Mercury transformations in the exhausts from lab-scale coal flames

A laboratory-scale pulverized coal flame generated exhausts from five coals which were processed at realistic quench rates and residence times with typical flyash loadings. Mercury levels were monitored in the coal feed, all particulate streams from the furnace, and in the gaseous effluent. Contributions from elemental and oxidized species to the mercury vapors were also determined. Reported extents of Hg oxidation were not proportional to coal-Cl levels. Only the coal with excessive Cl generated an abundance of oxidized Hg species. Extents of Hg oxidation did not increase for progressively longer residence times in the exhaust system, but were affected by the level of unburned carbon, suggesting an essential role for heterogeneous chemistry. The split between particulate and vapor Hg species shifted toward particulate-Hg for progressively cooler temperatures at the exhaust outlet. The levels of particulate-Hg were generally higher for the coals that generated less oxidized Hg vapors. Appreciable Hg sorption was observed at temperatures as hot as 500 °C.     

Removal of mercury from coal by mild pyrolysis and leaching behavior of mercury

Various types of coals were subjected to the pyrolysis at 300 and 400 °C (mild pyrolysis), and the removal of Hg from coal was determined. The removal efficiency of Hg greatly varied with coal type from 20 to 80%. The removal efficiency was dependent neither on the specific surface area of coal nor on the particle size of coal. The leaching of Hg from coal was tested using sulfur-containing chelating agents such as 3-mercaptopropionic acid (MPA). The Hg leaching efficiency also varied greatly with coal type. There was a good correlation between the degree of %Hg leaching in the MPA leaching and that of %Hg removal in the mild pyrolysis.     

Product compositions in rapid hydropyrolysis of coal

The rapid hydropyrolysis of a Montana lignite and a Pittsburgh Seam bituminous coal has been examined at a high heating rate (1000 °C s^?1). The results are interpreted in the light of those presented previously for pyrolysis of the two coals. A complex situation involving simultaneous chemical and mass transfer rate control is revealed, and it is apparent that the pyrolysis phase cannot be treated separately from the hydropyrolysis phase. Three key process variables, hydrogen pressure, temperature, and particle diameter, are seen to have major effects on the total yields of products. There appears to be an important tradeoff between high hydrogen pressures and high temperatures. Under the present conditions of almost zero vapour residence time at elevated temperatures, methane is the principal product of reaction between hydrogen and coal, and the yields of light aromatic liquids are small.     

Thermal stability of mercury captured by ash

The thermal stability of mercury captured by ash was studied by sampling ash throughout the collection train of two Kentucky power plants. Sampling occurred over multiple years and involved both fresh and archived samples. During one ash collection episode, sampling was from the combustion of a single pulverized coal feed. The other collections involved ash from blended feeds. Ash was collected from economizer, mechanical and electrostatic precipitator hoppers. Feed coals, rejects and bottom ash were also sampled. Fractions of all the samples were heated in a thermal analyzer to maximum temperatures increased sequentially from 100 to 500 °C in 100 °C increments. The mercury content of the spent material was then determined by analysis of the solids for Hg. From this data the thermal decomposition temperature of the captured mercury was determined. The total mercury captured by each sample, thermal stability of the mercury in relation to collection site, and correlations between mercury capture and chemical composition of the sample were also determined. The data showed that mercury was released between 300–400 °C for all ash samples. The thermal release of Hg between 300–400 °C was studied in greater detail by following the Hg release in several samples at 25 °C intervals from 300–400 °C. The concentration of mercury captured in the ESPs hoppers was greater than in the ash collected from the economizer or mechanical separators.     

Ambient-pressure thermogravimetric characterization of four different coals and their chars

Ambient-pressure thermogravimetric characterization of four different coals and their chars was performed to obtain fundamental information on pyrolysis and coal and char reactivity for these materials. Using a Perkin-Elmer TGS-1 thermobalance, weight loss as a function of temperature was systematically determined for each coal heated in helium at 40 and 160 °C/min under various experimental conditions, and for its derived char heated in air over a temperature range of 20 to 1000 °C. The results indicate that the temperature of maximum rate of devolatilization increases with increasing heating rate for all four coals. However, heating rate does not have a significant effect on the ultimate yield of total volatiles upon heating in helium to 1000 °C; furthermore, coupled with previous data⁹ for identical coal samples, this conclusion extends over a wide range of heating rate from 0.7 to 1.5 × 10⁴ °C/s. Using the temperature of maximum rate of devolatilization as an indication of relative reactivity, the devolatilization reactivity differences among the four coals tested that were suggested by this criterion are not large. For combustion in air, the overall coal/char reactivity sequence as determined by comparison of sample ignition temperature is: N. Dakota lignite coal ≈ Montana lignite coal > North Dakota lignite char > III. No. 6 bituminous coal ≈ Pittsburgh Seam bituminous coal > Montana lignite char > III. No. 6 bituminous char > Pittsburgh Seam bituminous char. The reactivity differences are significantly larger than those for devolatilization. The reactivity results obtained suggest that coal type appears to be the most important determinant of coal and char reactivity in air. The weight loss data were fitted to a distributed-activation-energy model for coal pyrolysis; the kinetic parameters so computed are consistent with the view that coal pyrolysis involves numerous parallel first-order organic decomposition reactions.     

Interactions between coking coals in blends

The thermoplastic behaviour of 78 binary blends of Australian coking coals was measured using proton magnetic resonance thermal analysis. Their measured extents of fusion were compared with the values calculated from measurements made on the component coals assuming additivity. Significant differences between calculated and measured results were found for most blends, though only at temperatures between 400 and 520 °C: the coals interacted at these temperatures in a way that affected their fluidity. Both positive and negative differences were observed. The magnitude of the differences increased both with increasing differences in rank between the coals and differences in fluidity between the coals. A statistical study of the differences showed that material that became fluid in coal at temperatures below about 360 °C did not appear to contribute to the interactions, which suggests that fluid material derived from liptinite plays a much smaller role in interactions than fluid material derived from vitrinite or inertinite. Additionally, the study indicated that the less fusible material in a blend slightly reduced the extent to which the associated more fusible material fused. It was not acting as an inert diluent.Fifteen blends of six Argonne premium coals were examined to see if the relationships found for Australian coals between the magnitude of the interaction and coal properties could be generalised. In most cases the agreement was good. However, at some temperatures, blends of Upper Freeport coal with lower rank coals were far less fluid than expected, suggesting that the fluidity of Upper Freeport coal is especially sensitive to these low rank coals.The general effect of interactions between coals of different ranks was to narrow the thermoplastic temperature range of the blend without reducing the maximum fluidity, in effect making the thermoplastic profile of the blend resemble the profile expected from an individual coal of the same average rank as the blend. The interactions are attributed to transfer of plasticising volatile material between the coals.     

停留时间对低阶煤快速热解产物分布、组成及结构的影响

利用自由落下床反应器,研究了快速热解过程中颗粒停留时间对神木烟煤和内蒙古褐煤热解过程的影响,并进一步延长快速热解新生半焦停留时间考察了半焦的二次热解过程。结果表明,快速热解过程中颗粒停留时间的增加促进了挥发分的析出,神木烟煤热解焦油产率持续增加,内蒙古褐煤热解焦油产率先增加后降低。停留时间对焦油品质有明显影响,随时间的增加,两种煤快速热解轻质油中苯类和苯酚类单环化合物含量均先增加后降低,进一步延长停留时间,多环芳烃化合物含量显著增加。快速热解半焦的二次热解主要促进了气体的生成,焦油产率和组成无明显变化,挥发分的进一步析出促进了半焦微孔的发展,神木烟煤和内蒙古褐煤半焦比表面积均显著增加。这表明,在以高品质焦油为目标产品时,采用较低的反应温度、较适宜的煤粒停留时间的快速热解工艺条件是可取的。     

CO2 gasification of Thai coal chars: Kinetics and reactivity studies

Two sized fractions (<75 μm and 150–250 μm) of Ban Pu lignite A and Lampang subbituminous B coals were pyrolyzed in a drop tube fixed bed reactor under nitrogen atmosphere at 500–900 °C. Gasification of coal chars with excess carbon dioxide was then performed at 900–1,100 °C. The result was analyzed in terms of reactivity index, reaction rate and activation energy. It was found that chars at lower pyrolysis temperature had highest carbon conversion, and for chars of the same sized fraction and at the same pyrolysis temperature, reactivity indices increased with gasification temperature. The lower rank Ban Pu lignite A had higher R _s values than higher rank Lampang subbituminous B coals. Smaller chars from both coals had higher R _s values, due to the higher ash content. At present, it can be concluded that, within the gasification temperature range studied, gasification rates of chars obtained at various pyrolysis temperatures showed a linear correlation with temperature. However, additional experiment is needed to verify the correlation.     

混煤热解过程中的表面形态

以管式电炉为热解室，改变热解终温，在惰性气氛下对无烟煤与烟煤的混煤进行快速加热条件下的热解。采用低温氮气吸附方法研究混煤焦表面形态的变化规律。通过对吸附等温线的分析，表明煤焦具有连续、完整的孔隙结构，无定形孔的存在使得吸附迴线存在不闭合的状态。随着热解终温的升高，混煤焦的比表面积先增加后减小；随着烟煤掺混比例的增加，混煤焦的微孔容积和表面积也先增加后减小，A1B2混煤焦具有最大微孔容积和表面积。对煤焦孔隙的分形研究发现煤焦孔隙分形维数与微孔结构关系密切。混煤焦表面形态的变化规律体现了混煤热解的独立性以及相互作用。     

The volatilization behavior of chlorine in coal during its pyrolysis and CO2-gasification in a fluidized bed reactor

The volatilization behavior of chlorine in three Chinese bituminous coals during pyrolysis and CO₂-gasification in a fluidized bed reactor was investigated. The modes of occurrence of chlorine in raw coals and their char samples were determined using sequential chemical extraction method. The Cl volatility increases with increasing temperature. Below 600 °C the Cl volatility is different, depending on the coal type and the occurrence mode of Cl. Above 700 °C, the Cl volatilities for the three coals tested are all higher than 80%. About 41% of the chlorine in Lu-an coal and 73% of that in Yanzhou coal are organic forms, and most of them are covalently-bonded organic chlorine, which shows high volatile behavior even at low pyrolysis temperatures (below 500 °C), while the inorganic forms of chlorine in two coal samples are hardly volatilized even at low pyrolysis temperatures (below 400 °C). The restraining efficiency of addition of CaO on chlorine volatility is greatly dependent on pyrolysis temperature. The optimal restraining efficiency can be obtained at temperature range from 450 to 650 °C during pyrolysis of Lu-an coal. The volatile behavior of Cl is mainly dependent on temperature. Above 700 °C high volatility of Cl is obtained in both N₂ and CO₂ atmospheres.     

Behavior of mercury release during thermal decomposition of coals

The mercury release behavior during thermal decomposition of three Chinese coals with different types was studied under nitrogen, carbon dioxide and air at temperatures of 800, 900, 1,000 and 1,100 °C. The thermal treatment experiments were carried out in a quartz tube reactor. Results showed that the release ratio of total mercury during thermal decomposition of coals increases with the increasing temperature. The order of the amount of mercury released under the three atmospheres is nitrogen<carbon dioxide<air for all three coals during thermal decomposition. This indicates that air and carbon dioxide can promote the mercury release due to their reactivity with coal. However, the order of amount of elemental mercury released under the three atmospheres is nitrogen>carbon dioxide>air for all three coals. The release behavior of the total mercury under air is independent of the coal type. Under the other two atmospheres the release behavior is distinguished by the coal type. This work was presented at the 7 ^th China-Korea Workshop on Clean Energy Technology held at Taiyuan, Shanxi, China, June 26–28, 2008.     

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.     

Gasification behaviour of Australian coals at high temperature and pressure

This paper presents gasification conversion data generated for a suite of Australian coals reacting with oxygen/nitrogen mixtures at 2.0 MPa pressure and at temperatures up to 1773 K, as part of a wider investigation into the gasification behaviour of Australian coals. The effects of O:C ratio, residence time and coal type on conversion levels and product gas composition were investigated under conditions relevant to those present in entrained-flow gasification systems. At higher temperatures, coal conversion levels are, as expected, higher, whilst product gas compositions continue to reflect the relevant gas phase equilibrium conditions. These gas phase equilibrium concentrations show strong dependence on the amount of carbon in the gas phase (i.e. coal conversion). The increased conversion achieved at high temperatures allows the contribution of coal-specific properties such as char structure and reactivity to be investigated in more detail than previously possible. Furthermore, at higher conversion levels the effects of coal type on product gas composition are more apparent than at lower conversion levels. These high temperature, high pressure gasification conversion data have been reconciled with high pressure bench-scale pyrolysis and char reactivity measurements, highlighting the significance of coal-specific effects of key gasification parameters.     

Coal flash pyrolysis: 2. Polymethylene compounds in low temperature flash pyrolysis tars

Pyrolysis of coals at low temperatures (< 600 °C) produces tars containing the precursors of the low molecular weight aliphatic hydrocarbons, such as ethylene and propylene, observed on flash pyrolysis of the coals at higher temperatures (700–800 °C). This is shown by further pyrolysis of these low temperature tars at high temperatures. Various methods, including isolation by h.p.l.c. were used to confirm the presence of straight chain paraffin and olefin pairs (C₁₄C₂₆ and above) in the low temperature tars. Pyrolysis of pure paraffins and olefins in this molecular weight range at temperatures > 700 °C produce ethylene, propylene and other cracking products similar to those obtained on flash pyrolysis of coal.     

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 removal of mercury from coal via sub-critical water extraction

The transformation behavior of mercury in two Chinese coals (WJP and HYS coal) during sub-critical water treatments was studied in a semi-continuous apparatus. Float–sink method, demineralization and sequential chemical extraction were used to study the occurrence mode of mercury in raw coals and extracted samples. The results show that with increasing temperature, pressure, water flow and extraction time, the removal of mercury increases. During sub-critical water treatment, the content of mercury associated with sulfates, monosulfides, disulfides, organic material, and insoluble forms decreases. The removal efficiency of mercury is almost 100% at 410 °C, 15 MPa, 0.58 l/h, and 60 min for HYS coal and 96.7% at 380 °C for WJP coal. Under the same temperature and pressure the mercury removal through pyrolysis is less than that through sub-critical water extraction which is an efficient method to remove most mercury from coal.     

Floatabilities of Treated Coal in Water at Room Temperature

Abstract

Experiments on equilibrium adsorption loadings of various probe cormpounds on 60–200 mesh Illinois #6 coal (PSOC-1539), Adaville #1 coal (PSOC-1544), Wyodak coal (PSOC-1545), and Pittsburh #8 coal (PSOC-1549) were erformed. The probe compounds incqude 2-methyl-1-pentanol (2MfP), 1-heptanol, benzene, and toluene. Equilibrium adsorption loadings of aromatic compounds such as toluene and benzene on the four chosen coals obey the Langmuir isotherm model up to 100 ppm in concentrations of probe compounds. Equilibrium adsorption Poadings of higher aliphatic alcohols such as 2M1P and 1-heptanol on the four chosen coals donot follow both the Langmuir isotherm model and the Freundlich empirical adsor tion model. Flotation of the coals, equilibrated with aqueous sofutions of 2M1P and 1-heptanol, increases linearly with equilibrium adsorption loadings of these probe compounds onthe coals.

The chosen coals were treated with nitrogen and air at 1 atm and 125–225°C for 24 h. Flotation experiments of the treated coals were conducted at room temperature, using distilled water only asa flotation medium. Flotation of Adaville #1 coal and Wyodak coal treated with nitrogen gas is higher than that of the untreated coals and increases with treatment temperatures. Flotation of Adaville #1 coal treated with air at 125–225°C is not significantl different from that of untreated coal. Flotation of Pittsburgg #8 coal treated with air is lower than that of untreated coal and decreases with treatment temperatures. Flotation of Illinois #6 coal treated with nitrogen only is higher than that of untreated coal. Flotation of Illinois #6 coal treated with nitrogen at 125–175°C increases with treatment temperatures, whereas flotation of Illinois #6 coal treated with nitrogen at 175–225°C decreases with treatment temperatures.     

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.     

Reactivity study of combustion for coals and their chars in relation to volatile content

To determine the effect of volatile matter on combustion reactivity, the pyrolysis and combustion behavior of a set of four (R, C, M and K coals) coals and their chars has been investigated in a TGA (SDT Q600). The maximum reaction temperatures and maximum reaction rates of the coals and their chars with different heating rates (5–20 °C/min) were analyzed and compared as well as their weight loss rates. The volatile matter had influence on decreasing the maximum reactivity temperature of low and medium rank coals (R, C and M coals), which have relatively high volatiles (9.5–43.0%), but for high rank coal (K coal) the maximum reactivity temperature was affected by reaction surface area rather than by its volatiles (3.9%). When the maximum reaction rates of a set of four coals were compared with those of their chars, the slopes of the maximum reaction rates for the medium rank coals (C and M coals) changed largely rather than those for the high and low rank coals (R and K coals) with increasing heating rates. This means that the fluidity of C and M coals was larger than that of their chars during combustion reaction. Consequently, for C and M coals, the activation energies are lower (24.5–28.1 kcal/mol) than their chars (29.3–35.9 kcal/mol), while the activation energies of R and K coals are higher (25.0-29.4 kcal/mol) than those of their chars (24.1–28.9 kcal/mol).     

Py-MS Study of Sulfur Behavior during Pyrolysis of High-sulfur Coals under Different Atmospheres

In this study sulfur pyrolysis behavior of two Chinese high sulfur coals and their treated coal samples was investigated by Py-MS at a heating rate of 5 °C/min from room temperature to 1025 °C under hydrogen, helium and 2% O₂-He. It is found that the internal and external hydrogen do not show hydrogenation ability at temperature below 400 °C, due to no H₂S formation at this temperature region for all the coal samples. At temperature higher than 400 °C, not only the indigenous hydrogen but also indigenous oxygen can react with sulfur-containing radicals to form H₂S or SO₂. The evolution of H₂S and SO₂ displays the same profiles in pyrolysis of ZY pyrite-free coal under He, further revealing that after the breakage of C-S bond in the organic sulfur structure in coal to form sulfur-containing radicals, which can equally react with indigenous hydrogen and oxygen. The similar tendency between evolution of CO₂ and SO₂ and the same ending temperature also shows that not only C-S but also C-C bond can be broken in pyrolysis of ZY coals under 2% O₂-He atmosphere. However, unlike SO₂ evolution, CO₂ emission increases in the temperature ranging from 500 °C to 800 °C in LZ raw and deashed coals, implying the breakage of C-C bond at high temperature, which might be related to their low coal rank and high pyrite content.