Effects of pressure in coal pyrolysis observed by high pressure TGA

High-pressure thermogravimetric analyzer was employed to investigate the effects of pressure on the thermal decomposition process, which is the very first step in most coal utilizing processes, and pyrolyzates from TGA were analyzed by on-line GC/MS. Results showed that pyrolysis of coal with steam under high-pressure conditions exhibited a slower reaction rate compared to the lower pressure conditions, and the effect is more evident at the high temperature region. Coal rank also exhibited a distinct effect on the pyrolysis rate such that a subbituminous coal showed a bigger effect by steam-addition and pressure than bituminous coals. Weathered coal sample illustrated a slower reaction rate compared to the unoxidized coal. In addition, the implication of pressure effects on pyrolysis has been described.     

The physical character of coal char formed during rapid pyrolysis at high pressure

Rapid pyrolysis was conducted in a drop tube reactor using seven coals under various operating conditions. In addition to dense char, porous chars (network char and cenospheric char) were formed by the rapid pyrolysis under certain conditions. Porous char was mainly composed of film-like carbon and skeleton carbon. The pyrolyzed coal char particles were characterized in detail. Morphology and bulk density of porous char were quite different from the dense char formed under the same conditions, but elemental composition and BET surface area were similar to each other. CO₂ gasification reactivity of porous char was lower than dense char in the later gasification stage, and this was ascribed to the low reactivity of skeleton carbon.     

Gas evolution kinetics of two coal samples during rapid pyrolysis

Quantitative gas evolution kinetics of coal primary pyrolysis at high heating rates is critical for developing predictive coal pyrolysis models. This study aims to investigate the gaseous species evolution kinetics of a low rank coal and a subbituminous coal during pyrolysis at a heating rate of 1000 °C s^− 1 and pressures up to 50 bar using a wire mesh reactor. The main gaseous species, including H₂, CO, CO₂, and light hydrocarbons CH₄, C₂H₂, C₂H₄, C₂H₆, C₃H₆, C₃H₈, were quantified using high sensitivity gas chromatography. It was found that the yields of gaseous species increased with increasing pyrolysis temperature up to 1100 °C. The low rank coal generated more CO and CO₂ than the subbituminous coal under similar pyrolysis conditions. Pyrolysis of the low rank coal at 50 bar produced more gas than at atmospheric pressure, especially CO₂, indicating that the tar precursor had undergone thermal cracking during pyrolysis at the elevated pressure.     

Influence of pressure on the release of inorganic species during high temperature gasification of coal

Alkali metal, sulphur, and chlorine species released during coal gasification are of concern, because they can lead to problems in colder parts of the plant. Therefore, hot gas cleaning technology is recently under development. This clean-up strategy requires a comprehensive knowledge of the release characteristics of inorganic compounds. The principal objective of this work was to provide details of the influence of pressure on the release of key chemical species, e.g. sodium, potassium, sulphur, and chlorine. Hence, a total of 19 different coals were investigated in lab-scale gasification experiments in an electrical heated pressurised furnace at absolute pressures of 2, 4, and 6 bar in an atmosphere of He/7.5v%O₂ at 1325 °C. Hot gas analysis was carried out by molecular beam mass spectrometry. The quantitative results showed a decreasing release of ³⁴H₂S⁺, ³⁶HCl⁺, ³⁹K⁺/³⁹NaO⁺, ⁵⁸NaCl⁺, , and ⁷⁴KCl⁺ with increasing pressure. The discussion was supported by thermodynamic calculations.     

Investigation on characteristics of pulverized coal dense-phase pneumatic conveying under high pressure

Experiments of dense-phase pneumatic conveying of pulverized coal were carried out in a test facility with a conveying pressure up to 4 MPa. The influence of fluidization nitrogen flow rate, the flow rate of supplementary nitrogen, and the pressure difference between sending hopper and receiving hopper on the solids to gas ratio and the solid mass flow rate was investigated. Test results indicate that with the increase in fluidization nitrogen flow rate, the solid mass flow rate increases, and the solids to gas ratio increases at first and then declines. When the fluidization of pulverized coal in the sending vessel becomes intensive, with the increase in supplementary nitrogen flow rate, the solids to gas ratio declines and the solid mass flow rate increases. And the solid mass flow rate increases linearly with the increase in pressure difference between two hoppers. The experimental results provide a database for the design and operation of a dense-phase pneumatic conveying system. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

The devolatilisation of millimetre sized coal particles at high heating rate: the influence of pressure on the structure and reactivity of the char

Most studies on the influence of pressure on the combustion of coal particles have shown that for a constant oxygen concentration, an increase of pressure leads to a decrease of combustion rate. Among the different phenomena, which can explain this behaviour, the influence of the devolatilisation pressure on the structure and reactivity of the char formed may be important. The aim of this paper was to obtain a quantitative characterisation of the physical and chemical structure of chars formed during pyrolysis under a large range of pressure. Experiments of single coal particle pyrolysis were conducted in a laser reactor with pressure ranging from 0.014 to 2.1 MPa in a nitrogen atmosphere. As expected, an increase of pressure lead to a decrease of the volatile matter yield, which can be related to the secondary reactions of volatile matter. A characterisation of the char was performed by gas adsorption methods: nitrogen adsorption, carbon dioxide adsorption and active surface area (ASA) measurement. True and apparent densities, porosities and swelling of the particles were also investigated. Although the volatile matter yield decreases, the porosity and the swelling of the char increases with increasing pyrolysis pressure. We observed an increase in surface area and microporosity with increasing pressures up to 0.6 MPa. The ASA surface also increases in this temperature range, but the ratio of ASA to CO₂ surfaces shows that the intrinsic reactivity of the surface decreases with increasing pyrolysis pressure.     

大庆重石脑油热裂解反应规律的研究

采用自建的小型裂解实验装置对大庆重石脑油进行了热裂解实验。考察了反应温度、停留时间和汽烃比对裂解产物分布的影响规律。结果表明,各操作条件对大庆重石脑油裂解产物分布有不同程度影响,反应温度影响最大,随反应温度的升高,乙烯产率单调增加,丙烯、丁二烯和三烯产率分别在不同的裂解深度出现最大值;停留时间和汽烃比的影响相对较小,随着停留时间的增大,三烯收率略有下降,焦油的产率显著上升;增大汽烃比,三烯收率的略有上升,焦油产率下降。在实验装置的优化条件范围内,总三烯产率最优值51%,其中乙烯产率最优值31%。     

Suppressing secondary reactions of coal pyrolysis by reducing pressure and mounting internals in fixed-bed reactor

Pyrolysis of Shenmu coal was performed in fixed-bed reactors indirectly heated by reducing operating pressure and mounting internals in the reactor to explore their synergetic effects on coal pyrolysis. Mounting internals particularly designed greatly improved the heat transfer inside coal bed and raised the yield of tar production.Reducing pressure further facilitated the production of tar through its suppression of secondary reactions occurring in the reactor. The absolute increase in tar yield reached 3.33 wt% in comparison with the pyrolysis in the reactor without internals under atmospheric pressure. The obtained tar yield in the reactor with internals under reduced pressure was even higher than the yield of Gray–King assay. Through experiments in a laboratory fixed bed reactor, it was also clarified that the effect of reducing pressure is related to volatile release rate in pyrolysis. It did not obviously vary tar yield at pyrolysis temperatures below 600 °C, while the effect was evident at 650 and 700 °C but became limited again above 800 °C. Under reduced pressure the produced tar contained more aliphatics and phenols but less aromatics.     

The effect of coal particle size on pyrolysis and steam gasification

For future power generation from coal, one preferred option in the UK is the air-blown gasification cycle (ABGC). In this system coal particles sized up to 3 mm, perhaps up to 6 mm in a commercial plant, are pyrolysed and then gasified in air/steam in a spouted bed reactor. As this range of coal particle sizes is large it is of interest to investigate the importance of particle size for those two processes. In particular the relation between the coal and the char particle size distribution was investigated to assess the error involved in assuming the coal size distribution at the on-set of gasification. Different coal size fractions underwent different changes on pyrolysis. Smaller coal particles were more likely to produce char particles larger than themselves, larger coal particles had a greater tendency to fragment. However, for the sizes investigated in this study ranging from 0.5 to 2.8 mm, the pyrolysis and gasification behaviour was found not to vary significantly with particle size. The coal size fractions showed similar char yields, irrespective of the different char size distributions resulting from pyrolysis. Testing the reactivity of the chars in air and CO₂ did not reveal significant differences between size fractions of the char, nor did partial gasification in steam in the spouted bed reactor. From the work undertaken, it can be concluded that pyrolysis and gasification within the range of particle sizes investigated are relatively insensitive to particle size.     

Study on the structure and pyrolysis characteristics of Chinese western coals

The structure and pyrolysis characteristics of three inertinite-rich Chinese western coals were researched and compared with one relative vitrinite-rich Chinese middle coal by means of XRD, TG-DTG and fixed-bed reactor. The results show that the atomic ratio O/C, aromaticity factor, even ring condensation number and ring condensation index increase and atomic ratio H/C decreases with increasing inertinite content in coal; inertinite contains more aromatic ring structure than that of vitrinite; the crystallite structure order of coal char increases slightly with increasing heat treatment temperature. The higher inertinite content in coal is, the lower pyrolysis reactivity of coal is at lower temperature, and yet they have obvious second pyrolysis reactivity in higher temperature. The pyrolysis reaction in primarily devolatilisation phase that comes mainly from the decomposition of containing hydrogen function groups and the secondary devolatilisation at high temperature is mainly the decomposition of stable containing oxygen function groups in coal matrix with higher inertinite.     

Molten waste plastic pyrolysis in a vertical falling film reactor and the influence of temperature on the pyrolysis products

Molten plastics are characterised with high viscosity and low thermal conductivity. Applying falling film pyrolysis reactor to deal with waste plastics can not only improve heat transfer efficiency, but also solve the flow problem.In this work, the pyrolysis process of molten polypropylene(PP) in a vertical falling film reactor is experimentally studied, and the influence of heating temperature on pyrolysis products is discussed. It has been found that with the temperature increases from 550 ℃ to 625 ℃, the yield of pyrolysis oil decreases from 74.4 wt%(± 2.2 wt%) to53.5 wt%(±1.3 wt%). The major compositions of the pyrolysis oil are C_9, C_(12) and C_(18), and β-scission reactions are predominant. The content of the light fraction C_6-C_(12) of pyrolysis oil is 69.7 wt%. Compared with other pyrolysis reactors, the yield of oil from vertical falling film pyrolysis reactor is slightly higher than that from tubular reactor,equal to that from rotary kiln reactor, and slightly lower than that in medium fluidised-bed reactor.     

Investigation on initial stage of rapid pyrolysis at high pressure using Taiheiyo coal in dense phase

Rapid pyrolysis of Taiheiyo coal was investigated with a laboratory-scale batch type reactor (BTR), which was specially developed to study various gasification processes at the conditions close to an industrial entrained bed gasifier. The experiments were carried out in helium at 1073 K, 7.1 MPa, varying reaction times from 1 to 80 s and coal/gas ratios in the range of 0.41-1.65 g/l. Extents of pyrolysis and profiles of product formation were discussed based on the results of char yield, gas and tar formation characteristics. It was observed that reaction of pyrolysis was significantly affected by coal/gas ratio. At high coal/gas ratios, pyrolysis was found to be retarded at initial stage. Pyrolysis products can be roughly divided into two groups. One is the ultimate products such as methane, carbon oxides, hydrogen, and benzene and the other is the intermediate products such as ethylene and toluene. Heat supply inside BTR was examined and the influence of thermal properties of atmospheric gases was investigated by carrying out pyrolysis in nitrogen and by comparing the results with that in helium. As a result, the heat capacity of atmospheric gas has less influence on pyrolysis process whereas heat conductivity of atmospheric gas as well as mixing conditions of gas and coal sample significantly affect the pyrolysis reaction.     

Combined effects of pressure and ion-exchangeable metallic species on pyrolysis of Victorian lignite

A set of ion-exchanged samples prepared from Loy Yang lignite was pyrolyzed in a wire-mesh reactor at elevated pressures from 1 to 36 bar. The tar yields from the pyrolysis of H-form (acid-washed) sample at a fast heating rate of 1000 °C s⁻¹ were drastically reduced by increasing pressure to 6 bar and then remained unchanged with further increase in pressure to 36 bar. This behavior of the tar yield was in sharp contrast to that from the raw lignite which showed a minimum with increasing pressure. The sensitivities of the tar yields to changes in the heating rate were also suppressed by increasing pressure. The tar yields from Ca-form and Na-form samples (prepared by ion-exchanging Ca and Na on the H-form sample, respectively) were not very sensitive to changes in the heating rate and pressure up to 11 bar. At 20 bar, the tar yields from the Na-from sample nearly doubled whereas from the Ca-form sample nearly halved compared to those respective values at 1 bar. Although increasing pressure is thought to cause changes in the intra-particle mass transfer processes of volatile precursors, the rate of formation of volatile precursors tends to dictate the kind of mass transfer process responsible for the release of volatiles. Therefore, depending on the pyrolysis condition, bulk diffusion or forced flow would dominate the mass transfer processes for the release of volatiles. The introduction of cations is thought to result in irreversible changes in the lignite structure and not only control the process of formation but also the amount of volatile precursors and in turn alter the effects of pressure. Valence and catalytic activity of cations seem to play important roles in determining pyrolysis products distribution at elevated pressures.     

The temperature's influence on the selectivity between HNCO and HCN from pyrolysis of 2,5-diketopiperazine and 2-pyridone

Two cyclic amides, 2-pyridone and 2,5-diketopiperazine (DKP), were pyrolysed at temperatures ranging from 700 to 1100 °C. Pyridone is the only one of the four main nitrogen functionalities found in coal that is likely to form HNCO under pyrolysis. DKP is a primary pyrolysis product from proteins, which are the main nitrogen source in biomass. The formation of HNCO from biomass has been suggested to originate from DKP and other cyclic amides. The aromatic 2-pyridone was thermally more stable than the non-aromatic DKP. Both amides formed HCN, HNCO and NH₃. The NH₃ yields, about 3-4% for 2-pyridone and 10% for DKP, were almost independent of temperature. The HCN yield on the other hand showed strong temperature dependence and increased with temperature for both of the cyclic amides. The HNCO yield decreased with increasing temperature for DKP over the whole temperature interval. For 2-pyridone, the pyrolysis was incomplete at the lowest temperature in the investigation. Between 900 and 1100 °C, the pyrolysis of 2-pyridone was complete and the HNCO yield decreased with increasing temperature. The HNCO/HCN ratio for both of the cyclic amides decreased with increasing temperature over the whole investigated temperature range. The finding in literature that the HNCO formation from cracking of coal tars produced a maximum HNCO yield at an intermediate temperature, is explained by the thermal stability of pyridone at low temperatures and the selectivity towards HCN at high temperatures.     

An experimental study of fragmentation of coals during fast pyrolysis at high temperature and pressure

An experimental apparatus has been developed in order to perform tests of primary fragmentation of solid fuels under severe heating conditions (up to 2200 K and 12 bar). Particles are laid on the strip and pyrolyzed under inert conditions, fragments are recovered and analyzed by a laser granulometer to assess the fragmentation propensity of the fuel.Experiments have been carried out at temperatures between 1400 K and 1900 K, heating rate of 5000 K/s, pressure in the range 1-12 bar. Four different coals have been studied: Gracem, Venezuelan, Omsky, and Kleincopje, classified respectively as anthracite, high and medium volatile bituminous coals.Results show that primary fragmentation at high heating rate and high temperature may result in the formation of relatively coarse fragments and sometimes in a multitude of fines. The probability of fragmentation and the propensity to form coarse versus small fragments varies from coal to coal. For a given coal fragmentation increases monotonously with temperature, whereas the effect of pressure is nonmonotonous.The role of different chemico-physical properties of coals on the pattern and the extent of primary fragmentation is discussed, in particular volatile matter content, ash melting point, rigidity and porosity of the carbon structure and swelling index.     

Formation of NOx precursors during the pyrolysis of coal and biomass. Part VIII. Effects of pressure on the formation of NH3 and HCN during the pyrolysis and gasification of Victorian brown coal in steam

Effects of pressure on the formation of HCN and NH₃ during the pyrolysis and gasification of Loy Yang brown coal in steam were investigated using a pressurised drop-tube/fixed-bed reactor. The NH₃ yield increased with increasing pressure during both pyrolysis and gasification. Increasing pressure selectively favours the formation of NH₃ at the expenses of other N-containing species. The changes in the yield of NH₃ with increasing pressure were mainly observed in the feeding periods both during pyrolysis and gasification and were closely related to the formation and subsequent cracking of soot both as a result of intensified thermal cracking of volatile precursors inside the particles and as a result of volatile-char interactions after the release of volatiles. While the corresponding HCN yield during pyrolysis showed little sensitivity to changes in pressure, the HCN yield during gasification in steam showed some increases with increasing pressure. Our data indicate that the direct hydrogenation of char-N by H radicals, favoured by the presence of steam, is the main route of NH₃ formation during pyrolysis and gasification. The direct conversion, either through hydrogenation or hydrolysis, of HCN into NH₃ on char surface during the pyrolysis and gasification of brown coal is not an important route of NH₃ formation.     

Desulfurization of coal through pyrolysis in a fluidized-bed reactor under nitrogen and 0.6% O2-N2 atmosphere

The raw Yima (YM) and Datong (DT) coal, their demineralized (YM-ash, DT-ash) and de-pyrite (YM-p, DT-p) coals were pyrolyzed in a fluidized-bed reactor to examine the sulfur removal efficiency. The effect of process parameters such as temperature, residence time and atmosphere were investigated. The results show that there is an optimal temperature and residence time for the maximum desulfurization, varying with type of coal and the thermal stability of organic sulfur. The alkaline-earth mineral in the raw coal plays an important role for the fixation of sulfur and makes desulfurization decrease. The interaction of pyrite with the organic matrix of coal is the dominant reason that leads to organic sulfur accumulation in char. YM has higher desulfurization of organic sulfur than DT due to more aliphatic sulfur in the raw coal. YM and DT were pyrolyzed in 0.6%O₂-N₂ mixed atmosphere, aiming at examining the effect of reactive gas on the sulfur removal during pyrolysis. The results show that sulfur removal is improved without great decrease in char yield. This indicates that small amount of O₂ in inert atmosphere can improve desulfurization efficiency. In addition, TG-MS runs in 0.6%O₂-Ar and Ar were carried out to check the sulfur-containing compounds in pyrolysis gas and further understand the desulfurization process.     

Investigation of an iron-based additive on coal pyrolysis and char oxidation at high heating rates

Iron-based catalysts have been shown to enhance coal pyrolysis and char oxidation at low to moderate temperatures and heating rates (< 1250 K and 1–1000 K/s). Such catalytic activity has not been demonstrated at high heating rates and temperatures approaching pulverized coal combustion applications. The effect of an iron-based additive on coal pyrolysis and char combustion was studied in a flat-flame burner system at high particle heating rates using a Kentucky bituminous coal. Pyrolysis and char reactivity of two treated coals with different catalyst loadings were studied and compared with the untreated coal. The total volatiles yield for the treated coals increased between 14 and 18% (absolute) on a dry ash-free basis compared to the untreated coal in experiments conducted at 1300 K. A first-order char oxidation model was used to compare the apparent char reactivities of the treated and untreated coals measured at 1500 and 1700 K. An increase in apparent char reactivity was observed for both treated samples.     

The influence of temperature on the yields of compounds existing in bio-oils obtained from biomass samples via pyrolysis

The influence of temperature on the compounds existing in liquid products obtained from biomass samples via pyrolysis were examined in relation to the yield and composition of the product bio-oils. The product liquids were analysed by a gas chromatography mass spectrometry combined system. The bio-oils were composed of a range of cyclopentanone, methoxyphenol, acetic acid, methanol, acetone, furfural, phenol, formic acid, levoglucosan, guaiacol and their alkylated phenol derivatives. Thermal depolymerization and decomposition of biomass structural components, such as cellulose, hemicelluloses, lignin form liquids and gas products as well as a solid residue of charcoal. The structural components of the biomass samples mainly affect the pyrolytic degradation products. A reaction mechanism is proposed which describes a possible reaction route for the formation of the characteristic compounds found in the oils. The supercritical water extraction and liquefaction partial reactions also occur during the pyrolysis. Acetic acid is formed in the thermal decomposition of all three main components of biomass. In the pyrolysis reactions of biomass: water is formed by dehydration; acetic acid comes from the elimination of acetyl groups originally linked to the xylose unit; furfural is formed by dehydration of the xylose unit; formic acid proceeds from carboxylic groups of uronic acid; and methanol arises from methoxyl groups of uronic acid     

SIS/石油树脂共混物高温阻尼性能的研究

通过动态力学分析研究苯乙烯-异戊二烯-苯乙烯嵌段共聚物（SIS）／石油树脂共混物的高温阻尼性能。结果表明，SIS有两个独立的tantδ峰；聚苯乙烯（PS）硬段含量越大，SIS的高温tantδ峰值越大，粘流温度越高，高温阻尼性能越好。SIS／石油树脂共混物的有效阻尼温度范围和tantδ峰值较大，高温阻尼性能较SIS好；石油树脂软化点升高，共混物的阻尼温度范围增大，且向高温方向移动。SIS-A／石油树脂（P140）共混比为40／60时，共混物在57～109℃范围内的阻尼性能较好。加入PS或PS／云母可使共混物的阻尼温度范围进一步向高温方向偏移。