Critical factors for the efficient conversion of biomass to energy & fuels by thermochemical processesArvelakis Stelios & Koukios Emmanouel School of Chemical Engineering
NTUA
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δεια Χρσης
• 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
• 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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• 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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• 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
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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Silicates (glass) Salts
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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
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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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
• Availability in large amounts
Disadvantages: • High moisture content
• Low grindability
• Low grindability
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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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
Disadvantages: • Partial energy loss during the pre-treatment process
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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
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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•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
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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and fuels production Thermochemical Conversion Process
Operating Pressure (bar)
Operating Temperature (oC)
Pretreatment-1
98
98
99
95
95
95
50
50
50
50
50
50
Atmospheric Combustion/Gasification
below 900oC
Atmospheric Combustion/Gasification
below 1300oC
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Pretreatment-2
99
99
90
95
95
95
50
50
50
50
50
50
Removal Rates (%)
Th er
m oc
he m
ic al
C on
ve rs
io n
P ro
ce ss
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Thermochemical Conversion
Chlorine Content
100% -99%
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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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Cl+K+Na in ash, wt%
to t
re le
o 1
(Ca)
Total weight loss as a function of the Cl+K+Na content
*New ash measurement 50
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Thermochemical TechnologiesCritical factors for the efficient conversion of biomass to energy & fuels by thermochemical processes
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Why Feedstocks Matter ?
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