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Dewatering and handling of pulp and paper mill sludge is challenging and often comprises more than half of the overall wastewater treatment costs at a mill. Primary sludge is usually more dewaterable than biosludge (waste activated sludge), and the ratio between both types of sludge largely determines the dewaterability of the sludge mix. This study has quantified the benefits of adding primary sludge to biosludge to enhance the dewaterability in terms of cake solids and filtrate/pressate solids content. Furthermore, the effects of the particle size distribution and the monovalent to divalent cation ratio of the various types of sludge were investigated. Biosludge and three types of primary sludge from a Canadian pulp and paper mill were mixed at various ratios and tested for dewaterability using a Crown Press®, a bench-scale device which simulates industrial belt press dewatering. Results show that after addition of primary sludge at up to 40% mass proportion (dry solids basis) the dewatered cake solids content increased substantially, from 10.4% to 18.7–19.9%, depending on the type of primary sludge added. Further addition of primary sludge led to comparably only small improvements. Primary sludge addition also resulted in decreased total suspended solids (TSS) concentrations in the combined gravity filtrate / Crown Press pressate. Primary sludge content of only 10% in the sludge mix was sufficient to decrease the TSS concentration from 1.8 g/L to 0.2 – 0.4 g/L. The study shows that only less than half of the sludge mix needs to consist of primary sludge, in order to achieve adequate dewaterability. This opens up the possibility of retaining more primary fiber within the pulping process, without compromising downstream sludge dewatering.

Wood ash is typically applied to soils to counteract acidification and base cation depletion, but the effects of this ash on the organic layer microbial community are rarely studied. We analyzed microbial responses to ash addition in two field-scale experiments in Ontario, Canada. One experiment was in a young boreal forest soil and the other was in an uneven-aged hardwood north-temperate forest soil. We also investigated the effects of a natural forest fire near the boreal experiment. In both cases, ash addition had no effect on overall microbial biomass and respiration, but increased the phylogenetic diversity of bacterial communities and the relative abundance of Bacteroidetes taxa, though effects on other bacterial taxa were site-specific. Eukaryotic effects also varied by experiment; at the boreal site, ash increased eukaryotic phylogenetic diversity and decreased the fungi:bacteria ribosomal marker ratio, but at the temperate site ash decreased eukaryotic diversity and did not affect the fungi:bacteria ratio. There was limited additional effect on the boreal soil microbial community of increasing ash addition from 0.7 to 5.7 t ha −1 , as determined by T-RFLP analysis, though soil pH in both experiments increased with higher addition rates. At the temperate site, ash effects were consistently stronger for fly ash than for bottom ash. In both experiments, only 14 unique bacterial taxa were found after ash addition, and the strongest driver of overall community composition was the forest type, not ash treatment. In contrast, the soil microbial community observed at the forest fire site was clearly different. Overall, these results indicate that wood ash addition has only minimal effects on the composition of the soil microbial community in sites across two distinct global forest biomes.

Agglomeration of bed materials at high temperature is one of the most important and challenging problems for fluidized-bed biomass boilers for thermal/power generation. Inorganic alkali components from the fuel can be problematic as they form low-melting alkali compounds. In the present study, the critical amount of liquid (molten ash in real biomass boiler operations) that would result in severe bed agglomeration and defluidization was studied in a small pilot-scale bubbling fluidized bed (BFB) rig, operated at elevated temperatures (415–420 °C) using low melting-point salt (KOH) to simulate molten ash. In the BFB fluidization system tested, the critical liquid amount of simulate molten ash that could cause the bed materials start to form agglomeration/channeling, and form severe agglomeration/channeling (poor fluidization) is approx. 0.5 wt.% and 0.8 wt.% at a lower fluidizing gas velocity (3.9 Umf) and a higher fluidizing gas velocity (5.9 Umf), respectively. With more low-melting-point compound (KOH) present in the BFB, more agglomerates of bed material are formed. Increasing the amount of liquid could change the fluidization behavior of group B (bubbling) particles towards group A (Aerated) and even C (Cohesive) depending on the amount of the low-melting-point compound in the bed material.

A multivariate data analysis (MVDA) was conducted to identify operating parameters that affect the sludge dewatering efficiency in a Canadian pulp and paper mill. The sludge is a mixture of waste activated sludge (WAS) and three different types of primary sludge. The dewatering efficiency was defined by three target parameters: the cake solids content after dewatering, the solids capture efficiency, and the costs of dewatering aids (chemicals) used per ton of dewatered sludge solids. The results show that the most influential parameters affecting the dewatering efficiency are 1) the type and amount of primary sludge added to the sludge mix, 2) the operating parameters of the dewatering equipment and 3) the sludge solids throughput. A high proportion of fiber-rich primary sludge in the sludge mix substantially enhances dewaterability. Operating the screw press at a low torque and/or at a high screw press speed lowers the dewatering performance. The pH adjustment of the sludge prior to dewatering also significantly affects dewaterability. The effectiveness of the dewatering aids decreases notably when the pH is not within the target range. Besides the processes in the dewatering plant, the activated sludge process and the associated amount and quality of the WAS also affect the dewaterability of the sludge mix. In particular, the dewatering efficiency seems to be seasonal. The higher temperature during the summer produces less WAS, and thus increasing the proportion of primary sludge in the sludge mix and improving dewaterability. Also, WAS with high microbial activity and firm flocs, as well as a low sludge volume index enhances dewaterability.

Agglomeration of bed materials at a high temperature is one of the most important and challenging problems for fluidized-bed biomass boilers for thermal/power generation. Inorganic alkali compounds derived from the biomass ash, mainly potassium (K) and sodium (Na) can be problematic as they form low-melting alkali compounds and may also react with the bed material (silica sand) forming low-melting alkali silicates. These low-melting alkali compounds, if surpassing a critical amount, could coat the sand particles to form agglomerates, eventually leading to partial or complete de-fluidization of the reactor. In the present study, the critical amount of liquid (molten ash in real biomass boiler operations) that would result in severe bed agglomeration and defluidization was studied a small pilot-scale cold bubbling fluidized bed (BFB) test rig filled with silica sand particles as bed materials equipped with non-invasive capacitance sensors and differential pressure transducers. In the cold BFB test rig, a solution of glycerol–water (30% v/v), employed to simulate molten ash in real biomass boiler operations, was injected to the bed at different quantities during the tests. It was found that in the present fluidization system the critical liquid amount causing bed agglomeration is likely 0.2 wt% (in relation to the weight of bed material loaded) and 0.7 wt% would cause severe channeling and de-fluidization conditions.

There have been limited studies on the potential use of enzymes for enhancing the dewaterability of biosludge. The mechanisms for such enhancement have not been investigated despite the environmental advantages of using enzymes over synthetic polymers for biosludge conditioning. In order to find enzymes with this potential, a screening of commercially available enzymes was carried out using capillary suction time to assess biosludge dewaterability. The only enzyme that showed dewatering improvements in the screening tests was a lysozyme which reduced the capillary suction time by 36% and increased the cake solids content from 5.6 to 8.9 DS%. Lysozyme aided in the flocculation of particles reducing the polymer demand from 11% to 6%. Active and inactive lysozyme exhibited a similar ability for enhancing sludge dewatering, indicating that the conditioning mechanism of lysozyme is similar to that of a flocculant.

Burning biomass fuels generates large quantities of ash products which contain plant nutrients, such as Ca, Mg, P and K. To return these nutrients back to forest soil, biomass ash could be spread onto forest soil. Although biomass ash land application can decrease soil acidity and restore soil nutrient balance, its high alkalinity presents some challenges in terms of direct land application. The high alkalinity affects the leachability of various ash components and adversely affects ground vegetation. In this paper, we will show how CO2 can be used to effectively decrease ash alkalinity. The use of CO2 aqueous solubility and bicarbonate reactivity will be shown to be essential. From the study, we found that the rate of CO2 absorption depends heavily on the moisture content of the ash. Thus, the amount of water that needs to be added to the cooled ash products is important. The rate is also influenced by the CO2 partial pressure in the gas, up to almost 15% by volume. This concentration of CO2 is commonly found in treated flue gas providing an opportunity for CO2 capture. The work also reports on the effect of carbonation on the leachability of ash nutrients.

This study investigated orange-peel pyrolysis due to solar radiation that was applied as a unique source of energy using a parabolic-trough solar concentrator. An optical analysis was conducted using a Monte Carlo ray-tracing method to provide a detailed tridimensional description of the optical performance of the thermo-solar system. The average irradiance of the pyrolytic reactor surface was 15.65 suns. The peak irradiance was very similar to that calculated using the ray-tracing method, confirming that the operational conditions were optimal. The heat balance was analyzed by applying optical and thermodynamic principles. The main heat losses were caused by the reflectivity of the biomass (37.85%) and the difference between the temperatures of the environment and the reactor (36.23%). The solar pyrolytic reactor reached a peak temperature of 465 °C at the middle of the focal line. The total weight loss of the orange peels was 79 wt.% at an average irradiance of 12.55 kW/m2. Furthermore, substances valuable to the energy, chemical and pharmaceutical industries were identified in the bio-oil that was produced during solar pyrolysis, such as (Z)-9-octadecenamide, diisooctyl phthalate, squalene, d -limonene, and phenol.

Difficulties in dewatering of biosludge result in economical and environmental issues for wastewater treatment plants. Various attempts have been made to overcome this problem by achieving some pretreatment on biosludge. The main purpose of all pretreatment methods is to modify the biosludge characteristics in such a way to boost settling of cells and solid particles of sludge, and to ease the release of water molecules from extracellular polymeric substances and cells and to facilitate flow of water through forming filter cake. The present work presents an overview of different properties of sludge and their measurement, the main reasons of sludge dewatering difficulty, the fundamentals of sludge dewatering and various proposed methods for sludge pretreatment. The advantages and drawbacks of different methods are described and the dominance of one over the others is discussed mostly with respect to energy requirement and environmental impacts. Some recommendations have been made for optimal application of each method.

Sootblowers used to control deposit accumulation on boiler tubes consume substantial amounts of costly high pressure steam; this cost motivates the study of sootblowers and development of improved sootblowing strategies. This paper reviews recent research done on sootblowing at the University of Toronto on three fronts. First, the breakup behavior of brittle deposits impacted by a supersonic gas jet was experimentally studied. Results reveal that crack formation is vital to deposit breakup: to effectively break a deposit, the jet must drill into the deposit and crack it in the short time that the deposit is exposed to the jet. Second, the interactions between a supersonic jet and various tube geometries were visualized to understand sootblower jet flow between boiler tube bundles. Images show that sootblower jet flow and penetration between platens is strongly affected by any interaction between the jet and the first tube of the platen. Finally, a CFD model of a turbulent supersonic sootblower jet was developed. The model yields results that are in good agreement with available laboratory and in situ sootblower measurements.

Failure of brittle solid bodies due to the impingement of a high velocity air jet on the body surface is studied, experimentally and theoretically. Using the linear elastic theory and stress distribution analysis, a general criterion for the failure of brittle materials impacted by a gas jet is derived. Several special cases of jet–solid body interaction including failure of thin and thick layers and cylindrical objects immersed in a crossflow gas stream are investigated and proper material failure criteria are developed. These criteria correlate the minimum jet peak impact pressure (PIP) required to break the material to the material's tensile strength and Poisson's ratio. A series of experiments were performed using a laboratory-scale apparatus. Gypsum cast on steel tubes forming cylindrical samples was used as the model brittle material. Experimental data and high-speed breakup movies are employed to understand the gas jet–solid body interaction and to validate the theoretical criteria developed for the material failure. It is deduced that the failure of cylindrical samples impacted by a gas jet is by the formation and propagation of cracks. However, when the impact jet diameter is small, the cracks cannot propagate, and the material is failed due to localized surface pitting. One of the practical applications of this research is in Kraft recovery boilers, where high velocity supersonic steam jets are employed to remove deposits accumulated on the outer surfaces of the steam tubes.

Three pretreatment methods were compared based on their ability to increase the extent and rate of anaerobic bioconversion of pulp mill secondary sludge to biogas. The pretreatment technologies used in these experiments were: (i) thermal pretreatment performed at 170 degrees C; (ii) thermochemical (caustic) pretreatment performed at pH 12 and 140 degrees C; and (iii) sonication performed at 20 kHz and 1 W mL(-1). Sludge samples were obtained from a sulfite and a kraft pulp mill, and biochemical methane potential (BMP) assays were performed using microbial granules obtained from a high-rate anaerobic digester operating at a pulp mill. Biogas production from untreated sludge was 0.05 mL mg(-1) of measured chemical oxygen demand (COD) and 0.20 mL mg(-1) COD for kraft and sulfite sludge, respectively. Thermal pretreatment had the highest impact on sludge biodegradability. In this case, biogas yield and production rate from sulfite sludge increased by 50% and 10 times, respectively, while biogas yield and production rate from kraft sludge increased by 280% and 300 times, respectively. Biogas yield correlated to soluble carbohydrate content better than soluble COD.

The linearized solution for an inviscid imperfectly-expanded supersonic axisymmetric jet has been extended to the case of a turbulent flow, by taking into account the mean Reynolds stresses. The analytical results agree reasonably well with experimental data available in the literature, and so indicate that the solution is a good approximation to the near-field of an imperfectly-expanded jet. This analytical solution could be used to improve semi-empirical models of broadband shock-associated noise in aeronautics. To cite this article: B. Emami et al., C. R. Mecanique 337 (2009).

The breakup by supersonic air jet of cylindrical brittle gypsum deposits formed on tube surfaces was studied using high-speed imaging. The breakup of symmetric deposits was visualized from the front and back simultaneously, and the effects of deposit asymmetry, jet/deposit attack angle, and jet duration and frequency, were investigated. Three jet types were considered: short and long duration single pulses, and a pulsating jet. For each experiment, the time to breakup, the breakup duration, and the breakup length were measured. Results indicate that the breakup of asymmetric deposits depends on orientation, and that attack angle affects the probability of breakup and breakup length. Results also indicate that soft deposits can be broken by any jet type, while harder deposits, especially those farther from the nozzle exit, require a longer exposure time to be broken. Pulsating jets do not improve breakup effectiveness. This work has relevance to cleaning of fireside deposits formed on heat exchanger tubes in boilers.

Mixtures of peat with bark and peat with straw were burned in a large lab-scale entrained flow reactor under controlled conditions, and deposits were collected on an air-cooled probe controlled at four to six different probe surface temperatures between 475 and 625 °C. The results show that the probe surface temperature has no effect on the deposition rate when peat is burned. When burning bark, either alone or in mixtures with peat, the deposition rate decreases with increasing probe surface temperature. When burning straw, either alone or in mixtures with peat, the deposition rate increases with increasing probe surface temperature up to 550 °C and remains constant at higher temperatures. The Cl content in the deposits decreases with increasing probe surface temperature, regardless of the mixture composition. In deposits obtained from burning peat–bark mixtures, K appears as K 2 SO 4 when the deposition rate is low and as KCl when the deposition rate is high. In deposits obtained from burning peat–straw mixtures, no clear relationship is found between the deposition rate and the contents of Cl, S and K in the deposits.

Model biofuel ash of well-defined size and melting properties was fed into an entrained flow reactor (EFR) to simulate the deposition behaviour of commercially applied biofuel mixtures in large-scale boilers. The aim was to obtain consistent experimental data that can be used for validation of computational fluid dynamics (CFD)-based deposition models. The results showed that while up to 80 wt% of the feed was lost to the EFR wall, the composition of the model ash particles collected at the reactor exit did not change. When model ashes were fed into the reactor individually, the ash particles were found to be sticky when they contained more than 15 wt% molten phase. When model ashes were fed in mixtures with silica sand, it was found that only a small amount of sand particles was captured in the deposits; the majority rebounded upon impact. The presence of sand in the feed mixture reduced the deposit buildup by more than could be expected from linear interpolation between the model ash and the sand. The results suggested that sand addition to model ash may prevent deposit buildup through erosion.

Mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor under controlled conditions, and deposits were collected on an air-cooled probe at a temperature of 550 °C. The fuel and deposit compositions were compared using chemical fractionation analysis and SEM/EDX. Chemical fractionation analysis was capable of explaining the relative fouling tendency of peat, bark, and straw. The composition of deposits obtained from firing peat, bark, and straw individually resembled the composition of their ashes. When firing peat–bark and peat–straw mixtures, it was found that the deposition rate only started to increase when the Cl/S molar ratio in the feed ash exceeded 0.15. The composition of the ensuing deposits resembled the deposits obtained from burning either bark or straw individually. For peat–bark mixtures it was concluded that the presence of S in the feed suppresses deposition by sulfating chloride compounds, leading to deposits that contain less Cl and have less molten phase. For peat–straw mixtures it was concluded that the deposition behaviour is governed by other mechanisms than the interaction of Cl and S.

Specified mixtures of peat with bark and peat with straw were burned in a lab-scale entrained flow reactor that simulates conditions in the superheater region of a conventional biomass-fired boiler. The deposition rates were recorded on an air-cooled probe that was inserted into the reactor at the outlet. For both mixtures, the deposition behaviour followed a non-linear pattern, which suggests that physico-chemical interaction between the types of ash has taken place. Peat seems to act as a cleansing agent in all mixtures with straw, while it acts as a cleansing agent in mixtures with bark only up to a share of 50 wt% bark. Between 50 and 100 wt% bark, it seems that peat adds to the deposition. The results indicate that it is possible to burn up to 30 wt% bark (renewable biofuel and pulp mill waste) and up to 70 wt% straw (renewable biofuel and agricultural waste) in mixtures with peat (CO2-neutral fossil fuel) without encountering increased deposition rates.

In this work, an advanced CFD-based ash deposition model is presented that allows for the quantitative prediction of the transport of fly ash particles near furnace walls and heat exchanger surfaces, combined with a detailed description of the thermochemical properties of the particles that consist of salt mixtures melting over a wide temperature range rather than at one specific melting point. The physical and chemical phenomena considered in this approach are: particle melting, particle impaction, particle sticking as a function of particle melt fraction, restriction of deposit layer growth by melt flow, and particle rebounding from the furnace wall or superheater surface. The model is validated with entrained flow reactor experiments at well-defined conditions. Three model substances consisting of mixtures of sulphates and chlorides are fired in the reactor with particle sizes typical for biomass combustion in fluidised bed combustors, ranging from smaller than 75 to 250 μm. The results indicate that while the proposed deposition model is capable of predicting deposit buildup on cooled surfaces, a clear distinction between physical and chemical effects contributing to deposit buildup for different particle size classes has to be made. For fly ash particles smaller 75 μm, all particles containing 15% molten phase or higher will stick to cooled surfaces as soon as they are transported to them and as long as the deposit does not grow above the steady state thickness. Larger particles may follow the same trend, but the high probability of them rebounding is a function of the impact angle and needs to be considered in the model.

The sintering behavior of eight recovery boiler dusts was evaluated by pressing them into pellets, heating them at temperatures from 300 to 700 °C, and recording the change in diameter of the pellets with time. The eight dusts sintered at widely different rates that did not correlate well with dust composition. Analysis of the rate of sintering showed that dusts containing chloride began to sinter via evaporation–condensation of NaCl, but the mechanism eventually changed to solid-state diffusion. Dusts with less chloride changed sooner to solid-state diffusion sintering. The apparent rate constants for sintering by evaporation–condensation of NaCl correlated well with the first melting point of the dusts. This correlation provides a method for estimating the relative sintering tendency of dusts, and for evaluating process operational changes to reduce the sintering rate of deposits.