Analytical assessment of a novel rotating fluidized bed solar reactor for steam gasification of char particles

doi:10.1016/j.solener.2016.10.047

Solar Energy

Volume 140, 15 December 2016, Pages 113-123

https://doi.org/10.1016/j.solener.2016.10.047 Get rights and content

Highlights

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A novel rotating fluidised bed solar gasifier is proposed.
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Increased fluidised bed surface area leads to improved radiation absorption.
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Particle conversion approaches completion as rotational speed increases.
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Particle conversion increases with feedstock particle radius.
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Balance of forces determines particle conversion and residence time.

Abstract

A novel rotating fluidised bed solar reactor for the gasification of carbonaceous materials is presented. A simplified single particle model, in which the particle gasification is coupled with three dimensional equations of motion, is developed and used to study the particle residence time and gasification conversion within the proposed reactor, together with their sensitivity to changes in key operating parameters, namely reactor rotational speed, initial feedstock particle radius, radiation intensity, and particle release position. The proposed reactor is shown to offer potential for increased particle residence time and conversion relative to conventional fluidised beds through adjustments of the operating parameters other than the particle release position. The results presented assist in developing an understanding of the operation of the proposed reactor and optimising its performance.

Introduction

Solar energy is becoming an increasingly attractive renewable energy source due to its abundance and low environmental impacts (Desideri and Campana, 2014). Concentrated Solar Thermal (CST) systems for application in electrical power generation are being applied in increasing numbers and scale due to their low cost for energy storage (Ho and Iverson, 2014). An alternative application of CST is for the solar thermal gasification of carbon rich materials which converts solid carbonaceous feedstocks into calorifically upgraded synthesis gas for use in power generation cycles or the Fischer Tropsch process for the production of liquid fuels (Piatkowski and Steinfeld, 2008). Solar thermal gasification has two distinct advantages over auto-thermal gasification; the net carbon emission is reduced, and the quality of syngas is improved due to the lack of combustion and associated by-products (von Zedtwitz and Steinfeld, 2005). Through life cycle analysis, Z’Graggen and Steinfeld (2008) estimated that solar energy could upgrade the calorific value of carbonaceous feedstock by up to 34% and lower the production cost of syngas by up to 14% per unit compared to auto-thermal gasification. However, a number of technical and economic challenges remain to be overcome before this technology can become commercially viable. Among these is the need for more efficient and reliable gasification reactors.

Solar thermal gasification of carbonaceous materials derived from biomass and fossil origins has been studied in a variety of solar reactors to determine their suitability for various feedstocks and operating conditions (Taylor et al., 1983, Trommer, 2006, Kodama et al., 2008, Z’graggen et al., 2006, Piatkowski and Steinfeld, 2008; Melchior et al., 2009). The directly irradiated configuration has received a lot of attention because it directly delivers high flux solar heat to the reaction site, resulting in relatively high temperature and fast reaction rate. The Solar Vortex Reactor (SVR) developed by Z’Graggen et al. (2006) was found to be one of the most promising concepts and currently has the highest lab scale solar to chemical energy conversion efficiency (Piatkowski et al., 2011). However, it suffers from the inability to preferentially control particle residence time based on particle size which could result in the premature conversion of small particles and incomplete conversion of large particles. Chinnici et al. (2015) modified the solar vortex reactor and proposed a new vortex reactor concept, referred to as the Solar Expanding Vortex Reactor (SEVR), which utilises an expansion cone in the vicinity of the tangential inlets to alter the vortex structure responsible for particle entrainment. Their CFD analysis showed that the altered vortex structure could significantly increase the sensitivity of particle residence time to particle size and reduce the number of particles depositing on the reactor window. Later, Chinnici et al. (2016) performed PIV experimental measurements in a lab scale SEVR to verify the findings of the CFD analysis. The SEVR’s ability to preferentially extend particle residence time greatly improves the conversion of feedstock particles at the reactor exit and the commercial viability of the reactor concept.

An alternative approach with increased particle residence time is the internally circulating bubbling fluidised bed solar reactor proposed by Kodama et al. (2008). In this reactor, feedstock particles are re-circulated in the fluidised bed under the influence of gravity and particle drag until they are reduced to sufficiently small sizes to be entrained by the fluidising gas. One key drawback of using a conventional fluidised bed for solar gasification is the limited bed surface area through which solar heat can be introduced to the bed. Conventional bubbling fluidised beds are typically tall and narrow in diameter which leads to a small top surface relative to the volume of the bed. The high flux solar heat required to adequately heat the entire bed volume has a high risk of overheating the particles at the top bed surface and creating undesirable melted lumps that were reported in the work of Kodama et al. (2008). A different configuration that increases the relative surface area for radiation absorption is the laterally irradiated fluidised bed confined by transparent a silica glass or quartz tube (Taylor et al., 1983, Kodama et al., 2002). However, Taylor et al. (1983) reported that using a transparent tube exposes the hot bed to the external environment, leading to increased re-radiation heat loss and reduced solar to chemical energy conversion efficiency. Both Taylor et al., 1983, Kodama et al., 2002 found that the use of either silica glass or quartz tube imposed a maximum temperature limit on the fluidised bed. At temperatures above 1000 °C, the quartz or silica glass tube was prone to permanent damage caused by overheating. Trommer (2006) compared the gasification of coke in both directly and indirectly irradiated fluidised beds and found that the heat transfer mode has negligible effect on chemistry and gasification conversion rate. Nevertheless, he noted that the directly irradiated configuration is more economical due to reduced parasitic heat loss. In the light of the above, the first aim of the present investigation is to introduce a novel concept of using rotating bubbling fluidised bed for solar gasification to address the problem of limited surface area for solar radiation absorption while maintaining reliability and also offering greater control over particle residence time for improved reactor reliability and efficiency.

Rotating fluidised beds, described in more detail below, have been proposed for non-solar applications to offer greater control of residence time and gas flow than conventional fluidised beds (Metcalfe and Howard, 1977). However, to the authors’ knowledge, no previous assessment has been reported of suitability of rotating fluidised bed for solar gasification. Hence, a further aim of the present investigation is to assess the potential benefits of the proposed concept in controlling particle residence time and conversion. This is achieved through (a) developing a numerical model with which the gasification conversion and entrainment trajectory of a single reactant particle can be investigated and (b) assessing the sensitivity of particle residence time and conversion to key operating parameters, namely rotational speed, location of particle release, particle diameter, and radiation intensity, with the aim of determining the underlying reasons.

Section snippets

Rotating fluidised bed gasification reactor concept

A schematic diagram of a conventional rotating fluidised bed reactor is presented in Fig. 1. The cylindrical gas distributor shown employs a porous distributor plate to radially inject the fluidising gas into the cavity. External walls, not shown in Fig. 1, enclose the cylindrical gas distributor to form a plenum that controls the direction of the pressure gradient for injecting the fluidising gas. The distribution of bed thickness along the axis of symmetry is typically constant in the range

Methodology

The reactor geometry is significantly simplified to remove the influence of complex geometrical features. The effects of inlet and outlet on particle elutriation and gasification are not considered. The modelled reactor domain consists only of the simple rotating fluidized bed shown in Fig. 1. The bed thickness is assumed to be constant in the Z direction, leading to a perfectly vertical bed surface profile. This assumption is considered to be reasonable based on the visual observation of

Baseline case

Under the conditions specified in Table 2, Table 3, the calculated particle trajectory of a 50 μm radius char particle is shown in Fig. 5. The outer solid circles indicate the top and bottom edges of the gas distributor at $r_{distributor}$ in the RFB cavity. The inner solid circle shows the exit port, located at $r_{exit}$ which is also the boundary of the outer irrotational region and inner viscous core. The particle is injected from the bed surface, at $r_{bed}$ , whose edges are defined by the dashed outer

Conclusion

The results of the present study confirm the suitability of the proposed rotating fluidised bed solar reactor to enable increased residence time and conversion of particles relative to a vertical solar fluidised bed. Precise control of the reactor is possible through adjusting relevant operating parameters. Some of the important findings are as follows:

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Rotational speed is found to be the most important operating parameter controlling particle conversion and residence time, influencing the

Acknowledgement

The authors would like to acknowledge the financial support provided by the Australian Solar Thermal Research Initiative.

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Analytical assessment of a novel rotating fluidized bed solar reactor for steam gasification of char particles

Highlights

Abstract

Introduction

Section snippets

Rotating fluidised bed gasification reactor concept

Methodology

Baseline case

Conclusion

Acknowledgement

Sol. Energy

Sol. Energy

Appl. Energy

Chem. Eng. Sci.

Sol. Energy

Renew. Sustain. Energy Rev.

Chem. Eng. Sci.

Fuel

Biomass Bioenergy

Chem. Eng. Process.

Combust. Flame

Powder Technol.

Chem. Eng. J.

Sol. Energy

Combust. Flame

Sol. Energy

Chem. Eng. Sci.

Chem. Eng. Sci.