Fate of Mercury in Volatiles and Char during in Situ Gasification Chemical-Looping Combustion of Coal.

Mercury emission is an important issue during in-situ gasification chemical-looping combustion ( iG-CLC) of coal. This work focused on experimentally "isolating" two elementary subprocesses (coal pyrolysis and char gasification) during iG-CLC of coal, identifying mercury distribution within the two subprocesses, and examining the effects of a hematite oxygen carrier (OC) on the mercury fate. The mercury measurement accuracy was carefully ensured by comparing online measurements (by a VM 3000 instrument) and benchmark measurements (by the standard Ontario Hydro Method, ASTM D6784) as well as repeated tests (10 times for each case). The mercury mass balance was 115% for the entire iG-CLC. A total of 44.7% of the mercury was released as the gas phase form within the coal pyrolysis process at a typical CLC operation temperature (950 °C), whereas 13.4% was released during the char gasification process. The release rate and amount of mercury were minimally affected by the presence of OC; however, the OC promoted the conversion of Hg0(g) to Hg2+(g). Only a small amount of mercury was absorbed by the OC and transported into the air reactor along with carbon residue, released as Hg0(g) and Hg2+(g) or remained in the OC and coal ash as particulate mercury.

Effect of the Conversion Degree on the Apparent Kinetics of Iron-Based Oxygen Carriers.

The role of the oxygen carrier is important in energy conversion processes with fluidized beds, particularly chemical looping technology. It is necessary to establish the relevant kinetics of oxygen carriers that can be applicable for various chemical looping processes. In this study, we analyzed the apparent kinetics of three iron-based oxygen carriers, namely, ilmenite, iron sand, and LD slag, during the conversion of CO, H2, and CH4 in a fluidized bed batch reactor. The effect of both the oxidation degree, presented as the mass conversion degree, and temperature was considered. The results show that the changing grain size (CGS) model is generally applicable in predicting the apparent kinetics of reactions between the investigated iron oxygen carriers and gaseous fuels even at lower oxidation degrees (3-5 wt % reduction). The activation energies of the investigated materials in the conversions of CO, H2, and CH4 obtained from the fittings of the CGS model are about 51-92, 55-251, and 72-211 kJ/mol, respectively. Both the mass conversion degree and temperature influence the reactivity of oxygen carriers in a directly proportional way, especially at temperatures higher than 925 °C. The results of this study are useful for reaction engineering purposes, such as designing a reactor, in chemical looping units, or in any other processes that use oxygen carriers as a bed material.

Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of Biomass-Experimental Investigation in a 10 kWth Pilot.

Alkali release was studied in a 10 kWth chemical looping pilot operated with a Linz-Donawitz (LD) slag oxygen carrier (OC) and three biomass fuels. Experiments were performed at three temperatures and in three operation modes: chemical looping combustion (CLC), chemical looping gasification (CLG), and oxygen-carrier-aided combustion (OCAC). Gas-phase alkali release was measured with a surface ionization detector (SID). Fuel reactor (FR) gas-phase alkali emissions increased with the temperature. This occurred as a result of increased evaporation of KCl and enhanced decomposition of alkali salts during char conversion. Air reactor (AR) alkali emissions were lower than in the FR and independent of the operating temperature. In comparison of operating modes, CLC and CLG modes resulted in similar gas-phase alkali emissions due to the similar extent of char conversion. In contrast, operation of the reactor system in OCAC mode resulted in significantly lower levels of gas-phase alkalis. The difference in alkali emission was attributed to the steam-rich atmosphere of CLC. The effect of steam was further investigated in CLC and OCAC tests. Lowering steam concentrations in CLC operation resulted in lower gas-phase alkali emissions, while introducing steam to the FR during OCAC operation resulted in higher alkali emissions. It was concluded that steam likely enhances gas-phase K release through a reaction of K2CO3 within the fuel char with steam to produce KOH(g). Solid sampling and analysis for K content was used along with SID measurements to develop a K mass balance for the reactor system. Mass balance results for the straw pellet fuel tests showed that LD slag OC absorbs approximately 15-51% of fuel K, 2.2% of fuel K is released to the gas phase, and up to 3.4% of fuel K is captured in the AR fly ash. The residual 40-80% of fuel K was determined to leave the FR as K-rich fly ash.

Effect of the Mass Conversion Degree of an Oxygen Carrier on Char Conversion and Its Implication for Chemical Looping Gasification.

Chemical looping gasification (CLG) is an emerging process that aims to produce valuable chemical feedstocks. The key operational requirement of CLG is to limit the oxygen transfer from the air reactor (AR) to the fuel reactor (FR). This can be accomplished by partially oxidizing the oxygen carrier in the AR, which may lead to a higher reduction degree of the oxygen carrier under the fuel conversion. A highly reduced oxygen carrier may experience multiple issues, such as agglomeration and defluidization. Given such an interest, this study examined how the variation of the mass conversion degree of ilmenite may affect the conversion of pine forest residue char in a fluidized bed batch reactor. Ilmenite was pre-reduced using diluted CO and then underwent the char conversion at 850, 900, 950, and 975 °C. Our investigations showed that the activation energy of the char conversion was between 194 and 256 kJ/mol, depending upon the mass conversion degree of ilmenite. Furthermore, the hydrogen partial pressure in the particle bed increased as the oxygen carrier mass conversion degree decreased, which was accompanied by a lower reaction rate and a higher reduction potential. Such a hydrogen inhibition effect was confirmed in the experiments; therefore, the change in the mass conversion degree indirectly affected the char conversion. Langmuir-Hinshelwood mechanism models used to evaluate the char conversion were validated. On the basis of the physical observation and characterizations, the use of ilmenite in CLG with biomass char as fuel will likely not suffer from major agglomeration or fluidization issues.