Chemical-looping combustion in packed-fluidized beds: Experiments with random packings in bubbling bed

Chemical-looping combustion (CLC) in packed-fluidized bed reactor was investigated. Experiments were carried out in a cylindrical laboratory-scale bubbling fluidized-bed reactor with an inner diameter of 78 mm and a hight of 1.27 m. Ilmenite concentrate particles in the size range 90–212 μm was used as oxygen carrying fluidizing solid. Two different types of random packings were used: aluminum silicate balls (ASB) with a diameter of 12.7 mm and bulk density of 1439 kg/m3 and 25 mm stainless steel thread saddles (RMSR) with bulk density of 204 kg/m3. The superficial gas velocity was 0.3 m/s. The fuels were CO and CH4. The bed temperature was 840 \ub0C for CO and 940 \ub0C for CH4. The height of the packed bed was kept constant at 1 m. The fluidized oxygen carrier bed height was varied from 2 cm to 40 cm. Results showed that fuel conversion in packed-fluidized beds is highly dependent on oxygen carrier bed height and the nature of the packing. Packed-fluidized beds with RMSR packing resulted in a significant improvement in fuel conversion, compared to a bubbling bed with no packing. With 30–40 cm bed height, CO conversion was ≈99.5% with RMSR packing and 91–96% without packing. The corresponding numbers for CH4 were ≈84% and ≈78%. Further, the RMSR packing has very high void factor (0.96). Thus, it should have limited effects on particle inventory, pressure drop and throughput. The most likely mechanism for improved fuel conversion is improved gas-solid mass transfer due to be reduced bubble size. The ASB packing has low void factor (0.43) and provided mixed results with respect to fuel conversion

Evaluation of bed-to-tube heat transfer in a fluidized bed heat exchanger in a 75 MWth CFB boiler for municipal solid waste fuels

Bed-to-tube heat transfer has been investigated for a tertiary superheater in a 75 MWth Circulating Fluidized Bed (CFB) boiler in Norrk\uf6ping, Sweden. The boiler is used for incineration of solid waste fuels. Two fluidized bed heat exchangers are located in loop seals, connecting the cyclones and the furnace. The heat exchangers are placed in series, with respect to the steam side, and in parallel, with respect to the particle side. The total heat transfer surface area is roughly 44 m2, distributed over 72 tubes. The total effect transferred most often is in the range 2–6 MW. The incoming steam temperature in the first superheater is 380–400 \ub0C, while the exiting steam temperature from the second is around 450\ub0, at 65 bar pressure. The bed temperature in the Fluidized Bed Heat Exchanger (FBHE) is 850–875 \ub0C. The analysis is based on operational data from two time periods (2002–2005 and 2014–2021). The two periods use different heat exchanger designs, following a retrofit in 2005. The aim of the study is to establish the bed-to-tube heat transfer coefficient in an industrial FBHE unit and investigate how it varies over different time periods, for two different bed materials and for two different designs. Also, the experimentally determined heat transfer coefficients are compared with an established heat-transfer correlation, for prediction of heat transfer from bubbling fluidized bed to tubes. Operation with two bed materials were evaluated, namely silica sand and crushed and beneficiated ilmenite. Both materials are classified as Geldart B particles. Air is used as fluidization gas in the FBHE. The analysis show, with a few exceptions, comparably low heat-transfer coefficients from bed to tube of 100–150 W/(m2K). The results were similar for silica sand and ilmenite, but the highest measured heat transfer coefficient was for a period with ilmenite. The heat transfer was lower than expected based on literature data from FBHE units and fluidized bed boilers in general, and much lower than bed-to-tube heat transfer coefficients from lab-scale experiments and empirically derived predictive expressions. The difference could be related to one or more of several factors, such as the effect of very small tube spacing, unknown thermal conductivity of one of the layers in the tube bundle, the effect of lateral particle flow and the effect of fouling due to ash layers forming on the tube surfaces. It is suggested that it should be possible to significantly increase the bed-to-tube heat transfer by increasing the tube pitch, which is expected to improve bed mixing without increasing the risk of corrosion

Development and Comparison of Thermodynamic Equilibrium and Kinetic Approaches for Biomass Pyrolysis Modeling

Biomass pyrolysis is considered as a thermochemical conversion system that is performed under oxygen-depleted conditions. A large body of literature exists in which thermodynamic equilibrium (TE) and kinetic approaches have been applied to predict pyrolysis products. However, the reliability, accuracy and predictive power of both modeling approaches is an area of concern. To address these concerns, in this paper, two new simulation models based on the TE and kinetic approaches are developed using Aspen Plus, to analyze the performance of each approach. Subsequently, the results of two models are compared with modeling and experimental results available in the literature. The comparison shows that, on the one hand, the performance of the TE approach is not satisfactory and cannot be used as an effective way for pyrolysis modeling. On the other hand, the results generated by the new model based on the kinetic approach suggests that this approach is suitable for modeling biomass pyrolysis processes. Calculation of the root mean square error (RMS), to quantify the deviation of the model results from the experiment results, confirms that this kinetic model presents superior agreement with experimental data in comparison with other kinetic models in the literature. The acquired RMS for the developed kinetic method in this paper varies within the span of 1.2 to 3.2 depending on temperature (400-600 degrees C) and various feedstocks (pine spruce sawdust, bagasse, wood bark, beech wood and paddy straw)

Techno-Economic Assessment of Chemical Looping Gasification of Biomass for Fischer-Tropsch Crude Production with Net-Negative CO2 Emissions: Part 2

This work presents a techno-economic analysis of a used as the primary gasification process for biofuel production through Fischer-Tropsch synthesis (FTS). Two different gas cleaning process configurations, cold-gas cleanup and hot-gas cleanup process trains, are explored, along with off-gas utilization possibilities, to study their influence on the process economics of an integrated CLG-FT process plant. Off-gas recirculation to increase Fischer-Tropsch (FT) crude production has a significant influence on reducing the levelized production costs for FT crude. The results indicate that the specific production cost estimated for a CLG-FT plant with a hot-gas cleanup train is roughly 10% lower than the case with a cold-gas cleanup train, while the total plant costs remain relatively the same for all plant configurations. In addition to this, the former has a considerably higher overall system energy efficiency of 63%, roughly 18% more than the latter, considering the co-production of FT crude, district heating, and electricity. The specific investment costs range from 1.5 to 1.7 M euro 2018/MWLHV, and the specific FT crude production cost ranges from 120 to 147 euro 2018/MWhFT. Roughly 60% of total carbon fed to the process is captured, enabling net-negative CO2 emissions. A CO2 price for negative emissions would significantly reduce the specific fuel production costs and would, hence, be competitive with fossil-based liquid fuels

Experimental Investigation of the Effect of Random Packings on Heat Transfer and Particle Segregation in Packed-Fluidized Bed

The heat transfer coefficient, pressure drop, and vertical segregation in a\ua0bubbling fluidized bed reactor containing random packings were investigated. The bed\ua0material was silica sand in the size range of 90−400 μm. Experiments were done for bed\ua0temperatures ranging from 400 to 900 \ub0C and superficial gas velocities up to 0.411 m/s.\ua0Five different types of packings were evaluated: (i) RMSR (25 mm stainless steel thread\ua0saddle ring), (ii) Hiflow (25 mm stainless steel pall ring), (iii) RR6 (6 mm ceramic\ua0Raschig ring), (iv) RR10 (10 mm ceramic Raschig ring), and (v) ASB (12.7 mmaluminum silicate balls). The RMSR packing showed an increase in the heat transfer\ua0coefficient (up to 1243 W/m2\ua0K), as compared to bubbling beds with no packings (up\ua0to 1124 W/m2\ua0K). Also, beds with RMSR and Hiflow packings had a lower pressure\ua0drop and vertical segregation compared to low void factor packings such as RR6, RR10,\ua0and ASB

Oxygen Carrier Aided Combustion in Fluidized Bed Boilers in Sweden - Review and Future Outlook with Respect to Affordable Bed Materials

Oxygen carriers are metal oxide particles that could potentially enhance both fuel conversion and heat distribution in fluidized bed combustion, resulting in e.g., lowered emissions of unconverted species and better possibilities to utilize low‐grade fuels. A related technology based on fluidized beds with oxygen carriers can separate CO2 without large energy penalties. These technologies are called oxygen carrier aided combustion (OCAC) and chemical‐looping combustion (CLC), respectively. In the past few years, a large number of oxygen carriers have been suggested and evaluated for these purposes, many of which require complex production processes making them costly. Affordable metal oxide particles are, however, produced in large quantities as products and byproducts in the metallurgical industries. Some of these materials have properties making them potentially suitable to use as oxygen carriers. Uniquely for Sweden, the use of oxygen carriers in combustion have been subject to commercialization. This paper reviews results from utilizing low‐cost materials emerging from metallurgical industries for conversion of biomass and waste in semi‐commercial and commercial fluidized bed boilers in Sweden. The paper further goes on to discuss practical aspect of utilizing oxygen carriers, such as production and transport within the unique conditions in Sweden, where biomass and waste combustion as well as metallurgical industries are of large scale. This study concludes that utilizing metal oxides in this way could be technically feasible and beneficial to both the boiler owners and the metallurgical industries

Chemical-Looping Combustion in Packed-Fluidized Bed Reactor – Fundamental Modeling and Batch Experiments with Random Metal Packings

The conversion of gaseous fuels during chemical-looping combustion (CLC) was investigated in a packed-fluidized reactor. The experimental set-up consisted of a cylindrical laboratory-scale bubbling fluidized-bed reactor with an inner diameter of 78 mm and a height of 1.27 m. Two types of fuel, Syngas (50/50% H2/CO) and carbon monoxide (100% CO) were used. Two different types of packings were assessed and compared with the reference case, which was a bubbling bed with no packings. The investigated packings were 25 mm stainless steel thread saddle rings (RMSR) with a bulk density of 195 kg/m3, and 25 mm stainless steel pall ring (Hiflow) with bulk density of 271 kg/m3. The height of the packed reactor section was kept constant at 1 m. Ilmenite concentrate particles in the size range of 90-212 \ub5m was used as oxygen carrier. The unfluidized bed height was varied between 10 and 60 cm. The results show that the fuel conversion increases as the bed height increases, and that the use of packings have positive effect on fuel conversion. For RMSR packings, the syngas conversion at 840\ub0C improves from 0.84 (for 10 cm bed height) to 1.00 (for 60 cm bed height). This should be compared to the bed with no packings, for which the corresponding improvement was from 0.69 to 0.98. The general pattern is consistent for all fuels, packings and bed heights. The results are interpreted as an improvement in gas-solid mass transfer when packings are used, mainly due to reduced bubble size. Fundamental analysis of the variance in pressure drop over the bed to estimate bubble diameter supports this interpretation. It is also shown that the mass-based first-order effective reaction contact factor kf improves up to 109% in the bed with RMSR packings, as compared to the bed without packings

Experimental Investigation of Oxygen Carrier Aided Combustion (OCAC) with Methane and PSA Off-Gas

Oxygen carrier aided combustion (OCAC) is utilized to promote the combustion of relatively stable fuels already in the dense bed of bubbling fluidized beds by adding a new mechanism of fuel conversion, i.e., direct gas–solid reaction between the metal oxide and the fuel. Methane and a fuel gas mixture (PSA off-gas) consisting of H2, CH4\ua0and CO were used as fuel. Two oxygen carrier bed materials—ilmenite and synthetic particles of calcium manganate—were investigated and compared to silica sand, an in this context inert bed material. The results with methane show that the fuel conversion is significantly higher inside the bed when using oxygen carrier particles, where the calcium manganate material displayed the highest conversion. In total, 99.3–99.7% of the methane was converted at 900 \ub0C with ilmenite and calcium manganate as a bed material at the measurement point 9 cm above the distribution plate, whereas the bed with sand resulted in a gas conversion of 86.7%. Operation with PSA off-gas as fuel showed an overall high gas conversion at moderate temperatures (600–750 \ub0C) and only minor differences were observed for the different bed materials. NO emissions were generally low, apart from the cases where a significant part of the fuel conversion took place above the bed, essentially causing flame combustion. The NO concentration was low in the bed with both fuels and especially low with PSA off-gas as fuel. No more than 11 ppm was detected at any height in the reactor, with any of the bed materials, in the bed temperature range of 700–750 \ub0C

Process Analysis of Chemical Looping Gasification of Biomass for Fischer-Tropsch Crude Production with Net-Negative CO2Emissions: Part 1

Large-scale biofuel production plants require an efficient gasification process that generates syngas of high quality (with minimal gas contaminants and inert gases) to minimize the extent of the syngas cleaning processes required for liquid biofuel production. This work presents process modeling of the chemical looping gasification (CLG) process for syngas production. The CLG process is integrated with a Fischer-Tropsch synthesis (FTS) process to produce Fischer-Tropsch (FT) crude with net-negative CO2 emissions, enabling process and system-level analyses of this novel biomass-to-liquid process. CLG resembles indirect gasification in an interconnected circulating fluidized bed reactor, where instead of inert bed material, a solid-oxygen carrier, such as mineral ores rich in iron or manganese oxides, is used. The oxygen carrier particles undergo oxidation and reduction in the air reactor and fuel reactor, respectively, thereby providing heat and oxygen for gasification. This work uses data from CLG experiments performed with steel converter slag as the oxygen carrier and investigates its potential when integrated with different downstream gas cleaning trains and the subsequent fuel synthesis process with the primary objective of quantifying and evaluating the performance of the integrated CLG-FT process plant. Syngas with a high energy content of 12 MJ/Nm3 (lower heating value basis) is predicted with a cold gas efficiency of 73%. CO2/CO ratios, higher than indirect biomass gasification, are also predicted in the raw syngas produced; thus, there exists an opportunity to capture biogenic CO2 with a relatively lower energy penalty in the subsequent gas cleaning stages. This work quantifies other key performance indicators, such as heat recovery potential, negative CO2 emission capacity, and FT crude production efficiency of the CLG-FT plant. A 100 MWth CLG plant produces roughly 677-696 barrels per day of FT crude, with net-negative emissions of roughly 180 kilotonnes of CO2 annually

Oxygen Carrier Aided Combustion (OCAC) of Wood Chips in a 12 MWth Circulating Fluidized Bed Boiler Using Steel Converter Slag as Bed Material

The novel combustion concept Oxygen Carrier Aided Combustion (OCAC) is realized by addition of an active oxygen-carrying bed material to conventional fluidized bed boilers. The active bed material is meant to become reduced in fuel-rich parts of the boiler and oxidized in oxygen-rich parts, thus potentially providing advantages such as new mechanisms for oxygen transport in space and time. In this study, oxygen-carrier particles prepared from so called Linz-Donawitz (LD)-slag are examined as active bed material in a 12 MWth Circulating Fluidized Bed (CFB) boiler. LD-slag is the second largest by-product in steel making and is generated in the basic LD oxygen converter process. The experimental campaign lasted for two full weeks. The fuel was wood chips. LD-slag worked well from an operational point of view and no problems related to handling, agglomeration or sintering were experienced, albeit the production of fly ash increased. The boiler temperature profile suggested that fuel conversion in the main boiler body was facilitated, but the effect did not readily translate into reduced emissions from the stack. Spraying an aqueous solution of ammonium sulphate directly into the cyclone outlet with the aim of rejecting alkali metals as alkali suphates was found to solve the problems related to carbon monoxide emissions, suggesting that the problems could be due to the poor ability of LD-slag to absorb certain ash components. Use of a mixed bed consisting of 10-50 wt% LD-slag, with the remaining part being silica sand for ash absorption, also worked well. It is concluded that LD-slag could be a very cheap and readily available oxygen-carrying bed material for use in fluidized bed applications