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Project ERA-NET Bioenergy “BIOFLEX!”

Prepared by: Christoffer Boman, Anders Rebbling, Markus Carlborg, Anna Strandberg

Place, Date: Wels, 28th February 2019

Fuel design concepts to reduce ash related problems and emissions - experiences from

single pellet combustion tests

Contents

■ Background

■ Introduction to ash chemistry and related problems

■ Fuel design concepts

■ Single-pellet studies in BioFlex

■ Results

■ Summary and conclusions

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Background

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Broadening of the biomass feedstock in small and me dium scale combustion plants …

� More challenging assortments such as energy crops, bio-residues and by-products …

� More ash, trace metals and nutrients introduced to the systems …

� More ash-related operational problems and increased emissions …

� More nutrients (especially P) to recycle …

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Introduction to ash chemistry …

Data from Vassilev 2009

Ash forming elements in woody biomass

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Introduction to ash chemistry … (cont.)

Ash formation, transformation and fractionation in a combustion system…!?

From: Anna Strandberg, PhD Thesis, UmU, 2018

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Classification of sintering degree and slag characteristics

Visual and subjective characterisation by hand on sieved material(i.e. morphology and ”hardness”)

Original: Öhman M, et al. Biomass and Bioenergy 2004; 27:585-596Revision 1: Lindström E, et al. Energy and Fuels 2007; 21:710-717Revision 2: Díaz-Ramírez M, et al. Energy and Fuels 2012; 26:3218-3229

Category 1 Category 2a Category 2b Category 3 Category 4

Non-sintered

(e.g. Ca/K-

oxides/carbonates)

Partly sintered

(e.g. K/Ca-carbonates)

Totally sintered

(e.g. K/Ca-silicates)

Introduction to ash chemistry … (cont.)

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Fuel design concepts

Fuel design is a general approach of using primary fuel based measures to increase the fuel quality and combustion performance in biomass combustion and gasification applications.

In this context:A novel tool to design the ash and to reduce/avoid unfavourable ash chemical transformations and its related operational and emission effects in biomass combustion processes

The aim of fuel design is to induce or promote chemical reactions between the fuels and/or additives to reduce the risk of ash related problems. Fuel design is implemented by blending the primary fuel with a secondary fuel or an additive that has to be chosen explicitly to get the best and desired effect

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Two approaches:

� Fuel blending

� Fuel additives

Fuel design concepts (cont.)

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Fuel design concepts (cont.)

Fuel blending: Can be divided as:

1. Biomass and Biomass2. Biomass and Peat3. Biomass and Industrial residues/fuels

- Rape seed cake/meal- Hydrolysis residues from ethanol production- Sewage sludge- …

4. Biomass and CoalStudied within BioFlex, although not further discussed in this presentation!

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Additives can be sorted into some main groups, depending on their chemical composition, additives containing;

1. Calcium2. Phosphorus3. Sulfur4. Aluminum5. Aluminum-Silicates

The clay kaolin(main component is the mineral kaolinite [Al2Si2O5(OH)4])

Fuel design concepts (cont.)

Studied within BioFlex, both in small-scale burners and single-pellet studies

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Fuel design concepts (cont.)

Fuel ash composition (in mole/kg) for the BioFlex fu els

Akali (+I)

Akaline earth metals (+II)

Metals (+II/+III)

Anion forming elements

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Al2Si2O5(OH)4 → Al2O3 ·2SiO2 + 2H2O

Kaolin is a white plastic clay used in many industrial applications

The main mineral is kaolinite that is transformed to meta-kaolinite upon heating (at 450-600°C):

Meta-kaolinite has a porous structure and is very capable of incorporating (react with) alkali vapours in biomass combustion processes:

Al2O3 ·2SiO2 + 2KOH → 2KAlSiO4 + H2O

Al2O3 ·2SiO2 + 2KOH + 2SiO2 → 2KAlSi2O6 + H2O

Al2O3 ·2SiO2 + 2KCl + H2O → 2KAlSiO4 + 2HCl

Al2O3 ·2SiO2 + 2SiO2 + 2KCl + H2O → 2KAlSi2O6 + 2HCl

Meta-kaolinite will at higher temperatures (>1000°C) transform further to spinel and mullite structures, less capable to capture alkali vapours.

Fuel design concepts (cont.)

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Single-pellet studies in BioFlex

Kaolin additivation within BioFlex:

Stoichiometic level: Assumes that there is a molar relation 1:1 between alkali in the fuel (K+Na) and aluminium in the kaolin.Note: kaolinite contains two Al atoms!

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Influences on “fuel ash” composition by kaolin addi tion

Single-pellet studies (cont.)

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Single-pellet studies (cont.)

The single particle (pellet) reactor at UmUAlso called Macro-ThermoGravimetric Analyser (M-TGA)

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Biswas et al, 2014

Experimental approach:

Single particle reactors; M-TGA (for pellets)

Time resolved study approach –quenching during devolatilisation(DV) and char conversion (CC).

Combustion at 950 °C in air.

20 pellets (single pellet test replicates) for each case.

Step-wise (off-line) characterisation (SEM-EDS, XRD, ICP-MS, etc.)

Single-pellet studies (cont.)

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Anaysis and evaluations:

Weighing on analytical balance (±1 µg) of each single pellet, char- and ash residue.

ICP-MS analysis of fuels and ashes/residues � quantitative elemental distribution.

SEM-EDS/WDS analysis � morphological information (imaging) and elemental distribution.

P-XRD analysis � phase composition of crystalline matter.

� Alkali release (fine particle formation) and melt formation (slagging tendencies)

Single-pellet studies (cont.)

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Results

Time-resolved fuel conversion and ash formation

Single pellet poplar combustion

DVX = X% of the devolatilization phase completed (time-based)

CCX = X% of the char conversion phase completed (time-based)

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Results (cont.)Semi-time resolved distribution (relative) of elements in

residual ash/char and the effects of kaolin addition

Poplar Poplar + kaolin

Wheat straw Wheat straw + kaolin

DVX = X% of the devolatilization phase completed (time-based)

CCX = X% of the char conversion phase completed (time-based)

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Results (cont.)

Average composition and elemental distribution of potassium (K) by SEM-EDS for Poplar (left) and Poplar with kaolin (right)

Average composition and elemental distribution of potassium (K) by SEM-EDS for Wheat straw (left) and Wheat straw + kaolin (right)

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Results (cont.)

Elemental mapping for Al, Si and K (by SEM-EDS) in ashes after full conversion (CC110) for Poplar + kaolin

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Results (cont.)

Semi-time resolved phase composition and transformations (by P-XRD) – preliminary results!

+ K-silicate melt

(Ca/Al-silicates)

(Ca/Al-silicates)

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Summary and conclusions

• Increased utilisation of challenging biomass assortments in combustion applications will imply the risk for ash related problems, environmental concerns and need for nutrient recycling!

• Fuel design concepts like fuel additives or blending have great potentials as efficient strategies to mitigate ash related problems and to reduce particle emissions.

• Within BioFlex, the concept of using kaolin as additive in small- and medium scale grate-fired biomass systems has been studied, tested and further developed.

• Detailed laboratory studies have generated mechanistic understanding of the ash formation/transformation processes during the fuel conversion and of influences of additives, thus proven to be a valuable complement to controlled burner experiments and full scale tests.

• This project has improved the basic knowledge on fuel design strategies and promoted the next steps in full-scale testing and implementation of developed concepts.

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Thank you for your attention

Project webpage: https://bioflex-eranet.eu/

Contact: Christoffer Boman, Umeå Universitychristoffer.boman@umu.se