Does Mechanical Screening of Contaminated Forest Fuels Improve Ash Chemistry for Thermal Conversion?

The effect of mechanical screening of severely contaminated forest fuel chips was investigated, focusing on main ashforming elements and slagging tendency and other properties with relevance for thermal conversion. In this study, screening operations were performed according to practice on an industrial scale by combining a star screen and a supplementary windshifter in six different settings and combinations. Mechanical screening reduced the amount of ash and fine particles in the accept fraction. However, the mass losses for the different screening operations were substantial (20−50 wt %). Fuel analyses of the non-screened and the screened fuels showed that the most significant screening effect was a reduction of Si and Al, indicating an effective removal of sand and soil contaminations. However, the tested fuel’s main ash-forming element’s relative concentration did not indicate any improved combustion characteristics and ash-melting behavior. Samples of the accept fractions and non-screened material were combusted in a single-pellet thermogravimetric reactor, and the resulting ashes’ morphology and elemental composition were analyzed by scanning electron microscopy−energy dispersive X-ray spectrometry and the crystalline phases by powder X-ray diffraction. Results from both these analyses confirmed that screening operations had no, or minor, effects on the fuels’ ash chemistry and slagging tendencies, i.e., the fuels’ proneness to ash melting was not improved. However, the reduction of ash and fine particles can reduce slagging and other operational problems in smaller and more sensitive combustion units

Bed material performance of quartz, natural K-feldspar, and olivine in bubbling fluidized bed combustion of barley straw

The present study investigates how three different silicate-based bed materials behave in bubbling fluidized bed combustion of a model agricultural residue with respect to ash composition, namely barley straw. Quartz, natural K-feldspar, and olivine were all used in combustion at 700 C-degrees, and the resulting layer formation and bed agglomeration characteristics were determined. Based on this, a general reaction model for bed ash from agricultural residues was proposed, taking into account the reactivity of the different silicates investigated towards the main ash-forming elements K, Ca, and Si. The proposed reaction model links bed material interaction with Krich bed ash to the degree of polymerization of the silicate bed material, where addition reactions occur in systems with high polymerization, predominately in quartz, and substitution reactions dominate for depolymerized silicates such as K-feldspar and olivine

Lignocellulosic residues for bioenergy : effects of storage, fuel design, and combustion characteristics

Utilization of lignocellulosic residues for bioenergy raises storage, handling, and combustion challenges. The ash-forming elements in biomass may cause ash-related problems during combustion, such as slagging, fouling, corrosion, or bed agglomeration. Most of these problems are linked to ash-chemical reactions involving alkali. The overall objective of this thesis was to evaluate the impact of storage, fuel design measures, and process adaptation for improving combustion characteristics of lignocellulosic residues to mitigate ash-related operational problems. This was done by monitoring storage-induced changes in bark influencing final material characteristics. Mechanical screening and Ca-additivation on forest and agricultural residues were done as a fuel design measure. In addition, the performance of different bed materials was evaluated in bubbling fluidized bed combustion. Significant changes in the lignocellulosic matrix near pile surfaces were observed during storage of bark, and these changes will not affect the combustion characteristics. Screening efficiently reduced the amount of ash but was associated with significant fuel mass losses and did not change the ash chemistry; thus, combustion characteristics and ash melting behavior can be expected to remain unchanged. The Ca-addition significantly increased the total defluidization temperatures but implied an elevated risk for forming corrosive species and high CO (g) concentrations. High interactions between fuel ash and quartz bed particles and low interactions between fuel ash, olivine, and feldspar bed particles were observed. The feldspar bed material reduced defluidization temperature due to the increased amount of alkali in the bed by diffusion of alkali from feldspar grains. Overall, a good understanding of the ash transformation reactions is needed to mitigate ashrelated problems. In addition, fuel design measures must affect the ash chemistry in the fuel to be worthwhile

Combustion characteristics of barley straw stored with CaCO3 during fluidized bed combustion using quartz and olivine as bed materials

In order to achieve a continuous feedstock supply for power plants a large quantity of harvested material must be stored for long-term periods. During storage, especially during right conditions of temperature, moisture and oxygen, solid agricultural fuels are susceptible to microbial activities. Microbial respiration not only leads to loss of organic carbon, but also to an increased temperature in the stored fuel which may lead to self-ignition. There is on-going work studying the effect of adding Ca as CaO or CaCO3 during storage of straw with the aim of creating an unfavorable alkaline microenvironment that prevents microbial growth during storage. The effect of this amendment during storage is still being explored, but the effect in the combustion step of the process in terms of determining the potential of Ca-additives to positively affect overall ash chemistry have yet to be certainly quantified. Utilization of agricultural crops for energy purposes in heat and power plants has proven to be challenging in numerous ways. Compared to woody fuels, agricultural fuels can cause severe ash-related operational problems in combined heat and power plants through by fouling, slagging and/or bed agglomeration due to their high concentrations of inorganic constituents. The objective of the present study is therefore to determine the fouling and bed agglomeration characteristics during fluidized-bed combustion of barley straw stored with different dosages of Ca to biomass with the main ash-forming elements shown in Figure 1, using two types of bed materials, quartz and olivine

Bed material performance of quartz, natural K-feldspar, and olivine in bubbling fluidized bed combustion of barley straw

The present study investigates how three different silicate-based bed materials behave in bubbling fluidized bed combustion of a model agricultural residue with respect to ash composition, namely barley straw. Quartz, natural K-feldspar, and olivine were all used in combustion at 700 °C, and the resulting layer formation and bed agglomeration characteristics were determined. Based on this, a general reaction model for bed ash from agricultural residues was proposed, taking into account the reactivity of the different silicates investigated towards the main ash-forming elements K, Ca, and Si. The proposed reaction model links bed material interaction with K-rich bed ash to the degree of polymerization of the silicate bed material, where addition reactions occur in systems with high polymerization, predominately in quartz, and substitution reactions dominate for depolymerized silicates such as K-feldspar and olivine.Validerad;2024;Nivå 2;2024-03-11 (hanlid);Full text license: CC BY</p

Does mechanical screening improve fuel properties? Effects of mechanical screening of stored logging residue chips on ash chemistry and other parameters relevant for combustion

Forestry and the forest industry plays an important role in the Swedish economy. From forest operations and at sawmills and pulp and paper mills several by-product assortments are generated and these are providing the basis for the highly developed Swedish bioenergy sector. Logging residues constitute a major resource and is utilized as fuel in heat and power plants. However, due to a relatively low heating value and high management costs, this resource is still underutilized. Logging residue chips have irregular particle size, high moisture content (30-60%) and high ash content (8-15 %) and these features cause most of the problems encountered during the operation of feeding systems and combustion processes. Ash, present both in endogen plant tissues and as extrinsic matter such as sand and clay minerals, is of especially big concern for small-size plants. In this on-going work screening of logging residue chips was performed. Different mechanical screening methods was applied with the aim to provide a homogenous fuel with a higher quality for combustion purposes. Through screening, the chemical fuel composition is also altered and this affects combustion behavior and ash chemistry. The objective of the present study is to, from a combustion process perspective with emphasis on ash chemistry, evaluate the overall effects of different screening procedures when applied on stored logging residue chips