Study of the interaction between a Mn ore and alkali chlorides in chemical looping combustion

Chemical looping combustion (CLC) is a novel technology for heat and power generation with inherent CO2 capture. Using biomass in CLC (bio-CLC), negative CO2 emissions can be attained. Biomass usually contains high content of alkalis (mainly K and Na) which can be problematic in the process, such as potential alkali-bed interaction, and this is the focus of current work. This work uses charcoal with and without the impregnation with alkali chlorides, KCl and NaCl. The results are compared to previous data from samples impregnated with K2CO3 and Na2CO3. A low-alkali braunite manganese ore is used as bed material to study the oxygen carrier interaction with the alkalis in cyclic experiments at 950 \ub0C in a quartz batch fluidized-bed reactor. As compared to charcoal without alkali impregnation, the impregnation with KCl, NaCl, K2CO3, and Na2CO3 can improve the rate of gasification by a factor of 4, 3, 10, 8, respectively. Partial-defluidization of the braunite particles was found with all the alkali-fuels, although the extent differed, e.g., K2CO3 and KCl resulted in earlier onset of defluidization than Na2CO3 and NaCl. Further, indications of partial defluidization were earlier and more permanent with the carbonates than the chlorides. Partial agglomeration with soft agglomerates of the bed was observed, while hard agglomerations were never seen. Accumulation of K, Na, Si, and Ca was found in the agglomerates after cycles with K2CO3-charcoal and Na2CO3-charcoal, while little K and Na was detected in the bridges between particles after the KCl and NaCl cycles. A significant fraction of the alkali added was found in the oxygen carrier, with 80% or more being retained for the Na salts, and around 40% for the K salts. There was no clear difference between chlorides and carbonates with respect to retention. The fresh and used braunite have very similar reactivity with CH4 and H2, whereas some decrease in reactivity is noticed with CO

Performance of an oxy-polishing step in the 100 kWth chemical looping combustion prototype

Unconverted fuel gases are normally present in the gas leaving the fuel reactor of a Chemical Looping Combustion (CLC) process. Depending on several factors, including oxygen carrier and fuel volatiles content, the unconverted gases represent 5–30% of the oxygen needed for full combustion. Further conversion of these fuel components is imperative to achieve adequate combustion and to fulfill the requirements for CO2\ua0storage. An oxy-polishing step using highly concentrated O2\ua0to fully oxidize the fuel components offers a straightforward one-step way to reach complete combustion. However, systematic and detailed investigation is lacking while it is essential for design, scale up and optimization. In this work, the oxy-polishing is studied in a post-oxidation chamber (POC) of a 100 kWth\ua0unit using different solid fuels and manganese ores in the CLC process. With various flows of air as oxidation agent, the POC performance was evaluated under stable operations in a wide range of operating conditions. An overall oxygen ratio was defined to analyze the effect of O2\ua0excess in the POC. Experimental results show that the oxidation of fuel gas from the fuel reactor can be greatly enhanced by air entering the POC, with the gas conversion being improved from 87 to 90% before the POC to as high as 99–100% after the POC. Full oxidation in POC can be accomplished with excess of O2. For the cases of incomplete oxidation, CO was normally found in higher concentrations than CH4. In a few cases close to optimum, CO and O2\ua0simultaneously have normalized concentrations below 0.5–1% with a low overall oxygen ratio of around 1.01. The POC performance was further compared to the results from a simple reactor model

Batch fluidized bed study of the interaction between alkali impurities and braunite oxygen carrier in chemical looping combustion

Chemical looping combustion (CLC) is a novel technology for heat and power generation with low-penalty CO2 capture. Using biomass in CLC (bio-CLC), negative CO2 emission can be attained. Alkali (mainly K and Na) in biomass can be problematic in bio-CLC, as it can interact with the oxygen carrier bed. The current work used charcoal and four charcoal samples impregnated with K2CO3, Na2CO3, KCl and NaCl, respectively, to study alkali interaction with a low-alkali braunite manganese ore oxygen carrier. The experiments were successfully carried out at 950\ub0C in a quartz batch fluidized-bed reactor. For each alkali-fuel sample, more than 30 cycles of redox were performed. Using the solid fuel impregnated with K2CO3, Na2CO3, KCl and NaCl, char gasification was improved by a factor of 10, 8, 4 and 3 as compared to the non-impregnated fuel. Partial-defluidization of the braunite particles was found with all the alkali-fuels, although the extent differed, e.g. K2CO3 and KCl resulted in earlier onset of partial defluidization than Na2CO3 and NaCl. Hard agglomeration was never observed, while soft partial agglomeration was seen. Accumulation of K, Si, and Ca in agglomerates and particle boundary was found after cycles with K2CO3- and KCl-charcoal, while Na, Si and Ca was found after the Na2CO3- and NaCl-charcoal cycles. The mechanism of agglomeration formation seems different for these alkali-charcoals. For K2CO3- and KCl-charcoal, it seems the potassium reacted with Fe and Mn in the braunite, forming low-melting point components and thus led to agglomeration. In the case of Na2CO3- and NaCl-charcoal, direct reaction with the braunite was not seen, and it seems as if other reactive species combined were formed, which acted as a binder between particles to form agglomerates. In addition, after cycles with the K2CO3- and Na2CO3- charcoals, 80% K and 40% Na were retained in the oxygen carrier particles. After the use with all the alkalis, the braunite reactivity with CH4, CO and H2 was similar to the fresh particles. It is clear that alkali species could react with the braunite oxygen carriers, and this could affect reactivity and fluidization tendency in the long run. Still, only soft agglomerates and partial defluidization were found, which may not be the case in a real CLC system operating at higher fluidizing velocities

Characterization of a sol-gel derived CuO/CuAl2O4 oxygen carrier for chemical looping combustion (CLC) of gaseous fuels: relevance of gas-solid and oxygen uncoupling reactions

A new sol–gel CuO/CuAl2O4 material was characterized in a thermogravimetric analyzer (TGA) for chemical looping combustion (CLC) with gaseous fuels, including the relevance of the oxygen uncoupling mechanism in oxygen transference was considered. This material possesses high reactivity and oxygen transport capacity, which combines the best features of the previously reported impregnated and spray-dried materials. During the cycles with N2 and air, CuO was fully decomposed into Cu2O in N2 and then regenerated to CuO in air, similarly to chemical looping with oxygen uncoupling (CLOU) for solid fuels. Decomposition of CuAl2O4 to CuAlO2 was quite slow, and the followed regeneration cannot be accomplished. Subsequently, the adequate and stable reaction rates of this material were examined in high numbers of cycles (>50 cycles) with gaseous fuels. The material undergone such cycles with gaseous fuels was then subjected to cycles with N2 and air. Segregation of CuO from Al2O3 in the CuAl2O4 was observed during gaseous fuels combustion, which produced more available oxygen for CLOU than the initial material. Finally, the relative importance of gas–solid reactions in CLC against oxygen uncoupling in CLOU was examined with the appearance of gaseous fuel.This work was supported by “National Natural Science Foundation of China (51390494)”, and “National Basic Research and Development Program (2011CB707300)”. Daofeng Mei is grateful for the support provided by the China Scholarship Council (CSC201306160054).National Natural Science Foundation of ChinaChinese Scholar CouncilPeer reviewe

Effects of Temperature, Operation Mode, and Steam Concentration on Alkali Release in Chemical Looping Conversion of BiomassExperimental Investigation in a 10 kWthPilot

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

Selecting and Testing of Cement-Bonded Magnetite and Chalcopyrite as Oxygen Carrier for Chemical-Looping Combustion

Combining iron and copper ores can generate an oxygen carrier that has a synergic effect of high temperature resistance and high reactivity. In this work, typical cements available in the market were studied as binders to bind magnetite and chalcopyrite to develop a suitable oxygen carrier for chemical-looping combustion (CLC). A first selection step suggested that an aluminate cement, namely CA70, could favor the generation of oxygen carrier particles having good crushing strength, good particle yield, and high reactivity. The CA70-bonded oxygen carrier was then subjected to cyclic tests with CH4, CO, and H-2 in reduction and in air oxidation at temperatures of 850, 900, and 950 degrees C with gas concentrations of 5, 10, 15, and 20% in a batch-fluidized bed reactor. The increase in temperature promoted the fuel conversion. At 950 degrees C, the conversions of CH4 and CO reached up to 80.4% and 99.2%, respectively. During more than 30 cycles, the oxygen carrier kept a similar reactivity to the fresh carrier and maintained its composition and physical properties. The oxygen transport capacity was maintained at 21-23%, and the phases were CuO, Fe2O3, Al2O3, and minor CaS. In the used sample, some grains were observed, but the morphology was not greatly changed. Agglomeration was absent during all the cycles, except for the deep reduction with H-2

Biogas upgrading through calcium looping: Experimental validation and study of CO2 capture

The calcium looping technology is one of the most promising technologies for capturing and storing CO2. This technology has been evaluated with a variety of sorbents and conditions in previous works, but the inlet CO2-ladden gas has typically been a flue gas from combustion, which typically has a composition of 10–20% CO2 and 80–90% N2. On the other side, the performance of the calcium looping process for CO2 capture of other gases (i.e., biogas or gases resulting from hydrothermal carbonization) remains largely unstudied. In this work, this knowledge gap is assessed through evaluating the performance of the calcium looping process for biogas (synthesized as 40% CO2, 60% CH4) in terms of carbonation conversion. This experimental study investigates the impact of: (1) using an inlet gas composition representative for biogas instead of combustion flue gas; (2) different biogas compositions; (3) the carbonation temperature; (4) the cooling-down and heating-up of the sorbent material between the reactor and ambient temperatures within cycles; (5) the atmosphere composition during calcination; and (6) the solids particle size. The main result obtained is that the overall CO2-capture performance of calcium looping improves when using biogas as inlet CO2-ladden gas, in comparison with combustion flue gas. One main contribution to this improved performance is shown to be the presence of secondary reactions (i.e., dry reforming, methanation). The impact of the CH4 to CO2 ratio tested is not remarkable, showing that the potentialities of the process in this aspect can be adapted to several biogas producing feedstocks

Oxygen Carrier and Alkali Interaction in Chemical Looping Combustion: Case Study Using a Braunite Mn Ore and Charcoal Impregnated with K2CO3or Na2CO3

Alkali is a problematic component in biomass and may create various operation issues in normal combustion as well as chemical looping combustion using biomass fuels (bio-CLC). To investigate the interaction of alkali with an oxygen carrier, a methodology has been developed where alkali salts are added with impregnated charcoal particles. This work studies the effect of K2CO3 and Na2CO3 on the fluidization/agglomeration behavior and reactivity as well as the interaction of a braunite manganese ore oxygen carrier with K and Na in a batch fluidized bed reactor. Charcoal impregnated with K2CO3 (K-charcoal) and charcoal impregnated with Na2CO3 (Na-charcoal) were used as solid fuels in the reduction step of the simulated CLC cycles. CH4 and syngas (50% CO + 50% H2) were periodically used to evaluate the reactivity of braunite before and after solid fuel experiments. In total, more than 50 cycles were performed for both K-charcoal series and Na-charcoal series tests, while some additional cycles with non-impregnated charcoal were conducted and considered as a reference. Partial agglomeration and partial defluidization were found after cycles with K-charcoal and Na-charcoal, and the use of K-charcoal tends to lead to the partial agglomeration/defluidization faster than the use of Na-charcoal. K, Na, Si, and Ca were found at a higher concentration on the surface of the agglomerated particles and can be assumed to be responsible for the partial agglomeration. The partial agglomeration with K-charcoal happened likely as a result of surface melting of the braunite particles, whereas the formation of the low-melting-point Na-Si-Ca system could be responsible for agglomeration in the Na-charcoal experiments. The concentration of K and Na in the braunite bed was found to increase during cycles with the alkali charcoals. In total, the added masses of K and Na were 0.8 and 1.2% of the bed, and around 40 and 80% of added K and Na were found, respectively, in the used oxygen carrier particles. Although partial agglomeration and accumulation were observed in the presence of these alkalis, the reactivity of used braunite was scarcely changed in comparison to the fresh sample

Investigation of LD-slag as oxygen carrier for CLC in a 10 kW unit using high-volatile biomasses

A steel slag from the Linz-Donawitz process, called LD-slag, having significant calcium and iron-fractions, was investigated as an oxygen carrier in a recently developed 10 kWth chemical-looping combustor with three high-volatile biomass fuels. In order to improve operability, the LD-slag was found to require heat-treatment at high temperatures before being used in the unit. In total, operation with the biomasses was conducted for more than 26 h at temperatures of 870–980 \ub0C. The fuel thermal power was in the range of 3.4–10 kWth. The operation involved chemical looping combustion (CLC), chemical looping gasification (CLG) and oxygen carrier aided combustion (OCAC). Around 12 h was in CLC operation, 13.3 h was conducted in CLG-conditions, while the remaining 0.7 h was OCAC. Here, the results obtained during the CLC part of the campaign is reported. Increased temperature in the fuel reactor and higher airflows to the air reactor both lead to better combustion performance. Steam concentration in the fuel reactor has little effect on the performance. The LD-slag showed higher oxygen demand (31.0%) than that with ilmenite (21.5%) and a manganese ore (19.5%) with the same fuel and normal solids circulation. However, with the LD-slag, there is possibility to achieve a lower oxygen demand (15.2%) with high solids circulation

Reactivity and lifetime assessment of an oxygen releasable manganese ore with biomass fuels in a 10 kWth pilot rig for chemical looping combustion

Finding a suitable oxygen carrier is crucial for the development of Chemical Looping Combustion (CLC). A new manganese ore was tested with different biomass fuels in a recently commissioned 10 kWth unit. The ore maintains the capability of generating O2 gas in N2 after continuous operations with the fuels, however, the concentration was relatively low within 0.45–1.0 vol% at 820 to 975 \ub0C. Influence of temperature, solids circulation and fuel power was examined for different fuels. Temperature increase enhances the carbon capture and reduces the oxygen demand, while the solids circulation and fuel power should be carefully controlled. Using biomass char the oxygen demand can be lowered to 2.6% while the carbon capture was close to 99%. The manganese ore showed a higher reactivity than the often-used ilmenite. Thus, a decrease of 8–10% in oxygen demand was achieved by using the manganese ore in comparison to ilmenite. During the 42 h of hot operation, defluidisation was not observed. Based on the analysis of the 35 fine samples collected, the initial attrition after first hours of operation was high, but gradually decreased to a relatively stable value of 0.27 and 0.12 wt%/h for hot and fuel operations, respectively, corresponding a lifetime of 370–830 h