Low-temperature Combustion Characteristics Research Articles

A Review on the Application of Circulating Fluidized Bed Fly Ash in Building Materials

Circulating fluidized bed (CFB)-based coal burning technology is a type of clean coal combustion technology. Owing to its characteristics of low temperature combustion and desulfurization in furnace, the CFB fly ash is quite different from ordinary fly ash. Based on the comprehensive understanding of physical and chemical characteristics of CFB fly ash, this review summarizes the research progress of the application of CFB fly ash in Portland cement, magnesium oxysulfate (MOS) cement, solid-waste–based cementitious material (SWCM), concrete, geopolymer, artificial aggregate, and other building materials. The study shows that CFB fly ash can improve the early physical and mechanical properties and reduce the drying shrinkage properties for cementitious materials. Its unique chemical composition endows it with good application performance in the MOS cement, geopolymer, and SWCM. However, unfavorable characteristics of CFB fly ash, such as loose and porous structure, self-hardening behavior, and expansibility greatly hinder its practical applications in building materials. Therefore, some measures, such as addition of rational water-reducing agent, ultrafine grinding, mixing of ordinary fly ash, controlling the dosage of CFB fly ash, and water-cement ratio, must be taken to improve the negative influence of CFB fly ash on the building materials, in order to expand its application range. This article summarizes the application and research status of CFB fly ash in building materials, aiming at further promoting the research studies on CFB fly ash resource utilization in building materials and improving the efficiency of CFB fly ash resource utilization.

Particulate characteristics of low-temperature combustion (PCCI and RCCI) strategies in single cylinder research engine for developing sustainable and cleaner transportation solution

In this experimental study, particulate matter (PM) characterizations of different low-temperature combustion (LTC) strategies have been compared with conventional compression ignition (CI) combustion for finding out a sustainable and cleaner transport solution. LTC strategies included premixed charge compression ignition (PCCI) and reactivity-controlled compression ignition (RCCI) combustion. Particulate sampling and characterization were carried out in a single-cylinder diesel engine. All engine tests were performed at 1, 2, 3, 4 bar brake mean effective pressure (BMEP) at 1500 rpm. CI and PCCI combustion experiments were performed using mineral diesel as the test fuel. However, mineral diesel and methanol were used as high reactivity fuel (HRF) and low reactivity fuel (LRF), respectively in the RCCI combustion strategy. For all combustion strategies, fuel injection pressure (FIP) was kept constant at 500 bar. However, the number of injections and start of injection (SoI) timings were varied to optimize the engine performance. Results showed that the RCCI combustion strategy emitted a relatively lower concentration of particles than the other two strategies (PCCI and CI). A relatively higher number concentration of accumulation mode particles (AMP) compared to nucleation mode particles (NMP) in the exhaust of the RCCI combustion strategy was an important finding of this study. Number-size and mass-size distributions of particles emitted from different strategies also exhibited the dominant concentration of particles in the CI combustion strategy. PM bound trace metal analysis was yet another critical aspect of this study, which showed that both RCCI and PCCI strategies emitted a relatively lower concentration of trace metals than the conventional CI combustion strategy. Parametric analysis of different PM characteristics and NOx-PM trade-off analysis also demonstrated the importance of LTC strategies over the conventional CI combustion strategy. Overall, this study demonstrated that all LTC strategies could be used for PM and NOx reduction; however, the RCCI combustion strategy was more dominant in NOx and PM reduction, in addition to having an excellent capability of using alternative fuel in the quest for developing sustainable transport solution.

Implementation of multi-component diesel fuel surrogates and chemical kinetic mechanisms for engine combustion simulations

Recent developments have led to the formulation of multi-component surrogates that closely match physical and chemical properties of diesel fuels. These surrogates have been characterized and extensively tested in constant pressure vessels as well as engine experiments. The adoption of such surrogates in simulations is an important step towards predictive engine simulations. However, significant challenges remain in adoption of such surrogates. The complex chemical kinetics associated with a multi-component fuel leads to an extremely large and stiff chemistry mechanism with thousands of species. Such chemistry mechanisms are prohibitive to be directly used in engine simulations using traditional solvers due to the excessively-high computational cost. This work describes some of the experimental investigations of a 4-component diesel fuel surrogate in constant volume combustion chambers that provide useful validation datasets for numerical models. In addition, a chemical kinetic mechanism for the multi-component diesel surrogate with 1544 species is proposed. A previously validated unsteady flamelet model, accelerated by a sparse solver and analytical Jacobian, is used to implement large chemistry mechanisms for realistic diesel engine simulations. A flamelet solver is used to generate multi-dimensional unsteady flamelet libraries for the multi-component surrogate for a range of conditions. This approach is then used in a large eddy simulation (LES) framework to model combustion of the 4-component surrogate in a constant pressure spray vessel over a range of ambient oxygen conditions. The current chemistry mechanism and computational methodology can capture the two-step ignition delay as well as flame lift-off length trends observed in the experiments. Some of the low temperature combustion characteristics are analyzed in detail. The computational setup and the validation results demonstrate the feasibility of using these latest multi-component fuel surrogates for engine simulations by leveraging multi-dimensional chemistry tabulation methods.Highlights:1. Detailed experiments with the multicomponent V0A surrogate were carried out in the High Temperature Pressure Vessel facility.2. A detailed chemistry mechanism with 1544 species was developed for the V0A surrogate.3. The LSODES based flamelet tabulation method was used to implement and validate these high-fidelity surrogates and mechanisms in engine relevant LES calculations.

Detection of transient low-temperature combustion characteristics by ion current – The missing link for homogeneous charge compression ignition control?

Homogeneous charge compression ignition is a promising low-temperature combustion mode because of its high efficiency and low emissions; however, its strong cycle-to-cycle coupling effect, which caused by the recirculation of exhaust gases, may entail problems with low combustion stability. In this study, a new concept that extracts more comprehensive combustion information in homogeneous charge compression ignition is proposed through the integration of ion current and in-cylinder pressure sensing. To analyze the correlations of combustion parameters and their relationships with the ion current parameters, steady-state measurements were conducted. Dynamic measurements were implemented to form a comprehensive database for artificial neural network training. To investigate the hypothesis that the ion current gives additional information beyond the pressure trace, black-box models based on experimental data are trained. The results show that the baseline model trained purely with the manipulated variables has the worst performance, while the model including both in-cylinder pressure and ion current derived parameters has the best predictability, with the overall root-mean-square error reduced by 2.5% in predicting combustion phasing, compared with in-cylinder pressure based model. It demonstrates that a significant improvement in model quality can be achieved by the combination of ion current and in-cylinder pressure sensing, which indicates that the ion current signal contains information that goes beyond a sole analysis of the pressure trace. By complementing the in-cylinder pressure, the use of the ion current as a “chemical sensor” for low-temperature combustion thus appears very promising for the stable control of homogeneous charge compression ignition combustion.

Low-temperature combustion characteristics of heavy oils by a self-designed porous medium thermo-effect cell

The combustion characteristics of two Xinjiang heavy oils were studied by the self-designed porous medium thermo-effect cell (PMTEC) that can monitor the temperature alterations caused by oxidation/combustion reactions occurring within a porous medium under dynamic air flow. The results indicated that the heavy oils encountered obvious low-temperature combustion (LTC) in the temperature range of about 280–360 °C, accompanied by a maximum temperature increase to roughly 600 °C and significant release of CO and CO2. Then, PMTEC was adopted to evaluate the effects of air injection rate and water saturation on crude oil combustion. The increase of the air injection rate was conducive to triggering the LTC reactions and enhancing the intensity of LTC. The highest air injection rate (1 L/min in this work) led to the occurrence of fairly intense LTC reactions, during which large amounts of oxygen and oil sample were consumed. The increase of water saturation produced a minor influence on the occurrence of LTC, but obviously weakened the intensity of LTC. In addition, the influence of four metallic salts on the combustion behavior of heavy oil was examined using PMTEC. Cobalt naphthenate exhibited the good catalytic effect for promoting the occurrence of LTC. The mixed catalyst (cobalt naphthenate + manganese naphthenate) was found to have a better catalytic performance than any used single metallic salt. This mixed catalyst has a good potential for application in in-situ combustion of the Xinjiang heavy oil.

Evaluation of comparative engine combustion, performance and emission characteristics of low temperature combustion (PCCI and RCCI) modes

In this study, a comparative investigation of engine combustion, performance and emission characteristics of low temperature combustion modes namely premixed charge compression ignition (PCCI) and reactivity controlled compression ignition (RCCI) with conventional compression ignition (CI) combustion mode was performed. Experiments were performed in a single cylinder research engine at constant engine speed (1500 rpm) and at four different engine loads (1, 2, 3 and 4 bar brake mean effective pressure (BMEP)). Baseline CI and PCCI mode combustion experiments were performed using mineral diesel as test fuel; while mineral diesel-methanol fuel pair was used as high-reactivity fuel (HRF) and low-reactivity fuel (LRF) respectively in RCCI mode combustion. Results showed that RCCI mode combustion was relatively more stable compared to baseline CI and PCCI combustion modes. At higher engine loads, RCCI mode combustion exhibited relatively lower knocking and combustion noise than other combustion modes. Performance characteristics showed that brake thermal efficiency (BTE) of RCCI mode combustion was comparable to baseline CI mode combustion, however, at higher engine loads, RCCI mode combustion resulted in relatively higher BTE compared to both baseline CI and PCCI combustion modes. Significantly lower EGT of RCCI mode combustion compared to baseline CI as well as PCCI combustion modes was another important finding of this study. Emission results showed that RCCI mode combustion emitted relatively lower oxides of nitrogen (NOx), but significantly higher hydrocarbons (HC) compared to baseline CI and PCCI combustion modes. A NOx-BTE trade-off analysis was also carried out, which demonstrated the suitability of RCCI mode combustion at all engine loads. Finally, a parametric analysis was carried out to compare the critical parameters of baseline CI, PCCI, and RCCI combustion modes at low and high engine loads, which exhibited improved engine performance and emission characteristics of low temperature combustion (LTC) modes, especially the RCCI mode combustion.

Influence of Different Core Mechanisms on Low-Temperature Combustion Characteristics of Large Hydrocarbon Fuels

The core mechanism is the cornerstone of combustion reaction kinetic models and has a crucial impact on the prediction accuracy of large hydrocarbon combustion mechanisms. At present, there remains...

Experimental study of low-temperature combustion characteristics of shale rocks

Relatively low temperature (500 °C) combustion has been applied to shale rocks collected from Lianmuqing, Xinjiang Province, China, to improve the permeability of shale without pore structure change caused by mineral decomposition. The shale rocks were firstly grinded into small particles, then burned in a furnace at a constant temperature of 500 °C for 5 min, 15 min, and 30 min, respectively. A cylindrical shale sample was also subjected to the combustion experiment. It was found that thermal cracking occurred along the height of the shale as combustion propagated from bottom to top. From the low-temperature nitrogen adsorption test, it was found that the pore diameters of shale samples were in the range of 2–50 nm, which were less than the mean free path of an oxygen molecule. Thus, the diffusion of gas inside the shale was Knudsen diffusion and the Knudsen number (Kn) was between 8 and 25. Moreover, the mean diameter of shale pores and the effective diffusion coefficient increased with increasing oxidization time. Whereas the surface area decreased after combustion, the diameter of the shale particles remained constant. So the density of the shale decreased with increase of combustion time. According to the porous media combustion model, the oxidization of shale particles was considered to be in Regime I, which is under kinetic control at isothermal combustion condition (500 °C). Furthermore, the effective diffusion coefficient was in a range of 3 × 10−6–6 × 10−6 m2/s. It increased with increasing combustion duration, especially during the first five minutes. The experiment results showed that low temperature combustion can effectively improve shale permeability to facilitate gas extraction from shale reservoir.

Evaluation of the Thermal NO formation mechanism under low-temperature diesel combustion conditions

Over the past two decades, the amount of exhaust gas pollutants emissions has been significantly reduced due to the severe emission legislation imposed in most countries worldwide. Initial strategies simply required the employment of simple after-treatment and engine control devices; however, as the restrictions become more stringent, these strategies are evolving in the development of different combustion modes, specially characterized by having low-temperature combustion characteristics. These new working conditions demand the need to check the suitability of the current NO predictive models that coexist nowadays under standard diesel combustion characteristics, paying closer attention to the Thermal mechanism. In order to do so, a common chemical-kinetic software was employed to simulate, for n-heptane and methane fuels, fixed local conditions (standard diesel and low-temperature combustion) described by constant pressure, relative mixture fraction, oxygen mass fraction and initial and final reaction temperature. The study reflects a common trend between all the studied cases, independently of the considered local conditions, making it applicable to more complex situations such as real NO formation processes in diesel sprays. This relationship was characterized by a fourth-degree polynomial equation capable of substantially improving the NO prediction by just using the Thermal NO predictive model.

Dependence of premixed low-temperature diesel combustion on fuel ignitability and volatility

A comprehensive study of fuel property effects in internal combustion engines is required to enable fuel diversification as well as the development of applications to advanced engines for operation with a variety of combustion modes. The objective of this paper is to investigate the effects of fuel ignitability and volatility over a wide range of premixed low-temperature combustion (LTC) modes in diesel engines. A total of 23 fuels were prepared from commercial gasoline, kerosene, and diesel as baseline fuels and with the addition of additives, to generate a cetane number (CN) range from 11 to 75. Experiments with a single-cylinder diesel engine operated in moderately advanced-injection LTC modes were conducted to evaluate these fuels. The combustion phasing is demonstrated to be a good indicator to estimate the in-cylinder peak pressure, exhaust gas emissions, and thermal efficiency in the LTC mode. Fuel ignitability affects the combustion phasing by changing the ignition delay. The predicted cetane number (PCN) based on fuel molecular structure analysis can be fitted to the ignition delays with a higher coefficient of determination than CN, suggesting good potential as a fuel ignitability measure over a wide range. The stable operating load range in the smokeless LTC mode depends more on the actual ignition delay or PCN rather than CN. With fixed injection timing and intake oxygen concentration, O2in, only when PCN < 40, the load range can be expanded significantly to higher loads. By holding the combustion phasing at top dead centre and varying intake oxygen concentration, the nitrogen oxides and smoke emissions become limitations of the load expansion for some fuels. The effects of fuel volatility on the characteristics of LTC are small compared to ignitability. Finally, the operational injection timing range and robustness of the LTC to fuel ignitability are examined, showing that the advantageous ignitability range becomes narrower, with fuel ignitability decreasing.