Henrik Thunman

This paper presents results from a comprehensive process integration study of different process alternatives for Bio-SNG production based on biomass gasification. The influence of the different conversion steps in the process chain – drying, gasification, gas cleaning, methanation, and gas upgrade – on the overall process performance is investigated. Process bottlenecks as well as heat and material integration aspects are highlighted. Using future energy market scenarios, the energetic, economic, and carbon footprint performance of different process configurations are evaluated from a system perspective. About 63 MW_LHV of Bio-SNG can be produced from a process converting 100 MW_th,LHV (20 wt-% moisture) of forestry residues. Drying of the feedstock from a natural moisture content of 50 wt-% using internal process heat recovery is shown to be important for increasing the process energy efficiency, while the choice of gasification and methanation technology is shown to be of minor im...

ABSTRACT During summer 2007 a 2–6 MWth indirect gasification section was integrated into the loop of the existing 82➀2 MWth circulating fluidized bed boiler at Chalmers University. With help of a particle distributor the gasification unit is connected to the loop after the cyclone. Hot bed material entrained from the boiler is so transferred to the gasifier providing the heat for the production of a nearly nitrogen free product gas. Non-gasified char is returned together with the bed material into the boiler and converted. Biomass can be fed into both sections; the boiler and the gasifier. The gasification is separated from the boiler via two loop seals and a particle distributer, directing particles either back to the boiler or into the gasification section. For that reason the CFB boiler can be operated even after the retrofit independently, just like before, or in combined combustion/gasification mode. This possibility keeps the risk for a retrofit low. As, furthermore, the investment costs for the integration are considerably lower than standalone gasification units of that size, the retrofit is an easy way to extend the potential of a CFB Boiler towards bi- and tri-generation (heat, power, fuel) and enter new markets. The first experimental season with the installation proved stable operation of both the boiler and the gasification. Furthermore, the full functionality in combustion only mode was shown. The gasifier was operated for 60 h with wood pellets and wood chips. First analysis of the producer gas composition shows high contents of methane, making the installation a good match for SNG production. The heating value of the gas is about 14.4 MJ/Nm3. Keywordsgasification-biomass-CFB-retrofit

Controlled experiments on the conversion rate of batches of mono-sized wood particles and wood pellets with similar moisture content, burning under substoichiometric conditions, show no clear influence of particle size. In addition, the measurements show that the fuel density has a very small influence on the conversion rate. Here, the size dependence (3×3×3, 4×16×19, 10×10×10, 30×30×30, and 20×50×80mm for parallelepipedic

Operation controllability and fluid dynamics were evaluated in a system of interconnecting fluidised beds. Results indicate that the solid circulation is controllable and possible to determine from pressure measurements. Sufficient gas tightness of the loop-seals and flexibility in controlling of solid fluxes was indicated.

ABSTRACT The use of untreated olivine as the bed material in a biomass gasifier is investigated in this work, in which activation of the material is the main focus. The experiments were carried out in the Chalmers 2–4-MWth indirect biomass gasification unit and comprised analyses of the gas composition and bed material, as well as changes in tar yield. Starting from the raw material, the first signs of activation, in the form of a reduction in the tar yield, were observed already during the second day of the operation. The tar yield continued to decrease with time, and by the fourth day it was reduced by 30%, as compared to the yield on the first day of the operation. Analysis of the bed samples showed accumulation of inorganics within the bed material, with a share of potassium being present in leachable form. Thermodynamic calculations support the indications from the experiment that potassium can be released under gasification conditions and may play an important role in the activation of olivine. To examine the impacts of S and silica on the activity of olivine, two experiments were conducted. The addition of S to the combustion side gave a positive effect in terms of the tar levels in the raw gasification gas. The addition of silica sand revealed, as expected given the affinity of potassium for silicone, negative influences on the tar yield and gas composition that could not be attributed to mere dilution, as compared with the gas produced during operation with pure olivine.

ABSTRACT Secondary catalytic gas conditioning is one strategy to eliminate tars formed in a producer gas during biomass gasification. However, most catalysts tend to lose their tar reforming activity after a short period of operation due to carbon formation. A novel technique for catalytic gas cleaning based on two interconnected fluidized beds has been investigated; this technique can be applied to all types of gasifiers. The idea is to reform the tar components into useful molecules—even at high tar contents—by means of a circulating catalyst. More precisely, the producer gas is cleaned with catalyst in one of the reactors, referred to as the fuel reactor, while the catalyst is continuously regenerated in another reactor, the air reactor (AR). The system described here is coupled with the Chalmers 2–4 MWth biomass gasifier while the AR is fed with nitrogen-diluted air. The effect of different catalysts on both the tar content and the gas composition was investigated. Some of the tested materials do not only reform tars, they also influence the H2/CO ratio in a beneficial manner; in particular, ratios closer to 3 in the reformed gas are favorable if subsequent methanation is implemented. In this paper, comparative results based on testing with manganese- and iron-based catalysts are presented. The former is a manufactured catalyst while the latter is a natural ore. Results suggest that both show satisfying ability for regeneration from carbon deposits. Higher temperature enhances tar decomposition during the experiment with both catalysts. Moreover, the iron-based catalyst enhances the water–gas shift activity, which in turn impacts the total amount of produced gas. On the other hand, the manganese-based catalyst seems to display a higher propensity for tar conversion.

ABSTRACT The possibilities to upgrade raw gas with the use of a manganese oxide have been investigated in an application for secondary tar cleaning of biomass-derived gas. Experiments were conducted in a reactor system where a novel technique that combines tar cleaning with catalyst regeneration is applied. Raw gas from the Chalmers non-catalytic steam biomass gasifier—containing roughly 32 gtar/Nm gas3—was fed to the tar cleaning reactor. The tar reforming qualities of the manganese oxide were evaluated in the reactor system using a mixture of 23 wt.% catalysts in silica sand at the temperatures 700 and 800°C. Experiments showed that the catalyst was continuously regenerated from carbon deposits and that the total amount of tars was decreased by as much as 44.5 % at a gas residence time of 0.4 s in the bed. The catalyst showed activity in water–gas shift reaction and the H2/CO ratio increased from 0.6 in the raw gas to a peak value of 1 in the reformed gas at 800°C. Only a slight decrease in methane and acetylene content was observed for both operating temperatures.

ABSTRACT Secondary catalytic tar cleaning has been evidenced as a promising technology for upgrading gas derived from biomass gasification. When applying this technology downstream a biomass gasifier, the tar fraction in the raw gas can potentially be reduced and the content of hydrogen be increased. In this work, experiments have been conducted in a chemical-looping reforming (CLR) reactor. The present reactor system features a circulating fluidized bed as the reformer section, which offers a higher gas-solid contact time than a bubbling bed configuration previously tested. All experiments were performed using raw gas from the Chalmers 2–4 MWth biomass gasifier as feedstock to the reactor system. The catalyst inventory consisted of a natural manganese ore, and its activity was evaluated at three different temperature levels—800, 850, and 880 °C—and with an oxygen content of 2.2 %, corresponding to a theoretical air-to-fuel ratio of 0.06. Experimental results showed that the manganese ore exhibits catalytic activity with respect to tar conversion, and a tar reduction of as much as 72 % was achieved at 880 °C. Moreover, this material showed high activity towards hydrogen production and overall, an interesting upgrading capacity toward this producer gas. An H2/CO ratio of nearly 3 in the produced gas can make this material potentially interesting for application in an SNG system. Regarding the analysis of the physicochemical characteristics, the material showed signs of agglomeration with traces of sand most likely resulting from previous sieving during particle preparation. Though, a positive aspect is that this occurred without impacting the catalyst activity.

ABSTRACT Both agglomeration of bed material and corrosion of heat transfer equipment are issues related to combustion of biomass in a fluidized bed boiler. The biomass-ash component potassium is considered a major contributor for both phenomena. In this study, the conventionally used bed material, silica sand, was replaced with up to 40 wt % by the natural ore ilmenite in Chalmers 12 MWth circulating fluidized bed (CFB) boiler. In this study the purpose was to evaluate the physical and chemical changes ilmenite undergoes during this process. Close observations revealed that ilmenite underwent segregation of iron to the surfaces and an enrichment of titanium in the particle core. The ash formed a calcium-rich double layer on the particle, surrounding the iron layer. A diffusion of potassium into the particle core was also seen which led to the formation of KTi8O16. In addition to evaluating how ash components interact with the material, the ilmenite was leached and investigated as a possible potassium capturer. Leaching experiments on the used ilmenite showed that calcium and potassium were leachable to a very limited degree, namely, to less than 0.2 and 1 wt %, respectively, of the total content. The diffusion of potassium into the core of the particle could reduce both agglomeration and corrosion issues and could thereby be of great value for the improvement of the resistance of the bed material agglomeration in the fluidized bed boiler.

The residence time of the fluid in a reactor can be analysed with at least three different computational methods:(a) Eulerian simulation of the residence time measurements;(b) solution of the Eulerian transport equation for residence time;(c) Lagrangian particle ...

ABSTRACT A secondary tar-cleaning process based on chemical-looping reforming (CLR) was investigated for upgrading biomass producer gas, derived from the Chalmers University of Technology 2-4 MW indirect gasifier. The experiments were conducted in a bench-scale CLR reactor using a manufactured nickel oxide (NiO) catalyst. Although Ni is a well-documented and efficient steam-reforming catalyst, it is susceptible to rapid deactivation under tar-rich conditions. The aim of this study was to explore the advantages of CLR as a gas-cleaning application, a process which offers continuous regeneration of the carbon deposits on catalysts. The tar-reforming performance of this Ni material and its influence on the gas composition and in particular its potential to increase the H2/CO ratio, were studied. The system was tested at reforming temperatures that ranged from 700 to 880 °C and at oxygen concentrations of 1.0% and 2.2% in the inlet feed to regenerator section. The results confirm the strong ability of the catalyst to reform tars. Higher process temperatures clearly promoted tar conversion, with 96% overall conversion at 880 °C (99% if benzene is excluded), as compared with 45% conversion at 700 °C. The hydrogen production was favored when temperature was raised, though, a maximum ratio of H2/CO of 2.2 was observed at 750 °C. Finally, no time-on-stream deactivation of the catalyst in the CLR was observed during the test, which lasted almost 7 h.

A simple method is proposed for the estimation of the effective lateral dispersion of fuel in a fluidized-bed combustor. The method is based on a combination of a model for drying and devolatilization of the fuel and cross-sectional measurements of the water concentration ...

ABSTRACT A novel concept for secondary catalytic tar cleaning with simultaneous regeneration of the catalysts is presented in this work. The process is demonstrated by an initial experiment with producer gas from the Chalmers biomass gasifier using ilmenite (FeTiO3) as catalytic bed material. The tar cleaning system was operated at 800 °C and fed with raw gas from the Chalmers biomass gasifier, in which silica sand was used as bed material and the gasification was performed with steam. The tar content of the gas emerging from the gasifier was roughly 30 gtar/Nm3gas. The experiment showed that the catalyst was continuously regenerated from carbon deposits, and the ilmenite reduced the total amount of tar by 35% at a gas residence time in the bed of 0.4–0.5 s. Branched hydrocarbons and phenols were more or less completely reformed, while there was an increase of stable aromatic rings (benzene, naphthalene). The catalyst showed high activity in water-gas shift reactions, and the H2/CO ratio was shifted from around 0.7 prior to the reactor to almost 3 after the reactor.

Grate firing is the most common way to burn bio-fuels in small-scale units. Different combustion modes are achieved depending on how fuel and primary air are introduced. In continuous systems fuel and air are usually fed in cross-current and counter-current flow. Here, combustion of wet biofuels is studied in a 31MW reciprocating grate furnace (a cross-current flow combustor), and additional

ABSTRACT The initial experiences of using an oxygen-carrying metal oxide, ilmenite, in the 12-MWth circulating fluidized bed (CFB) boiler/gasifier system at Chalmers University of Technology are presented. The rationale for the addition of ilmenite to the solids inventory is that ilmenite has the ability to alternately take up and release oxygen, and thereby improve the distribution of oxygen throughout the furnace. As a consequence, less air is needed to maintain low emissions from carbon monoxide (CO) and unreacted hydrocarbons (HC) during the combustion of volatile-rich fuels, such as biomass. One of the conducted experiments involved only the boiler, and the reference case corresponded to operation solely with silica-sand as the bed material, while in an additional three cases, ilmenite in various amounts was added to make up to 40 wt.% of the total bed inventory. During the experiments, the concentrations of CO and nitric oxide (NO) in the convection path of the boiler were measured. The addition of ilmenite to the silica-sand decreased the concentrations of CO and NO by 80% and 30%, respectively. Additional experiments were performed in which a concentrated stream of raw gas produced in the indirect gasifier was injected into the freeboard of the boiler. In one experiment, only silica-sand was used, while 12 wt.% ilmenite was added to the bed material in a separate experiment. The concentrations of CO and HC were measured at three different heights in the boiler and at nine positions over a cross-section of the furnace. The concentrations of CO and total HC in the furnace cross-section during concomitant gasification operation were reduced by the addition of ilmenite.

ABSTRACT Biomass gasification plays an important role in the emerging production of second-generation biofuels. One of the major challenges facing biomass gasification is to find simple and efficient ways to reform tar components. While the tar causes operational problems, it can be reformed to increase the chemical efficiency of the gasification process. With respect to tar reforming, catalytic materials are of special interest. Many of the materials that have been proposed as promising catalysts are metal oxide-based materials. However, metal oxides also have the ability to transport oxygen when subjected to alternating oxidizing and reducing atmospheres, similar to that which occurs in a dual fluidized bed gasification system. In this work, ilmenite was used as the catalytic material in the Chalmers 2–4 MWth dual fluidized bed gasifier to decrease the yield of tar. The ilmenite was mixed with the silica sand, which was used as the bed material, to investigate how the level of ilmenite affected chemical efficiency and tar yield. Furthermore, energy balance calculations were established to elucidate the general aspects of oxygen transport in dual fluidized bed gasification systems. The results presented in this paper reveal that adding low levels of ilmenite reduces the tar yield by 50%mass. However, the oxygen transport induced by ilmenite caused a reduction in the chemical efficiency of the gasifier and the heating value of the gas, compared to using 100% silica sand as the bed material. The impact of adding ilmenite was found to be dependent upon the operational conditions of the gasifier; a low fluidization velocity gave the highest reduction of the tar yield, whereas higher fluidization velocities led to increased levels of heavy components. Overall, the use of ilmenite as a catalyst for reduction of the yield of tar appears promising, provided that the level of oxygen transport can be restricted.

ABSTRACT The ability to model the combustion of biofuels in a fixed bed is evaluated by a sensitivity analysis. The analysis treats the uncertainty of model parameters related to heat transport, reaction rates, and composition of volatiles. The scatter of the parameters is estimated from the differences between several published correlations. The results are compared with measurement data and possible model simplifications are discussed. It is shown that the bed model is able to reproduce the ignition rate and the maximum temperature. Prediction of these properties is relatively insensitive to the uncertainty of most of the parameters. Gas concentrations within the bed are more difficult to predict. They are greatly influenced by the composition of the volatile gas released during devolatilization. Also the composition of the volatile gas has a significant influence on the ignition of the gas in the model, affecting the ignition rate, particularly at low airflows. Moreover, the investigation shows that treatment of radiation can be simplified, and the number of gas species included in the model can be restricted without significant losses of model generality.

ABSTRACT The modeling of the conversion of solid fuel in combustion or gasification systems needs a description of the composition of the volatile gases that leave a fuel particle of typical size in a fixed or fluidized bed during devolatilization. Much work has been published on release of volatiles, but a general model of the composition of volatile gases and a comprehensive presentation of the related thermochemical properties of the fuel are still missing. Here, a simplified model is presented whose structure is valid for any solid fuel. The model consists of a heat and mass balance complemented by empirical data. The empirical coefficients have to be specified for certain classes of fuel:  the fuels treated in this paper are one type of hardwood and one of softwood, having particle sizes in the range used in utility boilers. The species in the volatile gases are represented by the time mean mass fractions of CO2, CO, H2O, H2, light hydrocarbons, and heavy hydrocarbons. In addition, a comprehensive set of data is presented for such properties of wood that are needed for modeling of conversion in fixed and fluidized beds. The data are taken from the literature and from measurements carried out in the present work.

ABSTRACT Low-temperature gasification of biomass is a primary process step for production of biofuels or electricity using gas-fired engines or turbines. In addition to the desired product gas, the raw gas produced through gasification inevitably contains condensable hydrocarbons, known collectively as tar. The amount and composition of the tar have relevance for the efficiencies of the downstream processes. Tar can be measured using both online and off-line methods. However, many of these methods currently lack information regarding their implementation and accuracy levels for large-scale systems. In this work, the Solid-Phase Adsorption (SPA) method for measuring tar in industrial applications is evaluated. The individual steps of the method were examined for their effects on the overall performance of the analysis. Sample collection was found to be the most prominent source of error, and this was mainly due to human factors. Omitting to determine the temperature and pressure of the sampled gas contributed to this error, as the sampled volume of gas under normal conditions could not be correctly calculated. Inconsistencies in the treatment and storage of the collected samples were shown to affect the more volatile species with boiling points similar to that of benzene. The gas chromatography (GC) analysis was performed with satisfying accuracy. However, the reliability of the estimations of the average composition and dew-point of the tar mixture were dependent upon the amount of the identified species. The current implementation of the SPA method yields values with a relative standard deviation within 10% for the majority of the compounds in a given sample. However, in line with the result of previous studies, the tar species with boiling points between those of benzene and xylene (i.e., the BTX compounds) were measured with a lower accuracy than those of heavier tars.

ABSTRACT Two metal oxides, naturally occurring ilmenite (iron titanium oxide) and manufactured nickel oxide supported on an α-Al2O3 matrix (NiO/AL2O3), were compared as catalysts for secondary biomass gas upgrading. The experiments were conducted in a chemical-looping reforming (CLR) reactor, which combines biomass gas upgrading with continuous regeneration of coke deposits. The CLR system was fed with a tar-rich producer gas from the Chalmers 2–4-MW biomass gasifier, and the possibilities to reduce the tar fraction and to increase the yield of hydrogen were evaluated for temperatures between 700 and 880 °C. A system-wide molar balance was established to enable calculations of tar removal efficiency on a mass basis; these results were further compared with those for the more widely used tar-to-reformed gas ratio, yielding tar concentrations in units of grams of tars per normal cubic meter of product gas (gtar/Nm3gas). Both materials exhibited activity with respect to tar decomposition and increased the yield of hydrogen. In addition, both tar removal and hydrogen production were increased with increasing temperature. All the phenolic compounds and a large proportion of the one-ring branched tars were decomposed at 800 °C by the two catalysts, despite the fact that the tar load in the raw gas was as high as 30 gtar/Nm3gas. Results from the mole balance showed that it is important to specify on what basis the tar removal efficiency is calculated. The tar removal efficiency was calculated as 95% for the Ni/Al2O3 catalyst at 880 °C and as 60% for the ilmenite catalyst at 850 °C on tar-to-reformed gas basis. When the produced permanent gases were removed from the reformed gas, the same calculations yielded tar removal efficiency of 86% and 42%, respectively. The testing of serial samples of the effluent stream from the regeneration reactor for carbon oxides showed that coke was removed from the catalyst, and no deactivation by coke deposits was detected during the 8 h of operation of the CLR reactor.

A simplified model for the combustion of solid fuel particles is derived, relevant for particle sizes and shapes used in fluidized and fixed-bed combustors and gasifiers. The model operates with a small number of variables and treats the most essential features of the conversion of solid fuel particles, such as temperature gradients inside the particle, the release of volatiles, shrinkage,

A drying number, Dr, which is in effect a Damköhler number, is derived to account for the separation of the drying and devolatilization processes of a solid fuel particle. This number is a qualitative measure of the relative times for drying and devolatilization. It is intended for fuels entering an environment that has a considerably higher temperature than its saturation temperature. The number relates the kinetic rate of devolatilization to the rate of evaporation, which is controlled by heat transfer to an evaporation front. The information given by the drying number is useful for choosing an appropriate model for the drying and the devolatilization of a fuel particle. For example, if the drying number is small, drying and devolatilization can be modeled separately; if it is around unity, a more detailed description is needed; and if it is very high, simplifications can be introduced, because drying and devolatilization then follow each other, and the rate of conversion of moisture and volatiles in a moist particle is given by the rate of evaporation. Experiments confirm that the drying number provides this information.

Combustion of biomass for heat and power production is continuously growing in importance, because of incentives for replacing fossil energy resources with renewable ones. In biomass combustion, the moisture content of the fuel is an essential operation parameter, which often fluctuates for biomass fuels. Variation in moisture content complicates the operation of the furnaces and results in an uncertainty in