Utilization of biomass as feedstock
in dual fluidized bed steam
gasification is a promising technology for the substitution of fossil
energy carriers. Experience from industrial-scale power plants showed
an alteration of the olivine bed material due to interaction with
biomass ash components. This change results mainly in the formation
of Ca-rich layers on the bed particles. In this paper, a mechanism
for layer formation is proposed and compared to the better understood
mechanism for layer formation on quartz bed particles. Olivine bed
material was sampled at an industrial-scale power plant before the
start of operation and at predefined times after the operation had
commenced. Therefore, time-dependent layer formation under industrial-scale
conditions could be investigated. The proposed mechanism suggests
that the interaction between wood biomass ash and olivine bed particles
is based on a solid–solid substitution reaction, where Ca2+ is incorporated into the crystal structure. As a consequence,
Fe2+/3+ and Mg2+ ions are expelled as oxides.
This substitution results in the formation of cracks in the particle
layer due to a volume expansion in the crystal structure once Ca2+ is incorporated. The results of this work are compared to
relevant published results, including those related to quartz bed
particles.
Steam gasification of solid biomass in dual fluidized bed systems is a suitable technology for the production of chemicals, fuels for transportation, electricity, and district heating. Interaction between biomass ash and bed material leads to the development of Ca-rich bed particle layers. Furthermore, incomplete decomposition of biomass leads to the formation of tar components; among these are stable intermediate products such as 1H-indene and stable gaseous hydrocarbons such as methane. In this work, the influence of bed particle layers on the conversion of intermediate products such as 1H-indene and methane via steam reforming was investigated by conducting experiments in a lab-scale test rig. Satisfying conversion of 1H-indene into gaseous molecules (e.g., CO, CO 2 , H 2 ) was achieved with used, layered olivine, whereas fresh olivine showed significantly poorer performance. Since steam reforming was connected to the watergas-shift reaction for the tested hydrocarbons, investigations regarding carbon monoxide conversion in the presence of steam were conducted as well. Furthermore, a comparison of the influence of fresh and used bed material concerning the conversion of methane is presented, showing that methane is not affected by the bed material, independent of the presence of particle layers.
Layer formation mechanism of K-feldspar in bubbling fluidized bed combustion of phosphorus-lean and phosphorus-rich residual biomass, Applied Energy. 248 (2019) 545-554.
The use of biomass
as feedstock for gasification is a promising
way of producing not only electricity and heat but also fuels for
transportation and synthetic chemicals. Dual fluid bed steam gasification
has proven to be suitable for this purpose. Olivine is currently the
most commonly used bed material in this process due to its good agglomeration
performance and its catalytic effectiveness in the reduction of biomass
tars. However, as olivine contains heavy metals such as nickel and
chromium, no further usage of the nutrient-rich ash is possible, and
additional operational costs arise due to necessary disposal of the
ash fractions. This paper investigates possible alternative bed materials
and their suitability for dual fluid bed gasification systems focusing
on the behavior of the naturally occurring minerals olivine, quartz,
and K-feldspar in terms of agglomeration and fracturing at typical
temperatures. To this end, samples of bed materials with layer formation
on their particles were collected at the industrial biomass combined
heat and power (CHP) plant in Senden, Germany, which uses olivine
as the bed material and woody biomass as feedstock. The low cost logging
residue feedstock contains mineral impurities such as quartz and K-feldspar,
which become mixed into the fluidized bed during operation. Using
experimental and thermochemical analysis, it was found that the layers
on olivine and K-feldspar showed a significantly lower agglomeration
tendency than quartz. Significant fracturing of particles or their
layers could be detected for olivine and quartz, whereas K-feldspar
layers were characterized by a higher stability. High catalytic activity
is predicted for all three minerals once Ca-rich particle layers are
fully developed. However, quartz may be less active during the buildup
of the layers due to lower amounts of Ca in the initial layer formation.
The necessity of recycling anthropogenically used phosphorus to prevent aquatic eutrophication and decrease the economic dependency on mined phosphate ores encouraged recent research to identify potential alternative resource pools. One of these resource pools is the ash derived from the thermochemical conversion of sewage sludge. This ash is rich in phosphorus, although most of it is chemically associated in a way where it is not plant available. The aim of this work was to identify the P recovery potential of ashes from sewage sludge co-conversion processes with two types of agricultural residues, namely wheat straw (rich in K and Si) and sunflower husks (rich in K), employing thermodynamic equilibrium calculations. The results indicate that both the melting behavior and the formation of plant available phosphates can be enhanced by using these fuel blends in comparison with pure sewage sludge. This enhanced bioavailability of phosphates was mostly due to the predicted formation of K-bearing phosphates in the mixtures instead of Ca/Fe/Al phosphates in the pure sewage sludge ash. According to the calculations, gasification conditions could increase the degree of slag formation and enhance the volatilization of K in comparison with combustion conditions. Furthermore, the possibility of precipitating phosphates from ash melts could be shown. It is emphasized that the results of this theoretical study represent an idealized system since in practice, non-equilibrium influences such as kinetic limitations and formation of amorphous structures may be significant. However, applicability of thermodynamic calculations in the prediction of molten and solid phases may still guide experimental research to investigate the actual phosphate formation in the future.
A promising way to substitute fossil fuels for production of electricity, heat, fuels for transportation and synthetic chemicals is biomass steam gasification in a dual fluidized bed (DFB). Using lower-cost feedstock, such as logging residues, instead of stemwood, improves the economic operation. In Senden, near Ulm in Germany, the first plant using logging residues is successfully operated by Stadtwerke Ulm. The major difficulties are slagging and deposit build-up. This paper characterizes inorganic components of ash forming matter and draws conclusions regarding mechanisms of deposit build-up. Olivine is used as bed
This study aims to determine the fate of P during fluidized bed co-combustion of chicken litter (CL) with K-rich fuels [e.g., wheat straw (WS)] and Ca-rich fuels (bark). The effect of fuel blending on phosphate speciation in ash was investigated. This was performed by chemical characterization of ash fractions to determine which phosphate compounds had formed and identify plausible ash transformation reactions for P. The ash fractions were produced in combustion experiments using CL and fuel blends with 30% CL and WS or bark (B) at 790−810 °C in a 5 kW laboratory-scale bubbling fluidized bed. Potassium feldspar was used as the bed material. Bed ash particles, cyclone ash, and particulate matter (PM) were collected and subjected to chemical analysis with scanning electron microscopy−energy-dispersive X-ray spectrometry (SEM−EDS) and X-ray diffraction. P was detected in coarse ash fractions only, that is, bed ash, cyclone ash, and coarse PM fraction (>1 μm); no P could be detected in the fine PM fraction (<1 μm). SEM−EDS analysis showed that P was mainly present in K−Ca−P-rich areas for pure CL as well as in the ashes from the fuel blends of CL with WS or B. In the WS blend, P was found together with Si in these areas. The crystalline compound containing P was hydroxyapatite in all cases as well as whitlockite in the cases of pure CL and WS blend, of which the latter compound has been previously identified as a promising plant nutrient. The ash fractions from CL and bark blend only contained P in hydroxyapatite. Co-combustion of CL together with WS appears to be promising for P recovery, and ashes with this composition could be further studied in plant growth experiments.
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