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  • Combustion Theory: The Basics

 

Combustion Theory

Introduction

Welcome to the Combustion blog series by Preferred Utilities Manufacturing Corporation. To read the introductory post, click here.

This series was inspired by Local Law 87, an environmental regulation passed by New York City legislators. LL87 seeks to reduce the city’s emissions by 50% while increasing the overall efficiency of large residential buildings (over 50,000 gross sq. ft.).

With additional state and local governments instituting similar environmental regulations across the United States, combustion system design and theory is more important now than ever.

Whether you’re a building owner, plant operator, building designer, or system engineer, this blog series will help you make informed decisions on your projects, especially as they pertain to LL87 and laws like it.

Why listen to us?

Because we’ve been doing combustion since 1920. Our rotary-style burners, invented in the 1960s, are still in operation all across New York City–almost half-a-century later.

But we’ve learned a lot since then.

We’re not like a lot of other burner companies. We don’t cut corners. Our products aren’t flimsy and they don’t come cheap. They last. And they perform.

Ultra low emissions. High efficiency. High turn down. Rugged durability. We reached for these marks because we believe in what we do. We love combustion. We love doing it right.

If this sounds like you, then read on.

Basics

The most common industrial fuels are hydrocarbons. This means that they are predominantly composed of carbon and hydrogen. Table 1 lists some common fuels and gives typical values for the hydrogen and carbon contents as percentages by weight. Note that there are some other components besides hydrogen and carbon. Some of these, such as sulfur, are combustible and will contribute to the heat released by the fuel. Other components are not combustible and contribute no positive energy to the combustion process.

Combustion Theory - the basics [table 2]

Table 1

The Chemistry

Table 2 reviews the basic chemical equations, which represent the most common combustion reactions. Note that nitrogen (N2) is shown on both sides of the equations. Except for the formation of NOx (in the parts per million range) nitrogen does not react in the combustion process. The nitrogen must be considered in fan sizing and stoichiometry calculations. Each atom of carbon in the fuel will combine with two atoms of oxygen (or one molecule of O2) from the atmosphere to form one molecule of CO2. On a weight basis, each pound of carbon requires 2.66 pounds of oxygen for complete combustion resulting in the production of 3.66 lb of carbon dioxide.

Combustion Theory - the basics [table 1]

Table 2

Each pair of hydrogen atoms (or each molecule of H2) will combine with one atom of oxygen (or one half molecule of O2) to form one molecule of H2O, or water. On a weight basis, each pound of hydrogen requires 7.94 pounds of oxygen for complete combustion, resulting in the production of 8.94 pounds of water.

By the Numbers

The air we breathe is only about 21% oxygen by volume. For all practical purposes, the remaining 79% is nitrogen. Since oxygen is a little heavier than nitrogen, the percentages by weight are somewhat different. The percentage of oxygen by weight is 23%, and the remaining 77% is nitrogen. Thus, it requires about 4.35 pound of air to deliver one pound of oxygen. Table 3 shows the composition of air.

Combustion Theory - the basics [table 3]

Table 3

A typical gallon of No. 6 fuel oil weighs 8 pounds and is 87% carbon and 12 % hydrogen (the missing percent is sulfur, ash, water and sediment). This gallon contains 6.95 pounds of carbon and 0.96 pound of hydrogen. From the data presented earlier, we can compute that 18.49 pounds of oxygen are needed to burn the carbon and 7.62 pounds of oxygen must be provided to burn the hydrogen in this gallon of fuel oil. This represents a total requirement of 26.11 pounds of oxygen. Since air is only 23% oxygen by weight, it will take 113.5 pounds of air (26.1 ÷ 0.23) for the complete and perfect (0% excess air) combustion of this gallon of fuel. Assuming there are 13 cubic feet of air to the pound, 1476 cubic feet of air are required to burn each gallon of fuel. A 50 gallon per hour burner (about 200 boiler HP) would need nearly 74,000 cubic feet of air per hour (or 1230 scfm) to fire without any allowance for excess air.

The Real World

In the real world, however, there must always be more air supplied to the combustion process than the theoretical or stoichiometric air requirement. This is because no burner made is this “perfect”. This “extra” air is referred to as “excess air.” If 20% more than the theoretical air requirement is supplied, we say that the burner is operating at 20% excess air. Another way of stating the same thing is to say that the burner is operating with 120% “total air.”

Complete combustion of our one gallon of No. 6 fuel oil with 20% excess air would require 136 pounds of air. The 50 gallon per hour burner would actually require about 90,000 cubic feet of air per hour.

For any particular burner-boiler combination, there is an ideal “minimum excess air” level for each firing rate over the turn-down range. Greater air flows would waste fuel because of the increased mass flow of hot gases leaving the stack. Lesser amounts of air would cause fuel waste because the fuel would not be burned completely. Typically, burners require much higher levels of excess air when operating near their minimum firing rates than they do at “high fire.” Table 4 shows a typical relationship between percent firing rate and the excess air required to insure complete combustion of the fuel. In many cases, even though stack temperature might decrease at low fire, efficiency suffers because so much of the fuel energy is lost to heat this excess air.

Combustion Theory - the basics [table 4]

Table 4

 

Other posts in this series:

  • Understanding Local Law 87 – and laws like it
  • Combustion Theory: The Basics
  • Combustion Theory: Variables – Account for variations in oxygen and fuel
  • Combustion Theory: Efficiency – Calculate efficiency and losses
  • Combustion Theory: FGR – See how flue gas recirculation reduces NOx
  • Combustion Theory: Combustion Controls – Learn how cutting-edge tech can cut your emissions
  • Combustion Systems: Design – Basic principles to follow when designing your combustion system
  • Combustion Systems: Troubleshooting: Burner problems and their causes
  • Combustion Control: Strategies – Linkage vs. Linkageless, and why you should care

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