Thermochemical Conversion of Biomass

Hesam Fatehi

Division of Fluid Mechanics

Outline

• Introduction

• Biomass to bioenergy

• Fixed bed combustion

• Single particle model

• Result and discussion

Biomass

• Biomass is biological material from living, or recently living organisms, most often referring to plants or plant-derived materials*

• Biomass to energy – thermal conversion

• combustion, torrefaction, pyrolysis, and gasification

– chemical conversion

– biochemical conversion

CPH, Co-firing

High energy density solid fuel

Bio-oil and Char Syngas

Digestion gas *

http://en.wikipedia.org/wiki/Biomass

thermal conversion

Gas

Tar

Char

Heat

Biomass 𝐶6𝐻0.2−1𝑂6.2−9

𝑌𝑗𝑗

𝑗: 𝐶𝑂,𝐶𝑂2 ,𝐻2𝑂𝐶𝐻4 ,𝐻2 ,𝐶2𝐻4

Mainly 𝐶

How to Formulate Biomass

Biomass characteristics

Lu, H. and L. L. Baxter (2009). "Biomass Combustion Characteristics and Implications for Renewable Energy."

Biomass vs. Coal

Biomass particles are typically • Much larger than pulverized coal particles • Contain much greater volatile content • Much higher moisture level than coal. • Lower densities than coal particles, commonly differing by a factor

of 4–7 • Higher oxygen, lower carbon, and lower sulfur content • More widely ranging (0.1–25%) ash contents • Lower heating values (by about half on an as-delivered basis) • Much lower bulk energy densities (by more than an order of

magnitude) • Higher alkali content dominated by potassium

Sweden http://www.esru.strath.ac.uk/EandE/Web_sites/06-07/Biomass/HTML/sweden_case_study.htm

Key Statistics:

Area: 449,964 km²

Forest Cover: 227,000 km² (50.5%) Population: 9,110,972

Biomass Utilisation: 14% of primary energy use. 89 TWh in 2002.

Biomass Applications:

The utilization of biomass as a fuel source within the forestry industry accounts for

57% total biomass use.

District heating is 30% individual small scale domestic heating 13%.

approximately 200 district heating plants in Sweden with a total capacity

of 24GW.

Type of Biomass Predominantly Used:

Forestry by-products such as bark and woodchips.

Government Support:

The energy tax reform of 1991 introduced a carbon and energy tax. This instantly made biomass the preferred fuel for district heating applications since the cost of using fossil fuels rose from between 30-60%. In 2004 the carbon tax was 100€/tone CO2. 11% of R&D funding since 1975 directed at bio-energy.

Biomass to Energy

• Fixed Bed Combustion – underfeed stokers

• small-scale systems • cheap and safe • Only low ash content fuels such as wood chips

– grate firings • fixed grates, moving grates, rotating grates and travelling grates • fuels with high moisture and ash content as well as with varying fuel sizes.

• Fluidized Bed Combustion Systems – Bubbling Fluidized Bed (BFB) Furnaces

• air is fed into the chamber with a velocity of between 1 and 2.5m/s. • The bed normally has a temperature of between 800 and 900°C • practical option with larger plants with a nominal boiler capacity greater than 10 MWth.

– Circulating Fluidized Bed (CFB) Furnaces • air velocity 5-10m/s • CFB’s deliver very stable combustion conditions • cost is relatively high • problems involved with fuel size, • difficulties involved in running them at partial load. • boiler capacity of over about 30MWth.

Yin, C., et al. (2008). "Grate-firing of biomass for heat and power production." Progress in Energy and Combustion Science 34(6): 725-754.

Fluidized bed

Gómez-Barea, A. and B. Leckner (2010). "Modeling of biomass gasification in fluidized bed." Progress in Energy and Combustion Science 36(4): 444-509.

Fixed Bed Combustion Modeling

Yin, C., et al. (2008). "Grate-firing of biomass for heat and power production." Progress in Energy and Combustion Science 34(6): 725-754.

Length and Time scales involved

• (1) Fluid-flow

• (2) Particle

• (3) Multi-particle

Hermansson, S. and H. Thunman (2011). "CFD modelling of bed shrinkage and channelling in fixed-bed combustion." Combustion and Flame 158(5): 988-999.

Thermochemical Conversion of Biomass

• When biomass particle exposes to heat – Moisture evaporation

– Pyrolysis or devolatilization • Gaseous products can burn in the particle boundary

• Char (fixed carbon) is formed in this stage

– Char oxidation/gasification • Depending on oxygen level

– Particle shrinkage

– Change in thermo-physical properties • Conductivity, diffusivity, Specific heat, porosity, permeability

….

Governing Equations

• Assume the particle as a porous media

• Temperature of the gas and solid structure are in equilibrium

• Species can diffuse into the porous structure

• Due to formation of gas inside the particle pressure can increase

• Particle density remains constant

Electron microscope Images of char

http://biocharproject.org/education-2/biochar-electron-microscope-images/#!prettyPhoto

Governing Equation

Evaporation Process

• Heat flux model

– Assumption is evaporation is in a constant temp

– All the heat reaches to the particle goes to evaporation

• Equilibrium model

– equilibrium between water vapor and liquid water

Pyrolysis

• Thermochemical decomposition of organic material at high temperatures without the participation of oxygen

• Change of chemical composition and physical phase

• Product of pyrolysis – gaseous products

– bio-oil or tar

– solid residue or char

Kinetic of Pyrolysis Biomass Cellulose (%) Hemicellulose (%) Lignin (%)

Hardwood 40-55 24-40 18-25

Hardwood 50 33 17

Hardwood 39 35 20

Softwood 45-50 25-35 25-35

Softwood 41 24 28

Aspen Bark 52 27 21

Bagasse 36 47 17

Bagasse 43 33 24

Beech 48 28 24

Beech 78* 25

Birch 45 33 22

Birch 76* 21

Corn cobs 45 35 15

Corn stover 36 49 15

Crude Cellulose 92 0 8

Maple 40 38 22

Maple 48 26 26

Wheat straw 30 50 15

Oak 35 40 25

Oak 68.5* 28

Olive husk 22 33 45

Peat 15 19 66

Peat 10 32 44

Pine 50 27 23

Pine 50 27 23

Pine 65* 30

Pine 69* 24

Pine bark 34 16 34

Poplar 48 30 22

Poplar-aspen wood 47 35 18

Redwood 56* 33

Rice Straw 35 25 17

Rice husk 30 25 12

Spruce 69* 29

Sugarcane bagasse 40 24 25

Switchgrass 45 30 12

Wheat straw 36 46 18

Wheat straw 40 28 17

Biomass Components

Reactions

Gómez-Barea, A. and B. Leckner (2010). "Modeling of biomass gasification in fluidized bed." Progress in Energy and Combustion Science 36(4): 444-509.

Example of Results

Experimental data from: Lu, H., et al., Comprehensive Study of Biomass Particle Combustion. Energy & Fuels, 2008. 22(4): p. 2826-2839.

Example of Results

Example of Results

Other Issues

• Heavy metal release; Deposit formation and corrosion

Figures form: Yin, C., et al. (2008). "Grate-firing of biomass for heat and power production." Progress in Energy and Combustion Science 34(6): 725-754.

Alkali Metal Release

CH /O /CO4 2 2

CO2

Shielding ring

Other Issues

• Pollutant emissions:

– The incomplete combustion

• emissions of CO, hydrocarbons (CxHy), tar, poly aromatic hydrocarbons (PAH) and incompletely burned char

– HCl, SOx, heavy metals

– NOx; NH3, HCN, and NO

References

• http://en.wikipedia.org/wiki/Biomass • http://www.esru.strath.ac.uk/EandE/Web_sites/06-07/Biomass/HTML/combustion_technology.htm. • Di Blasi, C. (2008). "Modeling chemical and physical processes of wood and biomass pyrolysis." Progress in Energy

and Combustion Science 34(1): 47-90. • Doherty, W. O. S., et al. (2011). "Value-adding to cellulosic ethanol: Lignin polymers." Industrial Crops and Products

33(2): 259-276. • Gómez-Barea, A. and B. Leckner (2010). "Modeling of biomass gasification in fluidized bed." Progress in Energy

and Combustion Science 36(4): 444-509. • Hermansson, S. and H. Thunman (2011). "CFD modelling of bed shrinkage and channelling in fixed-bed

combustion." Combustion and Flame 158(5): 988-999. • Lu, H. and L. L. Baxter (2009). "Biomass Combustion Characteristics • and Implications for Renewable Energy." • Yin, C., et al. (2008). "Grate-firing of biomass for heat and power production." Progress in Energy and Combustion

Science 34(6): 725-754.

• http://biocharproject.org/education-2/biochar-electron-microscope-images/#!prettyPhoto

Thank you for your attention