Coal-biomass Blends Research Articles

Interactions between corncob and lignite during temperature-programmed co-pyrolysis

Interactions between feedstocks during pyrolysis of coal–biomass blends, and their impacts on product distribution and conversion behavior during utilization, need to be understood if we are to successfully develop systems for the effective co-utilization of biomass and coal. A novel two-stage fixed-bed reactor containing three quartz tubes was designed and used in this study to investigate these interactions, with particular emphasis on the impact on product distribution and char properties. The results show that interactions exist during co-pyrolysis of corncob and lignite blends, which increase overall tar yields and decrease overall gas yields compared with those obtained from pyrolysis of unblended feedstocks, with the interactions between corncob volatiles and lignite playing a dominant role. It is also shown that interactions between chars and volatiles can impact on the reactivity of chars to O2. Consistent with our expectations, there is a link between potassium content in char and its reactivity; however, it is found that for lignite chars in particular the potential impact of potassium is dominated by the changes of char chemical structure. The effect of potassium content is the opposite to that of char chemical structure, leading to significant complexity in determining the net result of any co-pyrolysis interactions on char reactivity. This highlights the importance of understanding these complex interactions to develop the industrial scale co-gasification systems and adjust the distributions of products.

Study of flow characteristics of biomass and biomass–coal blends

The flow characteristics of biomass and biomass–coal blends were demonstrated experimentally by investigating the shear properties, flow energy and aeration with a FT4 Powder Rheometer. The results show that the addition of biomass into coal has no effect cohesion strength but significantly increases the angle of internal friction. We propose an improved empirical correlation to describe the quantitative relationship between angle of internal friction and particle shape factor, which agrees well with the experimental data. With the increase of biomass mass fraction, the flow energy was found to exponentially increase and the flow of blends became unstable, which produce a critical mass fraction to obtain an optimum flow performance of biomass–coal, i.e., 10wt%. It is apparent that the flow behavior of biomass–coal blends is very sensitive to aeration, but torpid for biomass particles due to its high permeability. It indicates that biomass is not suitable for dense-phase pneumatic conveying.

Experimental and modeling study of pyrolysis of coal, biomass and blended coal–biomass particles

This paper reports a combined experimental and numerical investigation of the pyrolysis characteristics of coal, biomass, and coal–biomass blends. Coal and straw were grounded and pressed into spherical particles with diameter of 8mm, and blended coal–straw particles were prepared through mixing pulverized coal and straw before pressed into particles. Sample particles were suspended in the center of a single-particle reactor system and devolatilized under different temperatures. The analysis of the time history of the residual mass of particles of coal, straw, and coal–straw blends suggested an absence of synergistic effect between the coal and the straw. In addition, a one-dimensional, time-dependent particle model; based on the chemical percolation devolatilization (CPD) and bio-CPD models; was developed to simulate the pyrolysis of coal and straw particles. The model predictions agree will with the measured data. An extended CPD model was proposed to explain the co-pyrolysis characteristics of coal–biomass blends. Encouraging agreement was found between the predicted and the experimental results of pyrolysis of coal–straw blends.

Source apportionment of carbon during gasification of coal–biomass blends using stable carbon isotope analysis

Stable carbon isotope analysis, a unique analytical technique, has been utilized for distinguishing and quantifying the individual contributions of coal and biomass feedstocks in the generation of carbon containing gases during the gasification of their blends. For this purpose, corn stover (CS) and switchgrass (SG) were individually blended up to 30% by weight with two different Montana coals, namely, DECS-38 sub-bituminous coal (SB) and DECS-25 lignite coal (LG) and gasified at atmospheric pressure with varying O2/steam ratios. Gasifying at a constant O2/steam ratio while increasing percentage of biomass in the feedstock resulted in an increase of δ13C (‰) values for the carbon containing product gases. An increase in the concentration of oxygen in the feed stream at a constant biomass percentage leads to the depletion of 13CO and enrichment of 13CO2 in the blends with corn stover while an enrichment of both 13CO and 13CO2 was observed in blends with switchgrass. Based on isotope mass balance considerations, the contribution of corn stover was in the range of 1% – 40% for CO generation and 6% – 69% for CO2 generation. Switchgrass, on the other hand, contributed up to 53% for CO generation and only 46% for CO2 generation.

Characterisation and model fitting kinetic analysis of coal/biomass co-combustion

The combustion behaviors of biomass, coal and their blends were studied by thermogravimetric analysis. Combustion parameters such as ignition, burnout, peak rate, ignition index, and combustibility index were analyzed. The kinetic parameters were optimized based on experimental results by a double parallel reactions random pore model (DRPM) proposed in this paper. The results show that the combustion characteristic temperature of the biomass is lower and maximum rate of combustion is higher than that of anthracite coal. With the increase of biomass content, ignition temperature and burnout temperature of blends tended to decrease, while the ignition index and combustibility index increased. Compared with the original RPM model, the DRPM model could not only describe the combustion process with a single peak rate, but also the combustion of biomass-coal blends with two rate peaks. The combustion activation energies of blends were extracted by DRPM model in the present study.

Effect of storage time on the flowability of biomass-coal granular system

The influence of storage time on the flowability of biomass-coal blends is experimental and theoretical investigated. The results show that there exists exponential relationship between discharge rate and storage time in an appropriate rice straw mass fraction range (less than 10%). In addition, the mechanism of gravity discharge rate variation for biomass-coal blends is theoretically investigated by analyzing the porosity variation of particle bed. The mechanism of porosity variation in the binary granular system is theoretically analyzed on the basis of emergent gas, mutual compression and particle rearrangement, and a mathematical model has been developed which agrees well with the experimental results. Furthermore, relaxation effect is proposed, which is mainly induced by elasticity of rice straw particle and cohesion of blends. Relaxation time is the most important parameters determining this effect. The latter decreases with increasing of coal particle size and biomass mass fraction.

Gravity discharge characteristics of biomass-coal blends in a hopper

Adding biomass particles into pulverized coal may significantly improve the flowability of the cohesive pulverized coal particles. Experimental study of solids discharge flow from a cylindrical hopper with no aeration revealed an interesting “needle particle effect”, which was believed to be responsible for the smoother solids discharge and significantly increased discharge rates of the coal-biomass blends within the limits of the study. The mechanisms behind the needle particle effect in a gravity discharge system, including adherence, lowering cohesion, arch-breaking and transition of flow pattern, are analyzed and verified by the experimental results. The blends behaved different when different types of biomass particles were added. The observed flowability of the blends was strongly affected by the aspect ratio and surface roughness of the biomass particles. A revised model based on Crewdson’s equation is proposed to predict the gravity discharge rate of biomass-coal blends.

Co-Pyrolysis Characteristics and Product Distributions of Coal Tailings and Biomass Blends

In order to make better utilization of coal tailings (low quality production after coal preparation) as the resources, the pyrolysis characteristics and product distributions during co-pyrolysis of coal tailings together with biomass at different ratio (20%, 40%, 60% and 80%) were determined in thermogravimetric analysis (TGA) and a fixed bed reactor in this paper. Coal tailings (TC) selected was provided by Hexi coal in Shanxi province, and pine branch wastes (PBW) were used as biomass samples. The result of TGA experiments indicates that the temperature corresponding to the maximum pyrolysis rate exhibited a significant difference between TC and PBW, and the value of the calculated TGA and DTG curves is similar to the experimental ones. In a fixed bed experiments within a temperature range of 25-900°C, gas product yields of co-pyrolysis of TC and PBW are higher than those of the sum of them individually, while tar and char yields were on the contrary. It shows some synergetic effect exists during co-pyrolysis process of TC and PBW blends, and the maximum synergy exhibits with a PBW blending ratio of 40%. CO yield increases up to 30% at 400°C and CH4yield increases up to 11.33% at 700°C compared with the calculated value. These findings can potentially help to understand and predict the behavior of coal tailings/biomass blends in practical systems.

Investigations on Combustion and NO Emission Characteristics of Coal and Biomass Blends

The effects of biomass ratios, coal types, biomass types, and furnace temperatures on the combustion and NO emission characteristics of coal and biomass blends were investigated using a customized thermogravimetric analysis system which can measure the burning profiles and the NO emissions simultaneously, and at a constant furnace temperature. The results indicated that blending biomass with coals could improve the burning rate of coals, especially the burning rate of the anthracites. Also blending biomass could reduce NO emissions for all types of coals. With the increase of biomass ratio in the coal–biomass blends, the burning rate was found to increase, and both burnout time and the final NO emissions was decreased. It was found that the different volatile contents in biomass could have different impacts on the burning rate of the coal–biomass blends while the nitrogen and ash contents of the different biomass appeared to have different influences on NO emissions. Also, according to the results, with the rise of the furnace temperature, the burning rate of the coal–biomass blends would improve while the NO conversion ratio would increase gradually.

Theoretical and experimental investigation on angle of repose of biomass–coal blends

The angle of repose of granular materials has been extensively studied, for it is a very important macroscopic parameter in characterizing the flowability of granular materials. The primary objective of the work is to develop a linear model, and to propose a quantitative expression about the angle of repose (ΦR) of mixed granular system, which gives a qualitative understanding of the influence of physical parameters on mixed granular flow behavior. The results show that the variation amplitude of tangent ΦR is linear with the related particle physical properties (density, particle size and secondary particle mass fraction). It is shown that the most important parameters determining the mixed granular system angle of repose are density variation and surface roughness for the similar particle size. In addition, whether the particle size of base particle material or secondary particle varies or not, the linear relationship always exists. And the slope k is determined primarily by particle shape and surface friction coefficient.

Numerical Study of Cofiring Biomass with Coal in Cyclone Combustor

This paper investigates the flow and temperature distribution inside a cyclone combustor during combustion of coal and coal blend. The combustion under study is based on the actual cyclone combustor experiment rig used to test the performance of combustion when it be different coal-biomass blend is used. Experiment investigation on 100% coal combustion was also carried out and serves as a basis for the subsequent test for co-firing of different coal or biomass blend. Validation of temperature magnitude along cyclone furnace at 100% coal combustion condition shows good agreement between the measured and CFD results. Subsequent simulation of coal and biomass blend shows very good impact as it gave less error compared to experiment

Microwave Induced Co-Processing of Biomass/Coal Blends

Co-processing of biomass with coal has become a promising approach for the efficient utilization of biomass and a feasible option to reduce coal consumption. Normally, biomass and coal blends are normally processed using conventional heating methods, such as co-pyrolysis, co-firing and co-gasification. Due to the fact that microwave heating has many advantages against conventional heating methods in terms of its volumetric heating and instant heating nature, in this study, biomass and coal blends were co-processed under microwave heating to enhance possible synergistic effect to improve the yield and quality of products. Biomass and coal blends were prepared in different mass fraction. Both conventional pyrolysis and microwave induced co-processing tests were carried out. The solid, liquid and gas products were analysed using scanning electron microscopy (SEM) and gas chromatography – mass spectrum (GC-MS). Results showed that in conventional pyrolysis, mole fraction of CO2 is relatively high whilst that of H2 is relatively low. However, for microwave induced co-processing of biomass coal blends with a mass fraction of biomass around 10wt%, the mole fraction of H2, CO and CH4 could be as high as 85%. This indicates the existence of synergy when biomass is co-processed with coal. In the liquid product, it was also found that the distribution of liquid products under microwave induced pyrolysis is much narrow compared with that of liquid products produced under conventional pyrolysis.

Kinetic models for the oxy-fuel combustion of coal and coal/biomass blend chars obtained in N2 and CO2 atmospheres

The thermal reactivity and kinetics of five coal chars, a biomass char, and two coal/biomass char blends in an oxy-fuel combustion atmosphere (30%O2–70%CO2) were studied using the non-isothermal thermogravimetric method at three heating rates. Fuel chars were obtained by devolatilization in an entrained flow reactor at 1273 K under N2 and CO2 atmospheres. Three nth-order representative gas–solid models – the volumetric model (VM), the grain model (GM) and the random pore model (RPM) – were employed to describe the reactive behaviour of the chars. The RPM model was found to be the best for describing the reactivity of the high rank coal chars, while VM was the model that best described the reactivity of the bituminous coal chars, the biomass char and the coal-biomass blend char. The kinetic parameters of the chars obtained in N2 and CO2 in an oxy-fuel combustion atmosphere with 30% of oxygen were compared, but no relevant differences were observed. The behaviour of the blend of the bituminous coal (90%wt.) and the biomass (10%wt.) chars resembled that of the individual coal concealing the effect of the biomass. Likewise, no interaction was detected between the high rank coal and the biomass chars during oxy-fuel combustion of the blend.

Thermogravimetric study of the pyrolysis characteristics and kinetics of coal blends with corn and sugarcane residues

This paper investigates the non-isothermal thermokinetics of the co-pyrolysis of sugarcane bagasse and corn residue blended with coal. The individual devolatilization behaviour of each of the fuels obtained separately was compared with the behaviour of the biomass blends with coal at various mix ratios. Possible synergistic behaviour that points to the existence of chemical interactions during pyrolysis between the coal and biomass fractions of the blends could thus be quantified. Synergistic interactions in the blends resulted in higher than expected volatile yields. The interactions occurred between 300°C and 500°C, corresponding to the end of biomass devolatilization and the start of coal decomposition. Isoconversional kinetic analysis revealed that apparent activation energy values varied between 165–180, 162–190, 160–175, and 225–260kJmol−1 for sugarcane bagasse, corn cobs, corn stover, and coal in the 0.1 to 0.8 conversion range. A comparison of E(α) for single fuels and coal-biomass blends gave further evidence of synergistic behaviour as demonstrated by larger variation in E(α) during the decomposition of the blends, compared to the individual fuels.

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.

Quantitative characterization of pulverized coal and biomass–coal blends in pneumatic conveying pipelines using electrostatic sensor arrays and data fusion techniques

Quantitative data about the dynamic behaviour of pulverized coal and biomass–coal blends in fuel injection pipelines allow power plant operators to detect variations in fuel supply and oscillations in the flow at an early stage, enable them to balance fuel distribution between fuel feeding pipes and ultimately to achieve higher combustion efficiency and lower greenhouse gas emissions. Electrostatic sensor arrays and data fusion algorithms are combined to provide a non-intrusive solution to the measurement of fuel particle velocity, relative solid concentration and flow stability under pneumatic conveying conditions. Electrostatic sensor arrays with circular and arc-shaped electrodes are integrated in the same sensing head to measure ‘averaged’ and ‘localized’ characteristics of pulverized fuel flow. Data fusion techniques are applied to optimize and integrate the results from the sensor arrays. Experimental tests were conducted on the horizontal section of a 150 mm bore pneumatic conveyor circulating pulverized coal and sawdust under various flow conditions. Test results suggest that pure coal particles travel faster and carry more electrostatic charge than biomass–coal blends. As more biomass particles are added to the flow, the overall velocity of the flow reduces, the electrostatic charge level on particles decreases and the flow becomes less stable compared to the pure coal flow.

A rapid and accurate method for on-line measurement of straw–coal blends using near infrared spectroscopy

On-line measurement of biomass–coal blends is very important for developing appropriate subsidy policies for biomass co-firing power generation. Near infrared reflectance spectroscopy (NIRS) was used for on-line qualitative analysis of straw, coal and straw–coal blends and on-line quantitative analysis of straw content of straw–coal blends. A total of 80 straw samples, nine coal samples, 120 straw–coal blends samples with straw content from 70% to 99% (blends1) and 120 straw–coal blends samples with straw content from 1% to 30% (blends2) were prepared. Spectra were obtained using a Thermo Fisher Scientific Antaris Target FT-NIR spectrometer. Linear discriminant analysis method was used for qualitative analysis, correct classification percentages of straw, blends1, blends2 and coal were 89.87%, 79.66%, 94.92% and 100%, respectively. The ratio of standard error of performance to standard deviation (RPD) of quantitative analysis model was 3.52. It is concluded NIRS is a rapid and accurate method for on-line measurement of straw–coal blends.

An Experimental Study on Nitric Oxide Reduction through Coal-Biomass Blend Reburning

Nitric oxide (NO) reduction through coal, biomass and their blend reburning is conducted on a drop tube furnace experimental system and it is proved to be feasible of using the coal-biomass blend as the reburning fuel. For a high NO reduction efficiency, the excess air ratio in the reburning zone should be less than 0.9 and the reaction temperature is required to be higher than 1373K. 50-60% of the biomass percentage in the blend is enough for an acceptable efficiency. At the same time, for a thorough NO reduction process, the reburning fuel fraction and the residence time in the reburning zone must be guaranteed about 20% and 0.6-0.8s, respectively. Also, when the initial NO concentration is excess 600ppm, the efficiency is varied little at different NO concentrations.

Reaction kinetics and producer gas compositions of steam gasification of coal and biomass blend chars, part 1: Experimental investigation

Biomass and coal are important solid fuels for generation of hydrogen-rich syngas from steam gasification. In this work, experiments were performed in a bench-scale gasifier to investigate the effect of coal-to-biomass ratio and the reaction kinetics for gasification of chars of biomass, coal and coal–biomass blends. In the gasification of these chars, steam was used as the gasification agent, while nitrogen was used as a gas carrier. The gasification temperature was controlled at 850, 900 and 950 °C. Gas produced was analysed using a micro-GC from which carbon conversion rate was also determined. From the experiments, it is found that the coal and biomass chars have different gasification characteristics and the overall reaction rate decreases with an increase in the ratio of coal–to-biomass. The microstructure of the coal char and biomass char was examined using scanning electronic microscopy (SEM), and it was found that the biomass char is more amorphous, whereas the coal char has larger pore size. The former enhances the intrinsic reaction rate and the latter influences the intra particle mass transportation. The difference in mass transfer of the gasification agent into the char particles between the two fuels is dominant in the char gasification.

A Feasibility Study on Using near Infrared Spectroscopy to Classify Straw-Coal Blends

Rapid classification of biomass, coal and biomass-coal blends is very important for developing appropriate subsidy policies and methods for biomass co-firing power generation in China. The use of near infrared reflectance (NIR) spectroscopy to classify straw, coal and straw-coal blends was explored in this study. Eighty-one straw samples, 9 coal samples, 81 straw-coal blends samples with straw content from 91% to 99% (blends1) and 90 straw-coal blends samples with straw content from 1% to 30% (blends2) were prepared and separated into a calibration set and an external prediction set. Spectra were scanned by a Fourier transform-NIR spectrometer. Raw spectra of the samples were mean centred and compared using discriminant analysis. Overall, the correct classification percentages for straw, blends1, blends2 and coal samples were 98.8%, 90.1%, 83.3% and 66.7%, respectively. The results show that the NIR technique could provide excellent classification between straw and coal. The spectra of straw-coal blends with straw content less than 98% can be classified exactly from calibrations based on pure straw and the spectra of straw-coal blends with straw content greater than 20% can be well classified from calibrations based on pure coal. It is concluded that NIR is a feasible method for rapid classification of straw, coal and straw-coal blends.