Effects of alkali and alkaline earth metal species on the combustion characteristics of single particles from pine sawdust and bituminous coal

Ash problems and prevention measures in power plants burning high alkali fuel: Brief review and future perspectives

Large-scale utilization of high-alkali fuels is considered an effective solution for alleviating energy shortages and reducing CO2 emissions. However, combustion of high-alkali fuels in boilers releases alkali metals into the flue gas, which leads to severe ash deposition and corrosion on the heating surface. Consequently, research into the efficient use of highly alkaline fuels has been conducted in recent years. In this review, ash issues and measures for their prevention during high-alkali fuel combustion are summarized. First, the characteristics of fly ash produced from high-alkali fuel combustion are reviewed, and the form, migration, and deposition characteristics of alkali metals are summarized. Subsequently, research progress of high alkali fuel ash is introduced in detail. Mechanisms of slagging, fouling, corrosion on the heating surface and the selective catalytic reduction (SCR) unit deactivation are summarized. Prevention and control methods for the high-alkali fuel ash problem are then introduced. Finally, based on current research, existing problems and future development directions for high-alkali fuel research are proposed. Through this review, we hope to provide insights into the effective utilization of high-alkali fuels.

Study on the effect of biomass on sulfur release behavior from dyeing sludge incineration: Focusing on in-situ sulfur fixation mechanism based on model compounds

Although incineration is a recommended disposal strategy for dyeing sludge (DS), sulfurous gases problem is severe. Wood sawdust (WS) and rice husk (RH) are eco-friendly and CO₂-neutral additives to relieve sulfur emission from DS incineration. However, the interaction between organic sulfur and biomass is uninterpreted. This study explores the effect of WS and RH on the combustion behavior and sulfur evolution from organic sulfur model compound combustion via thermogravimetry (TG) with mass spectrometry (MS). Results indicated that the sulfone and mercaptan combustion activities in DS were more drastic than in other forms. WS and RH additives generally deteriorated the combustibility and burnout performance of model compounds. The combustion of mercaptan and sulfone in DS contributed to most gaseous sulfur pollutants, where CH₃SH and SO₂ were the predominant forms. WS and RH minimized the sulfur release from mercaptan and sulfone incineration, whose in-situ retention ratios reached 20.14 % and 40.57 %. The retention mechanism to sulfur could be divided into: (1) Diffusion stage: the closed structure of biomass residue restrained sulfurous gases from escaping. (2) Chemical reaction stage: multiple sulfation occurred and inhibited sulfur release. Ca/K sulfate and compound sulfates were predisposed and thermostable sulfur-fixing products for the mercaptan-WS and sulfone-RH co-combustion systems.

Effects of alkali and alkaline earth metal species on the combustion characteristics and synergistic effects: Sewage sludge and its blend with coal

A large amount of alkali and alkaline earth metal species (AAEMs) are contained in sewage sludge (SS) ash, which will affect the combustion characteristics and synergistic effects of the co-combustion of SS and coal. The main objective of this paper is to investigate the effects of K, Ca, Na and Mg on combustion characteristics and synergistic effects of the blend of SS and coal by TGA and single particle combustion methods. The ash-free sludge (AS), impregnated AS and the blends of AS and bituminous coal (BC) were prepared. A high speed camera was used to record the combustion process, which were processed to calculate the ignition delay time, burnout time and combustion temperature. The synergistic effect of co-combustion process was studied by comparing the experimental results with the theoretical calculation results. The synergistic effects main occur at 350-530 °C and the blends of AS and BC with 2 wt% Na has the strongest strength of synergistic effects. The K, Ca and Na are conducive to the shorter ignition delay time, burnout time, higher combustion temperature and stronger synergistic effects of the blends. However, Mg will increase the delay time and reduce the combustion temperature of char of the blends. There are optimal concentrations of AAEMs on the promotion of combustion and synergistic effects, and too high concentrations of AAEMs will have negative impacts. K and Na (alkali metals) have stronger promotion effects than Ca and Mg (alkaline earth metals) on combustion of volatiles and synergistic effects.

Investigation of the Characteristics of Catalysis Synergy during Co-Combustion for Coal Gasification Fine Slag with Bituminous Coal and Bamboo Residue

As a kind of solid waste from coal chemical production, the disposal of coal gasification fine slag poses a certain threat to the environment and the human body. It is essential for gasification slag (GS) to realize rational utilization. GS contains fewer combustible materials, and the high heating value is only 9.31 MJ/Kg, which is difficult to burn in combustion devices solely. The co-combustion behavior of the tri-fuel blends, including bituminous coal (BC), gasification slag (GS), and bamboo residue (BR), was observed by a thermogravimetric analyzer. The TGA results showed that the combustibility increased owing to the addition of BC and BR, and the ignition and burnout temperatures were lower than those of GS alone. The combustion characteristics of the blended samples became worse with the increase in the proportion of GS. The co-combustion process was divided into two main steps with obvious interactions (synergistic and antagonistic). The synergistic effect was mainly attributed to the catalysis of the ash-forming metals reserved with the three raw fuels and the diffusion of oxygen in the rich pore channels of GS. The combustion reaction of blending samples was dominated by O1 and D3 models. The activation energy of the blending combustion decreased compared to the individual combustion of GS. The analysis of the results in this paper can provide some theoretical guidance for the resource utilization of fine slag.

Combustion of single particles from sewage sludge/pine sawdust and sewage sludge/bituminous coal under oxy-fuel conditions with steam addition

Co-pelletization of sewage sludge (SS) and conventional fuels for combustion is considered to be a feasible SS disposal method. Oxy-fuel combustion is recognized as a promising technology to reduce the emission of CO₂. In practical applications, the combustion atmosphere in oxy-fuel boiler is O₂/CO₂/H₂O, which is different from that in the conventional boiler (O₂/N₂). Therefore, the effects of gas composition on the combustion characteristics of boiler fuels should be both taken into consideration. In this work, the SS/pine sawdust (PS) and SS/bituminous coal (BC) blended fuel particles were prepared, and the single particle combustion experiments were conducted in O₂/N₂, O₂/CO₂ and O₂/CO₂/H₂O atmospheres. The influences of SS blending proportion and gas composition on combustion characteristics of fuel particles were analyzed. The results reveal that increasing the blending proportion of SS from 20 to 40 wt% decreases the ignition delay time, burnout time and combustion temperature. The substitution of N₂ by CO₂ increases the ignition delay time and burnout time, while decreases the combustion temperature. Replacing CO₂ by 10 vol%, 20 vol% and 30 vol% H₂O decreases the ignition delay time and burnout time, while increases the combustion temperature.

Estimation of Energy and Emissions Properties of Waste from Various Species of Mint in the Herbal Products Industry

The paper presents the results of research on the physicochemical properties of plant biomass consisting of four mint species, these being Mentha × piperita L. var. citrata Ehrh.—‘Bergamot’, Mentha × rotundifolia L., Mentha spicata L., and Mentha crispa L. The research conducted consisted of the technical analysis of biofuels—determining the heat of combustion and the calorific value of the material under study, and the content of ash, volatile compounds, and humidity. In addition, elemental analysis was carried out for the biomass under study by determining the content of carbon, hydrogen, nitrogen, and sulfur. The research demonstrated that Mentha × piperita L. var. citrata Ehrh.—‘Bergamot’ had the highest energy potential with a gross calorific value of 16.96 MJ·kg−1, and a net calorific value of 15.60 MJ·kg−1. Among the tested materials, Mentha × rotundifolia L. had the lowest content of ash at 7.23%, nitrogen at 0.23%, and sulfur at 0.03%, and at the same time had the highest content of volatile fraction at 70.36%. When compared to hard coal, the estimated emission factors indicated a CO reduction of 29–32%, CO2 reduction of 28–31%, NOx reduction of 40–80%, SO2 reduction of 92–98%, and dust reduction of 45–61%, depending on the type of biomass used.

Investigation of thermal conversion characteristics and performance evaluation of co-combustion of pine sawdust and lignite coal using TGA, artificial neural network modeling and likelihood method

(Co-)combustion of pine sawdust (PS) and lignite coal (LC) were investigated using artificial neural networks (ANN), particle swarm optimization (PSO), and Monte Carlo simulation (MC) as a function of blend ratio, heating rate, and temperature via thermal conversion characteristics. The order of degraded compounds in terms of hemi-cellulosic and lignin-based compounds demonstrated the main oxidation and degradation mechanism of co-combustion of PS and LC. The best prediction (R² of 99.99%) was obtained by ANN28 model. Operating conditions of 90LC10PS, 425 °C, and 19 °C min^-1 were determined by PSO as optimum levels with TG value of 67.5%. Once three-replicated validation experiments were performed under PSO-optimized conditions, mean TG values ware observed as 67.5% with a standard deviation of ±0.4%. Consequently, MC was used to identify the stochastic variability and uncertainty associated with ANN models that were derived to predict TG values.

High-speed tracking of fast chemical precipitations

Heterogeneous reactions taking place in the aqueous phase bear significant importance both in applied and fundamental research. Among others, producing solid catalysts, crystallizing biomorphs or pharmaceutically relevant polymorphs, and yielding bottom-up synthesised precipitate structures are prominent examples. To achieve a better control on product properties, reaction kinetics and mechanisms must be taken into account especially in dynamic systems where transport processes are coupled to chemistry. Since the characteristic time scale of numerous precipitation reactions falls below 1 s within the relevant concentration range, unique experimental protocols are needed. Herein we present a high-speed camera supported experimental procedure capable of determining such characteristic time scales in the range of 10 ms to 1 s. The method is validated both experimentally and by performing 3D hydrodynamic simulations.

Catalytic performance of potassium in lignocellulosic biomass pyrolysis based on an optimized three-parallel distributed activation energy model

The pyrolysis kinetics of extractive tobacco stem and pretreated samples with different KCl impregnation ratios were investigated by the thermogravimetric experiment and an optimized three-parallel distributed activation energy model (DAEM). The significant fitting deviation for the cellulose pyrolysis and the unrealistic partial fitting curve for the hemicellulose pyrolysis were mitigated during the optimization process by applying the Avrami-Erofeev-DAEM and reducing the latent interferences. The optimized parameters with good fitting qualities (about 2%) were obtained. Furthermore, based on the experimental results (changes in reaction intensity and temperature), model calculations (differences in reaction order, activation energy, volatiles fraction, etc.), and the maximum residual error analysis (with a high catalytic reaction rate) regarding different KCl-to-biomass ratios, it was found that KCl kinetically promoted the hemicellulose pyrolysis, which can be utilized as the theoretical support for the industrial application.

Effect of torrefaction on rice straw physicochemical characteristics and particulate matter emission behavior during combustion

In this work, the effects of different torrefaction temperatures and durations on the physicochemical properties of rice straw (RS), and the emission characteristic of PM₁₀ (particulate matter with aerodynamic diameters of ≤10 µm) during torrefied RS combustion, were investigated. Results indicate that the release of Cl and K, and decomposition of the organic matrix demonstrated a promoting effect during torrefaction. However, the removal of Cl and K did not reduce the emission of PM₁. The emission concentration of PM₁ and PM_1-10 generated from torrefied RS was enhanced, and the yields of PM_1-10 was much higher than those of PM₁. The concentrations of K and Cl in PM_1-10 increased with torrefaction temperature, combined with the microstructure, indicating that the torrefaction pretreatment promoted the heterogeneous condensation of KCl vapour to form PM_1-10.