Comprehensive performance assessment of a continuous solar-driven biomass gasifier

Highlights

• A high-temperature solar biomass gasifier was operated continuously with various biomass feedstocks.

• The impact of operating parameters on syngas production capacity and reactor performances was evidenced.

• The H₂ and CO yields were improved by using optimum steam/biomass ratio and by increasing gas residence time.

• The gasification rates and syngas yields were enhanced by increasing temperature or biomass feeding rate.

• The biomass consumption rate matched the feeding rate at 1300 °C enabling reliable continuous operation.

Abstract

The experimental performance assessment of a continuous solar-driven biomass gasifier using real high-flux concentrated solar radiation as the source of process heat has been performed. A comprehensive parametric study considering different lignocellulosic biomass feedstocks (wood type), biomass feeding rates (0.6–2.7 g/min), steam/biomass molar ratios (1.6–2.8), carrier gas flow rates (2–3.3 Nl/min) and reaction temperatures (1100–1300 °C) was conducted for optimizing the syngas production capacity and evaluating the gasification performances. Different wood biomass feedstocks were continuously fed as particles and gasified with H₂O for producing syngas, thus successfully demonstrating the reliability of the reactor that was operated compatibly with different particle sizes and shapes. A small excess of water with respect to stoichiometry was beneficial for biomass gasification regarding the increase of H₂ and CO and the decrease of CH₄, CO₂ and C₂H_m production. An increase in the gas residence time resulted in the improvement of the syngas yields and quality. Significant enhancement of syngas yields and production rates through the rise of operating temperature was highlighted with activation energy in the range of 24–29 kJ/mol. Increasing biomass feeding rate improved the syngas yields and gasification rates, enabling efficient solar energy storage into syngas and enhancing the energy upgrade factor (U) above 1.20, the solar-to-fuel energy conversion efficiency (η_{solar-to-fuel}) above 29% and the thermochemical reactor efficiency (η_reactor) above 27%.

Introduction

Solar thermochemical processes offer several promising routes for solar fuels production. These thermochemical routes involve endothermic reactions that proceed at high temperature delivered by concentrating solar systems [1]. Among them, the solar-driven gasification of carbonaceous feedstock represents a promising pathway for producing synthesis gas (syngas containing mainly H₂ and CO) [2]. The carbonaceous feedstock is gasified with an oxidant such as H₂O/CO₂ by utilizing concentrated solar power as an external energy source to drive the gasification reaction. In this process, solar energy can be long-term stored into high-value syngas while upgrading the calorific value of the carbonaceous feedstock, by an amount equal to the enthalpy change of the endothermic gasification reaction [3]. In addition, the process is carbon-neutral when biomass is utilized as carbonaceous feedstock [4].

The chemistry of thermochemical gasification of carbonaceous feedstock basically involves two main steps. First, the pyrolysis represents the process in which carbonaceous materials decompose when heated without oxygen, thereby releasing incondensable and combustible gases, chars, and tars. These reactions are only slightly endothermic, thus requiring relatively low amount of energy with temperature in the range of 300–1000 °C [5]. Second, the char remaining after pyrolysis is gasified with steam or CO₂ to produce syngas at above 1000 °C [3,5]. This reaction is highly endothermic; therefore, solar energy is converted into chemical energy while chars are being gasified.

The overall stoichiometric steam gasification of solid carbonaceous materials can be represented as:

$${\mathrm{C}}_{\mathrm{x}}{\mathrm{H}}_{\mathrm{y}}{\mathrm{O}}_{\mathrm{z}}+\left(\mathrm{x}-\mathrm{z}\right){\mathrm{H}}_2\mathrm{O}\to \left(\frac{\mathrm{y}}{2}+\mathrm{x}-\mathrm{z}\right){\mathrm{H}}_2+\mathrm{xCO}$$

The advantages of the solar-driven allothermal versus conventional autothermal biomass gasification are various: (i) it provides higher syngas output per unit of biomass feedstock and it reduces CO₂ emissions as no portion of the biomass feedstock is burned for process heat, thus eliminating the need for additional biomass (~35–40%) and oxygen supply [6]; (ii) an energy-rich and high-quality syngas that is not contaminated by the combustion products is produced, thus avoiding the requirement of additional energy consumption in downstream gas separation systems [7]; (iii) the discharge of pollutants to the environment is avoided; (iv) the energy content of the feedstock is solar upgraded, resulting in the storage of intermittent solar energy into convertible and transportable chemical fuels; (v) the solar gasifier can be operated at high gasification temperatures (>1200 °C), thus resulting in faster reaction kinetics, higher syngas quality, and avoiding the presence of tars in the produced syngas [8]; (vi) the possible handling and processing of a wide variety of feedstocks is offered [9].

In order to overcome the intermittent solar radiation issue, a hybrid solar/autothermal gasification concept has been proposed for operating systems around the clock [10,11]. Muroyama et al. [12] have designed and tested a 1.5 kW_th solar-hybrid steam gasification prototype. In this reactor, pure O₂ was fed into a solar reactor to increase the temperature. Maximum cold gas ratio and solar-to-fuel efficiency of up to 1.16 and 22.1%, respectively, were observed.

Solar thermochemical reactor concepts can be broadly categorized as directly and indirectly irradiated solar reactors. On the one hand, directly-irradiated solar reactors transfer heat directly to the reaction site, thereby both allowing optimal radiative transfer to the feedstock and minimizing heat losses [13]. However, they require a transparent window for the entrance of highly concentrated solar radiation [14]. On the other hand, indirectly-irradiated solar reactors transfer heat to the reaction site through an opaque absorber wall, which is commonly made of SiC owed to its high emissivity, high conductivity and inertness at high temperature [12]. This further eliminates the need for a transparent window that is inclined to fouling, at the expense of additional heat transfer resistance though the opaque absorber. Regardless of the type of reactor, the material selection for the solar receiver/absorber is conditioned by the maximum operating temperature, thermal conductivity, radiant absorbance, chemical inertness, resistance to thermal shocks, and suitability for transient operation [1,9].

Relevant solar thermochemical reactor designs that have already been applied to solar gasification include packed-bed [3,[15], [16], [17], [18]], fluidized-bed [[18], [19], [20]], molten-salt [21,22], entrained flow [23,24], drop tube [[25], [26], [27]] and vortex flow [[28], [29], [30]] reactors. Packed-bed reactors are generally operated in batch-mode. These reactors accept a wide variety of feedstocks with bulk moisture content and can accommodate to different particle shapes and sizes thanks to long residence times. Nevertheless, they face challenges related to high condensable tars production, unreacted remaining products as well as heat and mass transport limitations [31]. Therefore, to essentially overcome such limitations, enhance solid-phase residence times and provide efficient gas-particles mixing, fluidized-bed reactors have been developed and tested [32]. These reactors, however, require small and relatively uniform particle size and shape to operate properly [18,33].

Additional design and development of solar-driven gasification reactors include the utilization of molten salts that offer the advantages of improved heat transfer, catalysis of gasification, reduced production of tars as well as thermal stability (inertia) for transient solar power input [22]. Hathaway and Davidson [34] demonstrated a 2.2 kW prototype molten salt solar gasification reactor in a continuous process. This reactor yielded a solar-to-fuel thermochemical efficiency of up to 30% and converted 47% of the carbon at 945 °C. In addition, entrained flow reactors have been developed for improving heat and mass transfer and operating in a continuous process; nevertheless, the drawback of such reactors is an excessively short residence time, and rather small particles (<1 mm) are required [35]. Drop-tube and vortex flow reactors have been designed for increasing both the residence time and heat and mass transfer that considerably influence the reaction rate. However, similar problems to the entrained flow reactors are encountered, and an acceptable size distribution of the feedstock is also required for these reactors [36,37].

In the present work, a continuous solar biomass gasifier based on the spouted bed concept was developed featuring high solid residence time in the directly-heated reactor cavity, continuous feeding and stirring of the biomass feedstock at high-temperature, thus offering continuous process operation. Spouted bed reactors have already been used for non-solar biomass pyrolysis and gasification [[38], [39], [40]]. They can accommodate to a wide range of feedstock sizes and shapes, and exhibit high heat and mass transfer rates due to efficient particles stirring and contact with the gas phase reactants. The suitability and reliability of this solar reactor concept for solar biomass gasification with H₂O/CO₂ has been experimentally proven [41]. The influence of operational conditions considering initial carbonaceous feedstock, biomass feeding rate, residence time, operating temperature and steam flow-rate needs to be experimentally studied in order to optimize syngas production and assess gasification performances. The detailed experimental assessment of the solar gasifier performances was thus conducted, and the operating parameters of most relevance were appraised regarding their effect on biomass conversion rates, syngas yields and energy conversion efficiencies. The tuning of processing conditions is a key challenge to provide insights into the maximum performance bounds of the solar gasifier and to match the continuous biomass supply rate with the consumption rate, thus improving gasifier capacity utilization and warranting long-term stable operation.