtures had been added. The results showed that element availability did not correlate with plant growth in the fertilised soil with. The largest concentrations of K (3534 mg/l), Mg (184 mg/l), P (1.8 mg/l), S (760 mg/l), Cu (0.39 mg/l) and Zn (0.58 mg/l) in soil pore water were found in the soil mixture with biosolids and MSWI fly ashes; however plants did not grow at all in mixtures containing the latter, most likely due to the high concentration of chlorides (82 g/kg in the leachate) in this ash. It is known that high salinity of soil can reduce germination by e.g. limiting water absorption by the seeds. The concentrations of As, Cd and Pb in grown plants were negligible in most of the soils and were below the instrument detection limit values. The proportions of biofuel fly ash and biosolids can be adjusted in order to balance the amount and availability of macronutrients, while the possible increase of potentially toxic elements in biomass is negligible seeing as the plant uptake of such elements was low.