Experimental Investigation of Volatiles-Bed Contact in a 2-4 MWth Bubbling Bed Reactor of a Dual Fluidized Bed Gasifier

The use of catalytic bed materials in fluidized bed gasifiers represents a promising primary measure to decrease the tar content of biomass-derived raw gas. For effective application of such in-bed catalysts, extensive contact must be established between the volatile matter released from the fuel particles and the bed material. However, the extent of the contact and, consequently, the potential of in-bed tar removal techniques are not well understood. In this work, the fraction of volatile matter that interacts with the bed in a large (i.e., throughput of 300-400 kg/h biomass) bubbling bed gasifier is quantified experimentally and the effect of fluidization velocity is investigated. The results show that a higher fluidization velocity enhances gas-solid contact, with 48-69% of the volatile matter coming in contact with the bed within the range of 6-10 times the minimum fluidization (umf)

Operational strategies to control the gas composition in dual fluidized bed biomass gasifiers

Steam gasification of biomass can increase the share of renewable energy and material resources in the energy sector, transportation and different industries. Prior its application, the raw gas produced in biomass gasifiers needs to be cleaned from impurities. In gasifiers operating at mild temperature, such as fluidized bed steam gasifiers, tar is an impurity of major concern due to the operational problems that it can cause. Tar species can condensate at temperatures as high as 300\ub0C, causing the clogging of pipes and coolers, deactivating downstream catalysts, and forcing unplanned shut-downs. Thus, it is necessary to control the tar and gas compositions in gasifiers to ensure the technical reliability of the technology. This work investigates measures to control biomass conversion in dual fluidized bed (DFB) steam gasifiers and, thereby, contribute to the rational operation and design of these types of units. A parametric experimental investigation of the influences of operating conditions on gas and tar compositions is presented. The examined parameters are: fluidization velocity; steam-to-fuel ratio (S/F); circulation rate of the bed material; temperature; and active bed materials. The bed materials tested include silica sand, olivine, bauxite, and feldspar, as well as the oxygen-carrying materials ilmenite and manganese. The work was carried in the Chalmers 2–4-MWth DFB gasifier using woody biomass as the fuel. The gasification technology applied in this work is similar to that of the existing gasifiers at the G\ufcssing, Senden, Oberwart, and GoBiGas plants.Within the operating window investigated, optimization of the bed material activity was the main tool for controlling tar conversion, which can be improved using additives. The levels of effectiveness of the in-bed catalysts were linked to the destruction of tar precursors. It is proposed that both homogeneous and heterogeneous catalysis of tar reactions occur in systems where alkali is expected in the gas phase. With active bed materials, temperature changes in the range of 700\ub0–830\ub0C were found to affect primarily the composition of the tar, and to a lesser extent, the tar yield. Finally, it is shown that a simple gasifier design with on-bed feeding ensures that at least 50% of the volatiles come in contact with the catalytic bed material when the bed is well-fluidized. Extensive experimental results and their implications for the design and operation of a DFB gasifier are discussed throughout this thesis

Behaviour of biomass particles in a large scale (2-4MWth) bubbling bed reactor

Biomass is regarded as an interesting fuel for energy-related processes owing to its renewable nature. However, the high volatile content of biomass adds a number of difficulties to the fuel conversion and process operation. In the context of fluidized bed reactors, several authors have observed that devolatilizing fuel particles tend to float on the surface of a gas-fluidized bed of finer solids. This behaviour, known as segregation, leads to undesired effects such as poor contact between volatiles and bed material. Previous investigations on segregation of gas-emitting particles in fluidized beds are conducted in small units and they are often operated at rather low gas velocities, typically between the minimum fluidization velocity (umf) and 2·umf. Therefore, it is not known to what extent such results are of relevance for industrial scale units and for higher fluidization velocities that are commonly used in large bubbling beds. In this work the behaviour of biomass particles in a large scale bubbling bed reactor is investigated. Tests were conducted at a wide range of fluidization velocities with three different bed materials of varying particle size and density. The fuel was wood pellets and the fluidization medium was steam, which makes the findings relevant for indirect gasification, chemical looping combustion (CLC) and bubbling bed combustion applications. The experiments were recorded by means of a digital video camera and the digital images were subsequently analysed qualitatively. The results show high level of segregation at fluidization velocity up to 3.5umf. Beyond this point fuel mixing was significantly enhanced by increasing fluidization velocities. At the highest fluidization velocity tested (i.e. >8umf), a maximum degree of mixing was achieved

Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite

Active bed materials are in this work investigated for in situ gas upgrading of biomass-derived gas. Previous research on in situ gas upgrading has focused on assessing gas quality, in terms of the concentrations of tar and permanent gases. Other aspects of fuel conversion, such as char conversion and the impact of oxygen transport on the final gas, are not as well documented in the literature on gasification. In this paper, the overall biomass conversion in a dual fluidized bed biomass gasifier is investigated in the presence of the catalytic material olivine and the alkali-binding material bauxite. The impact of these materials on fuel conversion is described as the combination of four effects, which are induced by the bed material: thermal, catalytic, ash-enhanced catalytic effect, and oxygen transport. Quartz-sand and ilmenite are here used as the reference materials for the thermal and the oxygen transport effects, respectively. Olivine and bauxite show activity toward tar species compared to quartz-sand. After 1 week of operation and exposure to biomass ash, the activities of olivine and bauxite toward tar species increase further, and the water gas shift reaction is catalyzed by both materials. Additionally, bauxite shows a stronger ability to increase char conversion than olivine. Under the conditions tested, olivine and bauxite have some degree of oxygen transport capacity, which is between those of quartz-sand and ilmenite. The oxygen transport effect is higher for bauxite than for olivine; nevertheless, the catalytic activities of the materials result in higher yields of H-2 than in a similar case with quartz-sand. The implications of the findings for the operation of dual fluidized bed gasifiers are discussed

Mapping the effects of potassium on fuel conversion in industrial-scale fluidized bed gasifiers and combustors

Potassium (K) is a notorious villain among the ash components found in the biomass, being the cause of bed agglomeration and contributing to fouling and corrosion. At the same time, K is known to have catalytic properties towards fuel conversion in combustion and gasification environments. Olivine (MgFe silicate) used as gasifier bed material has a higher propensity to form catalytically active K species than traditional silica sand beds, which tend to react with K to form stable and inactive silicates. In a dual fluidized bed (DFB) gasifier, many of those catalytic effects are expected to be relevant, given that the bed material becomes naturally enriched with ash elements from the fuel. However, a comprehensive overview of how enrichment of the bed with alkali affects fuel conversion in both parts of the DFB system is lacking. In this work, the effects of ash-enriched olivine on fuel conversion in the gasification and combustion parts of the process are mapped. The work is based on a dedicated experimental campaign in a Chalmers DFB gasifier, wherein enrichment of the bed material with K is promoted by the addition of a reaction partner, i.e., sulfur, which ensures K retention in the bed in forms other than inactive silicates. The choice of sulfur is based on its affinity for K under combustion conditions. The addition of sulfur proved to be an efficient strategy for capturing catalytic K in olivine particles. In the gasification part, K-loaded olivine enhanced the char gasification rate, decreased the tar concentration, and promoted the WGS equilibrium. In the combustion part, K prevented full oxidation of CO, which could be mitigated by the addition of sulfur to the cyclone outlet

Dual Fluidized Bed Gasification Configurations for Carbon Recovery from Biomass

Techniques that produce chemicals and fuels from sustainable carbon sources will have to maximize the carbon recovery to support circularity. In dual fluidized bed (DFB) gasification, to facilitate carbon recovery, the CO2\ua0from the flue gas can be concentrated using pure oxygen as an oxidant. The heat required by the process can also be provided electrically or by oxidizing an oxygen-carrying bed material, rather than combusting part of the char, thereby concentrating all of the carbon in the syngas. In this work, the three configurations of oxyfuel, electrical, and chemical-looping gasification (CLG) are compared to each other, as well as to the standard or “air” configuration, which corresponds to the combustion of char with air and the separation of CO2\ua0from both the flue gas and syngas. The configurations are compared based on their carbon distributions and energy demands for CO2\ua0separation. We show that the air and oxyfuel configurations lead to similar carbon distributions, whereas the CLG configuration gives the lowest carbon recovery in the form of an end product. The oxyfuel and CLG configurations show the lowest energy demands for CO2\ua0separation, while the air configuration exhibits the highest. The electrical configuration has the lowest potential to benefit from heat integration to cover this energy demand. An investigation into the optimal gasification temperature for the air and oxyfuel configurations shows that there is no driver for operation at high temperatures

Thermochemical conversion of polyethylene in a fluidized bed: Impact of transition metal-induced oxygen transport on product distribution

Thermochemical conversion in dual fluidized bed (DFB) systems is a potential alternative to the recycling of abundantly available plastic waste. The development of oxygen transport in DFB systems is in most cases unavoidable due to the transition metal content of the bed material as well as the metal fraction in the waste stream. This work investigates the influence of transition metal oxide-induced oxygen transport on the thermochemical conversion of high-density polyethylene, a model plastic feedstock, in a bubbling fluidized bed reactor. Conversion in the reactor at 700 \ub0C was investigated using four different bed materials that had different concentrations of iron oxide. The share of carbon oxides among the gaseous products increased with an increase in the iron oxide content of the bed material. The yield of light olefinic and paraffinic compounds decreased with increased iron oxide content of the bed. The presence of iron oxide in the bed material significantly increased the formation rates of aromatic compounds and solid carbon deposits on the bed material. The observed shift in the product distribution due to oxygen transport follows a dehydrogenation-type reaction mechanism

Developing a parametric system model to describe the product distribution of steam pyrolysis in a Dual Fluidized bed

Steam pyrolysis is a thermochemical process that converts carbon-based materials into valuable gases. In general, the products of the reaction are syngas (H2,CO,CO2), low-molecular-weight hydrocarbon gases (methane, ethylene, and propylene), pyrolytic gasoline and oils, monoaromatic and polyaromatic species (tar), and carbonaceous residues (char) with ashes. However, the intricacy of the reactions comprising the process, the diversity of the product species, and the constraints linked to the sampling and measurement equipment, create a highly complex system. In this work, a method for data representation is presented based on a special Parametric System Model (PSM) that portrays product species measurements in a way that provides relevant information and valuable insights into the process. The method incorporates generic knowledge of the chemical nature of the reactions to create a constrained system in which the data can be expressed in parametric terms with meaningful statistical functions. The evaluated data were obtained from a high-temperature steam pyrolysis process performed in the 2–4-MW Dual Fluidized Bed reactor at Chalmers University using polyethylene as feedstock. The quantities of the hydrocarbon species detected in the gas product were taken for the PSM as a probabilistic system that can be described with a set of distribution functions. The carbon, hydrogen and oxygen balances were taken into account to build a constrained set of equations to find the parameters of the functions. The resulting model was proven to be useful as a prediction tool to quantify unmeasured carbon group species and to estimate process variables, such as the oxygen transport of the bed material. Also, it was demonstrated the potential of the model as a method to identify and estimate inconsistencies in the measurements, which improve the quality of the characterization data. The modeĺs outcomes find application in providing critical information for the control and evaluation of pyrolysis process and downstream operation of biorefineries

Influence of in-bed catalysis by ash-coated olivine on tar formation in steam gasification of biomass

The use of catalytic bed materials has become a state-of-the-art solution to control the concentration of tar in fluidized bed biomass steam gasifiers. Ash-coated olivine is commonly applied as bed material, owing to its relatively high catalytic activity towards tar species. However, the mechanisms and conversion pathways influenced by the ash-coated olivine when applied as an in-bed catalyst are still not well understood. The present work aims at proving that the ash-layered olivine prevents the formation of biomass-derived tar at an early stage of their formation. Tests with olivine at different stages of activation and at different temperatures are carried out in the Chalmers 2-4MWth DFB gasifier. Detailed characterization of the tar and light hydrocarbon fractions are presented and discussed in relation to the sources of aromatic species. It is concluded that the ash-coated olivine prevents the formation of aromatic tar species by promoting the steam reforming of early tar precursors. Gas-phase interactions of the early tar precursors and bed material contribute to the tar reduction observed. The results indicate that olivine interferes the cyclization routes involving C2H2 and C3 hydrocarbons

Unraveling the hydrocracking capabilities of fluidized bed systems operated with natural ores as bed materials

Hydrocracking represents an alternative to the recycling of abundantly available plastic waste. Hydrocracking of polyethylene in a fluidized bed, at 750 \ub0C and 1 atm, was investigated in this work. Water dissociation, through the steam-iron reaction, was used as the source of hydrogen. Bauxite and olivine, containing reduced iron, were used as the bed materials in the reactor to drive the water dissociation reaction. The hydrogen-to-carbon (H/C) ratios of the products were compared to assess the hydrocracking potential. It was discovered that conversion of polyethylene on the surface of reduced bauxite effectively increased the H/C ratios of the products, as compared to bauxite in its oxidized form. Reduced olivine was ineffective at increasing the H/C ratios of the products in the presence of water dissociation. It is concluded that hydrocracking through hydrogen donation by steam is feasible in fluidized beds, provided that the bed material has the ability to transfer the hydrogen atoms to the hydrocarbon species