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

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

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.

INTRODUCTION Thermo-chemical conversion routes

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.

COMBUSTION OF BIOMASS Overview

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

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

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.

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

COMBUSTION OF BIOMASS Comparison of Direct Combustion Technologies Application Type Typical Size (MW) Fuels Ash (%) Water content (%) Manual Wood stoves 0.002 - 0.01 Dry wood logs < 2 5 - 20 Log wood boilers 0.005 - 0.05 Log wood, sticky wood residues 5 - 30 Pellets Pellet stoves and boilers 0.002 - 0.025 Wood pellets 8 - 10 Automatic Understoker furnaces 0.02 - 2.5 Wood chips, wood residues 5 - 50 Moving grate furnaces 0.15 - 15 All wood fuels and most biomass < 50 5 - 60 Pre-oven with grate 0.02 - 1.5 Dry wood (residues) < 5 5 - 35 Understoker with rotating grate 2 - 5 Wood chips, high water content 40 - 65 Cigar burner 3 - 5 Straw bales 20 Whole bale furnaces Whole bales Straw furnaces 0.1 - 5 Straw bales with bale cutter Stationary fluidized bed 5 - 15 Various biomass, d < 10mm Circulating fluidized bed 15 - 100 Dust combustor, entrained flow 5 - 10 Various biomass, d < 5mm < 20 Co-firing* total 50 - 150 5 -60 total 100 - 300 Cigar burner straw straw 5 – 20 Dust combustor in coal boilers total 100 – 1000 Various biomass, d < 2-5mm

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 60 - 75 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 1 - 10mm 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.

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 1250 - 1350 ºC 1000 - 1200 ºC 800 - 850 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 45 - 50% 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

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