Design Requirements for Pressurized Chemical Looping Reforming

A key issue in chemical looping reforming is to operate the process under pressurized conditions. Applicability of dual fluidized bed systems, currently used in atmospheric chemical looping processes, is affected by pressure. Critical design issues were studied and experimentally verified by cold flow model experiments. It turns out that it is important to achieve sufficient global solids circulation and to keep the pressure difference between the reactors low enough for proper operation of the loop seals

Bioenergy Recovery from Cotton Stalk

Cotton stalk (CS) plant residue left in the field following harvest must be buried or burned to prevent it from serving as an overwintering site for insects such as the pink bollworm (PBW). This pest incurs economic costs and detrimental environmental effects. However, CS contains lignin and carbohydrates, like cellulose and hemicelluloses, which can be converted into a variety of usable forms of energy. Thermochemical or biochemical processes are considered technologically advantageous solutions. This chapter reviews potential energy generation from cotton stalks through combustion, hydrothermal carbonization, pyrolysis, fermentation, and anaerobic digestion technologies, focusing on the most relevant technologies and on the properties of the different products. The chapter is concluded with some comments on the future potential of these processes

Feasibility of fluidized bed reactor systems for pressurized chemical looping combustion of natural gas

Chemical looping combustion (CLC) of natural gas at atmospheric pressure has been successfully demonstrated at technical laboratory scale (120 kWth) since 2009 (1,2). Scale-up studies to 10 MWth have been presented (3,4), but no pilot or demonstration projects have been executed so far. Instead, researchers have recently concentrated their activities on CLC of solid fuels (5). One reason for this is the deployment challenge for natural gas CLC power plants. State-of-the-art gas-turbine combined cycle (GT-CC) plants reach high electric efficiencies up to 60% and are superior to steam cycle concepts even if post combustion CO2 capture is applied. On the other hand, all the early studies on CLC focused on efficiency increase for GT-CC plants through application of CLC (6,7). The present contribution therefore seeks to address this mismatch and to assess the practical potential of natural gas CLC for power production. A principal baseline is the application of atmospheric pressure CLC and steam cycle power generation reaching electric efficiencies up to 45% dependent on steam parameters and plant size. In order to reach higher electric efficiencies, pressurized CLC and gas turbine cycles would need to be implemented. Fluidized bed systems have been proposed for CLC (8) and preferred so far for they combine good heat management and continuous operation. If CLC is used in combination with gas turbines, at increased pressure, the operation of fluidized bed systems is challenging. Process configurations have been compared based on mass- and energy balances and basic design calculations have been carried out based on fluidization engineering methods. It turns out that high gas turbine efficiencies can only be reached if turbine inlet conditions are sufficiently high and relative pressure losses are within reasonable limits. Based on the results of the present work, the efficiencies of CLC based power generation cycles remainsignificantly lower than standard GT-CC efficiencies without CO2 capture. An optimal range of operating conditions can be identified for operation of a pressurized CLC plant with increased efficiency and design considerations for a dual circulating fluidized bed reactor system are reported. Such systems are characterized by solids transport ducts and loop seals of increased dimensions relative to atmospheric pressure systems. Accordingly, also the fluidization gas (steam) demand for loop seals is relatively increased for pressurized systems. The outcome of this work may serve as a general basis for techno-economic evaluation of pressurized CLC systems for power generation. Please click Additional Files below to see the full abstract

Performance Characteristics of an 8 MW(th) Combined Heat and Power Plant Based on Dual Fluidized Bed Steam Gasification of Solid Biomass

The work focuses on a dual fluidized bed gasification technology for which a model has been developed and validated accompanying the operation of the 8 MWth biomass combined heat and power plant in Guessing/Austria. The reactor concept is a circulating fluidized bed system with a large steam-fluidized bubbling bed integrated into the solids return loop. The solids circulation rate is shown versus the riser exit velocity. Further, plant performance maps are presented for both electric and heat power output. The water content of the fuel is a major parameter with respect to plant performance. High fuel water content at high gas engine load means high gas velocities in the riser (erosion limit) and higher heat share in the produced energy

Heat transfer challenge and design evaluation for a multi-stage temperature swing adsorption (TSA) process

Functionalized solid amine-based temperature swing adsorption (TSA) processes have recently been proposed as a potential way to reduce the energy-penalty of post-combustion carbon capture processes (1). If TSA is to be carried out at large scale and with high energy-efficiency, continuously operated counter-current contactors are required for thermodynamic reasons. This could, generally, be achieved by using moving bed contactors. However, the heat exchange requirement of TSA is significant and heat transfer is poor in fixed and moving beds. Therefore, multi-stage fluidized bed contactors with counter-current flow of solids and gas phase and immersed heat exchange surfaces may solve the heat transfer challenge while maintaining the thermodynamic process requirements. Experiments have shown that adsorption and desorption kinetics of suitable functionalized amine sorbents are fast and equilibrium loadings are practically reached in the stages (1). Thus, heat exchange is the dominant limiting factor for a practical stage design in multi-stage fluidized bed TSA. The present work rationally develops design requirements for TSA stages based on the necessary heat exchange rates. The considered particles are Geldart Type B (diameter 200-300 µm, particle density 1000-1500 kg/m3). Scalability of the design proposal is considered and vertically orientated heat exchanger tubes are compared to horizontal tube bundles. The net movement and mixing of particles within the bubbling bed stage must be maintained in spite of the emulsified heat exchangers (possible dead zones in the area of the tube bundles). It is shown that the pressure drop of multi-stage fluidized bed TSA units for flue gas CO2 capture is practically determined by the heat exchange requirement and not by the space-time of the solids for the adsorption. Future work will employ a bubbling fluidized bed heat exchange testing device for optimization of the heat exchanger geometry with respect to heat transfer rates and particle residence time distribution in the stage. Heat exchange measurement devices have been presented recently in literature for horizontal tube bundles and Geldart Type A particles (2), but the importance of the heat exchanger issue in continuous fluidized bed TSA requires the detailed investigation for the Geldart B range, potentially considering the macroscopic particle movement relative to the heat exchangers within each individual TSA stage. Please click Additional Files below to see the full abstract

H2-Rich Syngas from Renewable Sources by Dual Fluidized Bed Steam Gasification of Solid Biomass

Steam gasification of solid biomass yields high quality producer gases that can be used for efficient combined heat and power production (CHP) and as a renewable resource for chemical syntheses. The dual fluidized bed steam gasification technology provides the necessary heat for steam gasification by circulating hot bed material that is heated in a second fluidized bed reactor by combustion of residual biomass char. The hydrogen content in producer gas of such gasifiers is about 40 vol% (dry basis). Addition of carbonates to the bed material and adequate adjustment of operation temperatures in the reactors allow selective transport of CO2 from gasification to combustion zone (Adsorption Enhanced Reforming – AER concept). An 8 MW (fuel power) CHP plant successfully demonstrates gasification in Guessing, Austria since 2002. A pilot plant (100 kW fuel power) has been recently operated to investigate the potential of the selective CO2 transport achieving a H2 content of 75 vol% (dry basis) in the producer gas. No significant increase in tar formation occurs despite the low gasification temperatures (600-700 °C). It can be shown, that the selective transport of CO2 yields high hydrogen contents in the producer gas and the possibility of operating at lower temperatures increases the efficiency of energy conversion

INVESTIGATION OF REFORMING ACTIVITY AND OXYGEN TRANSFER OF OLIVINE IN A DUAL CIRCULATING FLUIDISED BED SYSTEM WITH REGARD TO BIOMASS GASIFICATION

Natural olivine (Mg,Fe)2SiO4 is examined in a dual circulating fluidised bed (DCFB) reactor system of 120 kWth with regard to its reforming activity. Further, the oxygen transport capacity due to redox-cycling of the iron containing part of the olivine is considered. Based on a syngas composition derived from biomass gasification, the olivine is exposed to a surrogate gas mixture of H2, CO, CO2, CH4 and a tar compound (1-methylnaphthalene) at 850 °C. The results show the tar conversion at different tar loads. The investigations reveal that a low content of oxygen is transported by the olivine due to the redox-cycling in the reactor system

Experimental Study on Reforming Activity and Oxygen Transfer of Fe- Olivine in a Dual Circulating Fluidized Bed System

Fe-olivine was investigated in a dual circulating fluidised bed reactor system with focus on hydrocarbon reforming activity and effects of oxygen transfer. H2, CO2, CH4 and 1-methylnaphthalene were fed as a surrogate gas mixture to the reforming part. Oxygen transport was developed by solids circulation. Tar decomposition was marginally affected by partial oxidation. The overall degree of tar decomposition was found to be in the range of 70 to 80%