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.     

Study of the influence of pressure on enhanced gaseous hydrocarbon yield under high pressure-high temperature coal pyrolysis

The influence of pressure on the yield of gaseous hydrocarbon products derived from pyrolysis of Fushun and Xianfeng coals have been investigated in an anhydrous and confined system. Pyrolysis was performed in sealed gold tubes at 380 °C and under the pressures ranging from 50 to 250 MPa for 24 h. The results show that the effect of pressure on coal pyrolysis and product generation should not be ignored. For the Fushun and Xianfeng lignite, the yields of gaseous hydrocarbon generation increase by 9.1% and 12.7% when the pressure increases from 50 to 250 MPa, respectively. However, the yields of hydrogen gas decrease greatly with pressure. The hydrogen gas yields of Fushun and Xianfeng lignite decrease by 76.5% and 75.9%, respectively, when the pressure increases from 50 to 250 MPa. Yields of carbon dioxide gas of Fushun and Xianfeng coals were enhanced with increasing pressure by 7.4% and 8.9% respectively. Data of stable carbon isotope compositions reveal that the methane and ethane carbon isotope values are also affected by pressure, as they become heavier by approximately 1.2‰ (PDB) when the pressure is increased from 50 to 250 MPa. Simultaneously, the hydrogen isotope compositions of methane and ethane increase by 10.3‰ and 7.1‰, respectively. Our experimental results suggest that the increase in gaseous hydrocarbon yield is resulted from synthesis of carbon dioxide and hydrogen and pressure serves to facilitate the synthetic process.     

Behavior of trace elements during pyrolysis of coal in a simulated drop-tube reactor

Release behavior and chemical form distribution of As, Pb, Cr, Cd and Mn in Datong coal during pyrolysis was studied in a simulated drop-tube reactor at a heating rate of about 1000 °C/s, including effects of temperature (300-1000 °C), atmosphere (N₂ and H₂), and holding time (0.3-10 min). Results show that the bleeding ratios of As, Pb, Cr, Cd and Mn increase with increasing pyrolysis temperature and holding time. Reductive environment results in higher emission of the elements. Among the five trace elements, As, Pb and Cd show similar behavior with volatilities higher than that of Cr and Mn at 1000 °C. The five trace elements in the coal and coal-derived chars are separated into five fractions through an extraction procedure. Ion exchangeable form of the elements is not found in the coal and the chars, and the elements remained in the residue fraction is the most dominant occurrence form in the coal and the chars for As, Pb, Cd and Cr. All the forms for all the elements undergo transformation in the pyrolysis resulting in reduced content in the chars.     

Examination of catalytic roles of inherent metallic species in steam reforming of nascent volatiles from the rapid pyrolysis of a brown coal

In-situ steam reforming of tar from the rapid pyrolysis of a Victorian brown coal was studied, employing a single-stage drop-tube reactor and a particular type of two-stage reactor, in which the nascent tar underwent steam reforming and thermal cracking in the presence and absence of nascent char particles, respectively. Na was the most abundant inherent metallic species contained in the coal, and a significant proportion of Na (60–80%) was volatilized during the pyrolysis. However, the Na dispersed in the vapor phase seemed to have no significant catalytic effect on the steam reforming. Na, and/or Ca remaining on the surface of char particles were responsible for rapid and extensive steam reforming of the nascent tar into gases, resulting in tar yield decrease by nearly 90%. The presence of steam alone was effective for suppressing soot formation from the tar vapor by approx. 80%, but in the absence of char particles containing metallic species, the addition of steam led to an increase in the yield of poly-nuclear aromatics.     

Tar destruction and coke formation during rapid pyrolysis and gasification of biomass in a drop-tube furnace

This paper describes tar destruction and coke (or soot) formation of biomass in three different conversion processes: pyrolysis (in a pure nitrogen stream), steam gasification (in a mixture stream of steam and nitrogen), and partial oxidation (in a mixture stream of oxygen and nitrogen), over a wide temperature range from 600 to 1400 °C. A woody waste, hinoki cypress sawdust (HCS), was used as a feedstock, and an entrained drop-tube furnace (DTF) was applied to all experimental tests. It is found that raising the temperature remarkably decreases tar evolution. Steam and oxygen also have a positive effect on tar destruction. Benzene and toluene are the most difficult condensable tar species to destroy. The achievement of their complete destruction in the product gas requires extremely high temperatures above 1200 °C, regardless of the gasifying agents. The coke deposits from 900 °C and reaches a maximum formation at 1000 or 1100 °C. The results obtained in this study suggest that competition occurs between the secondary decomposition of hydrocarbon species and gasification reactions of the produced char and/or coke with gasifying agents in the temperature range of 900-1100 °C.     

The reactions of formation of selected gas products during coal pyrolysis

Pyrolysis examinations conducted under non-isothermal conditions as well at low heating rate can show that the processes of hydrogen and methane formation are the result of several constituent reactions. In the presented paper a number of these reactions has been determined separately for each of the above mentioned gaseous products. The kinetic parameters of the reactions as well as the yields of products have also been calculated. It has been found that, during coal pyrolysis, methane is formed as a result of six constituent reactions and hydrogen is produced as a result of five constituent reactions. The values of activation energy and frequency factor for the reactions in question were determined. These values fall within the range, which is typical of chemical reactions.     

Modeling changes of fractal pore structures in coal pyrolysis

Coal pyrolysis processes are numerically investigated in mathematically produced coal pore models which simulate real coal pores in the parameters of the porosity and fractal dimension. The simulations include FG-DVC chemical reaction model, gas molecular diffusion in pores, energy conservation model and coal swelling model. Numerical results are verified by experimental results qualitatively, and they revealed that both the porosity and volatile contents of the parent coal can affect the fractal dimension of the final char pores after pyrolysis linearly. A formula to predict the fractal dimension of char pores from its parent coal properties is obtained by curve fitting in numerous results.     

A comparison of basket willow and coal hydrogasification and pyrolysis

The presented work aimed at investigating the course of basket willow hydrogasification and comparing that process to the hydrogasification of bituminous coal. The examinations focussed on basket willow (Salix viminalis), bituminous coal of low degree of metamorphism and their blends at the mass ratio 1:1. Measurements of evolution kinetics of gaseous hydrocarbons during hydrogasification of those materials were conducted in the atmosphere of hydrogen under the pressure of 2.5 MPa. In the investigations, the non-isothermal method was employed. The examined samples were heated from ambient temperature up to 1200 K at the rate of 3 K/min.     

Nitrogen evolution during rapid hydropyrolysis of coal

The behavior of nitrogen evolution during rapid hydropyrolysis of coal has been investigated at temperatures ranging from 923 to 1123 K and hydrogen pressure up to 5 MPa using a continuous free fall pyrolyzer. Three coals have been tested in this study. The dominant nitrogen gaseous species is ammonia, together with a little amount of HCN because most of HCN is converted to NH₃ through secondary reactions. The results show that the evolution of nitrogen in coal is caused mainly by devolatilization at temperatures below 973 K, while the evolution of volatile nitrogen in char is accelerated with increasing temperature and hydrogen pressure. The mineral matter in coal act as catalysts to promote the evolution of volatile nitrogen in char to N₂ apparently at high temperatures of 1123 K, as found during pyrolysis of coal by Ohtsuka et al. A pseudo-first-order kinetic model was applied to the evolution of nitrogen in coal during rapid hydropyrolysis. The model shows the activation energy for the nitrogen evolution from coal is 36.6–58.6 kJ/mol while the rate of the nitrogen evolution depends on hydrogen pressure in the order of 0.16–0.24.     

Change of nitrogen functionality of N-enriched condensation products during pyrolysis

Changes in the nitrogen functionality of ¹⁵N-enriched condensation products prepared from glucose and ¹⁵N-glycine were investigated during pyrolysis at 600–1000 °C. The structural changes in the condensation products were studied by means of solid-state ¹³C and ¹⁵N NMR spectroscopies. During pyrolysis, the aliphatic moieties of the condensation products decomposed and evolved as gas and tar. At pyrolysis temperatures above 600 °C, almost all the carbon in the chars were converted to aromatic carbon. After pyrolysis, large amounts of nitrogen remained in the chars as char nitrogen (char-N), and about 30% of the nitrogen was eliminated from the chars as HCN and NH₃. With increasing temperature, the production of HCN and NH₃ increased and the amount of char-N decreased. By combining X-ray photoelectron spectroscopy and NMR results, detailed results for nitrogen fractions in chars were obtained. During pyrolysis, the fraction of unsubstituted pyrrole-N decreased and the fraction of quaternary-N increased. The fraction of pyridine-N remained almost constant at temperatures below 800 °C, but at 900 °C and above, the fraction of pyridine-N decreased. The fraction of substituted pyrrole-N showed minimum at 800 °C. On the basis of these results, structural changes of nitrogen functional groups during pyrolysis are discussed.     

Integration of coal pyrolysis process with iron ore reduction:Reduction behaviors of iron ore with benzene-containing coal pyrolysis gas as a reducing agent☆

An integrated coal pyrolysis process with iron ore reduction is proposed in this article. As the first step, iron oxide reduction is studied in a fixed bed reactor using simulated coal pyrolysis gas with benzene as a model tar compound. Variables such as reduction temperature, reduction time and benzene concentration are studied. The carbon deposition of benzene results in the retarded iron reduction at low temperatures. At high temperatures over800 °C, the presence of benzene in the gas can promote iron reduction. The metallization can reach up to 99% in20 min at 900 °C in the presence of benzene. Significant increases of hydrogen and CO/CO2 ratio are observed in the gas. It is indicated that iron reduction is accompanied by the reforming and decomposition of benzene. The degree of metallization and reduction increases with the increasing benzene concentration. Iron oxide can nearly completely be converted into cementite with benzene present in the gas under the experimental conditions. No sintering is found in the reduced sample with benzene in the gas.     

Birch wood particle shrinkage during rapid pyrolysis

A study of the shrinkage of cubic (∼5 mm) birch wood particles during pyrolysis is presented. The particles were rapidly injected into a preheated furnace with a constant temperature in the range 350-900°C. The size of the particles in longitudinal, tangential and radial directions was measured until no further mass loss occurs. The volume shrinkage was found to be 45-70% and the shrinkage in the different directions 5-25, 25-40 and 15-40% for longitudinal, tangential and radial directions, respectively. Longitudinal shrinkage commenced after about 60% mass loss and is not strongly dependent on heating rate or on cellulose chain scission. A maximum shrinkage was found for tangential and radial directions at 400 and 500-700°C, respectively, and above these temperatures the shrinkage decreases. The char yield decreases and the char structure becomes more deranged with increasing temperature. Empirical models of shrinkage as a function of conversion are presented.     

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.     

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.     

The effects of morphological changes and mineral matter on H2S evolution during coal pyrolysis

The H₂S release profiles of five important Colombian coals have been monitored using temperature programmed pyrolysis. It was found that there was no correlation between the amount of H₂S and the sulphur content of the original coals. Coals which had been treated to remove all of the mineral matter and inorganic sulphur showed a good correlation with the free swelling index of the coals. This was explained by the physical trapping of H₂S in closed porosity formed during pyrolysis. A similar result was found for demineralised coals with pyrite present. The H₂S for untreated coals showed no systematic variation with rank, the coal sulphur content or free swelling index. This was because certain coals were rich in finely dispersed calcium which could chemically prevent H₂S release.     

Effect of inorganic matter on reactivity and kinetics of coal pyrolysis

Two Chinese coals, Shenfu subbituminous coal and Huolingele lignite, were used to investigate the effect of mineral matter in coal on reactivity and kinetic characteristics of coal pyrolysis. The experiments were carried out by using thermogravimetry to check the pyrolysis behavior of raw coal, HCl/HF demineralized coal and demineralized coal with inorganic matter (CaO, K₂CO₃ and Al₂O₃) added, respectively. The results showed that inherent mineral in coal had no evident effect on the reactivity and kinetics of coal pyrolysis. CaO, K₂CO₃ and Al₂O₃ all had a catalytic effect on the reactivity of coal pyrolysis, their effects were closely related to temperature region and coal types. The pyrolysis process of all the samples studied can be described by three independent first order kinetic model. Addition of inorganic matter the activation energy decreased and the characteristic temperature of coal changed.     

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.     

Uneven distribution of sulfurs and their transformation during coal pyrolysis

Two Chinese coals, Liuzhi high pyrite coal with high ash content (LZ) and Zunyi high organic sulfur coal (ZY), were pyrolyzed in a fixed-bed reactor under nitrogen and hydrogen at temperature ranging from 400 to 700 °C. The effects of heat rate, temperature and gas atmosphere on sulfur transformation and sulfur uneven distribution were examined by XPS combined with traditional sulfur analysis method. The ratio of surface S to bulk S is used to describe the uneven distribution of sulfurs. It is found that oxygen is rich on the surface, while S in the bulk. The increasing ratio of surface S to bulk S with increasing temperature clearly indicates the sulfur transfer from the bulk to the char surface during pyrolysis. The ratios are higher at all temperatures studied for ZY coal than for LZ coal, which may be related to the higher ash content in LZ coal. The ratio of surface S to bulk S increases with increasing heating rate for LZ coal, while it decreases for ZY coal. In the presence of H₂, the S on the surface is much lower than that under N₂ and surface S in sulfidic, thiophenic and sulfoxide forms is totally disappeared for LZ coal at various temperatures and heating rates, while the surface S in thiophenic and sulfoxide forms is not totally disappeared for ZY coal, which may be related to the high rank of ZY coal. The ratio of surface S to bulk S decreases before 600 °C with increasing temperature for both coals in the presence of H₂, showing that gaseous H₂ can easily react with the surface S to form H₂S, while above 600 °C it increases because the supply of H₂ cannot match the rate of formation of HS free radicals at high temperature.     

Biodesulphurized subbituminous coal by different fungi and bacteria studied by reductive pyrolysis. Part 1: Initial coal

One of the perspective methods for clean solid fuels production is biodesulphurization. In order to increase the effect of this approach it is necessary to apply the advantages of more informative analytical techniques. Atmospheric pressure temperature programming reduction (AP-TPR) coupled with different detection systems gave us ground to attain more satisfactory explanation of the effects of biodesulphurization on the treated solid products.Subbituminous high sulphur coal from “Pirin” basin (Bulgaria) was selected as a high sulphur containing sample. Different types of microorganisms were chosen and maximal desulphurization of 26% was registered. Biodesulphurization treatments were performed with three types of fungi: “Trametes Versicolor” - ATCC No. 200801, “Phanerochaeta Chrysosporium” - ME446, Pleurotus Sajor-Caju and one Mixed Culture of bacteria - ATCC No. 39327. A high degree of inorganic sulphur removal (79%) with Mixed Culture of bacteria and consecutive reduction by ∼13% for organic sulphur (S_org) decrease with “Phanerochaeta Chrysosporium” and “Trametes Versicolor” were achieved.To follow the S_org changes a set of different detection systems i.e. AP-TPR coupled “on-line” with mass spectrometry (AP-TPR/MS), on-line with potentiometry (AP-TPR/pot) and by the “off-line” AP-TPR/GC/MS analysis was used. The need of applying different atmospheres in pyrolysis experiments was proved and their effects were discussed. In order to reach more precise total sulphur balance, oxygen bomb combustion followed by ion chromatography was used.     

Gas evolution during rapid,low-temperature pyrolysis of a North Dakota lignite

Low-temperature pyrolysis of low-rank coals is proposed as a possible technique for producing high-heating value solids. Results are presented showing gas yields and char compositions from low-temperature pyrolysis of a lignite at very short residence times. Tar evolution is observed even at temperatures less than 570K coinciding with the first release of CO₂, presumably from carboxyl groups.