Reburn Fuels Research Articles

Reburning of Animal Waste Based Biomass with Coals for NOx Reduction, Part II: Dairy Biomass (DB) and Coal–DB Blends

Concentrated animal feeding operations (both slaughter and dairy cattle) lead to land, water, and air pollution if waste storage and handling systems are not effectively managed. At the same time, cattle biomass (CB), which includes both slaughter/feedlot biomass (FB) and dairy biomass (DB), have the potential to be a source of green energy at coal-fired power plants. Part I presented results on NOx reductions with pure FB or Coal: FB blends as reburn fuels. Part II deals with results from reburning with pure DB or Coal: DB blends as reburn fuels. A mixture of NG with a small amount of NH3 was used to generate the baseline NOx of 400–420 ppm (or 185–194 g/GJ). NOx emissions were found to be reduced by as much as 96% when reburning with FB. The effects of reburn fuel type, equivalence ratio (ERRBZ) in the reburn zone, vitiated air, several injection configurations of reburn fuel and initial NO concentrations on NOx emissions were investigated. The ERRBZ shows a significant effect on the NOx reduction. The 20% heat input by reburning was the better operating condition for the long-term operation due to its ash production. The results reveal that reburn with DB fuels is an effective technology for NOx emission control when the initial NOx emission is higher than 275 ppm (or 127 g/GJ or 0.3 lb/MMBtu).

Reburning of Animal Waste Based Biomass with Coal for NOx Reduction, Part I: Feedlot Biomass (FB) and Coal:FB Blends

Cattle biomass (CB or manure from cattle) is proposed as reburn fuel under slightly fuel-rich conditions. The CB includes wastes from cattle feedlots (cattle grown in feedlots to slaughter weights of 450–640 kg) termed as Feedlot Biomass (FB) and cattle wastes from dairy farms termed as Dairy Biomass (DB). NOx emissions from coal-fired power plants can be reduced by using pure CB and Coal:CB mixtures as reburn fuels (10~30% by heat) injected after the primary combustion region. Experiments with Coal:CB mixtures as reburn fuels were performed using the 30 kW burner facility. Part I deals with results from experiments using pure FB and Coal:FB blends as reburn fuels while Part II presents results on extent of NOx reduction using pure DB and Coal:DB blends as reburn fuels (RF). In the current work, results on NOx emission are presented with FB and Coal:FB blends as RF. The parametric studies include: equivalence ratio in reburn zone (ERRBZ), vitiated air, angle of reburn nozzles, presence and absence of heat exchangers (HEXs), and baseline NOx concentration. The optimum operating conditions for FB reburning were found to be conditions with vitiation at ERRBZ = 1.1 with 45° upward counter-current injection in the presence of HEXs. NOx emissions were reduced by as much as 96%.

Effect of Re-burn Fuel Stream Location on NO Reduction in a Model Pulverized Coal Combustor

I is missing this work, a computational simulation has been performed to investigate the positional effect of reburn fuel injection on NO-reburn. Reburn fuel methane is injected across the coal injection plane at different axial positions of the combustor. Various major NO source mechanisms are considered for NO formation and NO reburn mechanism is used for NO depletion. Temperature profile, species concentration are also investigated, as both NO formation and depletion rate depends on these parameters. It has been observed that, a high temperature flame exists near coal inlet, when the reburn fuel injection plane is closer to coal inlet. On the other hand, the temperature of the flame near the coal inlet decreases when the reburn fuel injection position is far away from coal inlet region. Moreover, NO sources are observed near coal inlet region, when the reburn fuel is injected closer to coal inlet. On the other hand, only Fuel-NO is observed near coal inlet, when the reburn fuel is injected away from the coal inlet. Maximum NO reduction efficiency is observed at outlet plane when reburn fuel is injected closer to inlet, whereas a relatively lower NO reduction efficiency has been observed at outlet plane when reburn fuel is injected far away from coal inlet region.

A kinetic evaluation and optimization study on NOx reduction by reburning under pressurized oxy-combustion

Pressurized oxy-combustion is an emerging and more efficient technology for carbon capture, utilization, and storage than the first generation (atmospheric) oxy-combustion. NOx is a major conventional pollutant produced in pressurized oxy-combustion. In pressurized oxy-combustion, the utilization of latent heat from moisture and removal of acid gases (NOx and SOx) are mainly conducted in an integrated direct-contact wash column. Recent studies have shown that NOx particular inlet concentration should be maintained before direct contact wash column to remove NOx and SOx efficiently. As a result, minimizing NOx for environmental reasons, avoiding corrosion in carbon capture, utilization, and storage, and achieving effective NOx and SOx removal in direct contact wash columns are crucial. Reburning is a capable and affordable technology for NOx reduction; however, this process is still less studied at elevated pressure, particularly in pressurized oxy-combustion. In this paper, the kinetic evaluation and optimization study on NOx reduction by reburning under pressurized oxy-combustion was conducted. First, the most suitable mechanism was selected by comparing the results of different kinetic models with the experimental data in literature at atmospheric and elevated pressures. Based on the validated mechanism, a variety of parameters were studied at high pressure, i.e., comparing the effects of oxy and the air environment, different reburning fuels, residence time, H2O concentration, CH4/NO ratio, and equivalence ratio on the NO reduction. The results show that de-NOx efficiency in an oxy environment is significantly enhanced compared to the air environment. Improvement in the de-NOx efficiency is considerably higher with a pressure increase of up to 10 atm, but the effect is less prominent above 10 atm. The formation of HCN is significantly reduced while the N2 formation is enhanced as the pressure increases from 1 to 10 atm. The residence time required for the maximum NO reduction decreases as the pressure increases from 1 atm to 15 atm. At the higher pressure, the NO reduction rises prominently when the ratio of CH4/NO increases from 1 to 2; however, the effect fades after that. At higher pressure, the NO reduction by CH4 reburning decreases as the H2O concentration increases from 0 to 35%. The optimum equivalence ratio and high pressure for maximum NO reduction are 1.5 and 10 atm, respectively. This study could provide guidance for designing and optimizing a pressurized reburning process for NOx reduction in POC systems.

Thermodynamic and kinetic study of NO reduction by pyridine/pyrrole during biomass tar reburning: the quantum chemical method approach

NOx emission is a serious problem during fossil fuel combustion. Novel reburning fuels, namely, nitrogen-containing tar compounds, including pyridine and pyrrole were taken into consideration for NO reduction and investigated by quantum chemical method. Theoretical calculation results show that pyridine could first decompose into small species, such as CN and hydrocarbon radicals, then reduce NO to N2. The decomposition of pyridine should overcome quite high energy barrier and adsorb a large amount of heat. This pathway may occur at high temperature zone, such as core flame zone of pulverized coal fired boiler. As soon as pyridine decomposes, the active intermediates (CN radical, etc.) could reduce NO very quickly. The direct reduction of NO by pyridine molecule should overcome lower energy barrier, which could occur at wider temperature range. Pyrrole is less thermostable than pyridine; the energy barriers of pyrrole cracking and direct reduction of NO stay at same level. Therefore, the reburning efficiency of pyrrole may be better at lower temperature than pyridine. Theoretical calculations indicate that nitrogen-containing tar compounds produced during biomass pyrolysis/gasification could have good performance for NO reduction in pulverized coal fired boiler as well as biomass or MSW furnace.

Kinetic Study on NO Reduction Using Dimethyl Ether as a Reburning Fuel

This study experimentally and numerically investigates the reaction mechanism of NO reduction during reburning using dimethyl ether (DME) as the fuel. The experiments are conducted in a quartz reaction tube at barometric pressure in the temperature range of 800–1400 K, while detailed reaction paths with 73 species and 518 reversible reactions are accounted for by a kinetic study. The numerical results are in reasonably good agreement with the experimental results. The model predictions indicate that HCCO plays a principal role in NO reduction, while HCCO plays a minor role in DME decomposition. The most important reactions for NO reduction are HCCO + NO → HCN + CO2 and NCO + NO → N2 + CO2.

Experimental Study on Advanced Reburning for NOx Reduction by Pulsating Injection of Reburn Fuel

Reburning technology has been developed to adopt various commercial combustion systems. Fuel lean reburning is an advanced reburning method to reduce NOx economically without using burnout air, however it is not easy to get high NOx reduction efficiency. In the fuel lean reburning system, the localized fuel rich eddies are used to establish partial fuel rich regions so that the NOx can react with hydrocarbon radical restrictively. In this paper, a new advanced reburning method which supplies reburn fuel with oscillatory motion is introduced to increase NOx reduction rate effectively. To clarify whether forced oscillating injection of reburn fuel can effectively reduce NOx emission, experimental tests were conducted in vertical combustion furnace. Experiments were performed in flames stabilized by a gas burner. The natural gas is used as both main and reburn fuel and total thermal input is about 40kW. The forced oscillating injection of reburn fuel is realized by electronic solenoid valve, so that fuel rich region and fuel lean region is established alternately. In the fuel rich region, NOx is converted to N2 by reburning reaction, however unburned hydrocarbon and CO is oxidized in fuel lean zone and mixing zone at downstream where slightly fuel lean region is formed by mixing of two regions. The NOx reduction rate increases up to 41% by forced oscillating reburn motion. And this paper makes clear that in order to decrease NOx when oscillating reburn fuel injection system is adopted, the control of factors such as frequency and duty ratio is very important.

Syngas as a Reburning Fuel for Natural Gas Combustion

Abstract The paper aims to confirm the syngas application as a reburning fuel to reduce e.g. NO emission during natural gas combustion. The main aim of this modelling work was to predict pollutants generated in the exhaust gases and to indicate the influence of the syngas on the natural gas combustion process. The effect of residence time of fuel-air mixture was also been performed. Calculations were made with CHEMIKN-PRO for reburning process using syngas. The boundary conditions of the reburning process were based on experimental investigations. The addition of 5, 10, 15 and 19% of reburning fuel into natural gas combustion was studied. The effects of 0.001 to 10 s of residence time and the addition of 5, 10, and 15% of syngas on combustion products were determined. The performed numerical tests confirmed that co-combustion of the natural gas with syngas (obtained from sewage sludge gasification) in the reburning process is an efficient method of NOx reduction by c.a. 50%. Syngas produced from sewage sludge can be utilised as a reburning fuel.

Process Evaluation and Detailed Characterization of Biomass Reburning in a Single-Burner Furnace

Reburning technology is considered to be one of the most promising and cost-effective NOx reduction strategies for coal combustion systems. Although natural gas and coal have been the most widely studied reburn fuels, use of biomass in the reburning process is also attractive because of the nearly CO2-neutral character as well as the lower NOx and SO2 emissions of biomass, with respect to fossil fuels. Further studies on process evaluation and detailed characterization of biomass reburning, especially on the semi-industrial pulverized-coal furnace, are required before large-scale biomass reburning implementation can be realized. In the present study, process evaluation for the sawdust, rice husk, and dry sludge reburning have been performed on a semi-industrial pulverized-coal single-burner furnace. The detailed O2, NO, CH4, HCN, and NH3 concentration distributions in the reburn zone have been obtained through in-furnace measurements. The effects of the reburn fuel ratio and biomass carrier gas on the NO ...

Study on NO Reduction and Its Heterogeneous Mechanism through Biomass Reburning in an Entrained Flow Reactor

The effects of biomass types (including cotton stalk, wheat straw, rice husk, and rice straw), the stoichiometric ratio in the reburning-zone (SR2), the reaction temperature in the reburning-zone (t2), the particle sizes of biomass reburning fuels (dp), and the reburning fuel fraction (Rff) on NO reduction efficiency during biomass reburning were investigated systematically in an entrained flow reactor. The NO heterogeneous reduction mechanism resulting from the reburning of wheat straw and its char was analyzed. The results indicated that cotton stalk has the best performance of NO reduction, wheat straw is in second place, and rice husk and rice straw are less effective. In the range of t2 = 900–1100 °C NO reduction efficiency increases when the reburning-zone reaction temperature is increased at the same SR2. NO reduction efficiency increases insignificantly with a decrease in the particle size of the biomass with dp < 425 μm. NO reduction efficiency follows a pattern of first increasing and then decre...

Detailed measurements in a laboratory furnace with reburning

This article presents a detailed experimental characterization of the reburning process in a large-scale laboratory furnace. Natural gas, pine sawdust and pulverized coal were used as reburn fuels. Initially, the study involved the collection of in-flame combustion data, without reburning, in order to define appropriate locations for the injection of the reburn fuels. Next, flue-gas data were obtained for a wide range of experimental conditions using the three reburn fuels and, subsequently, detailed measurements of local mean O2, CO, CO2, HC and NOx concentrations, and gas temperatures have been obtained in the reburn zone for three representative furnace operating conditions, one for each reburn fuel studied. The flue-gas data revealed that the sawdust reburning leads to NOx reductions comparable or even higher than those attained with natural gas reburning, while coal reburning yields much lower NOx reductions. The detailed data obtained in the reburn zone indicates that the reburning process remains active throughout all the reburn zone in the cases of natural gas and sawdust reburning, while in the case of coal reburning its relatively low volatile matter content is insufficient to establish an effective reburn zone. In the cases of the sawdust and coal reburning the burnout levels remain approximately constant, regardless of the NOx emissions reduction, with the sawdust reburning leading to higher particle burnout performance than the coal reburning.

Effects of ash fouling on heat transfer during combustion of cattle biomass in a small-scale boiler burner facility under unsteady transition conditions

Combustion of cattle biomass (CB) as a supplementary fuel has been proposed for reducing emissions of NOx, Hg, SO2, and nonrenewable CO2 in large coal-fired power plants; however, its high ash content resulted in fouling and slagging problems when the CB was co-fired with coals during small- and pilot-scale tests. Ash depositions during combustion of the CB as a reburn fuel were investigated using a 30 kWt (100 000 Btu h−1) boiler burner facility with water-cooled heat exchangers (HEXs) under unsteady transition conditions and short-term operations. Two parameters were used to characterize the effects of the ash deposition: (1) Overall heat transfer coefficient (U) and (2) Burnt fraction (BF). A methodology was presented and empirically demonstrated for the effects of ash deposition on heat transfer under unsteady transition conditions. Experiments involving ash deposition during reburning the CB with coals were compared with experiments involving only ash-less natural gas. It was found that the growth of the ash layer during reburning the CB and coals lowered the heat transfer rate to water in the HEXs. In low-temperature regions, the thin layer of the ash deposition promoted radiation effects, while the thick layer of the ash deposition promoted the thermal resistance of the ash layer. A chemical analysis of the heavy ash indicated that the BF increased when a larger fraction of the CB was used in the reburn fuels, indicating better performance compared with coal combustion alone. However, the results of ash fusion temperature indicated the ash deposited during combustion of the CB and coals was more difficult to remove than the ash deposited during coal combustion alone. Copyright © 2010 John Wiley & Sons, Ltd.

Evaluation of Applying Low Calorific Fuel as Reburn Fuel in an Opposed Wall Fired Boiler

The alternative fuels, such as biomass, municipal wastes, and underground coal gasification gas, become attractive to the power plants as renewable energy sources or economical fuels. However, the alternative fuels usually have much lower heating value and different chemical compositions from those of coal and natural gas. Firing these alternative fuels in the boilers that are originally designed for coal firing or natural gas firing may cause unexpected boiler operating issues and/or thermal performance degradation. A careful evaluation study is often required prior to implementation. This paper presents the results of a study that evaluated the feasibility of using an underground coal gasification gas as a reburn fuel. The evaluation was done on Eskom’s Majuba Unit 5, a 710 MWe opposed wall-fired boiler, located in South Africa. The study utilized heat transfer analysis and computational fluid dynamics models to (1) evaluate the impacts of firing low calorific fuel on boiler efficiency and the boiler auxiliary system performance, (2) develop a conceptual gas reburn injection system, and (3) evaluate the impacts of gas reburn on the boiler thermal performance and boiler NOx emissions. The results indicate that the underground coal gasification gas can be an effective reburn fuel for the Majuba boiler with upgrades on the auxiliary systems.

Investigations of Ash Fouling with Cattle Wastes as Reburn Fuel in a Small-Scale Boiler Burner under Transient Conditions

Fouling behavior under reburn conditions was investigated with cattle wastes (termed as feedlot biomass [FB]) and coal as reburn fuels under a transient condition and short-time operation. A small-scale (30 kW or 100,000 Btu/hr) boiler burner research facility was used for the reburn experiments. The fuels considered for these experiments were natural gas (NG) for the ashless case, pure coal, pure FB, and blends of coal and FB. Two parameters that were used to characterize the ash “fouling” were (1) the overall heat-transfer coefficient (OHTC) when burning NG and solid fuels as reburn fuels, and (2) the combustible loss through ash deposited on the surfaces of heat exchanger tubes and the bottom ash in the ash port. A new methodology is presented for determining ash-fouling behavior under transient conditions. Results on the OHTCs for solid reburn fuels are compared with the OHTCs for NG. It was found that the growth of the layer of ash depositions over longer periods typically lowers OHTC, and the increased concentration of ash in gas phase promotes radiation in high-temperature zones during initial periods while decreasing the heat transfer in low-temperature zones. The ash analyses indicated that the bottom ash in the ash port contained a smaller percentage of combustibles with a higher FB percentage in the fuels, indicating better performance compared with coal because small particles in FB burn faster and the FB has higher volatile matter on a dry ash-free basis promoting more burn out.

Experimental evaluation and detailed characterisation of biomass reburning

Previous works demonstrate that biomass is a valid reburn fuel, displaying NOx reductions close to those obtained in natural gas reburning. However, available information on biomass reburning basically refers to parametric studies based on exhaust emissions, lacking a thorough description of the process. The present work is an attempt to provide a detailed experimental characterisation of biomass reburning, including parametric studies in propane and pulverised coal flames, as well as in-furnace spatial distributions of the relevant variables. The tests were performed in a semi-industrial-scale furnace. Both natural gas and sawdust were used as reburn fuels for a range of residence times (0.41–1.44 s) and stoichiometries (0.85–1.05) in the reburn zone. NOx and unburnt (CO and UHC) emissions were measured in all the tests. A detailed characterisation of the local temperatures and concentrations of CO, UHC, NOx, HCN and NH3 inside the reducing zone was accomplished for some selected cases.

Influence of Calcium Content of Biomass-Based Materials on Simultaneous NOX and SO2 Reduction

Pyrolysis products of biomass (bio-oils) have been shown to cause a reduction in NOx emissions when used as reburn fuels in combustion systems. When these bio-oils are processed with lime, calcium is ion-exchanged and the product is called BioLime. BioLime, when introduced into a combustion chamber, pyrolyzes and produces volatile products that reduce NOx emissions through reburn mechanisms. Simultaneously, calcium reacts with SO2 to form calcium sulfate and thus reduces SO2 emissions. This paper reports the characterization of composition and pyrolysis behavior of two BioLime products and the influence of feedstock on pyrolysis products. Thermogravimetric analysis (TGA) and 13C-CP/MAS NMR techniques were used to study the composition of two biomass-based materials. The composition of the pyrolysis products of BioLime was determined in a laboratory scale flow reactor. The effect of BioLime composition on NOx and SO2 reduction performance was evaluated in a 146.5 kW pilot-scale, down fired combustor (DFC). The effect of pyrolysis gas composition on NOx reduction is discussed. The TGAweight loss curves of BioLime samples in an inert atmosphere showed two distinct peaks corresponding to the decomposition of light and heavy components of the BioLime and a third distinct peak corresponding to secondary thermal decomposition of char. The study also showed that BioLime sample with lower content of residual lignin derivatives and lower calcium content produced more volatile compounds upon pyrolysis inthe combustor and achieved higher NOx reduction (15%). Higher yields of pyrolysis gases increased the NO reduction potential of BioLime through homogeneous gas phase reactions. Calcium in BioLime samples effectively reduced SO2 emissions (60-85%). However, addition of higher calcium content to the BioLime samples also appeared to inhibit the volatile yield and thereby lowered the NOx reduction.

THE EFFECTIVENESS OF REBURNING USING RICE HUSK AS SECONDARY FUEL FOR NO X REDUCTION IN A FURNACE

ABSTRACT The aim of the present work was to evaluate the effectiveness of the reburning process using biomass (rice husk) as reburn fuel in a large-scale laboratory furnace. For comparison purposes, tests were also conducted using natural gas and ethylene as reburn fuels. The paper reports new data for flue-gas emissions for a wide range of experimental conditions which quantify the effects of the reburn fuel fraction (energy basis), residence time in the reburning zone, and initial NO x concentration for the three secondary fuels on NO x reduction from the present combustor. The results show that at reburn zone residence times of about 0.7 s the reburning performance of the rice husk (1) is comparable to that of the natural gas reburning at high reburn fuel fractions, with almost 60% NO x reduction achievable at reburn fuel fractions of 25 and 30%, and (2) approaches those of the natural gas and ethylene at high initial NO x concentrations, with nearly 60% NO x reduction attainable at initial NO x concentrations between around 500 and 970 ppm. The results also reveal that there is a correlation between the extent of NO x reduction and particle burnout: the higher the reduction, the lower the burnout. However, the present results combined with previous data from the same furnace using coal as reburn fuel suggest that the use of rice husk as reburn fuel in full-scale equipment would have a lower impact in the carbon burnout as compared with coal.

The effect of the calcium in lignite on its effectiveness as a reburn fuel

The effect of the calcium present in lignite on the effectiveness of NO reduction in the reburning method has been studied in the 900–1100 °C temperature range. Lignites from three Polish mines were used. The experiments were carried out in a laboratory scale drop-tube furnace reactor. The demineralised lignites were loaded with Ca using: calcium acetate Ca(CH 3COO) 2, calcium hydroxide Ca(OH) 2 and calcium carbonate CaCO 3. For the reburning process the catalytic effect of the presence of calcium in lignite was found to be rather modest (approx. ±20%) in the temperature range studied. It was shown that up to 1000 °C the calcium added to the demineralised lignites improved their effectiveness as reburn fuels. The effect was almost reversed when the reburn temperature was raised to 1100 °C. With the impregnated lignites the extent of NO reduction appeared to be practically independent of the calcium compound used. Mineral matter in raw lignite exerts a similar catalytic effect to calcium introduced by impregnation.

Prediction of furnace and boiler performance: coal quality factor : Chudnovsky, B. et al. Proceedings of the International Technical Conference on Coal Utilization & Fuel Systems, 2002, 2, (27), 979–989

Previous works demonstrate that biomass is a valid reburn fuel, displaying NOx reductions close to those obtained in natural gas reburning. However, available information on biomass reburning basically refers to parametric studies based on exhaust emissions, lacking a thorough description of the process. The present work is an attempt to provide a detailed experimental characterisation of biomass reburning, including parametric studies in propane and pulverised coal flames, as well as in-furnace spatial distributions of the relevant variables. The tests were performed in a semi-industrial-scale furnace. Both natural gas and sawdust were used as reburn fuels for a range of residence times (0.41–1.44 s) and stoichiometries (0.85–1.05) in the reburn zone. NOx and unburnt (CO and UHC) emissions were measured in all the tests. A detailed characterisation of the local temperatures and concentrations of CO, UHC, NOx, HCN and NH3 inside the reducing zone was accomplished for some selected cases.

An experimental study on the combustion characteristics of a low nox burner using reburning technology

The combustion characteristics of a low NOx burner using reburning technology have been experimentally studied. The reburn burner usually has three distinct reaction zones which include the primary combustion zone, the reburn zone and the burnout zone by provided secondary air. NOx is mainly produced in a primary combustion zone and a certain portion of NOx can be converted to nitrogen in the reburn zone. In the burnout zone, the unburned mixtures are completely oxidated by supplying secondary air. Liquefied Petroleum Gas (LPG) was used as main and reburn fuels. The experimental parameters investigated involve the main/ reburn fuel ratio, the primary/secondary air ratio, and the injection location of reburn fuel and secondary air. When the amount of reburn fuel reaches to the 20-30% of the total fuel used, the overall NO reduction of 50% is achieved. The secondary air is injected by two different ways including vertical and parallel injection. The injector of secondary air is located at the downstream region of furnace for a vertical-injection mode, which is also placed at the inlet primary-air injection region for a parallel-injection mode. In case of the vertical injection of the secondary air flow, the NOx formation of stoichiometric condition at a primary combustion zone is nearly independent of the reburn conditions (locations, fuel/air ratios) while the NOx emission of the fuel-lean condition is considerably influenced by the reburn conditions. In case of the parallel injection of the secondary air, the NOx emission is sensitive to the air ratio rather than the fuel ratio and the reburning process often coupled with the multiple air-staging and fuel-staging combustion processes.