Thermochemical Technologies Critical factors for the efficient conversion of biomass to energy & fuels by thermochemical processes

Arvelakis Stelios & Koukios Emmanouel School of Chemical Engineering

NTUA

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Άδεια Χρήσης

Presentation Outline • Definition of Thermochemical Conversion Processes of Biomass Feedstocks

• Problems Associated with Combustion & Gasification

• Pretreatments applied to Coal & Biomass Fuels

• Critical Factors Associated with the Efficient Use of Biomass Feedstocks in Thermochemical Conversion Processes

• Biomass Torrefaction: A “Rising Star” Process

• Conclusions & Recommendations

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Definition of thermochemical processes

• Combustion -> Heat, Electricity, CHP

• Gasification -> Fuel Gas, Synthesis Gas

• Pyrolysis* -> Bio-oil as Fuel or for Chemicals

• Liquefaction (with Solvents) -> Same as Pyrolysis

• Carbonisation -> Char as Fuel, Fertiliser or Absorbent

• Torrefaction* -> Char as above or Pretreatment (prepyrolysis)

• Hot Briquetting -> Pellets as Solid Fuel or Pretreatment

• Other sources of Energy*: Plasma, Microwave, Ultrasound... * Processes about to reach commercial applications

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Why Feedstocks Matter ? They contribute significantly to the final production cost of the energy/industrial vectors They threaten the smooth technical performance of their conversion processes, being characterized by high variability of physical, chemical & biological properties Due to their bulky nature, they require the design of complex logistic chains, which are costly and open to various risks They create a long (and uncharted) interface between 2 Worlds: the Energy industry & markets; and large-scale Eco-bio-systems (agricultural, forestry, urban, etc.), raising sustainability issues (environmental, social, structural) …A major part of the BTU mission focuses on bio-feedstocks

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Problems associated with Fossil/biomass fuels

• Feeding problems (Investment in feeding systems required) • Ash-related problems • Heterogeneity problems • Mixing and Flow problems of different biomass/ biomass, biomass/coal blends • Emission problems (Particulate matter, NOx, etc.) • Transportation, Storage & Logistics problems

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Ash problems associated with Biomass fuels

• Slagging • Fouling • Agglomeration • Sintering • Corrosion • Catalyst poisoning

Main inorganics responsible for ash-related problems and gas-phase release in coal/biomass ash: K, Si, Cl, S, Na, Ca, P, Al

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• Ash in fuel generates particles • These particles may stick to the heat transfer surfaces • Deposits are a problem because: • They lower the rate of heat transfer to the water/steam cycle • They are potentially corrosive

Fossil/Biomass fuel combustion

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Release of Inorganics and Formation of Ash Species

Steps in thermal conversion of fuels: • Drying of the fuel • Pyrolysis of the fuel • Char burnout

• Combustion of volatile species (secondary alkali-S-Cl reactions - except for the sulfation of KCl to K2SO4 - are virtually unknown)

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Ash Formation Mechanisms

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4

0

Solid Melt T

Coal ash Co-firing Straw ash

Silicates (glass) Salts

Viscous Non-viscous

Cryst. Melt T

Mechanisms of Formation of Ash Melts

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Deposit Formation-1 Thermoforesis

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Deposit Formation-2 Condensation

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Deposit Formation-3 Chemical Reaction

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TubeFlue gasflow

Large ashparticle

Small ashparticles

TubeSticky ashdroplet

Dry ashparticle

Deposit Formation-4

Inertial impaction Deposit on tube wall

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Densification of Deposits/Agglomerates:

Sintering/consolidation of deposits/agglomerates

– Replacement of liquid silicate phases in deposits/ agglomerates – May be coupled to heat transfer through the deposit porosity and

may be modeled empirically

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Sintering in presence of a liquid phase dependent on the amount of liquid phase dependent on the viscosity of liquid phase

Ash Sintering Mechanism-1

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Reactive gas

Sintering through a solid-gas reaction dependent on the reaction rate dependent on the extent of reaction

Ash Sintering Mechanism-2

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Mechanism of Agglomeration in Fluidised Bed Systems

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Formation of Tars in Gasification Systems-1

Tar Content in the Producer Gas The product gas formed from biomass gasification contains the major components CO, H2, CO2, CH4, H2O, and N2, in addition to organic (tars) and inorganic (H2S, HCl, NH3, alkali metals) impurities and particulates. The organic impurities range from low molecular weight hydrocarbons to high molecular weight poly-nuclear aromatic hydrocarbons. The lower molecular weight hydrocarbons can be used as fuel in gas turbine or engine applications, but are undesirable products in fuel cell applications and methanol synthesis. The higher molecular weight hydrocarbons are known as “tar.”

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Formation of Tars in Gasification Systems-2

Tar Content in the Producer Gas A definition of tars is that “tars” are considered to be the condensable fraction of the organic gasification products and are largely aromatic hydrocarbons, including benzene. The diversity in the operational definitions of “tars” usually comes from the variable product gas compositions required for a particular end-use application and how the “tars” are collected and analyzed. Tars can condense in exit pipes and on particulate filters leading to blockages and clogged filters. Tars also have varied impacts on other downstream processes. Tars can clog fuel lines and injectors in internal combustion engines. Luminous combustion and erosion from soot formation can occur in pressurized combined-cycle systems where the product gases are burned in a gas turbine.

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Formation of Tars in Gasification Systems-3

Tar Content in the Producer Gas The producer gas from an atmospheric pressure gasification process needs to be compressed before it is burned in a gas turbine and tars can condense in the compressor or in the transfer lines. The removal of tars from the producer gas using catalysts as well as physical processes is problematic has high capital/operational cost that could reach 50% of the total gasification capital cost. Thermal tar cracking could be a more efficient and less costly tar elimination process but requires high temperatures (>1000oC) that in the case of biomass could cause significant ash-related problems. As a result a pretreatment technology that eliminates these problems could also assist significantly with the efficient removal of tars from the producer gas. 22

BIOMASS FUELS Woody fuels

Advantages: • Low to average ash problems

• Availability in large amounts

Disadvantages: • High moisture content

• High purchase cost

• Low grindability

• High transportation costs

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Energy crops

Advantages: • Environmental friendly

• Availability in large amounts

• Average transportation costs

Disadvantages:

• High alkali and chlorine content

• Low grindability

• High purchase cost

BIOMASS FUELS

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Agricultural residues/Food processing wastes

Advantages: • Low cost fuels

• Availability in large amounts

• Low transportation costs (Food processing wastes)

Disadvantages: • High alkali and chlorine content

• Low grindability

• In some cases high moisture content (Food waste)

BIOMASS FUELS

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Pre-treatment techniques for biomass fuels

• Fractionation (Conditions: D=1mm) • Leaching (Conditions: Use of Tap Water, Room Temperature, Biomass Specific Water/Mass Ratio, Application Time) • Fractionation + Leaching (Conditions: Combination of Former Two) • Torrefaction + Leaching (Conditions: Mild Decomposition in N2 Atmosphere, Temperature: 200-300oC, Time: Biomass Sample Dependent, Leaching Conditions)

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Why use pre-treatments in biomass fuels ?

Advantages: • Inexpensive

• Complete elimination of alkali, and chlorine in the pre-treated fuels. Substantial reduction of sulfur, phosphorus, and heavy metals such as mercury, lead, zinc, copper up to 70-80%

• Not sophisticated equipment required

• Elimination of ash-related problems

• Substantially lower grinding, mixing and transportation costs

Disadvantages: • Partial energy loss during the pre-treatment process

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Why apply the leaching pre-treatment in biomass fuels??

Advantages: • Inexpensive

• Complete elimination of chlorine, and reactive alkali, in the pre-treated fuels. Substantial reduction of sulfur, phosphorus, and heavy metals such as mercury, lead, zinc, copper up to 70-90%

• Not sophisticated equipment required

• Elimination of ash-related problems. Substantially lower transportation costs, Higher biomass availability

Disadvantages: • Leachate treatment after the pre-treatment process

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Biomass torrefaction

History: • First attempts made back in the 1930’s. The technology reached a demo stage during the 1980s

• Demo-scale technology developed by Pechiney in France for metallurgical applications (coke substitute)

• It has been also used in small scale for the production of charcoal for centuries around the World

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Main transformations during torrefaction

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Grinding energy after the torrefaction pre-treatment

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•A solid coal-like hydrophobic fuel, •A low to medium calorific value gas stream that can be combusted into a gas engine or be fed into the boiler. •Water uptake is limited to below 4% in the new fuel, •The coal-like solid bio-fuel contains 85-90% of the initial energy content, 60-70% of the initial mass.

Products of the torrefaction process

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SEM images of wheat straw samples: a) original straw, b)

torrefied straw

a b

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The Netherlands: ECN

Current status: • Laboratory scale testing has been conducted in the last 4 years, • Pilot scale testing is conducted, • Thoughts of commercial applications close to 2013, • Attempts are focused only on clean biomass fuels (woody biomass) mainly for co-combustion applications, • Efforts are being made with municipal waste materials.

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Germany: Choren industries Current status: • CARBO V technology: Uses higher temperatures (close to 500oC), cannot produce solid bio-fuel, • Pilot scale testing has been conducted, • Attempts are focused only on the gasification of clean biomass fuels (woody biomass) mainly for BTL and co-combustion applications, • Large commercial BTL plant under construction in Freiberg, Germany in cooperation with Volkswagen/Mercedes-Benz Groups.

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Sweden: Royal institute of technology, KTH, Stockholm

Current status: • Laboratory investigations are being performed, • No signs for pilot scale testing, • Attempts are focused only on the utilization of clean biomass fuels (woody biomass) mainly for co-combustion applications.

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Research focuses on: • Formation of second generation advanced solid bio-fuels through the physicochemical pre-treatment (torrefaction + leaching) and upgrade of low quality biomass, coal and waste materials • Production of solid bio-fuels for:

- Combustion, Co-combustion - Gasification, Co-gasification - Fast pyrolysis production of Bio-oil - Production of energy and liquid fuels

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Woody Biomass

•Torrefaction as pre-treatment • Grinding, blending with other biomass/coal fuels • Combustion, Co-combustion • Gasification, Co-gasification • Production of energy and liquid fuels

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Energy crops/ agricultural residues/food processing wastes • Torrefaction + Leaching as pre-treatment • Grinding, blending with other biomass/coal fuels • Combustion, Co-combustion • Gasification, Co-gasification • Production of energy and liquid fuels

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High-sodium poor coal/lignite

• Torrefaction+Leaching as pre-treatment • Grinding, blending with other biomass/coal fuels • Gasification, Co-gasification • Production of energy and liquid fuels

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Thermochemical pathways for energy and liquid fuels production • ACFB, PCFB combustion & co-combustion • PF combustion & co-combustion • ACFB, PCFB gasification & co-gasification • Entrained Flow: High Pressure (50bar), High Temperature (1800oC) Gasification & co-gasification • Combined cycle, Fischer-Tropsch (FT) synthesis

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Operating characteristics of boilers and gasifiers for energy

and fuels production Thermochemical Conversion Process

Operating Pressure (bar)

Operating Temperature (oC)

Atmospheric Combustion/Gasification

1 900

Atmospheric Combustion/Gasification

1 1300

Super Critical Combustion/OxyFuel

Combustion

1 1600+

Pressurized Combustion/Gasification

2-50 1000

High Temperature Pressurized Gasification

2-50 1600

Pyrolysis Production of Biooil/Char

1 600

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Suggested Removal Rates of Inorganics After the Leaching

Pretreatment-1

98

98

99

95

95

95

50

50

50

50

50

50

0 20 40 60 80 100 120

AtmosphericCombustion/Gasification

below 900oC

AtmosphericCombustion/Gasification

below 1300oC

PressurizedCombustion/Gasification 2-

50bar up to 1000oC

Removal Rates (%)

Ther

moc

hem

ical

Pro

cess

ReactivePhosphorusRemoval (%)Fuel

ReactiveSulfurRemoval (%)Fuel

ChlorineRemoval (%)Fuel

ReactiveAlkali MetalsRemoval (%)Fuel

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Suggested Removal Rates of Inorganics After the Leaching

Pretreatment-2

99

99

90

95

95

95

50

50

50

50

50

50

0 20 40 60 80 100 120

Super Critical Combustion/OxyFuelCombustion up to 1600oC

Pressurized Gasification 2-50 bar upto 1600oC

Pyrolysis Production of Biooil up to600oC

Removal Rates (%)

Ther

moc

hem

ical

Con

vers

ion

Pro

cess

ReactivePhosphorusRemoval(%) Fuel

ReactiveSulfurRemoval(%) Fuel

ChlorineRemoval(%) Fuel

ReactiveAlkali MetalsRemoval(%) Fuel

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Main properties of raw vs. Leached biomass

Thermochemical Conversion

Process

Original Biomass

Leached Biomass

Ash Content 100% -80%/+10% HHV 100% +5% -+20%

Chlorine Content

100% -99%

Reactive Alkali Metals Content

100% -99%

Reactive Sulfur Content

100% -50%

Reactive Phosphorus

Content

100% -50%

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Conclusions-1

Based on a detailed survey of the literature and long years of original research, we have identified a number of critical factors affecting the successful conversion of biomass feedstocks from agricultural and energy crop sources to energy and fuels by thermochemical methods. The list includes: * The type, composition, availability and sustainability of the bioresource; * The type and operating conditions of the conversion technology;

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Conclusions-2

* The composition and chemistry of the biomass ash, and particularly

- The melting point of the ash; plus the main - Ash-related problems, such as corrosion, deposition,

agglomeration and sintering; * The tar content of the producer gas; * The milling and feeding behavior of the biomass * The particle size of the feedstock; and * The mixing behavior of biomass.

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Conclusions-3

To meet the needs of the conversion and optimize the feedstock for one of more of the above factors, a number of biomass pretreatments appear to show promise for large-scale application, including: * Biomass leaching; * Biomass Torrefaction (pre-pyrolysis/low temperature gasification); and * Their combinations.

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ASH: FROM MYTHOLOGY TO SCIENCE • “It is only the organics that matter” • “Ash as a inert component” • Problems correlated with ash content • “Science too complex” to study/understand • “Phenomena too complex” to follow/predict • The ash melting point • Managing by mixing • Ash-bed material interactions • The promise of a catalytic role • The emergence of pretreatments • Towards an improved agenda of feedstock refining

Implications for Large-Scale Implementation

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0

10

20

30

40

50

60

0 5 10 15 20 25 30 35 40 45 50 55 60 65

Cl+K+Na in ash, wt%

tot

rele

a, 400 t

o 1

450o

C, %

H4

H1* H1

H8* H11

H12

H3

H2 H10 H8

H9

H7

H5

(Ca) (Ca)

(Ca) (Ca)

(Ca)

Total weight loss as a function of the Cl+K+Na content

*New ash measurement 50

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