Preferential Combustion Research Articles

Numerical calculation on combustion process and NO transformation behavior in a coal-fired boiler blended ammonia: Effects of the injection position and blending ratio

Ammonia (NH3), as a potential carbon-free alternative fuel, can be blended into coal-fired boiler to achieve significant pollution reduction and carbon reduction, but there are concerns about high NOx emissions due to high nitrogen content. According to the characteristics of coal/NH3 co-combustion, a dual-fuel co-combustion model with strong adaptability and high accuracy was established in this study through Chemkin software to study the influence of different injection positions and blending ratios on combustion characteristics and NOx generation process. Then, the co-combustion model was applied to the three-dimensional CFD calculation process of a 330 MWe front-fired boiler, and the combustion characteristics, NOx distribution and reaction process were calculated when cal. 20% NH3 was blended in the primary air. The results show that when cal. 20% NH3 is blended, the change of NO content mainly occurs in ignition zone and flame zone, and the transformation behavior of N in NH3 is optimized to a 15-step elementary reaction; The temperature distribution in the furnace is similar, and the average temperature at the furnace outlet decreases from 1033 °C to 988 °C, while NH3 have a preferential combustion reaction with air than coal, resulting in a decrease in the burnout rate of coal; The NOx concentration at the furnace outlet decreases from 355 mg/Nm3 to 281 mg/Nm3, which is 20.85% lower than that under the pure coal burning condition, and the variation range of O2 concentration and unburned NH3 concentration is small.

Effects of hydrogen fraction in co-injection gas on combustion characteristics of the raceway in low carbon emission blast furnace

Co-injection of coal and hydrogen-rich gas with oxygen blast at tuyere is a promising technology for low carbon emission blast furnace ironmaking, which favors the development of hydrogen-based reduction and affordable carbon capture. Effects of hydrogen fraction in the co-injection gas on the combustion characteristics in raceway are numerically studied. Results show that increase of hydrogen fraction in the co-injection gas accelerates the preferential combustion of the injected gas, which promotes the preheating and pyrolysis of pulverized coal. This compensates the oxygen-grabbing effect and improves the burnout of coal from 83.2% to 86.8% with the hydrogen fraction in co-injection gas increase from 20 vol% to 80 vol%. With the CO/H2 molar ratios in the co-injection gas of 8:2, 6:4, 4:6 and 2:8, the outlet CO/H2 molar ratios are 54:46, 42:52, 32:68 and 24:76 respectively. Combustion, gasification and water-gas shift reactions play conjoint roles in the redistribution of reducing gas species.

Effects of fire frequency on litter decomposition as mediated by changes to litter chemistry and soil environmental conditions.

Litter quality and soil environmental conditions are well-studied drivers influencing decomposition rates, but the role played by disturbance legacy, such as fire history, in mediating these drivers is not well understood. Fire history may impact decomposition directly, through changes in soil conditions that impact microbial function, or indirectly, through shifts in plant community composition and litter chemistry. Here, we compared early-stage decomposition rates across longleaf pine forest blocks managed with varying fire frequencies (annual burns, triennial burns, fire-suppression). Using a reciprocal transplant design, we examined how litter chemistry and soil characteristics independently and jointly influenced litter decomposition. We found that both litter chemistry and soil environmental conditions influenced decomposition rates, but only the former was affected by historical fire frequency. Litter from annually burned sites had higher nitrogen content than litter from triennially burned and fire suppression sites, but this was correlated with only a modest increase in decomposition rates. Soil environmental conditions had a larger impact on decomposition than litter chemistry. Across the landscape, decomposition differed more along soil moisture gradients than across fire management regimes. These findings suggest that fire frequency has a limited effect on litter decomposition in this ecosystem, and encourage extending current decomposition frameworks into disturbed systems. However, litter from different species lost different masses due to fire, suggesting that fire may impact decomposition through the preferential combustion of some litter types. Overall, our findings also emphasize the important role of spatial variability in soil environmental conditions, which may be tied to fire frequency across large spatial scales, in driving decomposition rates in this system.

Ketonuria may be associated with low serum amylase independent of body weight and glucose metabolism

Context: Ketonuria, which reflects a preferential combustion of lipids relative to carbohydrates, is often observed in lean rather than obese people. Clinical studies have shown that individuals with diabetes and/or obesity predispose to have low serum amylase (LSA).Objective: To investigate the association between ketonuria and LSA.Methods: We examined ketonuria assessed by dipstick urinalysis and clinical parameters including serum amylase in 3638 Japanese people aged 25–79 years who underwent a health-screening checkup.Results: There was an inverse relationship between body mass index (BMI) and serum amylase. The lowest serum amylase was observed in obese subjects (BMI ≥ 25.0 kg/m2) with positive ketonuria. Logistic regression analysis showed that ketonuria was significantly associated with LSA (<50 IU/L), which was not altered by the adjustments for relevant confounders including age, sex, BMI, and HbA1c.Conclusions: Current results suggest a relative unavailability of carbohydrates for energy production in individuals with LSA.

Estimating the heat transfer to an organic soil surface during crown fire

The Peatland Smouldering and Ignition (PSI) model was developed to quantify the heat transfer from a wildfire to an organic soil or moss surface in a Sphagnum–black spruce peatland. The Canadian Fire Behaviour Prediction system was used as a basis for the relationship between wind speed and rate of spread. Convection, conduction, and radiation processes were modelled before and during the arrival of the flaming front. The net heat flux to the soil from fire varied between 1.1 and 8.6 MJ m–2, with moderate-intensity fires transferring more energy to the surface compared with higher-intensity fires under higher winds. Radiative heat transfer to the soil surface both before the fire’s arrival and within the flaming front were the primary mechanisms of energy gain to the peatland surface. The role of convective and conductive cooling was no greater than 30% of gross energy gain. Peatland surface ignition in hummock and hollow microforms was modelled under normal and drought conditions. Hollow microforms dried out significantly during the course of a summer and increased their ignition vulnerability. Small-scale changes in slope and aspect of the peatland surface increased the amount of heat transferred by radiation by up to 30%, allowing some areas of higher soil moisture content to ignite. While no direct model validation is available, model outputs showing the preferential combustion of lichen and feathermoss, and the lack of ignition in Sphagnum in all but the most severe drought generally mimic observed ignitions patterns. The modelled peak of net energy input to the surface occurred at moderate wind speeds, suggesting that high-intensity fires do not necessarily lead to greater energy transfer and risk of smouldering combustion.

The Influence of Fire on the Radiocarbon Signature and Character of Soil Organic Matter in the Siskiyou National Forest, Oregon, USA

AbstractForest fires contribute a significant amount of CO2 to the atmosphere each year, and CO2 emissions from fires are likely to increase under projected conditions of global climate change. In addition to volatilizing aboveground biomass and litter layers, forest fires have a profound effect on belowground carbon (C) pools and the cycling of soil organic matter as a whole. However, the influence of fire on belowground organic matter cycling is not well defined and varies widely with fire severity. We measured soil organic matter (SOM) characteristics across a range of fire severities two years after the 2002 Biscuit Fire in southwest Oregon, USA, to address the following questions: (1) Which C pools are preferentially volatilized or transformed to charcoal under low-severity and high-severity fire? (2) How does fire change the distribution of SOM among density fractions and depths? (3) How does fire affect the general character of SOM including such variables as abundance, C:N ratio, 13C abundance, and 14C abundance? We examined soils from a mixed hardwood-evergreen forest across a range of burn severities: unburned, low severity, mixed severity, and high severity. Results indicated that increasing burn severity led to progressive loss of forest floor mass, but not to progressive loss of C from the mineral soil. Although fire significantly increased the charcoal content of the soils, fire did not significantly change the distribution of soil organic matter between heavy and free or light fractions. Other significant changes in soil organic matter characteristics included a progressive increase in nitrogen (N) with increasing burn severity, possibly due to the encroachment of N-fixing shrubs following the loss of native vegetation. Although qualitative changes in total root abundance following fire were noted, differences among burn severity treatments were not statistically significant. Increased concentrations of rock fragments in burned areas may be suggestive of erosion in these areas, consistent with previous studies documenting varying degrees of soil erosion following fire. In addition, although 13C abundances were similar among severely burned and unburned plots, soils from severely burned plots were significantly depleted in 14C in comparison to soils from unburned plots. This 14C depletion is most likely the combined result of erosion and preferential combustion of organics enriched in 14C relative to the bulk soil, perhaps reflecting a historical pattern of fire occurrence and severity across the landscape.

Investigation of HCCI combustion of diethyl ether and ethanol mixtures using carbon 14 tracing and numerical simulations

Despite the rapid combustion typically experienced in homogeneous charge compression ignition (HCCI), components in fuel mixtures do not ignite in unison or burn equally. In our experiments and modeling of blends of diethyl ether (DEE) and ethanol (EtOH), the DEE led combustion and proceeded further toward completion, as indicated by 14C isotope tracing. A numerical model of HCCI combustion of DEE and EtOH mixtures supports the isotopic findings. Although both approaches lacked information on incompletely combusted intermediates plentiful in HCCI emissions, the numerical model and 14C tracing data agreed within the limitations of the single zone model. Despite the fact that DEE is more reactive than EtOH in HCCI engines, they are sufficiently similar that we did not observe a large elongation of energy release or significant reduction in inlet temperature required for light-off, both desired effects for the combustion event. This finding suggests that, in general, HCCI combustion of fuel blends may have preferential combustion of some of the blend components.

Development of a Reactivity Test for Coal-Blend Combustion: The Laboratory-Scale Suspension-Firing Reactor

A laboratory-scale technique has been developed for assessing changes in the combustion reactivity of coal blends with blend composition. The test consists of the incomplete combustion of a batch of coal particles in a novel suspension-firing reactor. Residual chars are analyzed for composition and reactivity by nonisothermal thermogravimetry. Results from the bench-scale combustor were compared with observations from a full-sized power station and a single-burner pilot installation. Using the same sets of coals, the suspension-firing reactor gave the same order of reactivities as that observed in larger-scale equipment. Systematic preferential combustion of the higher-reactivity coal in blends has been identified; the extent of preferential combustion has been quantified from the thermogravimetric profiles of the residual chars. Our findings indicate that the reactivity of residual coal-blend chars is determined by the extent of enrichment of the lower-reactivity component in the blend. The coupled use of the new suspension-firing reactor and nonisothermal thermogravimetric analysis provides a powerful tool in comparing the combustion performance of different coal blends. Initial indications are that this simple system can be used as a predictive tool.

Factors influencing the stable carbon isotopic signature of methane from combustion and biomass burning

Factors controlling the δ13C of methane released by combustion include the combustion efficiency of the fire and the δ13C of the fuel. Smoldering fires produced13C‐depleted methane relative to hot, flaming fires in controlled forest and grassland burns and within a wood stove. Pine forest burns in the southeastern United States produced methane which ranged from −21 to −30‰, while African grassland burns varied from −17 to −26‰, depending upon combustion phase. African woodland burns produced methane at −30‰. In forest burns in the southeastern United States, the δ13C of methane released with smoldering was significantly13C depleted relative to methane released under hot flaming conditions. Methane released with smoldering was depleted by 2–3‰ relative to the fuel δ13C, but this difference was not significant. The δ13C of methane produced in a variety of wood stove conditions varied from −9 to −25‰ and also depended upon combustion efficiency. Similar results were found for methane produced by gasoline automobile engines, where the δ13C of methane varied from −9 to −22‰. For combustion occurring within the confining chamber of a wood stove or engine the δ13C of methane was clearly13C enriched relative to the δ13C of the fuel, possibly because of preferential combustion of12CH4in the gas phase. Significant quantities of ethylene (up to 25 to 50% of methane concentrations) were produced in southeastern U.S. forest fires, which may have consequences for physiological and reproductive responses of plants in the ecosystem. Methane production in these fires varied from 0.2 to 8.5% of the carbon dioxide production.

Silicon carbonitride derived from an organometallic precursor: Influence of the microstructure on the oxidation behaviour

The pyrolysis of the polymer issued from the octamethylcyclotetra-silazane (OMCTS) under nitrogen leads to amorphous silicon carbonitride residues containing high carbon levels. Their partial crystallization in α- Si 3 N 4 depends on the experimental parameters (temperature, time). If the pyrolysis is carried out in a stream of ammonia (a reactive gas), the carbon content of the ceramics was drastically reduced (<0·2 %wt), the remainder being α- Si 3 N 4. The study on the oxidation behaviour of these ceramic products investigated by TGA-MS allowed to point out the influence of pyrolysis conditions on the microstructure and, in particular, on the repartition of free carbon into the mineral residues. Oxidation of the partly crystallized SiCN products results from the preferential combustion of free carbon with CO and CO 2 departure, followed by a degradation of the silicon carbonitride with the release of gas such as CO, CO 2, NO, N 2 and traces NO 2 However, in the amorphous products, the carbon is oxidized with difficulty.

The effect of neutron-bombardment on the reactivity of graphite to oxygen

The effect of neutron -bombardment on the reactivity of graphite to oxygen has been investigated using a static apparatus operating at low pressure (10 −1 Torr). Bombardment introduces large numbers of unpaired electron centres, detectable by electron spin resonance (ESR), but these do not appear to function as active sites directly. Indeed in moderator specimens irradiated in carbon dioxide, the reactivity change was small enough to be obscured by mechanical damage introduced in the process of powdering the material. Irradiation of graphite in vacuum led to a marked increase in the rate of formation of carbon dioxide subsequently formed in reaction with oxygen and this is mainly attributed to preferential combustion of trapped interstitial species, the existence of which has come to light during the ESR work. The effect of neutron-irradiation on the formation of carbon monoxide is rather smaller and a different type of defect is believed to be responsible in this case, though its nature is not yet known.

The point of oxygen attack in the combustion of hydrocarbons I. The origin of carbon monoxide

An investigation is described of the slow combustion of some 14 C-labelled hydrocarbons. This work was designed to throw light on the point of initial attack in gaseous oxidation processes. Methods have been devised for the synthesis of 2-methylpentane labelled in each of the skeletal positions in turn. Combustion of these compounds with an insufficiency of oxygen was carried out at 242 and 380°C and the carbon monoxide formed was further oxidized by preferential combustion over copper oxide. The specific activity of the resulting carbon dioxide was then determined in the liquid phase after absorption in a solution of lithium phenyl. At both temperatures carbon monoxide was found to be derived principally from the 3- and 4-positions. Oxidation at a tertiary centre is unlikely, however, to lead to production of appreciable amounts of carbon monoxide and this may account for the low yield s from the 2-position. Nevertheless, it is concluded that the relative amounts of carbon monoxide formed from the 1-, 3-, 4- and 5-positions provide a reliable indication of the relative probabilities of initial attack at these points.

Societies and Academies

LONDON Royal Society, June 9.—William A. Bone: The combustion of hydrocarbons (with experiments) (Bakerian lecture). The principal researches and theories concerning the chemical mechanism of hydrocarbon combustion, from the time of John Dalton until now, are reviewed and discussed in three sections, namely: (1) nineteenth century discoveries, showing how the successive theories of preferential combustion, whether of hydrogen or carbon, failed; (2) development of the ‘hydroxylation’ theory (1900–15), together with the relevant experimental evidence from both slow and explosive combustion supporting it; and (3) recent developments—‘hydroxylation’ v. ‘peroxidation’, in which the two theories are discussed in the light of both old and new experimental evidence.