Mechanisms Behind the Positive Effects on Bed Agglomeration and Deposit Formation Combusting Forest Residue with Peat Additives in Fluidized Beds

Abstract

A compilation was made of the composition of peat from different areas in Sweden, of which a selected set was characterized and co-combusted with forest residue in controlled fluidized-bed agglomeration tests with extensive particle sampling. The variation in ash-forming elements in the different peat samples was large; thus, eight peat samples were selected from the compilation to represent the variation in peat composition in Sweden. These samples were characterized in terms of botanical composition, analyzed for ash-forming elements, and oxidized using a low-temperature ashing procedure, followed by characterization using scanning electron microscopy/electron-dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD). The selected peat samples had in common the presence of a small fraction of crystalline phases, such as quartz, microcline, albite, and calcium sulfate. The controlled fluidized-bed agglomeration tests that co-combusted forest residue with peat resulted in a significant increase in agglomeration temperatures compared to combusting forest residue alone. Plausible explanations for this were an increase of calcium, iron, or aluminum in the bed particle layers and/or the reaction of potassium with clay minerals, which prevented the formation of low-melting bed particle layers. The effects on particle and deposit formation during co-combustion were reduced amounts of fine particles and an increased number of coarse particles. The mechanisms for the positive effects were a transfer and/or removal of potassium in the gas phase to a less reactive particular form via sorption and/or a reaction with the reactive peat ash (SiO 2 and CaO), which in most cases formed larger particles (>1 μm) containing calcium silicon and potassium.

Composition of agglomerates in fluidized bed reactors for thermochemical conversion of biomass and waste fuels

“…The dominant reaction paths in the ash layers formed on silica sand bed material involve the formation of potassium and calcium silicates. The characteristic dual ash layer previously reported in the literature [25,26,34,44,46] is confirmed. This dual ash layer consists of an inner layer rich in potassium that is potassium silicate and an outer layer rich in calcium where calcium silicate is dominant.…”

“…Advanced thermodynamic equilibrium modelling has become a widely used method [15,17,26,45,46,48,53] it is an important tool for studying high temperature processes and ash-forming elements in biomass and waste combustion and gasification [26,41,[49][50][51]. The method is fast and cost effective.…”

Waste peat ash mineralogy and transformation to microporous zeolites

“…The few papers that discuss peat ash infer crystal phases from (a) elemental composition by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDAX) or X-ray fluorescence (XRF) only and/or (b) unseen XRD data; the reader is thus advised to treat such assertions with caution. For example, quartz, microcline, albite and calcium sulfate were reported to be present in peat ash samples, although no XRD results were shown [7,8].…”

“…The alkalinity of peat ash alters the soil quality of the landfill site such that only alkali tolerant plants can grow during the years immediately after disposal. The composition of peat ash varies with location but typically contains a mixture of silicon-, aluminium-, calcium-and iron-containing species, and a range of toxic elements including arsenic and cadmium [7][8][9][10]. While its elemental composition has been well studied, a definitive characterisation of peat ash mineralogy is lacking in the literature.…”

Syngas Production by Combined Biomass Gasification and in Situ Biogas Reforming

“…The bench-scale gasifier has been described in detail in previous work, 22 and therefore, only a brief description is given in the present paper. The addition of peat to the wood feedstock was made to avoid potential bed agglomeration at the highest temperature settings, because peat has previously been demonstrated to increase the agglomeration temperature in a fuel mixture.…”