1. Thermo-chemical Conversion Technologies The Basics
Training on
Technologies for Converting Waste Agricultural Biomass into Energy
Organized by
United Nations Environment Programme (UNEP DTIE IETC)
23-25 September, 2013
San Jose, Costa Rica
Thermo-chemical Conversion Technologies The Basics
Surya Prakash Chandak
Senior Programme Officer
International environmental Technology Centre
Division of Technology, Industry and Economics
Osaka, Japan

2. CONTENT Introduction Combustion of Biomass Overview
Volatiles and Fixed Carbon
Combustion Controlling Factors
Direct Combustion Technologies
Comparison of different types of Direct Combustion Technologies

3. INTRODUCTION
Thermo-chemical conversion methods represent one of the two main categories of biomass energy conversion technologies.
Among the two categories, these represents the most widely used energy conversion technologies in all form of fuels.
Could be categorized as:
Direct combustion, Gasification, Liquefaction and
Pyrolysis (carbonization, destructive distillation & fast pyrolysis)
Basically used to derive energy or intermediate fuel with improved properties from a primary fuel
Involve complex chemical reactions during which devolatilization and cracking take place.

4. INTRODUCTION
Thermo-chemical conversion routes

5. COMBUSTION OF BIOMASS
Biomass materials mainly comprised of cellulose, hemicelluloses and lignin, each consists of three basic elements: C, O and H.
Further, elements such as S and N could be present.
In such case, the chemical equation can be expressed in a very convenient form, as a stoechiometry formula written for one atom of carbon as CHxOyNzSu.
For pure and dry biomass fuels of the ligno-cellulosic type, nitrogen and sulfur are usually negligible and the chemical formula may be rewritten as follows: CHxOy with x ≅1.44 and y ≅0.66 describing the average composition of typical biomass used for combustion, i.e., wood, straw, or similar material.

6. COMBUSTION OF BIOMASS
Overview

7. COMBUSTION OF BIOMASS Overview Air Heat Conduction to Fuel
Fixed Carbon (Char)
Volatile Matter
Fuel
Heat
Glowing Combustion
Flaming Combustion
Flue Gas
Heat Radiation
to Fuel
Light
Infrared Radiation
Conduction
Convection
Radiation

8. COMBUSTION OF BIOMASS Volatiles and Fixed Carbon
Volatiles: Flaming combustion
Fixed carbon: Glowing combustion

9. Combustion Controlling Factors
COMBUSTION OF BIOMASS
Combustion Controlling Factors
Physical and chemical properties of the fuel;
Fuel/air ratio;
Temperature of the flame/envelope;
Mode of fuel supply;
Primary and secondary air supplies.

10. Direct Combustion Technologies
COMBUSTION OF BIOMASS
Direct Combustion Technologies
Grateless combustors,
Pile burning combustors,
Stoker burning combustors,
Suspension burners, and
Fluidized bed burners.

11. COMBUSTION OF BIOMASS Comparison of Direct Combustion Technologies
Application
Type
Typical Size (MW)
Fuels
Ash
(%)
Water content (%)
Manual
Wood stoves
Dry wood logs
< 2
5 - 20
Log wood boilers
Log wood, sticky wood residues
5 - 30
Pellets
Pellet stoves and boilers
Wood pellets
8 - 10
Automatic
Understoker furnaces
Wood chips, wood residues
5 - 50
Moving grate furnaces
All wood fuels and most biomass
< 50
5 - 60
Pre-oven with grate
Dry wood (residues)
< 5
5 - 35
Understoker with rotating grate
2 - 5
Wood chips, high water content
Cigar burner
3 - 5
Straw bales 20
Whole bale furnaces
Whole bales
Straw furnaces
Straw bales with bale cutter
Stationary fluidized bed
5 - 15
Various biomass, d < 10mm
Circulating fluidized bed
Dust combustor, entrained flow
5 - 10
Various biomass, d < 5mm
< 20
Co-firing*
total
5 -60
total
Cigar burner straw
straw 5 – 20
Dust combustor in coal boilers
total 100 – 1000
Various biomass, d < 2-5mm

12. Suspension Combustion
COMBUSTION OF BIOMASS
Comparison of different types of direct combustion technologies
Parameter
Pile Combustion
Stoker Combustion
Suspension Combustion
Fluidized Bed
Combustion
Grate
Fixed / Stationary Grate
Fixed or moving grate
No grate or moving grate
No grate
Fuel Size
Uniform size of the fuel in the range of mm is desired & % fines should not be more than 20%
Uneven fuel size can be used
Preferable for high % of fins in the fuel
Uniform size fuel in the range of mm
Difficult to maintain
good combustion due to :
Air fuel mixing is not proper,
Bed height is in stationary condition resulting in clinker formation,
Difficult to avoid air channeling
Due to intermittent ash removal system it is difficult to maintain good combustion.
The combustion is better & an improved version of pile combustion. Since most of the fuel is burnt in suspension the heavier size mass falls on the grate. If the system has a moving grate the ash is removed on a continuous basis & therefore the chances of clinker formation are less.
It is similar to stoker combustion, but since the fuel sizes is small & even the combustion efficiency is improved as maximum amount
of fuel is combusted during suspension.
Best combustion takes place in comparison with the other types since the fuel particles are in fluidized state & there is adequate mixing of fuel & air.

13. Suspension Combustion
COMBUSTION OF BIOMASS
Comparison of different types of direct combustion technologies
Parameter
Pile Combustion
Stoker Combustion
Suspension Combustion
Fluidized Bed
Combustion
Bed
temperature ºC ºC ºC
Moisture
High moisture leads to bed choking & difficult combustion conditions
Combustion condition not very much disturbed with 4-5 % increase in moisture
Same as Stoker Combustion
It can handle fuels with high moisture condition up to % but high moisture in the fuels is not desirable, & adequate precautions are to be taken up in the design stage itself.
Draft
Conditions
Natural Draft / Forced Draft/ Balance Draft
Forced Draft / Balance draft
Balance draft
Maintenance
Not much maintenance problems
Frequent problems due to moving grate
Variation in fines in fuel leads to delayed combustion thereby affecting the boiler tubes
Erosion of boiler tubes embedded in the bed is quite often

14. The End