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Despite recent progress, NO x is still a major problem in biomass combustion, especially when using fuels with high nitrogen contents. The aim of this work was to determine the minimum NO x emissions achievable by applying primary measures in small-scale biomass grate furnaces. For this purpose, a new benchmark for furnaces using double air staging was defined on the basis of three different plants. An experimental study with different biomass fuels was conducted in two of these plants to systematically improve this benchmark. The results show that the new NO x benchmark using double air staging is up to 39 % lower than values achieved with conventional single air staging. Furthermore, this new benchmark was improved by extending the reduction zone, additionally reducing the NO x by up to 30 %. Secondary flue gas recirculation and partial load further reduced the NO x emissions. By combining the primary measures investigated in the 70-kW furnace, NO x emissions were achieved that were up to 70 % lower than those from other small-scale furnaces described in the literature. This efficiency also applies to fuels with a high nitrogen content, so that secondary NO x reduction measures can be avoided in most cases in furnaces in the future by taking this approach. [Display omitted] • A new benchmark for NO x emissions in double air staged biomass furnaces was set. • Double air staging led to a major reduction in NO x compared to single air staging. • Long residence times in the reduction zone improved the NO x reduction efficiency. • NO x emissions below 100 mg ⸱ m−3 at 13 vol% O 2 were reached using straw pellets. • A novel furnace achieved up to 70 % lower NO x emissions than commercial boilers. [ABSTRACT FROM AUTHOR]

To gain reliable data for the development of an empirical model for the prediction of the local high temperature corrosion potential in biomass fired boilers, online corrosion probe measurements have been carried out. The measurements have been performed in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4-5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Within the test runs the flue gas temperature at the corrosion probe has been varied between 625 °C and 880 °C, while the steel temperature has been varied between 450 °C and 550 °C to simulate typical current and future live steam temperatures of biomass fired steam boilers. To investigate the dependence on the flue gas velocity, variations from 2 m·s −1 to 8 m·s −1 have been considered. The empirical model developed fits the measured data sufficiently well. Therefore, the model has been applied within a Computational Fluid Dynamics (CFD) simulation of flue gas flow and heat transfer to estimate the local corrosion potential of a wood chips fired 38 MW steam boiler. Additionally to the actual state analysis two further simulations have been carried out to investigate the influence of enhanced steam temperatures and a change of the flow direction of the final superheater tube bundle from parallel to counter-flow on the local corrosion potential. [ABSTRACT FROM AUTHOR]

• Fuel-flexible biomass combustion technology with low emissions developed. • Compact updraft fixed bed served as an efficient filter for inorganic particles. • Temperature and minor element distribution measured across fixed bed height. • Different potassium retention mechanisms identified for woody and herbaceous fuels. • Conflicting literature on potassium release in fixed beds for wood clarified. 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 NO X 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. [ABSTRACT FROM AUTHOR]

This paper presents the verification of a hybrid reaction mechanism (28 species, 104 reactions) by means of a kinetic study with a view to its application for the CFD-based prediction of gas phase combustion and NOx formation in biomass grate furnaces. The mechanism is based on a skeletal kinetic scheme that includes the subsets for H2, CO, NH3 and HCN oxidation derived from the detailed Kilpinen 97 reaction mechanism. To account for the CH4 breakdown two related reactions from the 4-step global mechanism for hydrocarbons oxidation by Jones and Lindstedt were adopted. The hybrid mechanism was compared to the global mechanism and validated against the detailed Kilpinen 97 mechanism. For that purpose plug flow reactor simulations at conditions relevant to biomass combustion (atmospheric pressure, 1200-1600 K) for approximations of the flue gases in a grate furnace at fuel lean and fuel rich conditions were carried out. The hybrid reaction mechanism outperformed the global one at all conditions investigated. The most striking differences obtained in predictions by the hybrid and the detailed mechanism at the residence times prior to ignition were attributed to the simplified description of the CH4 oxidation in the case of the former. The overall agreement regarding both combustion and NOx chemistry between the hybrid and the detailed mechanism was better at fuel lean conditions than at fuel rich conditions. However, also at fuel rich conditions, the agreement was improving with increasing temperature. Moreover, it was shown that an improvement in the prediction of NOx formation by the N-subset of the hybrid reaction mechanism can be achieved by replacing its C-H-O subset with that of the detailed one. [ABSTRACT FROM AUTHOR]

While reasonably accurate in simulating gas phase combustion in biomass grate furnaces, CFD tools based on simple turbulence-chemistry interaction models and global reaction mechanisms have been shown to lack in reliability regarding the prediction of NOx formation. Coupling detailed NOx reaction kinetics with advanced turbulence-chemistry interaction models is a promising alternative, yet computationally inefficient for engineering purposes. In the present work, a model is proposed to overcome these difficulties. The model is based on the Realizable k-ε model for turbulence, Eddy Dissipation Concept for turbulence-chemistry interaction and the HK97 reaction mechanism. The assessment of the sub-models in terms of accuracy and computational effort was carried out on three laboratory-scale turbulent jet flames in comparison with the experimental data. Without taking NOx formation into account, the accuracy of turbulence modelling and turbulence-chemistry interaction modelling was systematically examined on Sandia Flame D and Sandia CO/H2/N2 Flame B to support the choice of the associated models. As revealed by the Large Eddy Simulations of the former flame, the shortcomings of turbulence modelling by the Reynolds averaged Navier-Stokes (RANS) approach considerably influence the prediction of the mixing-dominated combustion process. This reduced the sensitivity of the RANS results to the variations of turbulence-chemistry interaction models and combustion kinetics. Issues related to the NOx formation with a focus on fuel bound nitrogen sources were investigated on a NH3-doped syngas flame. The experimentally observed trend in NOx yield from NH3 was correctly reproduced by HK97, whereas the replacement of its combustion subset by that of a detailed reaction scheme led to a more accurate agreement, but at increased computational costs. Moreover, based on results of simulations with HK97, the main features of the local course of the NOx formation processes were identified by a detailed analysis of the interactions between the nitrogen chemistry and the underlying flow field. [ABSTRACT FROM AUTHOR]

[Display omitted] Around 2.7 billion people worldwide have no access to clean cooking equipment, which leads to major health problems due to high emissions of unburned products (VOC, CO and soot). A top-lit updraft gasifier cookstove with forced draft was identified as the technology with the highest potential for reducing harmful emissions from incomplete combustion in simple cookstoves. The basic variant of the stove was equipped with a fan for efficient mixing of product gas with air and fired with pellets to increase the energy density of low-grade residues. The development was conducted based on water boiling test experiments for wood and rice hull pellets and targeted CFD simulations of flow, heat transfer and gas phase combustion with a comprehensive description of the reaction kinetics, which were validated by the experiments. Emphasis was put on the reduction of CO emissions as an indicator for the burnout quality of the flue gas. The optimisation was carried out in several steps, the main improvements being the design of a sufficiently large post-combustion chamber and a supply of an appropriate amount of primary air for a more stable fuel gasification. The experiments showed CO emissions <0.2 g/MJ del for wood and rice hull pellets, which corresponds to a reduction by a factor of about 15 to 20 compared to the basic forced draft stove concept. Furthermore, these values are between 5 and 10 times lower than published water boiling test results of the best available cookstove technologies and are already close to the range of automatic pellet furnaces for domestic heating, which are considered to be the benchmark for the best possible reduction of CO emissions. [ABSTRACT FROM AUTHOR]

• Development of a fuel flexible biomass combustion technology with low emissions. • NO X emissions minimized at reduction zone air ratios between 0.8 and 0.95. • Gas temperature, NO, HCN and NH 3 distribution measured within the reduction zone. • Unexpected opposite trend of HCN/NH 3 ratio compared to typical grate combustion. • Identified mechanisms to minimize NO X emissions with novel technology. A novel biomass combustion technology with a compact fixed-bed operated with a low oxygen content and double air staging was investigated. Minimized flue gas emissions at high fuel flexibility were achieved only with primary measures. The fuel nitrogen conversion mechanisms were investigated in detail in the secondary zone of a 30 kW lab-reactor, designed as efficient reduction zone. Experimental investigations were carried out to determine the distribution of gas temperatures, main dry product gas components as well as NO X precursors such as NH 3 and HCN along the height of the reduction zone. The objective was to determine and understand the various fuel nitrogen conversion mechanisms in the reduction zone that can minimize NO X emissions. It was found that the HCN/NH 3 ratio increases with the fuel nitrogen content. This corresponds to an unexpected opposite trend to typical biomass grate furnaces. It was concluded that it is crucial for the HCN/NH 3 ratio whether the released nitrogen tars are already cracked in the fixed-bed or only in the gas phase, as in the novel technology. Furthermore, the influence of gas temperature, air ratio, mixing, recirculated flue gas and residence time on the formation and reduction of NH 3 , HCN and NO is discussed. Finally, this novel technology achieves NO X emissions of<95 mg·m−3 and 175 mg·m−3 for woody and herbaceous fuels, respectively, which is well below the small-scale state-of-the-art for the respective N contents and it achieves fuel nitrogen conversions to NO X in flue gas of 35% and 25%, respectively. [ABSTRACT FROM AUTHOR]

In order to describe and predict the formation of ash deposits in biomass fired combustion plants, a mathematical model is being developed and implemented into the CFD code Fluent(r) as a post processing tool. At the present state of development the model covers the release of coarse ash particles and ash-forming vapours from the fuel bed, the transport to furnace and boiler surfaces and the deposition of particles and vapours. Changes in the flue gas composition due to chemical reactions are considered by performing thermodynamic equilibrium calculations in order to determine the gas phase composition. The build-up of the deposit layer depending on wall temperature, deposit porosity and chemical composition is also taken into account. First results of modelling deposit formation for the furnace and fire tube of a grate fired combustion unit (boiler capacity 440 kWth) show plausible results but have yet to be validated. [ABSTRACT FROM AUTHOR]

In situ adaptive tabulation has been used to store equilibria states in a tree-like structure to be used in prediction of local condensation rates in CFD simulations of heat exchanger tubes. An interface to several tools to compute chemical equilibrium in flue gas compositions was developed. The results show a reduction of computation times at least by a factor of 8 (Table 4). Relative tolerances of 10-4 are sufficient to interpolate the table. Simulations indicate that mesh resolution is more important than a smaller tolerance. [ABSTRACT FROM AUTHOR]

High-temperature corrosion in biomass fired boilers is still an insufficiently explored phenomenon which causes unscheduled plant shutdowns and hence, economical problems. To investigate the high-temperature corrosion and deposit formation behaviour of superheater tube bundles, online corrosion probe as well as deposit probe measurements have been carried out in a specially designed fixed bed/drop tube reactor in order to simulate a superheater boiler tube under well-controlled conditions. The investigated boiler steel 13CrMo4–5 is commonly used as steel for superheater tube bundles in biomass fired boilers. Forest wood chips and quality sorted waste wood (A1–A2 according to German standards) as relevant fuels have been selected to investigate the influence on the deposit formation and corrosion behaviour. The following influencing parameter variations have been performed during the test campaigns: flue gas temperature between 650 and 880 ° C , steel temperature between 450 and 550 ° C and flue gas velocity between 2 and 8 m/s. One focus of the work presented is the detailed investigation of the structure and the chemical composition of the deposits formed as well as of the corrosion products. A further goal of the work presented was the development of an empirical model which can be used within CFD simulations of flow and heat transfer to calculate and evaluate the local corrosion potential of biomass fired plants already at the planning stage. The corrosion probe measurements show a clear dependency on the parameters investigated and the empirical function developed reproduces the measured corrosion behaviour sufficiently accurate. Since the additional calculation time within the CFD simulation is negligible the model represents a helpful tool for plant designers to estimate whether high-temperature corrosion is of relevance for a certain plant or not, when using fuels with similar compositions and the steel 13CrMo4–5. [ABSTRACT FROM AUTHOR]

• Combined experiments/kinetic simulation performed with urea-based SNCR in a biomass furnace. • Parameter study conducted of urea amount (NSR), temperature and residence time. • Over 90 % NO x reduction achieved at the optimum temperature of 950 °C. • 70% NO x reduction reached in steady state when complying with NH 3 legal limits. • Different NH 3 :HNCO ratios analysed for the urea decomposition. The aim of this work was to assess the potential NO x reduction achieved with urea-based SNCR in a small-scale biomass grate furnace operated with miscanthus pellets and forest biomass residues. Therefore, experiments were performed to test the main parameters of SNCR efficiency, namely the urea amount (normalised stoichiometric ratio), temperature and residence time, and the results were compared with those of a kinetic simulation applying ideal reactors in conjunction with a detailed nitrogen chemistry mechanism. The urea decomposition was estimated using different NH 3 :HNCO ratios, which were compared in the course of this work. The experimental and simulation results confirm an optimum temperature of around 950 °C, enabling a NO x reduction of over 90 % for both investigated fuels in this biomass furnace. However, at steady state, the NO x reduction was limited to 60–70 % due to the legal limits for NH 3. At lower temperatures or residence times, a significant increase in NH 3 emissions was observed. Due to the use of urea in this SNCR system, an increase in N 2 O emissions was detected, and the simulations also predicted non-negligible HNCO emissions. For a simulation of these two species, a suitable NH 3 :HNCO ratio for urea decomposition is needed. Finally, the optimum operating point for achieving 60–70 % NO x reduction and for complying with the NH 3 legal limits shows that a great potential for using SNCR exists in small-scale biomass furnaces, if the required temperature and residence time can be provided. [ABSTRACT FROM AUTHOR]

The relevance of the greenhouse gas N 2 O was investigated in two small-scale biomass combustion plants, a 30-kW fixed bed and a 70-kW moving grate furnace, both of which use double air staging. Furthermore, the 70-kW furnace was simulated with ideal reactor models, applying a detailed nitrogen chemistry mechanism to gain insights into N 2 O formation mechanisms that occur during double air-staged combustion. The experimental and simulated results confirm the existence of correlations between CO, NO x and N 2 O. For all investigated fuels, N 2 O reached the highest levels (increase of up to 300 %) when high CO and low NO x concentrations were present. Therefore, caution is advised when looking for minimum NO x emissions, due to the risk of having high N 2 O emissions, an aspect which is often overlooked. A deeper analysis of the N 2 O formation mechanisms revealed that N 2 O is mainly generated from HCN in the oxidation zone and it is thermally dissociated at high temperatures. Finally, a comparison between the results of both plants showed that the concern about high N 2 O emissions at minimum NO x concentrations is not a reactor-specific problem; therefore, it is necessary to ensure that CO and N 2 O are kept at a low level during NO x reduction with primary measures. • N 2 O is a concern in NO x reduction with primary measures. • High amounts of N 2 O occur especially at low NO x and high CO concentrations. • N 2 O is mainly formed from HCN in the oxidation zone at low temperatures. • A high temperature in the oxidation zone is needed for low CO and N 2 O emissions. [ABSTRACT FROM AUTHOR]

The aim of this work is to utilize various biogenic fuels without ash slagging and to significantly reduce NO X and particulate matter emissions in comparison to modern combustion technologies. For this purpose, a novel small-scale multi-fuel biomass grate furnace technology was developed and experimentally investigated. It employs a low oxygen concentration in the fixed-bed and a double air staging, including the supply of flue gas recirculation. In this way slagging is prevented on the grate, reducing the release of ash-forming volatiles, NO X emissions are minimized in the reduction zone and an efficient flue gas burnout is achieved in the tertiary zone. Wood pellets and chips as well as miscanthus briquettes were investigated. The measured total particle emissions showed a reduction of 68% for pellets and 70% for wood chips compared to typical small-scale furnaces. Furthermore, a reduction of NO X emissions of 39% for wood chips, 40% for wood pellets and 45% for miscanthus briquettes was achieved compared to typical small-scale furnaces. The experimental parameter study provided fundamental insights into the various mechanisms involved in this novel technology, which is close to market introduction, and proved its high fuel flexibility and great potential for particulate matter and NO X emission reduction. • Development of a fuel flexible biomass combustion technology with low emissions. • Compact bed design with low oxygen concentration worked as an efficient dust filter. • Supply of recirculated flue gas below the grate leads to high fuel flexibility. • Lowest NO X emissions due to an air ratio in the secondary zone between 0.8 and 0.9 • Supply of recirculated flue gas above the grate leads to even lower NO X emissions. [ABSTRACT FROM AUTHOR]

This work presents a three-dimensional Computational Fluid Dynamics study of a small-scale biomass combustion system operating with low primary air ratios. The Eulerian Biomass Thermal Conversion Model (EBiTCoM) was adapted to incorporate a pyrolysis mechanism based on the detailed Ranzi-Anca-Couce (RAC) scheme. Two scenarios were simulated using woodchips with 8% and 30% moisture content, and the results were validated against experimental data, including in-flame and bed measurements. The model accurately predicted bed temperature profiles and the influence of fuel moisture content on the pyrolysis and drying fronts, as well as on the distribution of volatiles and temperatures above the solid fuel bed. For the 8% moisture content case, the average gas temperature above the bed is approximately 700 °C, while for the 30% case, it drops to around 400 °C. The lower temperatures hinder the tar cracking reaction, resulting in a 25% higher tar content in the producer gas for the 30% moisture content fuel. The lower part of the bed consists of a thick layer of char undergoing reduction reactions, similar to that of an updraft gasifier. The developed model can accurately simulate biomass combustion systems with solid fuel beds consisting of numerous particles, while maintaining low computational requirements. [Display omitted] [ABSTRACT FROM AUTHOR]

• 1D model of fixed-bed biomass conversion with updraft configuration. • Investigations of fuel flexibility in boilers with such a bed conversion system. • Temperature profiles and producer gas compositions were accurately predicted. • Fuel moisture content, up to 30% w.b., does not affect the char conversion zone. • Temperatures on the grate can only be reduced with flue gas recirculation. Fixed-bed biomass conversion with a low primary air ratio and a counter-current configuration has a high feedstock flexibility, as it resembles updraft gasification, and the potential to reduce emissions when integrated in biomass combustion systems. A 1D bed model was validated with experimental results from a biomass combustion boiler with such a bed conversion system, predicting with a good accuracy the temperatures in the reactor and producer gas composition. The model was applied for different cases to investigate the fuel flexibility of this combustion system, including the influence of moisture content and the maximum temperatures achieved in the bed. It was shown that with variations in fuel moisture content from 8 to 30% mass w.b. the producer gas composition, char reduction to CO or maximum temperatures at the grate were not affected due to the separation of the char conversion and pyrolysis/drying zones. Flue gas recirculation was the only possible measure with the tested configuration to reduce the maximum temperatures close to the grate, which is beneficial e.g. to avoid slagging with complicated fuels. A higher tar content was obtained than in conventional updraft gasifiers, which is attributed to the absence of tar condensation in the bed due to the limited height of the reactor and the integration in the combustion chamber. The presented model can support the development of such combustion technologies and is a relevant basis for detailed CFD simulations of the bed or gas phase conversion. [ABSTRACT FROM AUTHOR]

• Model for K release during pyrolysis, KCl evaporation and carbonate dissociation. • Validated with single particle experiments with different fuels and temperatures. • Correct prediction of online K release at different stages and final release. The release of potassium during biomass combustion leads to several problems as the emissions of particle matter or formation of deposits. K release is mainly described in literature in a qualitative way and this work aims to develop a simplified model to quantitatively describe it at different stages. The proposed model has 4 reactions and 5 solid species, describing K release in 3 steps; during pyrolysis, KCl evaporation and carbonate dissociation. This release model is coupled into a single particle model and successfully validated with experiments conducted in a single particle reactor with spruce, straw and Miscanthus pellets at different temperatures. The model employs same kinetic parameters for the reactions in all cases, while different product compositions of the reactions are employed for each fuel, which is attributed to differences in composition. The proposed model correctly predicts the online release at different stages during conversion as well as the final release for each case. [ABSTRACT FROM AUTHOR]

This publication focuses on the experimental investigation of a novel small-scale fuel flexible biomass combustion technology with a fixed-bed employing a low oxygen concentration. It was obtained through a low primary air ratio and the additional supply of recirculated flue gas. The plant was operated with spruce wood chips, which contained three different mass fractions of water, and miscanthus pellets. All relevant components of the released gas above the fixed-bed were measured, as well as the 3D bed temperature distribution. The balances confirmed a high experimental data consistency. Therefore, it was possible to determine the location of the four different conversion zones inside the fixed-bed: drying, pyrolysis, char gasification and char oxidation. The reduction of CO 2 to CO in the char reduction zone worked efficiently across the entire grate area. Furthermore, the results showed that the water mass fraction of the fuel did not influence the dry product gas composition, but significantly affected the location for the release of pyrolysis products such as tars. It was found that the low oxygen concentration in the fixed-bed combined with flue gas recirculation was an effective method to reduce bed temperatures and therefore its inorganic emissions while significantly increasing feedstock flexibility. The investigations provided fundamental findings on the conversion and release behavior of the new technology under real operating conditions and are very useful for further experimental work and CFD simulations targeting the reduction of PM and NO X emissions. • Development of fuel flexible biomass combustion technology with low emissions. • Detailed investigation and characterization of fuel conversion and release behavior. • Compact bed design with low temperatures and high content of tar in released gas. • Supply of recirculated flue gas below the grate leads to high fuel flexibility. • Data consistency checked by mass and energy balance calculation. [ABSTRACT FROM AUTHOR]