Download We are going to start this class of by reviewing the idea of fire as a

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 of fire. 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. 1
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. 2
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 CO2 within the photosynthesis reaction occurs in the chloroplasts for most higher plants. 3
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
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 of thermo 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. 4
One key physical principle to point out here is the first law of thermo 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
We will also have to balance our chemical equations for oxidation /combustion as well. 5
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 (and/or monoxide) and heat energy. 6
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. 7
Another example of the consumer reaction is combustion. Combustion is also a chemical reaction which destructively 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. 8
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
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. 9
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 of fuel 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. 10
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. 11
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. 12
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
about pilot ignition. This is equivalent to the heat leg of the fire triangle. 13
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 if the 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
This step of the process represents the heat leg of the triangle interacting with the fuel leg. 14
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. 15
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. 16
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. 17
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. 18
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. 19
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
When the combustion process no longer produces enough heat to sustain the breakdown of the fuel extinction occurs. Extinction is a transition point signaling the end of combustion. 21
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. 22
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
between the amount of heat energy produced by combustion and the amount of energy required to ignite the next piece of fuel (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
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. 24
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 of fire and heat transfer can provide us with valuable insights in to fire effects. 25
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. 26
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 Kw/m2 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
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 I am 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
different thersholds
of temperture for crown scorch. for crown scorch
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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
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