Air-staging Conditions Research Articles

Agglomeration behaviour of various biomass fuels under different air staging conditions in fluidised bed technology for renewable energy applications

The most common technology for thermochemical conversion of solid fuel through the process of gasification, such as coal, varieties of residues from biomass and agriculture are the bubbling fluidised bed or fluidised bed systems. However, there are some disadvantages within these systems including the formation of eutectic mixtures, increased process of melting and bed agglomeration. Hence, the current study aimed to evaluate the agglomeration behaviour of various biomass fuels. Characteristics of five biomass fuels (miscanthus, peanut, straw, domestic wood and industrial wood) in a bubbling fluidised bed (BFB) combustor were studied by SEM-EDX, XRD and XRF analysis of bed materials and ash samples. The influence of bed temperatures on the fuel ash composition and bed particles was evaluated, as well as fuel ash components, their interaction with the particles from the bed sand and morphology of the bed material. The combustion efficiency ranged from 95.5 to 99.5% with the highest value obtained for miscanthus under SA conditions. The peanut and straw had the highest K content, whereas the industrial wood ash contained the highest Ca content compared to other biomasses. The XRF results showed that most of the potassium (K) from the wheat straw biomass was converted to K2O with 19%. No bed agglomeration was noticed without staging-air (WSA) and with staging-air (SA) combustion conditions, even with temperatures up to 850 °C during the testing period. Overall obtained results demonstrated that agglomeration is unlikely expected to be a major problem during fluidised bed combustion of tested biomass fuels.

Minimization of inorganic particulate matter emissions with a novel multi-fuel combustion technology that enhances inorganic retention in a compact updraft fixed-bed

A novel biomass combustion technology was investigated that operates at a low oxygen content under fixed-bed and double air staging conditions. This technology was used to achieve extremely low NOX and particle matter emissions in a 30 kW lab-scale reactor, displaying high fuel flexibility and no slagging. In this experimental work, the aim was to minimize inorganic particulate matter emissions, this aim was achieved by enabling the very low release of inorganics such as K from the fixed bed, which operates like a compact updraft gasifier. The elemental composition of the employed fuels, emitted dust particles, and fuel particle samples taken at three different heights within the fixed bed, and the bed temperatures were measured. The main objective in this study was to determine and understand the different processes of inorganic matter release that take place within the compact fixed bed. The results show that 98% and 99.7% of the K could be retained in the fixed bed for wood chips and miscanthus pellets, respectively, thus minimizing the particulate matter emissions. Different processes in the context of K release within the fixed bed could be identified for silica rich/agricultural and calcium rich / woody fuels, respectively and inconsistencies in the literature on these mechanisms could be resolved. In the case of miscanthus pellets, K is retained in silicates, and no accumulation of K, Cl and S occurs in the fixed bed above. In the case of wood chips, on the other hand, there is an unexpected K accumulation in the fixed bed, which is due to the release of K in the hot oxidation zone and the subsequent formation of large amounts of K chlorides and sulfates by condensation in the cooler upper region. Furthermore, for woody fuels, bounding or intercalation of K into the char matrix plays a more important role than the formation of carbonates in avoiding K release from the bed.

Effects of Ammonia Solution and Pyrolysis Gas on NOx Emission from a 75 t/h Pulverized Coal Boiler

To explore methods of reducing NOx emission from pulverized coal boilers, the effects of injecting ammonia solution and pyrolysis gas into the furnace on NOx emission were experimentally investigated on a 75 t/h pulverized coal boiler. Results show that the deep air staging with 30% separated over fire air (SOFA) creates a high temperature and strong reducing atmosphere in the reducing zone, providing the prerequisites for NOx reduction by ammonia solution and pyrolysis gas. Compared with deep air staging itself, NOx emission can be reduced by 16.7% when ammonia solution is injected from the reducing zone with a normalized stoichiometric ratio of 2.0. However, NOx reduction efficiency is largely affected by its injection position. Similarly, NOx emission is decreased by 28.2% through injecting pyrolysis gas with its calorific value of 10% into the furnace, while a further increase of pyrolysis gas input will not increase NOx reduction efficiency. When ammonia solution and pyrolysis gas are simultaneously injected into the furnace under deep air staging conditions, the overall NOx reduction efficiency reaches 92.0% and NOx emission is decreased to 39.1 mg/m3. Considering the increasingly strict NOx emission standard, these findings can provide theoretical and practical guides to the future NOx reduction in pulverized coal boilers.

Effects of exhaust tube vortex on the in-furnace phenomena in a swirl-stabilized pulverized coal flame

This paper presents an experimental investigation of the effects of the exhaust tube vortex (ETV) on pollutant emissions and carbon burnout performance. The novelty in this study is to improve understanding of combustion behavior with the ETV structure via detailed in-furnace measurements (temperature, gas-phase species concentration, carbon burnout, emissions, and etc.) in a 16-kWth coal-fired furnace. The experiment considered two different swirling flames (co- and counter-swirl) with four air staging positions. A hollow tubular structure such as the ETV structure has been created under the co-swirl condition. The values of NOx emissions are about 430 ppm, but the CO emission is lower and carbon burnout is approximately 2% higher than that of the counter-swirl unstaging condition at the furnace outlet. For the air staging conditions, the air staging position 3 (SL3) maintained the generated ETV structure stronger in the primary combustion zone. The SL3 condition produced the lowest CO emission at about 275 ppm, and a relatively low emission of NOx showed with the highest carbon burnout performance of approximately 94.5% or more. The SL3 condition is the most favorable condition for the synergistic effect of air staging technology on the ETV structure with a low-NOx swirl-stabilized burner.

Experimental study on combustion characteristics of pulverized coal preheated in a circulating fluidized bed

A new technique to preheat pulverized coal in a circulating fluidized bed was adopted. This technique consists of two stages: the pulverized coal is first self-preheated in a circulating fluidized bed, and then the preheated fuel gas and char particles are burned out in a down-fired combustor under air-staging conditions. Experiments were conducted with two types of coals, two air ratios in the circulating fluidized bed and different air distributions in the combustor. The preheating and combustion processes ran stably under all experimental conditions. For both types of coals, the preheating temperatures in the circulating fluidized bed could be adjusted broadly within 800–950°C, and the ratios of primary air could be lowered to 0.15. The combustion efficiency and nitrogen conversion were remarkably influenced by the air distribution, and were controlled above 98% and below 10% respectively, although the two targets could not be simultaneously achieved.

Numerical simulation of the combustion characteristics and NOx emission of a swirl burner: Influence of the structure of the burner outlet

Numerical simulations on coal combustion and NOx emissions were performed for a centrally fuel-rich pulverized-coal swirl burner. By varying cone lengths of primary air, inner secondary air and outer secondary air, the burner outlet structure effect was disclosed in detail. The new findings which can deepen the understanding of the swirl burner performance are as follows. The ignition performance and NOx reduction capacity of burner increases as primary air cone length decreases. As the inner secondary air cone length increased, ignition performance of burners becomes poor and the NOx reduction capacity increases. The ignition performance and NOx reduction capacity of burners become stronger with increases in outer secondary air cone length. Under the conditions including that (i) outer secondary air cone length is three times of the outer secondary air duct diameter, (ii) inner secondary air cone length is 50–100% of outer secondary air cone length and (iii) primary air cone length is 0mm, the burner presents an excellent performance on NOx emissions and pulverized-coal combustion characteristics with air-staging conditions.

Ash behaviour and emission formation in a small-scale reciprocating-grate combustion reactor operated with wood chips, reed canary grass and barley straw

The emissions and ash behaviour during combustion of wood chips, or co-combustion of two solid agricultural fuels (reed canary grass and barley straw) with wood chips, were studied. In addition, the sensitivity of the results towards different air-staging conditions was investigated using pure wood chips. The experiments were carried out in a 40-kW combustion reactor equipped with a reciprocating-grate burner. The addition of the reed canary grass to the wood chips increased only slightly the emission of fine particles (PM1), nitrogen oxides (NOX) and sulphur dioxide (SO2); while carbon monoxide (CO), hydrogen chloride (HCl), organic carbon (OC), elemental carbon (EC) and the geometric diameter (GMD) of the particles either decreased or remained unchanged. However, the number of particles emitted increased 2-fold in the reed canary grass combustion compared to the pure wood chips. In contrast, the addition of straw to wood chips substantially increased the emissions of PM1, CO, EC, SO2 and HCl. The straw-originating particles were mostly crystalline KCl, and their number emission was clearly reduced, but their size was larger compared to the case with pure wood chips. The distribution of the combustion air had only a very minor influence on the release of the major ash species, whereas the effect was significant for the release of specific trace metals and the products of incomplete combustion. Finally, the partitioning of ash-forming elements with various fuels was evaluated based on chemical analyses of the fuel, bottom ash and fine fly ash fractions.

Evaluation of staged air and overfire air in regulating air-staging conditions within a large-scale down-fired furnace

To understand the deep-air-staging combustion performance and evaluate effects of staged air and overfire air (OFA) in regulating deep-air-staging conditions within a 600-MWe down-fired supercritical boiler, industrial-size measurements were performed in turn at three settings (i.e., damper opening partners of 30%:15%, 50%:15%, and 30%:40% for staged air and OFA, respectively). It was found that the staged-air effect on combustion and NOx emissions was opposite to that of OFA. At a shallow OFA opening of 15%, the furnace attained low carbon in fly ash of 4.47–5.24% and high NOx emissions of 1234–1360 mg/m3 at 6% O2. Under these circumstances, deepening air-staging conditions by increasing the staged-air damper opening from 30% to 50% essentially favored the NOx formation and improved burnout rate. With the staged-air opening fixed at 30%, opening OFA from 15% to 40%, on the contrary, acted as a positive role in strengthening air-staging conditions, i.e., reducing NOx emissions to levels of about 1000 mg/m3 at 6% O2 and raising combustible loss. The results suggested in the down-fired furnace, regulating deep-air-staging conditions to sharply reduce NOx emissions relied on opening OFA rather than staged air.

The effect of limestone on SO2 and NOX emissions of pulverized coal combustion preheated by circulating fluidized bed

A series of pulverized coal combustion experiments were carried out in order to investigate effects of limestone addition on SO2 and NOX emissions. The process takes place in two stages: first, the coal is preheated in a circulating fluidized bed with limestone addition; second, combustion of the resulting fuel gas and preheated char particles takes place in a down-fired combustor under air staging conditions. Four types of coal were used in the experiments including lignite, bituminous, anthracite, and semi-coke. Results showed that SO2 emission decreases after the addition of limestone to the circulation fluidized bed for all types of coal due to the reaction between H2S and CaO. The addition of limestone to the circulating fluidized bed was observed to have little effect on NOX emissions for low-volatile-content samples (anthracite and semi-coke), where as high-volatile-content coals (lignite and bituminous) displayed significant decreases in NOX emissions after the addition of limestone in this technique.

Combustion Flexibility of a Large-Scale Down-Fired Furnace with Respect to Boiler Load and Staging Conditions at Partial Loads

Down-fired boilers, designed especially for industry-firing low-volatile coals such as anthracite and lean coal, suffer similarly from various problems, such as poor burnout, high NOx emissions, and asymmetric combustion. A new down-fired combustion technology was developed specially for these problems and finally trialed in a 600-MWe supercritical boiler. To evaluate the flexibility of this technology application with respect to boiler load and air-staging conditions at partial loads, industrial-size measurements (taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in the flue gas, and carbon in fly ash) were performed within the furnace at different boiler loads (i.e., 370, 450, and 600 MWe) and staged-air damper openings (i.e., 10, 30, and 45% at 450 MWe), respectively. It was found that a relatively symmetric gas temperature distribution pattern developed for all settings, despite asymmetric burner operation models at 450 and 370 MWe. Decreasing boiler load reduce...

Combustion and NOx Emission Characteristics with Respect to Staged-Air Damper Opening in a 600 MWe Down-Fired Pulverized-Coal Furnace under Deep-Air-Staging Conditions

Deep-air-staging combustion conditions, widely used in tangential-fired and wall-arranged furnaces to significantly reduce NOx emissions, are premature up to now in down-fired furnaces that are designed especially for industry firing low-volatile coals such as anthracite and lean coal. To uncover combustion and NOx emission characteristics under deep-air-staging conditions within a newly operated 600 MWe down-fired furnace and simultaneously understand the staged-air effect on the furnace performance, full-load industrial-size measurements taken of gas temperatures and species concentrations in the furnace, CO and NOx emissions in flue gas, and carbon in fly ash were performed at various staged-air damper openings of 10%, 20%, 30%, and 50%. Increasing the staged-air damper opening, gas temperatures along the flame travel (before the flame penetrating the staged-air zone) increased initially but then decreased, while those in the staged-air zone and the upper part of the hopper continuously decreased and increased, respectively. On opening the staged-air damper to further deepen the air-staging conditions, O2 content initially decreased but then increased in both two near-wall regions affected by secondary air and staged air, respectively, whereas CO content in both two regions initially increased but then decreased. In contrast to the conventional understanding about the effects of deep-air-staging conditions, here increasing the staged-air damper opening to deepen the air-staging conditions essentially decreased the exhaust gas temperature and carbon in fly ash and simultaneously increased both NOx emissions and boiler efficiency. In light of apparently low NOx emissions and high carbon in fly ash (i.e., 696-878 mg/m(3) at 6% O2 and 9.81-13.05%, respectively) developing in the down-fired furnace under the present deep-air-staging conditions, further adjustments such as enlarging the staged-air declination angle to prolong pulverized-coal residence times in the furnace should be considered to improve the deep-air-staging combustion configuration.

An Experimental Study on NOx Emissions in Combustion of Pulverized Coal Preheated in a Circulating Fluidized Bed

This article describes an experimental study on NOx emissions in pulverized coal combustion after preheating and air staging. The process takes place in two stages: the pulverized coal is first preheated in a circulating fluidized bed, and then the resulting fuel gas and char particles are burned in a down-fired combustor under air-staging conditions. Experiments were conducted with three types of coal, three coal particle sizes, four preheating temperatures, and three air ratios in the circulating fluidized bed. With this process, combustion was stable, the temperature was uniform in the combustor after coal preheating, and the burning efficiency was high. The combination of air-staging combustion with pulverized coal preheated in a circulating fluidized bed was effective in reducing NOx emissions. NOx emissions from combustion of bituminous coal, anthracite coal, and semicoke were 368, 256, and 458 mg/N m3 (@6% O2), and the conversion rates of nitrogen in the fuel (fuel-N) were 28.66%, 23.94%, and 57.36%, respectively. The NO emissions and fuel-N conversion rate initially decreased then increased with increasing preheating temperature. The NOx emissions and fuel-N conversion rate decreased with increasing pulverized coal size and air ratio in the circulating fluidized bed.

Effects of Air Staging Conditions on the Combustion and NOx Emission Characteristics in a 600 MW Wall Fired Utility Boiler Using Lean Coal

A three-dimensional numerical investigation is presented on the coal combustion and NOx emission in a supercritical 600 MW wall fired utility boiler fed with lean coal. The distributions of velocity, temperature, and species were obtained using a finite volume method and some validated submodels. The influence of air staging condition, i.e., overfire air (OFA) ratio and OFA port position, on combustion and especially on NOx emission was studied. The temperature of the burner zone and the carbon content in fly ash increased with increasing OFA ratio, while the NOx emission decreased with increasing OFA ratio. An increase of the OFA port position resulted in a decrease of NOx emission and an increase of carbon content in fly ash. In addition, high burnout rate of pulverized coal and low NOx emission can be achieved in the present boiler, in which OFA was injected from Port 3 with a ratio of 34.2%. This study provides new insights into the physical and chemical processes in a wall fired utility boiler fed with lean coal and also illustrates a method to reduce NOx emission.

Overall Evaluation of Combustion and NOx Emissions for a Down-Fired 600 MWe Supercritical Boiler with Multiple Injection and Multiple Staging

To achieve significant reductions in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers, a deep-air-staging combustion technology was trialed in a down-fired 600 MWe supercritical utility boiler. By performing industrial-sized measurements taken of gas temperatures and species concentrations in the near wing-wall region, carbon in fly ash and NOx emissions at various settings, effects of overfire air (OFA) and staged-air damper openings on combustion characteristics, and NOx emissions within the furnace were experimentally determined. With increasing the OFA damper opening, both fluctuations in NOx emissions and carbon in fly ash were initially slightly over OFA damper openings of 0-40% but then lengthened dramatically in openings of 40-70% (i.e., NOx emissions reduced sharply accompanied by an apparent increase in carbon in fly ash). Decreasing the staged-air declination angle clearly increased the combustible loss but slightly influenced NOx emissions. In comparison with OFA, the staged-air influence on combustion and NOx emissions was clearly weaker. Only at a high OFA damper opening of 50%, the staged-air effect was relatively clear, i.e., enlarging the staged-air damper opening decreased carbon in fly ash and slightly raised NOx emissions. By sharply opening the OFA damper to deepen the air-staging conditions, although NOx emissions could finally reduce to 503 mg/m(3) at 6% O2 (i.e., an ultralow NOx level for down-fired furnaces), carbon in fly ash jumped sharply to 15.10%. For economical and environment-friendly boiler operations, an optimal damper opening combination (i.e., 60%, 50%, and 50% for secondary air, staged-air, and OFA damper openings, respectively) was recommended for the furnace, at which carbon in fly ash and NOx emissions attained levels of about 10% and 850 mg/m(3) at 6% O2, respectively.

Experimental study on combustion and NOx emissions for a down-fired supercritical boiler with multiple-injection multiple-staging technology without overfire air

A deep-air-staging combustion technology was previously developed to achieve reduction in NOx emissions and to eliminate strongly asymmetric combustion found in down-fired boilers. Recently, one of two down-fired 600MWe supercritical utility boilers using this technology (without applying overfire air) began commercial operations. To understand coal combustion and NOx emissions characteristics within the furnace, full-load industrial-size experiments were performed at different air-staging conditions with measurements taken of gas temperatures in the burner region, gas temperatures and species concentrations in the near wing-wall region, carbon content in fly ash, and NOx emissions. As expected, the furnace performance characterized by relatively timely coal ignition, symmetric combustion, and low levels of carbon in fly ash, developed in the furnace at all three settings. Deepening the air-staging conditions could reduce NOx emissions by one-fifth, but varied slightly carbon in fly ash. In view of the still high NOx production (i.e., 1036mg/m3 at 6% O2), adding an overfire air system which was essentially a part of the technology, was recommended for the boiler to significantly reduce the present NOx emissions.

NOx control in coal combustion by combining biomass co-firing, oxygen enrichment and SNCR

There has been renewed interest in evaluating NOx emission control by selective non-catalytic reduction (SNCR) combined with biomass co-firing to meet impending enforcement of NOx emission limits for power generation plant. Oxygen enrichment for the concentration of CO2 in the flue gas has been observed in this work to have benefits for NOx emission control. This paper presents new information on the effect of combining biomass co-firing with SNCR under various oxygen enriched and air-staging conditions performed in the 20 kW combustion facility. Biomass has a higher tendency to generate CO and produced better reductions in NOx emission with and without using SNCR. NO reduction of around 80% were attained using SNCR for 15% and 50% blending ratios of biomasses at 21% overall O2 concentration for unstaged combustion. Whereas, a range of 40–80% NO reductions were attained for coal (Russian Coal) and 15% co-fired biomasses with 3.1–5.5% overall O2 concentration at 22–31% levels of flame staging. Moreover, it was found that better NOx removal efficiency was attained for higher NOx emission baselines under both oxygen enriched and normal firing conditions. However, SNCR NOx control for both coal or coal-biomass blends was observed to produce higher NOx reductions during O2 enrichment, believed to be due to the self-sustained NOx reduction reactions. Hence, NOx control by SNCR, oxygen enriched co-firing in power station boilers would result in lower NOx emissions and higher CO2 concentration for efficient scrubbing with better carbon burnouts.

The effect of air staged, co-combustion of pulverised coal and biomass blends on NO x emissions and combustion efficiency

Co-firing of biomass residues with coal is continuously increasing in it’s application in coal-fired boilers for electricity production. In this study, co-firing experiments were performed using a Russian coal with a range of biomasses, shea meal (SM), cotton stalk (CS), sugarcane bagasse (SB T), sugarcane bagasse (SB R) and wood chips (WC) as biomasses in 5%, 10% and 15% thermal fractions to evaluate their potential as substitute fuel and an agent for NO x control. It was found that the addition of biomass increased NO reduction under both un-staged and air-staged conditions. However, NO reductions obtained under optimum conditions of primary zone stoichiometry (SR 1 = 0.9) and over-fire air (OFA) injection port location 3, were found to be significantly higher than un-staged co-firing for the same biomass thermal share in the fuel blend. It was found that the addition of biomass has a positive effect on carbon burnout under the optimum conditions that were determined in the study. A 10% biomass blending ratio (BBR) was found to be optimum for air-staging conditions. When co-fired under optimum air-staged conditions, a 10% BBR of sugarcane bagasse (SB R), shea meal (SM), wood chips (WC), cotton stalk (CS) and sugarcane bagasse (SB T) in coal gave NO reduction of 49%, 51%, 53%, 60% and 72%, respectively.

The Effect of HCl and SO2 on NOx Formation in Coal Flames

The effect of HCl and SO2 on NOx formation in coal flames was investigated experimentally and kinetically in an entrained flow combustion reactor (EFCR). Two bituminous coals (German “Goettelborn” and a Polish coal) were used as fuels with a feeding rate of 1 or 1.5 kg/h. HCl or SO2 is added into the primary air for injection into the reactor. Experimental results indicate that when HCl is added in the range from 0 to 610 ppmv, NOx emissions are reduced by 0−20%. The measured NOx profiles along the reactor length clearly show that the addition of HCl reduces NOx formation. Under air-staging conditions, HCl addition has almost no influence on NOx emissions presumably because of the different reaction mechanisms. The effect of added SO2 on NOx emissions is less significant than the effect of added HCl. A detailed kinetic mechanism is used to model the reactions, and the controlling reactions are analyzed.

Influence of coal quality on combustion performance

Three coals have been fired in a tangentially and an opposed fired full scale power plant (about 400 MW e) and in a pilot scale test rig (160 kW t) in order to investigate the influence of coal quality on nitrogen oxide (NO) formation and unburned carbon in relation to furnace design. In-flame and furnace gas temperatures have been measured, and substantial differences in temperature between the coals were observed in the full scale furnaces. Identical trends in NO emission as a function of coal type were obtained for the three furnaces. The emissions correlated well with the high temperature volatile yield obtained from heated wire mesh analysis. Under air staging conditions the pilot scale test rig was able to reproduce quantitatively the amount of NO from the tangentially fired plant, which operates with over fire air. This is probably due to the relatively small influence of the near burner mixing pattern on NO formation under these conditions. Emissions from the opposed fire plant with all combustion air introduced through the burners could only be qualitatively reproduced by the pilot furnace. Under single stage conditions the test rig provided higher NO levels. Carbon in ash levels did not show any correlation between the coals and the furnaces. An engineering, mathematical model has been developed describing radiation heat transfer and coal combustion in full scale furnaces. The model has been validated against measured temperatures and the amount of carbon in fly ash. The model was well able to predict average temperature and carbon in ash levels, but failed to predict the influence of coal quality on both temperature and carbon in ash. This may be due to a lack of knowledge on char reactivity and deactivation as a function of temperature and coal type. A brief parametric study has been performed on important model parameters.

98/01452 Measurement of the concentration of ammonia and ethene in the combustion chamber of a circulating fluidised-bed boiler: Åmand, L.-E. et al. J. Institute of Energy, March 1997, 70, 25–30

This paper presents the results of measuring the ammonia and ethene concentration profiles in the combustion chamber of a circulating fluidised-bed boiler. The boiler was operated in three air-staging conditions with peat as fuel, and with the addition of lime. Two sampling techniques were used for the analysis of ammonia: gas-sampling probe connected to a Fourier transformed infra-red (FTIR) instrument, and a gas-quenching (GQ) probe where the sample was quenched directly in the probe tip by a trapper solution. Both techniques are accurate enough to distinguish the influence on air staging of the level of ammonia in the combustion chamber. Comparison of the concentration profiles for ammonia measured by the two procedures shows similar results. The levels of ammonia and ethene measured by FTIR follow each other in the combustion chamber for all three staging conditions. The FTIR spectra were evaluated by three methods: spectral subtraction, differential absorbance and the multiple variable analysis method named 'partial least square' (PLS). Almost equal levels of concentration of ammonia and ethene were found, regardless of the method employed. The measurement error from reactions with ammonia at the GO probe tip was estimated in a simple model.