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Fire Ecology and Management

by: Ms. Alene Howell

Fire Ecology and Management FOR 426

Ms. Alene Howell
GPA 3.99

Chad Hoffman

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Chad Hoffman
Class Notes
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This 29 page Class Notes was uploaded by Ms. Alene Howell on Friday October 23, 2015. The Class Notes belongs to FOR 426 at University of Idaho taught by Chad Hoffman in Fall. Since its upload, it has received 31 views. For similar materials see /class/227825/for-426-university-of-idaho in Natural Resource Ecology And Mgmt at University of Idaho.

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Date Created: 10/23/15
ssi FOR 426 Flre as a Physlcal Process Chad Hoffman We are going to start this class of by reviewing the idea of fire as a physical and chemical process By understanding the chemical and physical mechanisms which allow fire to release the energy stored in the bonds of fuel we can gain valuable insights into the effects offire Following this section we will focus further on fire as an ecological process and then investigate the effects of fire on specific ecosystem attributes such as plant populations wildlife and air and stream quality Outline for this lecture 0 Producer and Consumer Chemical Reactions Fire as a Chemical Reaction The phases of combustion Heat transfer mechanisms Specifically we are going to discuss the chemical reactions involved in producing and consuming combustible material and than discuss the process of combustion followed by heat transfer mechanisms involved in wildfires Throughout this lecture we are going to focus on the chemical and physical processes involved and try to relate these to specific laws of physics and chemical and mathematical equations Production Reaction o Photosynthesis Is the process in which carbon dioxide coz and Water H20 are used to produce carbohydrates and evolve oxygen 02 in the presence of light and chlorophyll Pmmsvmansls mm Mum mum nloxm wm Imaue ram Photosynthesis is the process by which light energy is captured by green plants and used to synthesize reduced carbon compounds from carbon dioxide and water The role of photosynthesis can not be overemphasized since nearly all the energy entering the biotic portion of the biosphere is derived from photosynthesis including all the combustible material in wildland fires The capturing of light energy and the fixation of C02 within the photosynthesis reaction occurs in the chloroplasts for most higher plants u sum Photosynthesls Continued 6H20 6C02 gt C6H1206 602 Photosynthesis Requires 6Water 6Carbon Dioxide Radiant energy Photosynthesis Produces Glucose oxygen and some water These are then turned into other compounds such as starch fat Cellulose Resins etcquot More about these and other compounds later The basic chemical reaction that occurs requires 6 water molecules 6 carbon dioxide molecules and radiant energy from the sun This reaction produces glucose and water and oxygen as by products The glucose molecules are than turned into other compounds such as starch fats cellulose and resins Like many other processes in plant physiology this consists of several sequential steps First light energy in the form of photons excite electrons of molecules causing them to change form a lower to a higher state This causes an electron to be transferred out of the reaction center This reaction leaves the system with enough energy to split water in the cell into oxygen which is released into the air The hydrogen ions and electrons released in the cell then bind to carbon dioxide to form sugar molecules One key physical principle to point out here is the first law ofthermo dynamics mass cannot be created nor destroyed As an example of this in our photosynthesis reaction we could ensure that we have kept all the mass Notice we have 12 hydrogen molecules on the left 6 H2 side and 12 on the right side thus we have maintained the hydrogen mass in this reaction If we did this for each element in the reaction we would have the same number on both sides We call this type of equation a balanced chemical equation A key physical and chemical concept Conservation of mass mass cannot be createddestroyed although it may be rearranged in space and changed into different types of particles This implies that for any chemical process in a closed system the mass of the reactants must equal the mass of the products One key physical principle to point out here is the first law ofthermo dynamics mass cannot be created nor destroyed As an example of this in our photosynthesis reaction we could ensure that we have kept all the mass Notice we have 12 hydrogen molecules on the left 6 H2 side and 12 on the right side thus we have maintained the hydrogen mass in this reaction If we did this for each element in the reaction we would have the same number on both sides We call this type of equation a balanced chemical equation We will also have to balance our chemical equations for oxidation combustion as well The Consumer Reaction The Consumer Reaction J A mm 19 sugar Olygilli my mite Ma eri lgy Image from Cantrell 2004 o The consumer reaction disassembles the products of photosynthesis and releases the energy stored between carbon atoms and between carbon and hydrogen atoms 0 This reaction produces carbon dioxide water and heat energy The consumer reaction disassembles the products of photosynthesis and releases the stored energy between the carbon atoms and between the carbon and hydrogen atoms The end result of the consumer reaction is carbon dioxide water and energy in the form of heat The consumer equations include your metabolism decomposition from fungi in a forest and combustion in the form of a forest fire In all cases the reactions are disassembling the products of photosynthesis to create some heat energy carbon dioxide andor monoxide and heat energy The Consumer Reaction o Metabolism Chemical and physical reactions with in an animal which help maintain growth and function Catabolism destructive larger molecules are broken down into smaller ones Anabolism constructive small molecules are assembled into larger ones 0 The catabolism process ultimately produces heat which helps maintain body temperature and adenosine triphosphate ATP In addition water and carbon dioxide are produced Your bodies metabolism is one example of the consumer reaction Metabolism is a chemical and physical process which helps maintain growth and function in an animal There are two separate parts to the metabolism process Catabolism which is destructive and anabolism which is constructive The catabolism process releases heat during the destruction of molecules as well as carbon dioxide and some water Z7 i L ngfzs The Consumer Reaction o Combustion Is also a chemical reaction which consumes the products of photosynthesis Example Combustion Engine 0 Uses a carbon based fuel and mixes it with 02 0 Spark plug provides a A 7 heat source jquot And byproducts are 7 released Image from dkimagescom Another example of the consumer reaction is combustion Combustion is also a chemical reaction which destructiver breaks down the products of photosynthesis As an example lets think of a internal combustion engine For an engine to operate we need 2 things fuel and a heat source In your car for example you use gasoline which is a carbon based fuel that originated from the producer reaction This fuel is than ignited by a chemical reaction called combustion after a heat source is applied by the spark plug The end result of this process is heat energy and some by products or emmissions The consumer reaction o The difference between combustion and other types of consumer reactions In combustion the reaction occurs so fast that the temperature rises substantially to the point where visible light is emitted form the reaction zone So what is the difference between combustion and the other types of consumer reactions They all release heat water and carbon dioxide right so what changes The main difference is the rate at which heat is generated and dissipated away form the reaction area In combustion the reaction occurs so fast that the temperature rises substantially to the point where visible light is emitted form the reaction zone In slower reactions that require weeks or months to go to completion release heat so slowly that the temperature never increases above the surrounding by more than a degree or so You can think of rusting as an example of this we KEY concept in COMbUS iO and chemistry o The first law of thermodynamics energy can be transformed but it can not be created nor destroyed At this point is a good time to reflect back on the first law of thermodynamics that is energy can not be created nor destroyed Lets think for a moment about the cows In this picture they are consuming fuel and ultimately releasing energy but if we removed the cows and lit a fire in this field would we produce more energy per unit offuel consumed Well the answer is of course we wouldn t remember that the heat energy released is stored in the bonds of the material so all fire is doing is rapidly releasing the energy The total amount of energy released per unit fuel would ultimately be the same The Consumer Reaction o Metabolism slow consumer reaction So as a recap the consumer reactions all function in the same way they break apart the products of photosynthesis to produce some heat energy some carbon dioxide and some water The difference is in how fast the energy is released Fire Is a Chemical Reaction 0 Fire is a rapid chemical reaction that combines fuel and oxygen to produce heat and light this chemical reaction is called combustion For combustion to occur three elements need to be in place Next lets move on and start discussing the process of combustion Remember that to have a combustion reaction we need three things a fuel element heat energy and oxygen Note that there are chemicals which will undergo combustion without oxygen present but they are not of any real concern for us in wildland fire We have all been introduced to this thought before in the fire triangle The process of combustion o The process of combustion begins with an endothermic reaction where heat from some source is supplied to the fuel a This heat source can be external such as a match or lightning known as a pilot heat source Or ignition can happen spontaneously due to chemical reactions such as fermentation o In this class we will primarily discuss fire from a pilot ignition So now lets take the ideas in the fire triangle and break them down into a process which leads to combustion occuring This process begins with an endothermic reaction where heat from some source is supplied to the fuel This heat source can be external such as a match or lightning or occur internally External heat sources are referred to as pilot heat sources where as internal sources occur through spontaneous combustion In this class we will primarily be thinking about pilot ignition This is equivalent to the heat leg of the fire triangle g7 E 7i L ngfzs 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 awe pilot heat Image fmm Cantrell 2004 Once a heat source is applied the heat will be transferred to the fuel element through some mechanism of heat transfer we will talk about these more later As this occurs the fuel will begin to dehydrate Dehydration occurs as the temperature of the 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 We will consider this the next step in leading towards combustion We will also assume that ifthe moisture is not driven off then combustion can not proceed This step of the process represents the heat leg of the triangle interacting with the fuel leg Pyrolysis o Pyrolysis is the thermal breakdown of the fuel particles Long polymeric molecules are broken down into smaller molecular weight gasses and semivolatile tar and solid char pa rti cl es me gaycud 39 tar droplets Cvfr 39 Image from Cantrell 2004 DiOI heal As the water molecules are driven off as steam the temperature will start to rise as this occurs the compounds contained in the fuel element will begin to breakdown into smaller molecular weight materials These smaller compounds will exit the physical fuel element and form a cloud of flammable gasses around the fuel This step continues with the interaction between the heat leg and the fuel leg of the triangle Ignition o Ignition is a transition point 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 combustion 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 We call this point ignition Before any type of ignition can occur we must have an interaction between the fuel leg which includes both the physical fuel and the combustible gasses and the oxygen leg of the triangle We will look at this more in a minute Flaming Combustion 0 During flaming combustion the volatile compounds released in the pre ignition 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 The mixing of oxygen and fuel occurs above the surface of the fuel element A closer look at flames o Diffusion flames the fuel and oxidizer or separate and must mix before ignition can occur Eamon dlDXIUB luu mm L39L IHDUSIIUquot oxygen macnan Zora zano Images from Cantrell 2004 Lets 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 During flaming combustion we can see that the fuel leg and oxygen leg are interacting in the combustion reaction zone Z7 i L ngfzs Glowing Combustion 0 During smoldering or glowing combustion the combustion reaction zone collapses to the fuel surface a The heat generated from glowing combustion pyrolzes adjacent fuel which sustains the process it also forms a whitish mineral ash 9 Cum 139 110N113 Image From Cantrell 2004 If the release of combustible particles from the fuel element is lowered the reaction zone can collapse to the fuel surface If this 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 Smoldering 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 oxygen Image from Cantrell 2004 When the heat produced during the breakdown of material no longer produces visible light but still has enough heat to continue the breakdown of the fuel element we have smoldering combustion This is often associated with the formation of a white ash layer which prevents the oxygen from mixing with the fuel elements and slows the reaction rate considerably This is why smoldering and glowing combustion have such low rates of spread 20 Extinction o Extinction is the termination of the combustion process It occurs when not enough heat is available to sustain the combustion process When the combustion process no longer produces enough heat to sustain the breakdown ofthe fuel extinction occurs Extinction is a transition point signaling the end of combustion 21 Phases of Combustion Summary o Pre ignition Dehydration Pyrolysis Ignition o Combustion Flaming combustion Glowing combustion Smoldering combustion Extinction To summarizes this information we can think of 4 distinct phases of combustion The pre ignition phase where dehydration and pyrolysis occur a transition stage where the reaction switched from endothermic to exothermic A combustion stage where heat energy is released from the fuel element and extinction where the combustion process ceases to exist it Assiwg zs Types of Heat Transfer 3 types of heat transfer Conduction heat flow in and trough solids Convection exchange of heat between a moving fluid air is a fluid and a solid surface Radiation exchange of heat energy by electromagnetic waves So far in this review we have looked at the chemical principles involved in combustion as well as the process which leads to different types of combustion Now lets look at how energy is transferred from one point to the next point The transfer of energy in wildfires is important for several reasons First the energy produced by fuel element burning can be considered the heat source for the next fuel element in the combustion process here we think of fire spread as being the relationship between the amount of heat energy produced by combustion and the amount of energy required to ignite the next piece offuel this is similar to Rothermels fire spread model We can also be concerned with heat transfer because it allows us to design silvicultural prescriptions to reduce tree mortality in wildfires or prevent a specific type such as a crown fire to occur It can also be useful in understanding fire effects such as on soils Basically we have 3 modes of heat transfer conduction which is the transfer of heat within a solid convection which is the transfer of heat between a solid and a moving fluid and radiation which is the transfer of heat from by electromagnetic waves 23 Heat transfer Lundunwn lnlnxlr luu Ilcil rg quotmum Innlculk x In quottunic u ncmmllx in IL lnh hrquot mmn up 1 ma quot1 1mm llk mun 9 mlrrulr A moltmlr R magnum I It A wink Innwrpn through um m in mg the um am m cm W admit A unmade R 39 n Fucrgllnl mulcullr Aumn mnmg imm tn mokx uk I In rumlcr Italian 3 Mark Here are some examples of these modes of heat transfer In conduction energy is passed from one molecule to another in radiation energy travels through space and In convection energy is carried directly to molecule b Examples of conduction and fire effects 0 Soil temperatures Lurnn39 I 3 1 lentsz um lemma lawn mun lot 1 I w yuan Numam 1 in mm Hm A good example of how conduction can be important in fire ecology is in modeling the effects of fire on soil microorganisms The two graphs here were both based on a simple algebraic conduction model In the left hand graph we have predicted the maximum temperature over time for several different depths in the soil Where as in the right hand figure we have graphed the maximum temperature at different depths as the fire front moves over an area The green line in this picture denotes the lethal temperature for organisms assumed to be 60 degrees C In this case we can see that understanding the physical nature offire and heat transfer can provide us with valuable insights in to fire effects 25 7 L FQR 26 Radiation Sun Twaemturz 5773 v hams See m n Earth Distance from he mm 1 J95 10quot Now lets move on and look at radiation for a few minutes Radiation is the transfer of energy through space by electromagnetic waves A great example of this is the sun s energy being transferred to the earth The same process which governs this energy movement also governs the transfer of energy between a fire and a firefighter Example of Radiation Observed effeclDr39sL1k 1990 Radial heal flux kW 51139 1 o Mmmun for inde nite 5km exposure 6 4 Pam flex 8 x luu expnsl e 10 4 Fun arm 1 seconds 9 me 1 5 Volatiles om wood maybe rgmred by pilomfler prolonged exposure 16 o Blmenug ofslun arm 5 seconds 290 bed ignites spontaneously after prolonged 5 o pom e Fibreboaxd ignites spoumleously m 5 seconds Rm WInquot As an example of radiation lets investigate the consequences of exposure to radiant heat flux on humans In the top table you can see a range of radiant heat flux s from 1 Kwm2 to 52 and the corresponding effects The figure below uses a simple radiative model to predict the radiative flux from a flame front 20m wide and 5m high You can also see that the area in side the dotted square represents the heat flux in which we would be in danger without any protection from the flames 27 Convection o Convection heat transfer through movement of liquid or air unammp a w c The final mode of heat transfer we will discuss is convection Convection is the transfer of heat energy by the movement of liquid or air Convective heat transfer can relate to fire effects by predicting the height at which leaf or needle damage will occur during a fire The figure you see here shows the predicted temperature at different heights above a fire Note that there have been many empirical and physical models developed to do this lam showing you two of them here the red line is an equation base don flame temperature and the blue line is base don fire line intensity There are also 3 colored lines which represent 3 different thersholds of temperture for crown scorch 28 Final Thoughts 0 Combustion is simply a chemical reaction that rapidly decomposes the products of photosynthesis The process of combustion can be thought of as a several steps occurring in a sequence There are three modes of heat transfer which can be used to understand how fire effects the ecosystem As we end our first lecture I want to emphasize a few points Primarily that fire is a chemical and physical process which has effects on ecosystem components and we can utilize knowledge about the mechanisms driving this process to predict and understand what effects will occur We can also think of combustion as a fairly linear process in which certain events must occur we will talk about this more later Throughout this class I want to emphasize to you to try and link the physical mechanisms that is the chemical and physical process to the effects fire has on an ecosystem our management of those ecosystems and the rational behind silvicultural operations As we move on in this class we will introduce fire as an ecological process as well and hopefully further connect the dots between the ecological and physical process of fire 29


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