Synergistic effect for co-coking of sawdust and coal blending based on the chemical structure transformation

Biomass was used as additives in coal blending for making coke in terms of widening the alternative raw materials and reducing CO₂ emissions. To obtain the influences of biomass incorporation on the semicoke formation, the chemical structure transformation as well as the gas evolution during sawdust (SD)/coal blending (BC) co-coking were investigated using in-situ Fourier transform infrared spectroscopy coupled with mass spectrometry (In-situ FTIR-MS). Meanwhile, the role of biomass in the semicoke formation was also characterized by several analytical techniques. The transformation of the five main functional groups between SD and BC exhibited the largest difference, and the synergistic effect based on the chemical structure transformation was also proposed for the SD/BC blends co-coking. The synergistic effect based on the chemical structure transformation was divided into two stages during semicoke formation. One stage occurred at 100–280 °C that was assigned to the physical effect that inhibited the BC decomposition. Another stage happened at 280–500 °C that was mainly attributed to the hydrogen transfer that enhanced the aromatization of semi-coke. In addition, it was also noted that the thermoplastic properties decreased proportionately to the quantity of the SD, and the non-agglomeration between BC and SD was clearly observed by SEM.     

Characteristics of co-combustion and kinetic study on hydrothermally treated municipal solid waste with different rank coals: A thermogravimetric analysis

This study presents an investigation on the influence of hydrothermally treated municipal solid waste (MSW) on the co-combustion characteristics with different rank coals, i.e. Indian, Indonesian and Australian coals. MSW blends of 10%, 20%, 30% and 50% (wt.%) with different rank coals were tested in a thermogravimetric analyser (TGA) in the temperature range from ambient to 700 °C under the heating rate of 10 °C/min. Combustion characteristics such as volatile release, ignition and burnout were studied for the blend fuel. Different ignition behavior was observed depending on the blends composition and the coal rank. The result of this work indicates that the blending of MSW improves devolatization properties of coal. But it was found that the co-combustion characteristics of MSW and coal blend cannot be predicted only from the pyrolytic and or devolatization phenomena as the other factors such as the coal quality also plays a vital role in deciding the blends co-combustion characteristics. The TGA combustion profiles showed that the combustion characteristics of blends followed those of parent fuels in both an additive and non-additive manners. These experimental results help to understand and predict the behavior of coal and MSW blends in practical applications.     

Pulverized coal burnout in blast furnace simulated by a drop tube furnace

Reactions of pulverized coal injection (PCI) in a blast furnace were simulated using a drop tube furnace (DTF) to investigate the burnout behavior of a number of coals and coal blends. For the coals with the fuel ratio ranging from 1.36 to 6.22, the experimental results indicated that the burnout increased with decreasing the fuel ratio, except for certain coals departing from the general trend. One of the coals with the fuel ratio of 6.22 has shown its merit in combustion, implying that the blending ratio of the coal in PCI operation can be raised for a higher coke replacement ratio. The experiments also suggested that increasing blast temperature was an efficient countermeasure for promoting the combustibility of the injected coals. Higher fuel burnout could be achieved when the particle size of coal was reduced from 60–100 to 100–200 mesh. However, once the size of the tested coals was in the range of 200 and 325 mesh, the burnout could not be improved further, resulting from the agglomeration of fine particles. Considering coal blend reactions, the blending ratio of coals in PCI may be adjusted by the individual coal burnout rather than by the fuel ratio.     

Techniques to determine ignition,flame stability and burnout of blended coals in p.f. power station boilers

The blending of coals has become popular to improve the performance of coals, to meet specifications of power plants and, to reduce the cost of coals. This article reviews the results and provides new information on ignition, flame stability, and carbon burnout studies of blended coals. The reviewed studies were conducted in laboratory-, pilot-, and full-scale facilities. The new information was taken in pilot-scale studies. The results generally show that blending a high-volatile coal with a low-volatile coal or anthracite can improve the ignition, flame stability and burnout of the blends.This paper discusses two general methods to predict the performance of blended coals: (1) experiment; and (2) indices. Laboratory- and pilot-scale tests, at least, provide a relative ranking of the combustion performance of coal/blends in power station boilers. Several indices, volatile matter content, heating value and a maceral index, can be used to predict the relative ranking of ignitability and flame stability of coals and blends. The maceral index, fuel ratio, and vitrinite reflectance can also be used to predict the absolute carbon burnout of coal and blends within limits.     

混煤着火模型研究

在混煤中的组合煤各自保持各自着火特性的基础上，应用热力着火理论，建立混煤着火计算模型，只要已知各分煤的煤质特性和化学动力不参数，就可以计算混煤的着火温度。采用该模型对混煤在一维沉降炉上的着火实验进行模拟，通过与实验结果的对比，说明计算模型与实验结果的趋势符合较好。计算结果进一步证明，由两种着火特性差别较大煤种组成的混煤，其着火温度主要取决于易着火煤的着火特性。     

Co-liquefaction behavior of a sub-bituminous coal and sawdust

The co-thermolysis and co-liquefaction properties of Shenhua coal and sawdust were investigated in this study. The synergistic effect between Shenhua coal and sawdust in co-liquefaction was probed. TG/DTG analysis suggests that the sawdust, which has lower pyrolysis temperature, can promote the thermolysis of Shenhua coal, resulting in more volatile matter to be released from coal molecular structure during the co-thermolysis process. This will result in the larger weight losses of their mixture compared to the corresponding weighted mean values of individual pyrolysis. The individual liquefaction of Shenhua coal and sawdust shows that sawdust has higher liquefaction activity compared to Shenhua coal. It gives much higher liquefaction conversion and oil yield than Shenhua coal at the same liquefaction conditions. Co-liquefactions of Shenhua coal and sawdust at different conditions were carried out. The results suggest that there exists an obviously synergistic effect during the co-liquefaction, and this synergistic effect is the function of liquefaction conditions. At high liquefaction temperatures and long reaction times, the synergistic effect decreases because of the increase of liquefaction activity of coal and lack of hydrogen donating ability of the system at the conditions, resulting in the increase of the rate of retrogressive condensed reactions. The largest enhancements in conversion of 16.8% and oil yield of 11.4% comparing with corresponding calculated weighted mean values of the individual liquefaction of Shenhua coal and sawdust were obtained at 400 and 380 °C, respectively in the co-liquefaction with 1/1 blending ratio of coal/sawdust.     

Experimental investigation on ignition and burnout characteristics of semi-coke and bituminous coal blends

Ignition and burnout characteristics of semi-coke and bituminous coal blends were investigated by thermogravimetric analyzer and drop tube furnace. The results showed that the ignitability index and the comprehensive combustion characteristic index of the blends decrease as the blending proportion of semi-coke increases, but the average activation energy of the blends increases gradually. Ignition mode of bituminous coal is changed from homogeneous to hetero-homogeneous ignition with the increasing of semi-coke content in the blends. When the mixing proportion of semi-coke is lower than 45%, the burnout rate is lower than the weighted value in the early stage of combustion and gradually higher than the weighted value with the development of combustion process. However, the burnout is always lower than the weighted value to mix with 67% semi-coke. Increasing furnace temperature from 850 °C to 1050 °C can improve the mid-term reaction process, alleviate the negative effects of semi-coke on the co-combustion process and increase the burnout rate. So less than 45% semi-coke blending ratio and increasing furnace temperature are recommended for semi-coke and bituminous coal co-combustion.     

An investigation into the effect of fast heating on fluidity development and coke quality for blends of coal and biomass

The addition of biomass to coking coals can reduce operational costs and carbon emissions but also reduces fluidity development. The use of heating rates up to 20 °C min⁻¹ in the softening stage of coal has been investigated using high-temperature small-amplitude oscillatory-shear (SAOS) rheometry to improve the fluid characteristics of binary blends of two coking coals with Scots pine. The effects of biomass concentration and particle size, biomass torrefaction, pellet mass and thermal pre-treatment of the blend on fluidity development and semicoke strength have also been studied. Fluidity increased with an increase in heating rate and an increase in the final temperature for fast heating. Relationships were found between the minimum complex viscosity of the blend, the heating rate and the concentration of biomass, which have been used to propose an equation to calculate the heating rate necessary to achieve optimum fluidity for a particular blend with biomass. The fluid characteristics of the blend were not affected to a great extent by the particle sizes of the biomass studied (<500 μm and >500 μm) or the torrefaction of the biomass (250 °C for 1 h in N₂), were increased by an increase in pellet mass, and were destroyed by blend pre-heating. The semicoke strength of the blend with a mass fraction of 10% Scots pine and fast heating (10 °C min⁻¹) proved to be higher than that of the coal alone with slow heating (3 °C min⁻¹) and resulted in a 3% reduction in non-renewable carbon emissions.     

The effect of air staged combustion on NOx emissions in dried lignite combustion

Experiments were carried out in a multi-path air inlet one-dimensional furnace to assess NOx emission characteristics of the staged combustion of BRXL lignite and its dried coals. The impact of moisture content, multiple air staging, pulverized coal fineness and burnout air position on NOx emissions under deep, middle and shallow air-staged combustion conditions. Moreover, the impact of blending coals on NOx emissions was investigated in this paper. The unburned carbon concentration in fly ash was also tested. Experimental results based on the combustion of BRXL lignite and its dried coals show that NOx emissions can be reduced drastically by air-staged combustion. NOx emissions reduce with the increase of the air that is staged and the distance between the burner and burnout air position. Dried coal of BRXL lignite emits a smaller amount of NOx than that of BRXL lignite. However, the dried degree of BRXL lignite is closely related to R₉₀ fineness. Dried coal with optimal moisture content yields least NOx emissions. When deep or middle staged combustion was adopted, the application of multi-staged combustion is conducive to NOx reduction. However, when shallow staged combustion was adopted, NOx emissions are higher in multi-staged combustion than that in single-staged combustion with M_S = 0.54. Thus, the existence of a certain concentration of O₂ in reduction zone would significantly reduce NOx emissions. The blending coals that dried coals of BRXL lignite were blended with bituminous coals emit a larger amount of NOx than that of the dried coal alone. NOx emissions decrease with the increase of the proportion of dried coal in the blending coal. Moreover, the unburned carbon concentration in fly ash of dried coal in staged combustion is lower than that of BRXL lignite in staged combustion. On the whole, the dried coal of BRXL lignite is conducive to NOx reduction in staged combustion.     

Interaction between biomass and different rank coals during co-pyrolysis

Effects of biomass on the pyrolytic decomposition of different rank coals were investigated by non-isothermal Thermogravimetric Analysis (TGA) method from ambient to 900 °C with a heating rate of 40 °C/min under nitrogen. Hazelnut shell (HS) which is a woody biomass species was added as much as 10 wt% to coals such as peat, lignite, bituminous coal, and anthracite to obtain coal/biomass blends for co-pyrolysis runs. Effects of HS present in the blends were evaluated regarding the apparent decomposition rates and the char yields. It was found that the addition of thermally reactive HS led to some increases in the volatilization rates of coals especially at temperatures below 500 °C. Besides, the char yields revealed unexpected variations in case of low rank coals. Although, HS addition did not play significant role on the char yields of bituminous coal and anthracite, considerable deviations from the theoretical char yields were detected in the case of peat and lignites. The presence of HS led to increasing char weight for peat, while the char weights for lignites decreased seriously. These variations were interpreted, and it can be concluded that these variations cannot be explained by simple additive behavior, and the existence of synergistic interactions should be taken into account.     

Co-gasification between coal/sawdust and coal/wood pellet: A parametric study using response surface methodology

This study had compared raw biomass and pre-treated biomass co-gasified with coal with the aim of investigating the reliability of pre-treated biomass for enhancing gasification performance. Sawdust (SD) and wood pellet (palletisation form of sawdust - WP) and blends of these two feedstocks with sub-bituminous coal (CL), were gasified in an air atmosphere using an external heated fixed-bed downdraft gasifier system. Response surface methodology (RSM) incorporating the central composite design (CCD) was applied to assist the comparison of all operating variables. The three independent variables were investigated within a specific range of coal blending ratios from 25% to 75%, gasification temperature from 650 °C to 850 °C and equivalence ratio from 0.20 to 0.30 against the dependent variables, namely the H₂/CO ratio and higher heating value of the syngas (HHVsyngas). The results revealed the H₂/CO ratio and a higher heating value of the syngas of more than 1.585 and 6.072 MJ/Nm³, respectively. Findings also showed that the H₂/CO ratio in the syngas from CL/WP possessed a higher value than the CL/SD. In contrast, CL/SD possessed a higher heating value for syngas with about 1% difference compared to the CL/WP. Therefore, co-gasified coal with wood pellets could potentially be a substitute for sawdust.     

煤粉与生物质混燃的低温着火特性

利用自制的管式炉恒温热重测量实验台研究了掺混比、温度、煤种以及生物质种类等因素对煤粉与生物质混燃时低温着火特性的影响,并对煤粉与生物质混燃时的低温着火活化能进行了计算.结果表明:随着掺混比的增大,混合物的燃烧速率加快且燃尽程度提高;温度升高能改善煤粉与生物质混合物的燃烧特性;掺混生物质对难燃煤的着火特性影响比对易燃煤更明显;对于某一煤种,掺混水分和挥发分含量高的生物质,燃烧初期的失重速率加快;掺混灰分含量越多的生物质,在燃烧后期对煤粉的促燃作用越差;燃烧反应活化能随着生物质掺混比和温度区间的增大而减小.     

Study on combustion characteristics of blended coals

Power plants in China have to burn blended coal instead of one specific coal for a variety of reasons. So it is of great necessity to investigate the combustion of blended coals. Using a test rig with a capacity of 640 MJ/h with an absolute milling system and flue gas online analysis system, characteristics such as burnout, slag, and pollution of some blended coals were investigated. The ratio of coke and slag as a method of distinguishing coal slagging characteristic was introduced. The results show that the blending of coal has some effect on NO _x but there is no obvious rule. SO _x emission can be reduced by blending low sulfur coal. Translated from Proceedings of the CSEE, 2005, 25(18): 97–103 [译自: 中国电机工程学报]     

褐煤混煤燃烧特性的热重分析法研究

利用热重分析法对三种褐煤及其混煤的热解、着火和燃尽等燃烧特笥及其活化能进行了研究。试验结果表明，混煤中的单一煤种在混煤燃烧过程中基本保持各自的着火和燃尽特性；而其热解特性和活化能与掺混比例有关。提出了能够表征单煤及混煤燃烧性能的综合燃烧特性指数SN。     

Synergy effects of co-firing wooden biomass with Bosnian coal

The paper presents synergy effects found during the co-firing of wooden biomass with Bosnian coal types in an experimental reactor. The co-firing tests used spruce sawdust in combination with Kakanj brown coal and a lignite blend of Dubrave lignite and Sikulje lignite. Coal/biomass mixtures at 93:7 and 80:20 wt% were fired in a 20 kW pulverized fuel (PF) entrained flow reactor. Over 20 test trials were performed to investigate ash deposition behavior and emissions under different conditions, varying the process temperature, excess air ratio, and air distribution. During the tests, the temperature in the experimental facility varied between 880 and 1550 °C, while the excess air ratio varied between 0.95 and 1.4. There was sufficient combustion efficiency under all co-firing regimes, with burning out at 96.5–99.5% for brown coal–sawdust co-firing. Synergy effects were detected for all co-firing regimes with regard to SO₂ emission, as well for slagging at the process temperature suitable for the slag tap furnace. CO₂ emissions were also calculated for the blends tested and significant reductions of CO₂ found, due to the very low ranking of Bosnian coals.     

Co‐gasification performance of coal and petroleum coke blends in a pilot‐scale pressurized entrained‐flow gasifier

Co‐gasification performance of coal and petroleum coke (petcoke) blends in a pilot‐scale pressurized entrained‐flow gasifier was studied experimentally. Two different coals, including a subbituminous coal (Coal A) and a bituminous coal (Coal B), individually blended with a petcoke in the gasifier were considered. The experimental results suggested that, when the petcoke was mixed with Coal A over 70%, the slagging problem, which could shorten the operational period due to high ash content in the coal, was improved. It was found that increasing O₂/C tended to decrease the syngas concentration and better operational conditions of O₂/C were between 0.6 and 0.65 Nm³ kg^?1. For the blends of Coal B and the petcoke, the slagging problem was encountered no more, as a result of low ash content in both Coal B and the petcoke. The better co‐gasification performance could be achieved if the blending ratio of the two fuels was 50%, perhaps resulting from the synergistic effect of the blends. With the aforementioned blending ratio, the optimal condition of O₂/C was located at around 0.65 Nm³ kg^?1. The co‐gasification was also simulated using Aspen Plus. It revealed that the simulation could provide a useful insight into the practical operation of co‐gasification. Copyright © 2011 John Wiley & Sons, Ltd.     

NOx and H2S formation in the reductive zone of air-staged combustion of pulverized blended coals

Low NO_x combustion of blended coals is widely used in coal-fired boilers in China to control NO_x emission; thus, it is necessary to understand the formation mechanism of NO_x and H₂S during the combustion of blended coals. This paper focused on the investigation of reductive gases in the formation of NO_x and H₂S in the reductive zone of blended coals during combustion. Experiments with Zhundong (ZD) and Commercial (GE) coal and their blends with different mixing ratios were conducted in a drop tube furnace at 1200°C–1400°C with an excessive air ratio of 0.6–1.2. The coal conversion and formation characteristics of CO, H₂S, and NO_x in the fuel-rich zone were carefully studied under different experimental conditions for different blend ratios. Blending ZD into GE was found to increase not only the coal conversion but also the concentrations of CO and H₂S as NO reduction accelerated. Both the CO and H₂S concentrations inblended coal combustion increase with an increase in the combustion temperature and a decrease in the excessive air ratio. Based on accumulated experimental data, one interesting finding was that NO and H₂S from blended coal combustion were almost directly dependent on the CO concentration, and the CO concentration of the blended coal combustion depended on the single char gasification conversion.Thus, CO, NO_x, and H₂S formation characteristics from blended coal combustion can be well predicted by single char gasification kinetics.     

Co-Carbonization of Two Anthracites with a Fat Coal or Two Pitches

The blends of two anthracite powders (YQ and JC) with a fat coal (C4) or a petroleum pitch (PP) or a coal tar pitch (CTP) in different proportions were co-carbonized at 3°C/min up to 1000°C in an experimental 1 Kg coke oven. Coke yield, coke particulate size, coke micro-strength and coke cracking strength were measured respectively. Coke optical textures were watched under a microscope. The results show that as anthracite proportion increases, coke yields of all blends improve; > 0.8 mm lump coke yields of blends with CTP or PP decline slightly, blends with C4 drop heavily; coke micro-strengths do not change sharply, and coke cracking strength of blends with C4 or PP decrease more than blends with CTP. C4 produces fine-grained mosaics, and two anthracites are mainly fusinite and fragments, PP is coarse-grained mosaics, and CTP is chiefly flow or domain textures. Independent optical textures were observed in cokes. There exist evident borders between the two contact optical textures which were produced by different components, and a few phenomena that domain or flow textures penetrating into fusinite appeared in the blends. It seems that CTP is the best adhesives for blending with anthracites for producing high quality cokes.     

An evaluation on rice husks and pulverized coal blends using a drop tube furnace and a thermogravimetric analyzer for application to a blast furnace

To evaluate the potential of pulverized coals partially replaced by rice husks used in blast furnaces, thermal behavior of blends of rice husks and an anthracite coal before and after passing through a drop tube furnace (DTF) was investigated by using a thermogravimetry (TG). For the blends of the raw materials in the TG, fuel reaction with increasing temperature could be partitioned into three stages. When the rice husks were contained in the fuel, a double-peak distribution in the first stage was observed, as a consequence of thermal decompositions of hemicellulose, cellulose and lignin. A linear relationship between the char yield and the biomass blending ratio (BBR) developed, reflecting that synergistic effects in the pyrolytic processes were absent. This further reveals that the coal and the rice husks can be blended and consumed in blast furnaces in accordance with the requirement of volatile matter contained in the fuel. After the fuels underwent rapid heating (i.e. the DTF), a linear relationship from the thermogravimetric analyses of the unburned chars was not found. Therefore, the synergistic effects were observed and they could be described by second order polynomials. When the BBR was less than 50%, varying the ratio had a slight effect on the thermal behavior of the unburned chars. In addition, the thermal reactions of the feeding fuels and of the formed unburned chars behaved like a fingerprint.     

Oxygen enriched co-combustion of biomass and bituminous coal

The oxy-fuel co-combustion behavior of two herbaceous biomass species (Bermuda grass and cornstalk) with bituminous coal was investigated by thermal gravimetric analysis (40°C/min). The incorporation of Bermuda grass or cornstalk could improve combustion indices of the bituminous coal. Once blending the biomass with bituminous coal, ignition temperatures of blends could be advanced by about 100–170°C. With increasing the oxygen concentration or blending ratio, the comprehensive performance index of the most blends and their parent samples increased. For the 80%grass/20%coal blend, there was a strong synergistic effect in its parent samples at 60% oxygen concentration.