Fluid-Dynamic Investigations in a Cold Model for a Dual Fluidized Bed Biomass Steam Gasification Process: Optimization of the Cyclone

Gasification of biomass is an attractive technology for combined heat and power production. Although a great deal of research and development work has been carried out during the past decade the commercial breakthrough for this technology is still not in sight. The optimisation of the operation costs influences significantly the economic efficiency. Especially bed material consumption constitutes a major part of expenses in fluidised bed systems. Thus, cyclones for circulating fluidized beds need to be designed properly. A dual fluidized bed steam gasifier is in commercial operation at the biomass CHP (combined heat and power plant) in Güssing, Austria since 2002. Some erosion and caking in the cyclone of the CHP plant could be observed with increasing hours of operation. The influences of these effects as well as the influence of the solid circulation rate between the two units on the separation efficiency were investigated by fluid-dynamic investigations using a cold model. The results show that due to erosion and caking elutriation rates are increased, especially for smaller particles. However, the cyclone achieves fractional separation efficiencies of more than 99.9%

CO-GASIFICATION OF COAL AND WOOD IN A DUAL FLUIDIZED BED GASIFIER VARIATION OF FLUIDIZATION CONDITIONS AND LOAD RATIO

Within this work the use of coal has been tested in a dual fluidized bed 100 kW - steam blown- gasifier originally designed for wood. With adjustments like fluidization conditions and fuel load, it is possible to gasify coal in load-ratios from 0 % to 100 % with this technology

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

Co-Gasification of Biomass and Coal in an 8MW Dual Fluidized Bed Steam Gasifier

Gasification of biomass is an attractive technology for combined heat and power production. Co-gasification of biomass and coal was tested in an 8 MW dual fluidized bed steam gasifier with coal ratios up to 22% on an energy basis. Hydrogen levels in the producer gas increased with the addition of coal as well as ammonia, hydrogen sulfide and tars. Addition of coal to the system stabilized the process and improved gas quality

Cold flow modelling of char concentration in the recirculated bed material stream of a dual fluidized bed steam gasification system

The dual fluidized bed (DFB) steam gasification technology of biomass was developed at Vienna University of Technology and is well-established for transforming biomass into a product gas which can be used for further applications. The DFB steam gasification reactor consists of a gasification chamber (bubbling bed, fluidized with steam) and a combustion chamber (turbulent bed, fluidized with air). Biomass is fed into the gasification chamber and gets in contact with the bed material, typically Olivine, at about 840°C. The released volatiles leave the gasification reactor as product gas. A part of the solid residue, called char, flows with the bed material via a chute to the combustion chamber where it is burnt with air. The bed material is heated up, separated from the flue gas stream in a cyclone and flows back to the gasification reactor via a loop seal where it provides the heat for devolatilization and drying of the biomass. The movement of the char is crucial since a sufficient amount has to flow to the combustion chamber and burn to provide enough energy for bed material heat-up. Up to now little is known about the char concentration in the bed material recirculation stream (or short recirculation stream) and its influencing variables. Therefore, a cold flow model, operated with ambient air, was constructed to study the influence of various parameters on the char concentration in the recirculation stream. Bronze is used as bed material since is matches closest to the scaling criteria. The char is also scaled; polyethylene is used as model char. The cold flow model, see Figure 1 for the flowsheet, consists of a “gasification chamber” which corresponds to the gasification chamber in the hot plant and is as well operated as a bubbling bed. Via a chute the recirculation stream moves to a rotary valve which enables to set a fixed recirculation rate and make it independent from the following pneumatic conveying. Then, gas and solids are separated in a cyclone and the recirculation stream finally flows back to the gasification chamber. After the loop seal samples are taken for investigation of the model char concentration in the recirculation stream. In the present study the influence of fluidization rate in the gasification chamber, bed material recirculation rate and model char mass in the system on the char concentration in the recirculation stream are investigated. It was found that the model char particles show a flotsam behavior. Higher fluidization rates increase the model char concentration in the recirculation stream because of better mixing, whereas the bed material recirculation rate has only little influence. Doubling and tripling the overall char mass in the system did not lead to a doubling or tripling model char concentration in the recirculation stream. The present observations are helping to better understand the ongoing phenomena inside of the dual fluidized bed gasification reactor and provide knowledge to further optimize it. Please click Additional Files below to see the full abstract

Synergetic Utilization of Renewable and Fossil Fuels: Dual Fluidized Bed Steam Co-gasification of Coal and Wood

AbstractGasification of biomass and coal is an attractive technology for combined heat and power production, as well as for synthesis processes such as the production of liquid and gaseous biofuels. The allothermal steam blown gasification process yields a high calorific product gas, practically free of nitrogen. Originally designed for wood chips, the system can also handle a large number of alternative fuels. To demonstrate the influence on the system performance of fuels that have a different origin, wood pellets, as the designated feedstock, and hard coal as an example fossil fuel were fed into the DFB gasifier with a fuel blend ratio of 20% coal in terms of energy. A fuel power of 78kW and a steam to fuel ratio of 1.0kg/kgdb were achieved. The system was operated at gasification temperatures between 830 and 870°C. This paper points out the influence of the temperature on the system

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

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