Training on Technologies for Converting Waste Agricultural Biomass
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 ü Overview ü Volatiles and Fixed Carbon ü Combustion Controlling Factors ü Direct Combustion Technologies ü Comparison of different types of Direct Combustion Technologies 2
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. 3
INTRODUCTION § Thermo-chemical conversion routes 4
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 CHx. Oy. Nz. Su. – 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: CHx. Oy 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. 5
COMBUSTION OF BIOMASS § Overview 6
COMBUSTION OF BIOMASS § Overview Air Flaming Combustion Volatile Matter Air Flue Gas Heat Radiation to Fuel Glowing Combustion Fuel Heat Conduction to Fuel Heat Flue Gas Heat Conduction Light Convection Infrared Radiation Fixed Carbon (Char) Air 7
COMBUSTION OF BIOMASS § Volatiles and Fixed Carbon – Volatiles: Flaming combustion – Fixed carbon: Glowing combustion 8
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. 9
COMBUSTION OF BIOMASS § Direct Combustion Technologies • Grateless combustors, • Pile burning combustors, • Stoker burning combustors, • Suspension burners, and • Fluidized burners. 10
COMBUSTION OF BIOMASS § Comparison of Direct Combustion Technologies Applicatio n Manual Pellets Wood stoves Typical Size Fuels (MW) 0. 002 - 0. 01 Dry wood logs Ash (%) <2 Water content (%) 5 - 20 Log wood boilers 0. 005 - 0. 05 Log wood, sticky wood residues <2 5 - 30 Pellet stoves and boilers 0. 002 - 0. 025 Wood pellets <2 8 - 10 Type Understoker furnaces 0. 02 - 2. 5 Wood chips, wood residues All wood fuels and most biomass Dry wood (residues) <2 5 - 50 Moving grate furnaces 0. 15 - 15 < 50 5 - 60 <5 5 - 35 Wood chips, high water content < 50 40 - 65 Straw bales Whole bales Straw bales with bale cutter Various biomass, d < 10 mm <5 <5 <5 < 50 20 20 20 5 - 60 Circulating fluidized bed 15 - 100 Various biomass, d < 10 mm Dust combustor, entrained flow 5 - 10 Various biomass, d < 5 mm Stationary fluidized bed total 50 - 150 Various biomass, d < 10 mm < 50 <5 < 50 5 - 60 < 20 5 -60 < 50 <5 5 - 60 20 <5 < 20 Pre-oven with grate Understoker with rotating grate Automatic Cigar burner Whole bale furnaces Straw furnaces Stationary fluidized bed Circulating fluidized bed Co-firing* Cigar burner straw 0. 02 - 1. 5 2 -5 3 -5 0. 1 - 5 5 - 15 total 100 - 300 Various biomass, d < 10 mm straw 5 – 20 Straw bales total 100 – Dust combustor in coal boilers Various biomass, d < 2 -5 mm 1000 11
COMBUSTION OF BIOMASS § Comparison of different types of direct combustion technologies Parameter Pile Combustion Stoker Combustion Suspension Fluidized Bed Combustion Grate Fixed / Stationary Grate Fixed or moving grate No grate or No grate moving grate Fuel Size Uniform size of the fuel in Uneven fuel size can be Preferable for high Uniform size fuel in the range of 60 - 75 mm is used % of fins in the range of 1 - 10 mm desired & % fines should fuel not be more than 20% Combustion Difficult to maintain The combustion is better It is similar to Best combustion takes good combustion due to : & an improved version stoker combustion, place in comparison –Air fuel mixing is not of pile combustion. but since the fuel with the other types proper, Since most of the fuel is sizes is small & since the fuel particles –Bed height is in stationary burnt in suspension the even the are in fluidized state & condition resulting in heavier size mass falls combustion there is adequate clinker formation, on the grate. If the efficiency is mixing of fuel & air. –Difficult to avoid air system has a moving improved as channeling grate the ash is removed maximum amount –Due to intermittent ash on a continuous basis & of fuel is removal system it is therefore the chances of combusted during difficult to maintain good clinker formation are suspension. combustion. less. 12
COMBUSTION OF BIOMASS § Comparison of different types of direct combustion technologies Parameter Bed temperature Moisture Draft Conditions Maintenance Pile Combustion Stoker Combustion 1250 - 1350 ºC 1000 - 1200 ºC Suspension Combustion 1250 - 1350 ºC High moisture leads to bed Combustion condition Same as Stoker choking & difficult not very much disturbed Combustion conditions with 4 -5 % increase in moisture Natural Draft / Forced Draft/ Balance Draft Not much maintenance problems Fluidized Bed Combustion 800 - 850 ºC 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. Balance draft Forced Draft / Balance draft Frequent problems due Variation in fines Erosion of boiler tubes to moving grate in fuel leads to embedded in the bed delayed is quite often combustion thereby affecting the boiler tubes 13
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