Steel converter slag as an oxygen carrier

Thermal conversion of fuels can be used to produce heat and power in addition to chemicals. In order to be aligned with climate targets, it is necessary that such systems do not emit carbon dioxide to the atmosphere. Carbon capture and storage (CCS) can be used together with fuel conversion systems to prevent CO2 from entering the atmosphere. If CCS is used together with biomass-based fuels, it is possible to achieve a net-flow of carbon dioxide out of the atmosphere, so called negative emissions.Chemical looping technologies for combustion (CLC) and gasification (CLG) are technologies which can be used for heat, power and chemical production with no or low penalties for carbon capture. In any chemical looping applications, a functional oxygen carrier is essential. The oxygen carrier is normally a metal oxide based material that can transport oxygen from one reactor to another. However, when fuel is introduced into the system, ash can react with the oxygen carrier and decrease its operational lifespan, especially reactive ash from biomass and low-grade fuels. Therefore, there is growing interest in low-cost oxygen carriers that can contribute to making the process economically feasible. Low-cost oxygen carriers can be obtained from ores or as byproducts of the steel industry. Of particular interest is steel converter slag, which is also known as Linz-Donawitz (LD) slag. LD slag is generated in significant amounts, contains sufficient amount of iron oxide (that can act as an oxygen carrier) and available at a low cost.This work presents a comprehensive overview of the chemistry and behavior of LD slag when it is implemented as an oxygen carrier in chemical-looping applications. The material has been investigated in laboratory reactors, in addition to pilot and semi-industrial units, and LD slags interactions with different fuel components, ash, alkali salts, sulfur and tars have been investigated.It is concluded from this work that LD slag can be viable as material for both CLC and CLG processes with biomass. In contrast to other bed materials, such as silica sand or the commonly investigated iron-based oxygen carrier ilmenite, the slag has limited reactivity with reactive alkali components. This results in more alkali being available in the gas phase, which is beneficial for tar cracking and for the gasification rate of the solid char. The high content of calcium in the LD slag is also favorable in terms of gasification and ash interactions. Calcium oxide catalyzes both the water-gas shift reaction and is catalytic towards tar cracking. A high level of calcium also increases the melting points of both the K-Ca-P and K-Ca-Si matrixes. However, the structural integrity of the material is lower compared to, for example, ilmenite, resulting in more fines being generated during the process. Overall, LD slag is a potential oxygen carrier that is suitable for chemical-looping processes that utilize low-grade fuels

Interaction behavior of sand-diluted and mixed Fe-based oxygen carriers with potassium salts

Oxygen carriers, in the form of metal oxide particles, are bed materials that transport oxygen in solid form in fluidized bed applications, such as in chemical-looping applications. In biofuel applications, it is well known that alkali from the fuel ash can react with the bed material and cause, among other operation issues, agglomeration. When using oxygen carriers in a fluidized bed, it is likely that the bed material is either a mixture of different metal oxide materials or partly diluted with sand. This is to improve or combine the chemical properties of the materials used or simply for economic reasons. This work investigates how three potassium salts K2CO3, K2SO4 and KH2PO4 interact with the oxygen carriers: Steel converter slag (LD slag), ilmenite, mixtures of the two and each carrier diluted with silica sand. The salts were used as model compounds that can occur in biofuel ash. The set-up used was a fixed bed where a small sample of bed material is mixed with a potassium salt equivalent to 4 wt-% of potassium. The mixture was then exposed to reducing (H2 in steam) conditions at 900 \ub0C during several hours in a tubular furnace. This provides a worst-case scenario for solid–solid interaction in a fluidized bed. If a solid–solid reaction does not take place in this setup, it will most likely never occur in a fluidized bed. When LD slag and ilmenite were combined, the potassium from the salts would prefer to accumulate in the ilmenite rather than the LD slag. Ca from LD slag interacted with KH2PO4 resulting in a less severe agglomeration than when ilmenite was used separately with the same salt. When the oxygen carriers were diluted with silica sand, potassium salt interaction resulted in agglomeration for both the oxygen carriers with all potassium salts. K2CO3 and K2SO4 formed potassium silicates, while KH2PO4 formed a phosphorus-containing melt. When LD slag was present, phosphorus was located in a K-Ca-P phase that was not present if ilmenite was present

Steel Converter Slag as an Oxygen Carrier-Interaction with Sulfur Dioxide

Steel converter slag, also called Linz-Donawitz (LD) slag, has been considered as an oxygen carrier for biofuel chemical looping applications due to its high availability. In addition to its content of iron which contributes to its oxygen-carrying capacity, LD slag also contains a significant amount of calcium. Calcium, however, is known to interact with sulfur, which may affect the usability of LD slag. To get a better understanding of the interaction between sulfur and LD slag, batch scale experiments have been performed using solid and gaseous fuel with or without sulfur dioxide, together with LD slag as an oxygen carrier. The reactivity and sulfur interaction were compared to the benchmark oxygen carrier ilmenite. Sulfur increases the gasification rate of biofuel char and the conversion of CO for both LD slag and ilmenite. However, no effect of sulfur could be seen on the conversion of the model tar species benzene. The increased gasification rate of char was suspected to originate from both surface-active sulfur and gaseous sulfur, increasing the reactivity and oxygen transfer of the oxygen carrier. Sulfur was partly absorbed into the LD slag particles with calcium, forming CaS and/or CaSO4. This, in turn, blocks the catalytic effect of CaO towards the water gas shift reaction. When the SO2 vapor pressure was decreased, the absorbed sulfur was released as SO2. This indicates that sulfur may be released in loop-seals or in the air reactor in a continuous process

Interactions between potassium ashes and oxygen carriers based on natural and waste materials at different initial oxidation states

One of the most essential features of an oxygen carrier is its ability to be oxidized and reduced in order to transfer oxygen in a chemical looping system. A highly reduced oxygen carrier can experience multiple performance issues, such as decreased reactivity, agglomeration, and defluidization. This is crucial for\ua0processes that require limited oxygen transfer from the air reactor to the fuel reactor. Meanwhile, biomasses as environmentally friendly fuel options contain ashes, which would inevitably react with oxygen carriers and exacerbate the performance issues. To mimic the interactions between a highly reduced oxygen carrier and biomass ash compounds, four iron-based oxygen carriers, based on natural ores and waste materials, and three potassium salts, K2CO3, KH2PO4, and K2SO4, were investigated in a tubular reactor under an atmosphere consisting of 2.5% H2 and 10% steam in Ar and N2 at 900\ub0C for 3 h. The results from the X-ray diffraction (XRD) material analysis showed that both initially fully oxidized and highly reduced materials reach the same oxidation state after the experiment. Based on the scanning electron microscopy coupled with energy dispersive X-ray spectroscopy results, K from K2CO3 and K2SO4 diffuses in the oxygen carrier particles, while K from KH2PO4 always forms a distinct layer around the particles. The initial oxidation state of an oxygen carrier surface affects the interactions with the potassium salt only to minor extents. Thus, the final state of the material and its performance in a large-scale process are only occasionally and mildly affected by its initial oxidation state

Steel converter slag as an oxygen carrier for chemical-looping gasification

Chemical Looping Gasification (CLG) is a dual fluidized bed gasification technique where an oxygen carrier is used as bed material instead of sand. An optimized process could have several advantages, including i) one concentrated CO2 stream, amiable for carbon capture, ii) less tar formation, iii) additional reaction pathways for syngas production, iv) less corrosion and v) CO2 is generated in one stream from the fuel reactor that could be captured. Steel converter slag, also called LD slag, is a by-product from the steel industry which, besides iron, contains significant fractions of Ca, Mg, Al and Mn in a complex matrix of phases. The low cost and presence of known catalytic solid phases in the slag makes it interesting as an oxygen carrier in CLG. In this work, LD slag was investigated using a batch reactor with gaseous and solid fuel as well as with TGA. It was found that during gasification with LD slag, the material can i) transfer oxygen to the fuel, ii) catalyze the water-gas-shift reaction, iii) react with CO2 forming carbonates and iv) split water to hydrogen. The overall result was a raw gas with a higher H2/CO ratio for LD slag than the other tested materials

Vanadium recovery from steel converter slag utilised as an oxygen carrier in oxygen carrier aided combustion (OCAC)

This study investigates vanadium extraction from steel converter slag from the LD (Linz-Donawitz) process. This slag has been used as an active bed material in a biomass boiler in a combustion technique called oxygen carrier aided combustion (OCAC). This usage in the boiler could be compared to the roasting step in the common roasting-leaching method for vanadium extraction. Leaching of LD slag prior to use as an oxygen carrier is undesirable as the materials active in OCAC are removed. This study successfully leached the slag following use in the combustion process. Two methods of leaching were utilised to compare the OCAC slag against traditional methods of vanadium extraction; a continuous flow leaching procedure and a microwave-assisted leaching procedure. It was found that a vanadium extraction efficiency of 22.1% could be achieved from the OCAC slag using 5 M sulphuric acid as a leaching solution following 30 min water leaching followed by 30 min of continuous acid leaching. Using the microwave-assisted method and further optimising leaching conditions, a final efficiency of 49.1% was achieved with 4 M sulphuric acid and particle size within the range of 44–74 μm

Negative emissions of carbon dioxide through chemical-looping combustion (CLC) and gasification (CLG) using oxygen carriers based on manganese and iron

Carbon capture and storage (CCS) is an economically attractive strategy for avoiding carbon dioxide (CO 2 ) emissions from, e.g., power plants to the atmosphere. The combination of CCS and biomass combustion would result in a reduction of atmospheric CO 2 , or net negative emissions, as plant growth is a form of sequestration of atmospheric carbon. Carbon capture can be achieved in a variety of ways, one of which is chemical looping. Chemical-looping combustion (CLC) and chemical looping gasification (CLG) are two promising technologies for conversion of biomass to heat and power or syngas/methane with carbon capture. There have been significant advances made with respect to CLC in the last two decades for all types of fuel, with much less research on the gasification technology. CLG offers some interesting opportunities for production of biofuels together with carbon capture and may have several advantages with respect to the bench mark indirect gasification process or dual-bed fluidized bed (DFBG) in this respect. In CLG, an oxygen carrier is used as a bed material instead of sand, which is common in indirect gasification, and this could have several advantages: (i) all generated CO 2 is present together with the syngas or methane in the fuel reactor outlet stream, thus in a concentrated stream, viable for separation and capture; (ii) the air reactor (or combustion chamber) should largely be free from trace impurities, thus preventing corrosion and fouling in this reactor; and (iii) the highly oxidizing conditions in the fuel reactor together with solid oxide surfaces should be advantageous with respect to limiting formation of tar species. In this study, two manganese ores and an iron-based waste material, LD slag, were investigated with respect to performance in these chemical-looping technologies. The materials were also impregnated with alkali (K) in order to gauge possible catalytic effects and also to establish a better understanding of the general behavior of oxygen carriers with alkali, an important component in biomass and biomass waste streams and often a precursor for high-temperature corrosion. The viability of the oxygen carriers was investigated using a synthetic biogas in a batch fluidized bed reactor. The conversion of CO, H 2 , CH 4 , and C 2 H 4 was investigated in the temperature interval 800–950\ua0\ub0C. The reactivity, or oxygen transfer rate, was highest for the manganese ores, followed by the LD slag. The conversion of C 2 H 4 was generally high but could largely be attributed to thermal decomposition. The K-impregnated samples showed enhanced reactivity during combustion conditions, and the Mangagran-K sample was able to achieve full conversion of benzene. The interaction of the solid material with alkali showed widely different behavior. The two manganese ores retained almost all alkali after redox testing, albeit exhibiting different migration patterns inside the particles. LD slag lost most alkali to the gas phase during testing, although some remained, possibly explaining a small difference in reactivity. In summary, the CLC and CLG processes could clearly be interesting for production of heat, power, or biofuel with negative CO 2 emissions. Manganese ores are most promising from this study, as they could absorb alkali, giving a better conversion and perhaps also inhibiting or limiting corrosion mechanisms in a combustor or gasifier

Understanding the Interaction of Potassium Salts with an Ilmenite Oxygen Carrier under Dry and Wet Conditions

This study describes how potassium salts representative of those in bio ash affect the reactivity of the oxygen carrier ilmenite under moist and dry conditions. Ilmenite is a bench-mark oxygen carrier for chemical-looping combustion, a technique that can separate CO2 from flue gases with minimal energy penalty. Different potassium salts were mixed with ilmenite to a concentration of 4 wt % potassium. The salts used were K2CO3, K2SO4, KCl, and KH2PO4. Experiments were performed at 850 \ub0C under alternately oxidizing and reducing conditions in a dry atmosphere or in the presence of steam. Analyses of the oxygen carrier regarding changes in reactivity, structure, and composition followed the exposures. This study showed that salts such as K2CO3, K2SO4, and KCl increase the reactivity of the ilmenite. For the samples mixed with KCl, most of the salt was evaporated. KH2PO4 decomposed into KPO3, forming layers around the ilmenite particles that lead to agglomeration. Additionally, the KPO3 layer was more or less nonpermeable for CO and decreased the reactivity toward H2 significantly in both dry and wet conditions. This decreased reactivity indicates that the concentration of phosphorus in biofuel may have a significant effect on oxygen carrier degradation. It was also observed that the presence of steam changed the chemistry drastically for the nonphosphorus-containing salts. Alkali salts may react with steam, forming volatile KOH that evaporates partly. KOH may also form K-titanates by reaction with the oxygen carrier, leading to segregation of iron and titanium phases in the ilmenite. \ua