Comprehensive study of the influence of total pressure on products yields in fluidized bed gasification of wood sawdust

doi:10.1016/j.fuproc.2010.04.001

Fuel Processing Technology

Volume 91, Issue 10, October 2010, Pages 1222-1228

https://doi.org/10.1016/j.fuproc.2010.04.001 Get rights and content

Abstract

Wood sawdust gasification experiments were performed in a steam fluidized bed at 800 °C between 2 and 10 bar. An evolution of gas yields with time was measured during the tests, and especially an increase of hydrogen and carbon dioxide yields. This test duration effect was ascribed to char build-up in the bed. As tests proceed, the contribution of char steam gasification to gas yield increases, and the catalytic effect of char on hydrocarbons and tar conversion and on water–gas shift reaction is enhanced.

As total pressure increases from 2 to 10 bar, hydrogen, carbon dioxide and methane yields increase by 16%, 53% and 38% respectively, whereas carbon monoxide yield decreases by 33%. The changes in gaseous yields with pressure can be partly explained by the influence of pressure on gas phase reactions (acceleration of water–gas shift kinetics and change in hydrocarbon reactions). The increase of methane yield with pressure is rather suggested to be linked to a change in secondary pyrolysis reactions scheme under high pressure.

Introduction

Performing biomass thermal conversion under high pressure has several advantages, especially:

▪
High pressure allows reducing the size of the gasification reactor.
▪
By performing gasification at high pressure, a step of gas compression could be avoided in the process leading to liquid or gaseous fuel synthesis. Indeed, Fischer–Tropsch synthesis is generally operated at pressures ranging from 20 to 40 bar [1]. Methanol and DME synthesis also need elevated pressures (50–300 bar and 15–100 bar respectively [2]). On the other hand, the methanation step leading to Synthetic Natural Gas could be operated between 1 and 10 bar [2]. Moreover, before synthesis, the gas cleaning step can be performed at high pressure, as for example with the Rectisol process [3] operated at the industrial scale up to about 60 bar.

Pressurized gasification of biomass has been performed at the industrial scale in Värnamo [4] and at the Institute of Gas Technology (IGT) [5]. The fluidizing gas was air (Värnamo, IGT) or a mixture of steam and oxygen (IGT). Biomass gasification was studied in several pressurized fluidized bed pilot plants [6], [7], [8], [9], [10], [11], under steam, nitrogen and oxygen mixture, or air sometimes mixed with steam. Several results obtained around 850 °C with peat or miscanthus, in a flow of air and steam, show that methane content in the gas increases with pressure between 5 and 10 bar [6], [10]. The increase of the methane content was of 20% between 5 and 10 bar in [6] and of about 30% between 4 and 7 bar in [10]. The same trend was observed under air at 820 °C between 1 and 5 bar [11]. At the same time, as pressure increased, tar content decreased by a factor of 2. The char yield was observed to increase with pressure, especially above 10 bar [9].

Our objective is to study in a comprehensive way the influence of pressure on wood sawdust gasification in a steam fluidized bed. For this purpose, several experiments were performed in a pressurized fluidized bed between 2 and 10 bar.

Section snippets

Description of the facility

The fluidized bed facility has been designed to study biomass steam gasification up to 1000 °C and 40 bar.

The facility (main dimensions in Table 1) is composed of a vertical part on which a horizontal part (for biomass feeding) is connected. The internal reactor and the feeding screws are surrounded by an external vessel made of stainless steel and designed to sustain a pressure of 40 bar (Fig. 1). The vessel is cooled by hot and pressurized water flowing into a coiled tube welded onto the outer

Output gas yields evolution during a test

For each test, the same type of gas yields evolution with time is noticed. Examples of major and minor gases yields evolution during a test at 7 bar are represented in Fig. 2, Fig. 3 (in kg/kg of dry biomass). The carbon dioxide yield was divided by 10 to be able to be represented on the same scale as the other major gases yields. After 1 h of biomass feeding, the methane and carbon monoxide yields are rather constant (Fig. 2). Hydrogen and carbon dioxide yields respectively increase by 12% and

Conclusion

Wood gasification experiments in a fluidized bed at 800 °C showed that the gaseous yields were influenced both by test duration and by total pressure in the reactor (between 2 and 10 bar).

The test duration effect was connected to char build-up in the bed. As tests proceed, the contribution of char steam gasification to gas yield increases, and the catalytic effect of char on hydrocarbons and tar conversion and on water–gas shift reaction is enhanced.

The changes in gaseous yields with pressure can

References (20)

M.J.A. Tijmensen et al.
Exploration of the possibilities for production of Fischer Tropsch liquids and power via biomass gasification
Biomass Bioenergy
(2002)
H. Weiss
Rectisol wash for purification of partial oxidation gases
Gas Separation & Purification
(1988)
R.G. Lundqvist
The IGCC demonstration plant at Värnamo
Bioresource Technology
(1993)
R.A. Knight
Experience with raw gas analysis from pressurized gasification of biomass
Biomass Bioenergy
(2000)
E. Kurkela et al.
Air gasification of peat, wood and brown coal in a pressurized fluidized-bed reactor. I. Carbon conversion, gas yields and tar formation
Fuel Processing Technology
(1992)
W. de Jong et al.
Thermochemical conversion of brown coal and biomass in a pressurised fluidised bed gasifier with hot gas filtration using ceramic channel filters: measurements and gasifier modelling
Applied Energy
(2003)
W. de Jong et al.
Biomass and fossil fuel conversion by pressurised fluidised bed gasification using hot gas ceramic filters as gas cleaning
Biomass Bioenergy
(2003)
V. Thonglimp et al.
Vitesse minimale de fluidisation et expansion des couches fluidisées par un gaz
Powder Technology
(1984)
C. Dupont et al.
Study about the kinetic processes of biomass steam gasification
Fuel
(2007)
Z. Abu El-Rub et al.
Experimental comparison of biomass chars with other catalysts for tar reduction
Fuel
(2008)

There are more references available in the full text version of this article.

Cited by (40)

Effect of temperature and pressure on the transformation characteristics of inorganic elements in cotton straw ash
2023, Fuel
The transformation characteristics of inorganic elements in biomass ash are crucial for the efficient and continuous operation of gasifiers. Cotton straw ashes produced initially at 500 °C were heated under different temperatures and pressures, and then the resulting ash samples were analyzed by XRF, XRD, and SEM-EDS. The results indicated that increasing reacting temperature aggravated ash melting and the ash loss rose linearly, reaching up to 18.1 % at 1000 °C and 0.1 MPa. Reversely, high pressure effectively declined ash loss and melting degree. Under 2 MPa and 4 MPa, the weight loss at 1000 °C was only 8.0 % and 4.3 %, respectively. High pressure decreased ash loss by inhibiting the decomposition and vaporization of low-temperature minerals (K₂SO₄, Na₈Al₆Si₆O₂₄SO₄, CaCl_2, and CaSi₂) and promoting the reactions among minerals in ash. At 1000 °C, KCl was present in ash, and its content rose with increasing pressure at the same temperature. The SEM-EDS images revealed that the structure of ash varied from melted to adhesion with increasing pressure at 1000 °C. KCl particles were present in large ash particles at 1000 °C and 2 MPa. As the temperature increased from 700 °C to 1000 °C, K₂O content decreased more than 5.0 %, while that of CaO increased 6.0 % at 0.1 MPa. At 4 MPa, the K₂O and CaO increased and reduced 0.6 %, respectively. Compared to atmospheric pressure, pressurization significantly reduced the release of inorganic elements from ash.
Syngas production strategies from biomass gasification: Numerical studies for operational conditions and quality indexes
2020, Renewable Energy
Citation Excerpt :
The partial pressure inside the gasifier is enhanced in the presence of steam so, as SBR is raised, water-gas, water-gas shift and steam reforming reactions are favoured which leads to the production of higher amounts of H2 and CO2 and lower yields of CO [66]. This has likewise been reported by other authors [67,68]. Along the parametric study for the assessed feedstocks, the evaluation of tar was also held in order to match the best experimental conditions, simultaneously leading to the higher LHV and to the lower tar amount.
Gasification is an enhanced thermal technique, as it affords a versatile synthetic gas (syngas) from which it is possible to obtain various commodity assets such as electricity, fuels and chemicals while conveniently disposing of biomass or waste residues.
To the best of the authors’ knowledge, there are no publications thoroughly relating syngas quality features with its possible applications. The report of quality indexes for syngas is quite scarce once they depend on quality indicators which are difficult to reconcile and simultaneously adjust to meet the expected syngas characteristics. Therefore, in an attempt to fill this gap, this work reports the gasification modelling for miscanthus (M), peach stone (PS) and a mixture of polyethylene terephthalate and vine pruning (PET-VP) in order to determine syngas quality indicators and potential applications. The operational conditions were optimized through a parametric study for the achievement of high-quality syngas, varying temperature (T), equivalence ratio (ER) and steam to biomass ratio (SBR) and assessing the lower heating value (LHV), cold gas efficiency (CGE) and tar content.
The main trends for the assessed parameters show improved calorific content for higher temperatures (800 °C – 850 °C), lower ER (0.2–0.25) and moderate SBR values (around 2.0). PET-VP displayed the higher LHV (around 9 MJ/Nm³) except when SBR was varied. In this case, M afforded enhanced calorific content (6.5 MJ/Nm³) which rapidly decreased for higher ratios. Conversion efficiencies of 70–80% were seen for the studied feedstocks and tar content was seen to decrease for higher temperatures, higher ER and higher SBR. PET-VP enabled the manufacture of chemicals and fuel gas, while M and PS originated syngas capable of producing synthetic fuels.
Gasification studies of low-grade Indian coal and biomass in a lab-scale pressurized circulating fluidized bed
2020, Renewable Energy
The present research is focused on the gasification study of a lab-scale pressurized circulating fluidized bed (PCFB) gasifier considering low-grade Indian coal and biomass such as sawdust and rice husk with a mean particle size of 600 $μ m$ as feed materials. Syngas composition, lower heating value (LHV), dry gas yield (Y), carbon conversion efficiency (CCE), and cold gas efficiency (CGE) are evaluated based on the experiments in the range of pressure 1–4 bar. It is observed that the concentration of CH₄ increases with pressure for all the three feed materials. The concentration of CH₄ is found to be increasing from (3.65–4.86 vol%) and (2.98–3.46 vol%) for sawdust and rice husk, respectively. Similarly, the LHV is found to be increasing with pressure for coal and sawdust. There is a significant increase of 12% in LHV with an increase in pressure from 1 to 4 bar for both coal and sawdust. The CGE is found to increase by 34, 51, and 61% for rice husk, coal and sawdust, respectively with the increase in pressure from 1 to 4 bar.
Production of a fuel gas by fluidised bed coal gasification compatible with CO<inf>2</inf> capture
2020, Fuel
A continuously fed, laboratory scale spouted bed gasifier has been used to study oxy-fuel gasification of German lignite. In this paper, the influence of different gasification agents and bed temperature on the process performance, during tests at atmospheric and elevated pressure are studied. Two gasification agents have been used, CO₂ (with different CO₂/C ratios) and mixtures of CO₂/steam. The results show that despite the relatively slow CO₂-char reaction, good gasification performance could be achieved with German lignite by adjusting the operating conditions at atmospheric pressure: complete carbon conversion, high energy conversion and a medium heating value fuel gas (8–10 MJ m⁻³). The CO₂/C ratio was found to have a large effect on the gasification performance. Increasing the ratio increased the carbon conversion, but the CO₂ conversion decreased. At 950 °C, maximum carbon conversion was already achieved with pure CO₂, therefore using steam at this temperature could not increase the conversion, but did increase the H₂/CO ratio in the fuel gas. At 850 °C, replacing 25% of CO₂ with steam increased the carbon conversion to the level achieved at 950 °C without steam. Replacing more than 25% of CO₂ with steam increased the H₂/CO ratio further. Therefore, with the addition of steam, the operating temperature could be reduced from 950 °C to 850 °C while maintaining the gasification performance. The changes of gasification performance with steam addition at pressures up to 10 bar_a followed the same trends achieved at atmospheric pressure.
Fluidized bed air gasification of solid recovered fuel and woody biomass: Influence of experimental conditions on product gas and pollutant release
2019, Fuel
The aim of this work is to study the air gasification of solid recovered fuel (SRF), and to compare it to the gasification of woody biomass (beech wood sawdust, and waste wood). This study focuses on the influence of several operating parameters: temperature – between 800 and 910 °C –, O₂/C ratio – between 0 (pyrolysis conditions) and 0.34, and addition of steam. Experiments are performed in a bubbling fluidized bed at 1.5 bar, with a solid feeding rate of 1–3 kg/h. The yield and composition of the product gas is particularly looked at, together with the repartition of carbon into the different types of products (gas species, tar molecules). Sulphur release to the gas phase is also investigated.
Temperature has mainly an influence on H₂ and CO yields, which is attributed to char gasification enhancement. The addition of steam in fluidising gas induces an increase in oxygenated gas species yields (CO + CO₂), related to char gasification improvement. It does not have any reforming influence on light hydrocarbon and tar, with even a slight inhibiting effect. Woody biomass and SRF show significant differences in product yields, which are qualitatively explained by the initial elemental composition and material content of each type of fuel.
From a process point of view, the conversion efficiency improvement by varying the gasification conditions for SRF is limited compared to the higher efficiency obtained with woody biomass. Co-gasification of biomass – possibly waste wood – and SRF, could be a way to improve the overall efficiency, and limit pollutant content.
Co-gasification and recent developments on waste-to-energy conversion: A review
2018, Renewable and Sustainable Energy Reviews
Biomass is currently seen as a promising renewable energy source, which can be sustainably utilized in the production of fuels and electric energy adding no carbon dioxide to the environment. Co-gasification has unveiled its potential amongst thermal techniques, as a result of the valuable products obtained, strengthening a solid position in the conversion of residues. Thus, the prevention of a complete depletion of non-renewable sources is supported and the effects of their utilization alleviated.
Extensive literature review was conducted and, few reports on co-gasification of biomass and wastes were found. In this context, this review addresses their thermal conversion, highlighting issues related to the equipment, operating conditions and physicochemical phenomena involved in such a complex process. Among other conclusions, the most important finding of this work was the synergy often encountered between the two feedstocks, proving co-gasification can overcome several of the individual gasification issues enhancing products quality and yields over biomass or wastes alone, and attesting its environmental-friendly character, with lower greenhouse gas emissions. It was also possible to depict some trends on the effect of biomass and waste blending ratios, as well as elucidating some of the mechanisms involved in their interaction. These are majorly explained by the response of molecules during pyrolysis and by hydrogen transfer from waste polymers to biomass derivatives. Experimental conditions were also assessed, fluidized beds being reported as the most suitable reactors for biomass and wastes, under several different possible combinations of operational parameters. A critical discussion is presented, aiming to contribute to a more profound understanding of this matter, its key points and noteworthy potential.

View all citing articles on Scopus

View full text

Comprehensive study of the influence of total pressure on products yields in fluidized bed gasification of wood sawdust

Abstract

Introduction

Section snippets

Description of the facility

Output gas yields evolution during a test

Conclusion

Biomass Bioenergy

Gas Separation & Purification

Bioresource Technology

Biomass Bioenergy

Fuel Processing Technology

Applied Energy

Biomass Bioenergy

Powder Technology

Fuel

Fuel