Biomass Char Combustion Research Articles

A Systematic Approach to Determining the Kinetics of the Combustion of Biomass Char in a Fluidised Bed Reactor

A Systematic Approach to Determining the Kinetics of the Combustion of Biomass Char in a Fluidised Bed Reactor

Digital twin of biomass/coal co-firing circulating fluidized bed boiler by using computational fluid dynamics simulation

Combining biomass fuel with coal in circulating fluidized bed (CFB) boilers is attractive as the increasing of energy demand. The characteristics of biomass/coal co-firing in the industrial boiler are then investigated using computational fluid dynamics (CFD). In this study, fuels of interest are coal, wood chips, and bark. The fuel proportions were set for investigating their effects on the combustion using a simplex centroid mixture design. CFD simulations consist of a standard k-epsilon viscosity flow model, Euler multiphase fluid flow model, and a simplified biomass-char combustion reaction model. The predictive responses of the model are bed temperature, heat flux, and carbon dioxide (CO2) and carbon monoxide (CO) mass flows. The simulation results showed that the operation with coal gave the highest temperature and heat flux, followed by wood chips and bark. On the other hand, the operation with biomass releases fewer gas emissions, CO2 and CO, than those of the operation with coal. The type and proportion of boiler fuel can be easily determined using two methods: predictive equation and ternary contour plot graph, which can be likened to the digital twin of biomass/coal co-firing boiler.

Chemical Looping Combustion of a Biomass Char in Fe2O3-, CuO-, and SrFeO3−δ-Based Oxygen Carriers

This research focuses on the combustion of biomass char in fluidized beds of various particulate solids, which, under the conditions of the reaction, were either inert or capable of supplying oxygen to reactions. The latter were termed oxygen carriers. The solids used were SiO2, as an inert material, and three oxygen carriers: (1) Fe2O3 prepared from a natural pyrite ore, (2) CuO supported on mayenite, and (3) SrFeO3−δ strontium ferrite perovskite. Combustion experiments were undertaken by introducing a sample of partially devolatilized biomass (commercial “biochar”) to a hot bubbling bed (inner diameter of 30 mm), fluidized by a mixture of oxygen and nitrogen, then analyzing the composition of the off-gas and the burnout time of the char sample. In the temperature range investigated in this work (1023–1168 K), CuO and SrFeO3−δ but not Fe2O3 thermally decomposed, releasing gaseous O2 [so-called “chemical looping oxygen uncoupling” (CLOU)]. Hence, to make the combustion conditions comparable to various oxygen carriers, all experiments were performed using a fluidizing gas with a fixed partial pressure of O2 (pO2) of ∼0.015 bar. Despite the same nominal pO2, the occurrence of the oxygen uncoupling reaction increased the total net amount of O2(g) available in the process, affecting external mass transfer of O2 to the char particle and accelerating its rate of combustion. The time needed to totally combust 0.1 g of biochar particles in different beds at 1168 K followed the trend CuO < SrFeO3−δ < Fe2O3 ≈ silica sand. The difference in the performance of CuO and SrFeO3−δ was ascribed to the lower oxygen availability via CLOU in perovskite compared to copper oxide. Interestingly, combustion in the bed of Fe2O3 particles took a similar amount of time as combustion in the inert bed of SiO2, despite iron oxide playing an active role in the process. The finding is explained by Fe2O3 reacting with CO produced from incomplete char combustion, which results in the reduced oxide competing with char for O2(g) and effectively decreasing the local pO2.

Analysis of the rate of combustion of biomass char in a fluidised bed of CLOU particles

This research concerns the combustion of biomass char in a fluidised bed using Chemical Looping Combustion with Oxygen Uncoupling (CLOU). To evaluate the influence of the CLOU material on the rate of combustion, an analytical model has been developed, based on a simplified scenario, i.e. gaseous mass transfer external to the fuel particle was taken as a stagnant system, ignoring advective flow. The combustion of a char particle was modelled as a shrinking particle. Results from the model were compared with experiments performed by combusting char from birch-wood in a fluidised bed (i.d. 30 mm) of an active oxygen carrier (CuO supported on mayenite) or inert SiO2 sand. The experiments were undertaken with a partial pressure of oxygen, pO2, close to the equilibrium pressure of O2 of the Cu-based oxygen carrier. Despite the same pO2 for both experiments, the presence of the reaction of oxygen uncoupling from the oxygen carrier resulted in a significant increase in the combustion rate of char. As a result, at 1173 K, the burn-out time of 0.1 g of char particles with the oxygen carrier was around five times faster than with SiO2. The results from the analytical model of CLOU agreed with the experimental observations despite the simplified assumption of a stagnant system, viz. a system in which the mass transfer boundary layer, δ, is infinite. This is because the char combustion in CLOU depends, in fact, on gaseous mass transfer across an effective boundary thickness, δe, rather than a conventional boundary thickness for the system, δ. At 1023 K when the oxygen uncoupling did not occur, the enhancement in the rate of reaction due to the presence of the CLOU particle was less significant. The model was used to investigate the apparent enhancement of the combustion rate in CLOU, and possible explanations, including improved mass transfer, are discussed.

Formation of NO and N2O during Raw and Demineralized Biomass Char Combustion

The formation of nitrogen oxides (NO and N2O) during raw and demineralized biomass char combustion and the reduction of NO over biomass char were investigated. The biomass fuels were pine wood, straw, waste wood, bran, dried distillers grains with solubles (DDGS), and sunflower seed. Fixed-bed combustion experiments were performed at 800 °C in 10 vol % O2, while NO reduction experiments were conducted at temperatures from 800 to 900 °C and NO inlet concentrations from 400 to 1500 ppmv. The chars were characterized by means of inductively coupled plasma emission spectroscopy, Brunauer–Emmett–Teller, scanning electron microscopy and energy-dispersive X-ray, and X-ray photoelectron spectroscopy. The conversion of char-N to NO decreased with an increase in the initial char-N content, partly explained by the increased inherent conversion of char-N to N2O. The reduction of NO over char exhibited no correlation to the surface functionalities and content of nitrogen and oxygen at the investigated conditions. The NO reduction reactivity was strongly dominated by the content and association of ash-forming elements in the chars. The NO reduction reactivity of pine wood, waste wood, and straw chars correlated reasonably well with the (K + Ca)/C molar ratio, while the chars with a high phosphorous content, i.e., bran, DDGS, and sunflower seed chars, differed by showing a significantly lower reactivity. The inhibition effect of phosphorous on NO reduction reactivity was likely caused by the formation of less catalytically active potassium species (such as KPO3) in biomass char.

Conversion of char-N into NOx and N2O during combustion of biomass char

Char-N is the part of the nitrogen bound in the fuel (fuel-N) that remained in the char matrix during biomass pyrolysis, which is the initial of combustion. Therefore, char-N is responsible for the emission of NOx and N2O during combustion. In this study, biomass char was produced by pyrolysis of rice straw (RS) and pine sawdust (PS) under Ar atmosphere at 500–900 °C. The N-functionalities and the surface area of the chars were characterized by X-ray photoelectron spectroscopy and Brunauer–Emmett–Teller analysis, respectively. Then, combustion experiments of biomass char were conducted in Ar/O2 to explore the conversion of char-N and the proportion of fuel-N released as char-NO (char-NO/fuel-N). Besides, biomass char was additionally combusted in N2/O2 as comparison. Results showed that NO was the dominant N-containing product and little N2O was found. NO2 was not observed. Both NO and N2O were formed by direct oxidation of char-N. No obvious difference was observed during char combustion in N2/O2 and Ar/O2 environments. With the increase in temperature, the NO concentration generally increased whereas the reverse was observed for char-NO/fuel-N. The concentration of NO clearly increased as the O2 percentage increased, while NO/char-N and char-NO/fuel-N were not obviously affected. The heating time showed negligible influence on char-N conversion. The increase in the char mass led to dramatically decrease in the conversion of char-N and char-NO/fuel-N. However, no clear correlation could be revealed between the char-N conversion and the evolution of N-functionalities as well the surface area of the char.

Kinetic study of biomass char combustion in a low temperature fluidized bed reactor

The purpose of this work is the kinetic study of biomass char combustion in a low temperature fluidized bed reactor. This char was obtained from fast pyrolysis of beech stick in an annex batch fluidized bed reactor at 923K and atmospheric pressure. Operating conditions of the combustion were thoroughly characterized so that the reaction takes place in isothermal conditions and a constant oxygen partial pressure. The kinetic study was performed for temperatures up to 643K, oxygen partial pressures ranging from 5065 to 21273Pa and cylindrical char particles (D=4mm and L=9mm). The Volumetric Model was found to be in very good agreement with experimental data. Values of activation energy and reaction order with respect to oxygen are respectively equal to 144kJ/mol and 0.59. The reaction scheme during char combustion showed that char first reacts with oxygen to form CO which is further oxidized either at the particle surface or in the gas phase to produce CO2. Besides, it was found that the char reactivity in combustion is higher in a fluidized bed reactor compared to TGA. This was explained by diffusional effects of oxygen and CO oxidation within the crucible in the TGA.

Kinetic study and modelling of char combustion in TGA in isothermal conditions

The purpose of this work is the kinetic study of biomass char combustion in isothermal conditions in TGA. This char was obtained from fast pyrolysis of beech bark pellet in a fluidized bed reactor at 850°C and atmospheric pressure. Kinetic study of isothermal char combustion was performed for temperatures up to 400°C, oxygen partial pressures ranging from 5065 to 21,273Pa and a char particles size of 25μm. Mass transfer effects around and within the crucible were thoroughly characterized by naphthalene vaporization. Oxygen diffusion was found to have no effect on char combustion for temperatures below 400°C. A novel method including the transfer function of the TGA which describes the variation of oxygen partial pressure just after switching the gas from inert to reactive in the TGA was taken into consideration in the kinetic modelling. Two kinetic models (the Grain Model and the Random Pore Model) were used to determine kinetic parameters. The Grain Model was found to be in very good agreement with experimental data. Values of activation energy and reaction order with respect to oxygen are respectively equal to 124kJ/mol and 0.74. Besides, the maximum combustion rate commonly observed in the literature during char combustion was found to be the result of the non-uniform oxygen partial pressure in the TGA at the initial stage of the char combustion.

Experimental Study on Biomass Char Combustion in O&lt;sub&gt;2&lt;/sub&gt;/CO&lt;sub&gt;2&lt;/sub&gt; Atmosphere

Experiments of biomass char combustion in 10%O2/90%CO2, 21%O2/79%CO2, 10%O2/90%N2 and 21%O2/79%N2 atmospheres were performed in the fixed-bed reactor system at the temperature of 800°C-1200°C. The results show that at the same temperature, NO emission is higher in O2/N2 atmosphere than that in O2/CO2 atmosphere. The higher the O2 concentration is, the higher the NO yield is. As the temperature rises, the NO yield rises at first and then decreases at the same O2 concentration and atmosphere.

Survey of influence of biomass mineral matter in thermochemical conversion of short rotation willow coppice

Short rotation willow coppice (SRC) has been investigated for the influence of K, Ca, Mg, Fe and P on its pyrolysis and combustion behaviours. These metals are the typical components that appear in biomass. The willow sample was pretreated to remove salts and metals by hydrochloric acid, and this demineralised sample was impregnated with each individual metal at the same mol g–1 biomass (2˙4 × 10–4 mol g–1 demineralised willow). Characterisation was performed using thermogravimetric analysis (TGA), and differential thermal analysis (DTA) for combustion. In pyrolysis, volatile fingerprints were measured by means of pyrolysis–gas chromatography–mass spectrometry (PY–GC–MS). The yields and distribution of pyrolysis products have been influenced by the presence of the catalysts. Most notably, both potassium and phosphorous strongly catalysed the pyrolysis, modifying both the yield and distribution of reaction products. Temperature programmed combustion TGA indicates that combustion of biomass char is catalysed by all the metals, while phosphorus strongly inhibits the char combustion. In this case, combustion rates follow the order for volatile release/combustion: P>K>Fe>Raw>HCl>Mg>Ca, and for char combustion K>Fe>raw>Ca–Mg>HCl>P. The samples impregnated with phosphorus and potassium were also studied for combustion under flame conditions, and the same trend was observed, i.e. both potassium and phosphorus catalyse the volatile release/combustion, while, in char combustion, potassium is a catalyst and phosphorus a strong inhibitor, i.e. K impregnated>(faster than) raw>demineralised »P impregnated.

Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part 1 — Model development and sensitivity analysis

A comprehensive model for steady state combustion of biomass char in a fast fluidized bed is presented, although the model is capable to deal any fluidization regime. The air needed for combustion is staged as bottom, primary and secondary air. The combustor is divided into two sectors: a dense (bottom) zone and a transport (upper) zone. The model consists of sub-models for bed hydrodynamics, conversion and conservation. The hydrodynamic description of the bottom bed is based on the modified two phase theory while the freeboard is based on a simplified core-annulus approach. Biomass char is modelled as partially volatile fuel. Char composition is a model parameter and is defined by weight fractions of C, H and O. The model includes the release of volatiles from biomass char and their subsequent combustion. The combustion reactor also serves as a sink for spent organic solvent. Char mass balance and energy balances are solved globally in each zone. The sensitivity analysis shows that the model is not very sensitive to changes in operating parameters. The model is robust to deal with solid, liquid and gaseous fuel combustion.

Model for biomass char combustion in the riser of a dual fluidized bed gasification unit: Part II — Model validation and parameter variation

The two-phase combustion model for biomass char combustion in a riser of a dual fluidized bed gasification unit that has been presented in part I is validated using the data obtained from the 8 MWth dual fluidized bed reactor at Guessing/Austria. The model is capable of calculating the average temperatures in all zones, the gas phase composition, solid hold up, char feed rates and air ratio. The model predictions for the temperature profile along the riser and for the exiting gas composition are in good agreement with the measured values. The simulation results show that the residual char from the gasifier is only partly converted in the riser for char particles larger than 0.6 mm. Un-combusted char is circulated back into the gasification reactor. Parameter variations show that the exact location where additional liquid fuels are introduced in the middle zone of the riser does not affect the global behaviour of the combustion reactor. Based on the simulation results it is proposed that external supply of char (additional) may be a very effective method for reducing producer gas recycling to the riser, which is currently necessary to obtain the desired gasification temperatures.

The influence of fine char particles burnout on bed agglomeration during the fluidized bed combustion of a biomass fuel

The combustion of biomass char in a bubbling fluidized bed is hereby addressed, with specific reference to the influence that the combustion of fine char particles may exert on ash deposition and bed agglomeration phenomena. Experiments of steady fluidized bed combustion (FBC) of powdered biomass were carried out with the aim of mimicking the postcombustion of attrited char fines generated in the fluidized bed combustion of coarse char. Experimental results showed that the char elutriation rate is much smaller than expected on the basis of the average size of the biomass powder and of the carbon loading in the combustor. Samples of bed material collected after prolonged operation of the combustor were characterized by scanning electron microscopy (SEM)–EDX analysis and revealed the formation of relatively coarse sand–ash–carbon aggregates. The phenomenology is consistent with the establishment of a char phase attached to the bed material as a consequence of adhesion of char fines onto the sand particles. Combustion under sound-assisted fluidization conditions was also tested. As expected, enhancement of fines adhesion on bed material and further reduction of the elutriation rate were observed. Experimental results are interpreted in the light of a simple model which accounts for elutriation of free fines, adhesion of free fines onto bed material and detachment of attached fines by attrition of char–sand aggregates. Combustion of both free and attached char fines is considered. The parameters of the model are assessed on the basis of the measured carbon loadings and elutriation rates. Model computations are directed to estimate the effective size and the peak temperature of char–sand aggregates. The theoretical estimates of the effective aggregate size match fairly well those observed in the experiments.

Determination of Biomass Char Combustion Reactivities for FBC Applications by a Combined Method

Combustion reactivities of five wood chars were measured at 300−900 °C to produce suitable kinetic data to mass balance equations of a fluidized-bed combustion model. Two different techniques were used: char burnout time measurements in a fluidized-bed reactor and low-temperature char reactivity measurements in a thermogravimetric analyzer (TGA). A combined method is proposed here to avoid the limitations of both experimental techniques, especially for very reactive chars. In this method, the intrinsic activation energy for each char was determined in TGA at low combustion temperatures and the pre-exponential factor was obtained by fitting the experimental values of burnout times. Good agreement between experimental and calculated results was achieved.

Fluidized Bed Combustion of a Biomass Char (Robinia pseudoacacia)

Fluidized bed combustion of char from a biomass, Robinia pseudoacacia, was investigated in a bench scale combustor. Different experimental techniques have been adopted to characterize the combined role of combustion and comminution phenomena (primary, secondary, and percolative fragmentations, attrition by abrasion) in determining fixed carbon conversion and the rate of carbon elutriation. Comparison of experimental results obtained under steadily oxidizing conditions and under alternating oxidizing/inert conditions suggested mechanistic aspects of the fluidized bed combustion of biomass char. Fixed carbon combustion was almost always complete. Conversion occurred to a large extent via the generation of carbon fines followed by postcombustion during their residence time in the bed. Approximately half of the initial fixed carbon followed this pathway, the remainder being directly burnt as coarse char. The prevailing mechanism of carbon fines generation in the bed was percolative fragmentation rather than attrition by abrasion. In spite of the extensive generation of elutriable carbon fines, the combined effect of high fuel reactivity and of relatively long fines residence times in the reactor determined the large combustion efficiency. It is inferred from experimental results that char fines adhesion onto bed solids might be relevant to the observed phenomenology.