Christina G. Vassileva’s research while affiliated with Bulgarian Academy of Sciences and other places

Toxic Hg species pose a global ecological threat due to the significant anthropogenic emissions of Hg to the atmosphere, particularly during coal combustion. An investigation about the content, association and modes of occurrence of Hg in diverse biomass ash (BA) varieties and its behaviour during biomass combustion was performed. These BAs belonging to woody, herbaceous, agricultural and aquatic biomass groups were examined with different chemical and mineralogical methods, as well as thermal and leaching procedures. The Hg contents in BAs are low and vary between 0.0032 and 0.0452 ppm (mean 0.0225 ppm). About 60–99% (mean 87%) and 63–100% (mean 91%) of Hg in biomass were volatilized at 500 °C and 700 °C, respectively, and only limited amounts of Hg were captured by BAs enriched mostly in salts such as carbonates, oxyhydroxides, phosphates, sulphates, and chlorides. The affinity of residual Hg in the BA system is towards relatively stable Fe-, P-, Ti- Al-, K-, and Si-bearing minerals (especially K aluminosilicates and Fe oxides), as well as less stable chlorides (particularly sylvite). The potential mode of Hg occurrence in BA is likely in the form of impurities in the above minerals. Alternative renewable fuels poor in Hg such as appropriate sustainable biomasses (0.001–0.043 ppm Hg) are suggested to partially or completely substitute the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants and thus minimize the ecological problems related to this element.

Toxic Hg species pose a global ecological problem due to the significant anthropogenic emissions of Hg into the atmosphere, especially during coal combustion. A study about the content, association and potential modes of occurrence of mercury and their ecological significance in eight biomass varieties belonging to woody, herbaceous, agricultural and aquatic biomass groups was conducted based on a combination of different chemical and mineralogical analyses and leaching procedures. The Hg contents in these biomasses vary between 0.003 ppm and 0.043 ppm (mean 0.013 ppm), which are about an order of magnitude lower than the Clarke value of coal (0.10 ppm). It seems that Hg occurs in both inorganic and organic matter of biomass, as the preferable association and potential modes of occurrence of this element include inorganic matter, hemicellulose, and water-soluble Cl-, S-, N-, and Na-bearing chlorides, sulphates, and nitrates such as halite, sylvite, arcanite, Ca nitrate, and nitrocalcite in the biomass system studied. Alternative renewable and C-neutral solid fuels poor in Hg such as appropriate sustainable biomasses are suggested to substitute partially or completely the industrial coals enriched in Hg (0.14–0.57 ppm) and used in Bulgarian thermoelectric power plants to minimize the ecological problems related to this element.

A short overview on the content, association, and significance of toxic Hg in 9 coal types and their fly ashes (FAs) from 12 Bulgarian thermoelectric power stations (TPSs) was conducted by a compilation of reference and our own data obtained by a combination of different chemical and mineralogical analyses, and separation procedures. The Bulgarian and Ukrainian coals studied are enriched in Hg (0.14–0.57 mg/kg) occurring in both organic and inorganic associations. The most abundant coals in Hg have higher S contents and ash yields, and are enriched in Fe sulphides, calcite, and Ca and Fe sulphates, as well as some clay minerals and feldspars. The dominant quantity (about 50–98%) from the fuel Hg was not captured by the coal ashes in TPSs. The significant Hg capture potential (38–50%) show FAs enriched in char, Ca and Fe sulphates and oxides, and Ca carbonates. It was found that the Hg concentrations in some FA water leachates are significantly higher in comparison with the Clarke values for fresh water and could provoke environmental risks. Alternative and sustainable biomass poor in Hg is suggested to substitute totally or partially the industrial coals used in Bulgarian TPSs to avoid the Hg problems.

The addition of biomass such as straw in the coal gasification process is one of the strategies for clean and efficient utilization of coal with decreasing CO2 emissions. However, the abundance of chlorine and alkaline metals contained in the corn straw is the key factor to aggravate the corrosion, deposition, and slagging of the co-gasifier. The effects of temperature (700–900 °C), residence time (2–12 min) and the addition ratios of corn straw on the migration behavior of Cl are studied during co-gasification of Shenmu coal and corn straw. The results show that the released amount of Cl from coal increases with the increase in gasification temperature and the extension of gasification time, while the amount of Cl in the coal gasification residue decreases initially and then stabilizes. On the other hand, the released amount of Cl from the corn straw increases with the increasing temperature and the extension of time below 800 °C, but both the temperature and time extension show limited effect on the released amount of Cl above 800 °C. It is also identified that the migration and volatilization behavior of Cl is related to the mass proportions of coal and biomass during the co-gasification process. When the addition ratios of corn straw are 16.7% and 50%, the released amount of Cl from the coal-biomass mixtures can be inhibited, while the released amount of Cl can be promoted when corn straw is added to coal-biomass mixture at the ratio of 37.5%.

The relatively high contents of some ash-forming elements (Na, K, Ca, Mg, P) in biomass ash can cause serious ash-related problems (deposition, slagging, corrosion, etc.) during the co-gasification of coal and biomass, influencing the stable operation of gasifier. The sintering behavior of Jincheng coal ash (JCA) and a phosphorus-rich Jatropha seed cake ash (JSCA) were investigated by in-situ heating stage microscopy, while the sintering mechanism was revealed by phase studies employing X-ray powder diffraction and thermodynamic modelling. Results indicated that the sintering temperature of blended ashes reached the minimum at the 20 wt% blending ratio of JSCA in the coal-biomass ash mixture. The high sintering temperatures of JCA and JSCA were due to the extensive mullite and quartz crystallization in the high acidic JCA system and the widespread formation of K–Ca phosphate and K–Mg phosphate in the high basic JSCA system, respectively. The transition of the phosphorus from fluxing effect to refractory effect on the sintering of ash mixture was identified at the 20 wt% blending ratio of JSCA. It was also found that the role of mullite and quartz decreased, while the role of anorthite, gehlenite, leucite, and calcium phosphate increased with increasing JSCA content, leading to the lowest sintering temperature. Finally, the variation trend of sintering temperatures against the JSCA additions was in accord with the change of the liquidus temperatures that was calculated by thermodynamic modelling. These observations can be used for the potential prediction of the ash sintering temperature in the coal-biomass gasifier.

The content and modes of occurrence of phosphorous (P) in eight biomass ashes (BAs) were studied. It was found that BAs are enriched in P as its concentration varies in the range 0.3–4.4% (mean 1.7%). The highest contents of P (>2 %) are characteristic of ashes produced from agricultural residues, namely plum pits, sunflower shells and corn cobs. It was identified that the modes of P occurrence in BAs are related mostly to apatite and whitlockite. Hence, BAs having renewable and C-neutral origin could be alternative and very perspective resource for the potential recovery of P and its utilization in industry in contrast to finite and non-renewable phosphate rocks.