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Fuels Inventory and Management

by: Ms. Alene Howell

Fuels Inventory and Management FOR 451

Ms. Alene Howell
GPA 3.99

Chad Hoffman

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Chad Hoffman
Class Notes
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This 24 page Class Notes was uploaded by Ms. Alene Howell on Friday October 23, 2015. The Class Notes belongs to FOR 451 at University of Idaho taught by Chad Hoffman in Fall. Since its upload, it has received 15 views. For similar materials see /class/227824/for-451-university-of-idaho in Natural Resource Ecology And Mgmt at University of Idaho.


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Date Created: 10/23/15
Intrinsic Fuel Properties Part 1 Chad Hoffman The goal of us looking at the intrinsic fuel properties is ultimately so we can discuss the flammability of vegetation We are starting with this topic for a few reasons First an understanding ofthe physical and chemical makeup of burning material will aid us in thinking about fire at larger scales Second although the material ofthis section is more difficult it will provide us a basis from which to work from for the remainder ofthe class With these ideas in mind we should recognize that this information is highly focused on the burning of one fuel element at a time For example how does a ponderosa pine needle burn compared to a lodgepole pine needle To answer these questions we will have to look into the chemical and physical principles of combustion with some detail For most of this lecture I will focus on the properties of wood if only because so much work has gone into this area but where information exists I will expand away from wood to look at needles grasses and shrubs as well O A look back on combustion o Combustion 0 Is a chemical reaction which consumes the products of photosynthesis Example Combustion Engine 0 Uses a carbon based fuel and mixes it with O2 Spark plug provides a heat source Byproducts are released Image from dkimagescom In the most simple terms combustion is a chemical reaction which breaks down some material called the fuel In our case the fuel material is often a product of photosynthesis note that in this class we will not cover anything about synthetic materials One example of the combustion process is a engine In this case a carbon based fuel gasoline is mixed with oxygen in your engine and heated by a spark plug This reaction ultimately releases heat energy as well as some byproducts E Dehydration o Dehydration of the fuel occurs as the temperature of the fuel is raised to the point where the water molecules are converted to gas and driven off of the fuel water Oa pilot heat Image from Cantrell 2004 Every combustion reaction begins with a heat source In most cases this heat source is called a pilot heat source because it is independent ofthe fuel element we are interested in Once a heat source is applied the heat will be transferred to the fuel element As this occurs the fuel will begin to dehydrate Dehydration occurs as the temperature ofthe fuel is raised to the point where the water molecules in the fuel element are converted to a gas steam and are released from the fuel I Pyrolysis o Pyrolysis is the thermal breakdown of the fuel particles Long polymeric molecules are broken down into smaller molecular weight gasses and semi volatile tar and solid char particles fuel gas cloud lmage frnm39 Cantrell 70m plol heat After the water molecules are removed from the fuel element pyrolysis will begin Pyrolysis is the thermal breakdown of long chain molecules into semi volatile tar and solid char particles As these gasses are released from the fuel element they form a combustible gas cloud around the fuel Ignition o Ignition is a transition point between the pre ignition process of combustion and combustion its self At this point the heat supplied by the pilot source is no longer needed After pyrolisis there is a transition stage between the endothermic reaction and the exothermic reaction called ignition This transition point happens when the chemical reaction no longer needs a pilot heat source and begins to release enough heat energy to support the chemical reaction m Flaming Combustion 0 During flaming combustion the volatile compounds released in the preignition phases mix with oxygen ignite and create a visual flame Image from Cantrell 2004 After ignition occurs the fuel element can have several different types of combustion occurring Flaming combustion occurs when the volatile compounds released in pyrolysis mix with oxygen causing the fuel gas cloud to ignite and create a visual flame m A closer look at flames o Diffusion flames the fuel and oxidizer are separate and must mix before ignition can O u r 3750 dioxide carbon Mamie gem quot co M oxygen AN 0 mm zone combuslion oxygen r 39iClIOl ZONE zone images from romrell 700A Let39s take a closer look at the flame produced during flaming combustion In wildfires the flames are classified as diffusion flames because the fuel and oxygen are separate and must mix before ignition can occur The area where this mixing process occurs is called the combustion reaction zone Since these flames require the fuel and oxygen to mix the flames are not directly in contact with the fuel element To test this you can look at the burning head of a match or candle Glowing Combustion 0 During smoldering or glowing combustion the combustion reaction zone collapses to the fuel surface 0 The heat generated from glowing combustion pyrolzes adjacent fuel which sustains the process it also forms a whitish mineral ash rwa carbon built0 lmag frnm39 CnnIrPIl 7mm lfthe release of combustible particles from the fuel element is lowered the reaction zone can collapse to the fuel surface lfthis occurs flaming combustion will stop and glowing combustion will occur During glowing combustion the heat generated can sustain the combustion process it also forms a whitish mineral ash During this process the fuel element is still producing visible light from the heat release s m Smolderlng Combustion o Smoldering combustion occurs when no flame or glowing combustion is present but enough heat is available to continue to breakdown the fuel tarry smoke ax gen 9 mineral 399 ash barrier J lt Image from Cantrell 70nd When the heat produced during the breakdown of material no longer produces visible light but still has enough heat to continue the breakdown ofthe fuel element we have smoldering combustion A note on pyrolsis o In almost all cases pyrolysis is necessary to yield products which can volatilize from the surface of the fuel element and enter the flame The composition of the volatiles which enter the flame is dependent upon the chemical nature of the solid fuel In most cases the material which is combusted in a flame are the products from the pyrolysis ofthe fuel element Therefore understanding the chemical nature of fuels is directly related to the behavior of the flame A brief look at fuel chemistry 0 Polymeric materials consist of long chains of repeated units which are derived from simple molecules known as monomers These materials often have high molecular weights 0 Pyrolysis breaks the bonds which link the monomers which releases materials with lower molecular weights The vapor which is released from flames comes from the breakdown of polymeric materials These materials are essentially long chains of repeated units called monomers Polymeric materials are broken down in pyrolysis into materials with lower molecular weights 3 39 a Mode for generating fuel vapors Sublimation evaporation melting evaporation Different modes by which fuel vapors are generated from a solid fuel Source Introduction to Fire Dynamics 2nd edD Drysdale 1999 New York John Wiley amp Sons Limited Note that the vapors released from a fuel element are ultimately what are consumed in a flame There are several ways in which a solid fuel element can produce these vapors In some cases the solid will melt first and then the liquid will produce the vapors forflaming combustion such as in a candle Sublimation occurs when a material is transformed directly into a gas However most solid fuels do not evaporate directly into a vapor Most solid fuels such as wood will go through pyrolysis where the components are being decomposed due to heat This process yields volatile vapors and solid char g quot A question to think about 0 Does wood actually burn 30 based on where we are at this point in time in this class I would like to ask you a question Does wood really burn Take a few minutes to think about this The quick answer is yes wood burns however if you think about the process we have been discussing the wood is simply decomposed and the vapors burn Particle flammability o Flammability 4 components Ignitability is the time until ignition once exposed to a heat source Sustainability is the ability to sustain fire once ignited Combustibility is the rate of burning after ignition Consumability is the proportion of mass consumed At this point in time lets step back and remember the question we are looking at Ultimately what we are investigating here is the flammability of a fuel particle independent of the plant structure That is how do the intrinsic properties of the fuel element influence flammability Flammability is made up of four components The first three were defined by Hal Anderson in 1970 and consumability was later added by Martin and others in 1994 g quot Chemical components of plants a broad groups Phenolic compounds Lignin and tannin 23 33 cellulose 3550 Hemi cellulose 15 25 0 Extractives 245 in the foliage Mineral ash very low concentrations and are not combustible The most obvious intrinsic fuel property is its chemical makeup In plants we can often think about there being five broad groups of chemicals Cellulose and hemi cellulose phenol compounds such as Iignin and tannin extractives and mineral ash m cellulose o Is made up of polymers of hexose sugar D glucose in a linear structure Produce much of the vapor fuel consumed in flaming combustion Cellulose Chlon 7 if 0 CH H H OH We q me o l 39ln o H o i1 f oEo H OH CH2OH H on cup Celloblou u 3mm Cellulose Cellulose is the major polymer in plant material making up about 50 ofthe plant by weight The pyrolysis of cellulose is responsible for much ofthe vapor material consumed in flaming combustion m hemicellulose o The structure of hemicellulose is similar to that of cellulose but is based on pentose sugars instead of glucose Structure of H emicellulose As i 3 REG 0 Mw1o voltloh m C u E c Mala Sa wood H emxcellulose Gdulogucommnm DP 200 4 Glucase ti 1 Manmse o Hemicellulase is a rmo G mates 5 bmchedpal vmer Ac U Acequ Group Hemi cellulose is similar in structure to cellulose except it is based on pentose sugar The behavior of hemi cellulose during pyrolosis is also similar to cellulose m Lignin and tannins o Lignin is an aromatic polymer that does not degrade as easily as cellulose and hemicelluose R o f l H300 quot OCHajl OH Lignin Monomer Lignin is an aromatic polymer that does not degrade as easily as cellulose and hemi cellulose during pyrolysis We will look at this later on in the lecture g quot Extractives o Extractives consist of organic compounds such as sugars resins alcohol hydrocarbons and fatty acids 0 Can be highly flammable and are often the first to be volatilized during the preignition phase of combustion Extractives consist of organic compounds such as sugars resins alcohol hydrocarbons and fatty acids These materials can be highly flammable and are often the first products to be volatilized during pre ignition phase of combustion m Mineral ash 0 Minerals such as Si Ca Mg K are found in low concentrations and are not combustible Pure mineral elements such as silica and magnesium are found in low concentrations within plants These compounds are often not combustible with the heat generated during a wildland fire 20 A a Physical properties of wood 0 Density of a material is defined to be the mass per unit volume a The density that makes up a wood cell wall has been found to be around 15 gcm3 However a piece of wood also contains air within the cell so most woods have a density of less than 1 gcm3 Pine needles have been been reported to have particle densities of around 6 gcm3 I would now like to start looking at a few of the physical intrinsic properties of fuel Namely I want to look at the density and specific gravity of wood Density is defined as the mass per unit volume Wood cells have a density of around 15 gcm3 however a piece of wood also has air in the cell so the actual density when measured typically is below 1 gcm3 Pine needles have higher densities typically around 6 gmcm3 The particle density of many materials can be found in the scientific literature and reference textbooks 21 I Specific gravity 0 Specific gravity is a measure of the amount of solid cell wall substance this is sometimes called a relative density 0 It is the ratio of density of a substance to the density of water 0 The specific gravity of wood can range from 035 to 065 for most woods in the US Exotic woods have a wider range from 004 to 140 Specific gravity is a measure ofthe amount of solid cell wall substance this term is often referred to as relative density This value is physically the ratio of density of a substance to the density of water The specific gravity of wood ranges from 035 065 for most woods in the US Exotic woods have a wider range from 004 to 140 22 g quot Thermal properties of wood 0 Thermal conductivity k o Is a measure of the rate of heat flow in response to a temperature gradient In wood k depends upon the direction of heat flow with respect to the grain Let39s now look at a few ofthe thermal properties of wood Thermal conductivity is a measure ofthe rate in which heat flow will occur in response to a temperature gradient For wood species this rate will depend upon the direction of heat flow with respect to the grain orientation 23


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