Written by: |
Helmut
Spieser - Engineer/OMAF |
Table of
Contents
- Why
Burn Corn?
-
Table 1. Heat Energy of On-Farm Fuel Sources
-
Basics of Corn Stoves
-
Types of Stoves
-
Cost of Heating with
Corn
-
Table 2. Heat Content and Heating Efficiency of Various
Fuels
-
Limitations
of Burning Corn for Heat
-
Stove Buying Criteria
A number of
manufacturers now make stoves which will burn shelled corn.
Although similar to wood stoves, these new stoves have been
specifically designed to burn a dry granular fuel, such as
shelled corn. Corn burning stoves usually have a combustion air
fan and a fuel stoker, both of which are not common in standard
wood stove construction.
Why burn corn?
In Ontario we
have an abundant source of dry shelled corn. The corn used as a
fuel in stoves does not have to be Grade No. 1, but can be of
lower quality. There are however two requirements of this corn
fuel:
- The shelled
corn must be dry, preferably 15% moisture content or less.
Corn which is higher in moisture will have a lower heat
value per unit weight than "dry" corn. Moist corn may also
cause flow problems through the fuel loading auger.
- The shelled
corn must be free of fines. Dirty corn which has a lot of
fines and cob pieces will cause problems with the fuel
loading auger.
The storage,
moving and handling of grain corn has evolved to a point where
there are very few unknowns. Every year farmers harvest, dry,
convey, and store millions of bushels of corn. The equipment to
do all these things is readily available. Consequently putting a
corn storage system together for a home heating set up is
possible with augers, conveyors and storage bins which are
readily available.
Two reasons it is
so attractive as a heat source are that dry shelled corn is so
easily handled and in plentiful supply. Shelled corn also has a
high heat energy per unit weight. Here's how shelled corn
measures up to other solid fuels.
Table
1. Heat Energy of
On-Farm Fuel Sources
Shelled Corn |
7000 BTU/lb
(16,200 kJ/kg) at 15% Moisture Content |
Straw |
6550
BTU/lb (15,200 kJ/kg) Air Dried |
Corn
Stover |
7540
BTU/lb (17,500 kJ/kg) Air Dried |
Wood |
8000
BTU/lb (18,500 kJ/kg) Air Dried |
You can see from this table that shelled corn has heat energy
close to that of wood.
Basics of Corn Stoves
Corn stoves are
specifically designed to burn a granular fuel. Because this fuel
is metered into the burning chamber, most stoves have a storage
hopper to contain a supply of fuel. In some ways corn burning
stoves are very similar to pellet burning stoves. In both cases,
corn and pellets are very dense. Consequently, neither of these
fuels will burn readily in an open pile in a fire chamber.
To get these
fuels to burn, some manufacturers use a small combustion chamber
into which the corn is fed and combustion air is pumped through.
The corn can be either dribbled into this combustion chamber
from above, or it can be stoked into the chamber from below by
means of an auger. The feed rate of this auger can be adjusted
to regulate the amount of corn burned, which in turn controls
the amount of heat produced. The second requirement for burning
to occur is oxygen. In order to support combustion, oxygen is
blown into the combustion chamber by means of a small fan. The
combustion air is usually brought in from outside, not room air.
This combustion chamber is actually quite small and could easily
fit into a child's lunch box.
As corn burns it
produces a clinker. Because of the small size of the combustion
chamber the clinker should be removed daily. With practice, the
removal of the clinker can be done without having to shut down
and then relight the stove. A specially designed poker is used
to upend the clinker, then tongs are used to remove it.
Inside the stove,
a heat exchanger is used to remove heat from the flue gases and
heat the room air. A fan is used to move the room air through
the stove where it is warmed. This fan may also help in moving
the heat further away from the stove.
A different style of corn stove also exists which does not use
augers to feed in the corn or fans to provide combustion air or
move heated air to the room. By careful design, these stoves
will burn corn at the bottom of a hopper and radiate heat to the
surrounding room. Unlike the previous type where electricity is
used to stoke the fire and move the heat to the room, these
stoves are not affected by electrical power outages.
The type of flue
pipe required to vent the exhaust gases from the stove will
depend on the design of the stove or corn burning appliance.
These flue pipes can range from those commonly used in wood
stoves to through-the-wall vent pipes which actually preheat the
combustion air by removing heat from the flue gases. Follow the
manufacturer's recommendations regarding the type of flue pipe
required. It is best to keep the flue pipe as short and straight
as possible (keep the number of elbows to a minimum) to maximize
the stove's performance.
Types of Stoves
A number of
different manufactures are presently making corn burning stoves.
They are available in a variety of sizes and styles. Here are
some of the configurations available:
- Stove (some
can be modified as a fireplace insert)
- Space heater
- Hot air
furnace
- Hot water
boiler
The size of fuel
hoppers also varies greatly. This hopper size can range from
holding one day to ten days supply of fuel.
One thing to
consider with the freestanding stove or space heaters is the
surface temperature of exposed metal parts. This is especially
important if there are small children in the house.
Cost of Heating with Corn
Before you can
accurately compare heating with corn to other heating fuels you
have to look at a number of factors. Not only is price of the
fuel important, but also the heating efficiency of the heating
system and the energy content of a unit of each fuel.
Here is a formula
which you can use to calculate your cost per Million BTU's of
useable energy. This formula takes into account all these
factors:
- Cost per
unit of fuel
- Energy
content per unit of fuel
- Seasonal
heating efficiency
Cost per Million
BTU's of useable energy =
(Cost Per Unit Of Fuel x 1,000,000) ÷ (Energy Content Per Unit
Of Fuel (BTU) x Seasonal Heating Efficiency)
Where: Cost per
unit of fuel is in Dollars Energy Content Per Unit of Fuel in
BTU's Seasonal Heating Efficiency is in decimal form ie (70% =
0.7)
Example: Lets
look at an example where you are using corn at $2.50 per bushel
in a stove which has a seasonal heating efficiency of 60%. What
is the cost per million BTU's of useable energy?
Corn Cost = $2.50
per bushel
Energy content per bushel = 7000 BTU/lb x 56 lb/bu. = 392,000
BTU
Seasonal Heating Efficiency = 60% = .6
Dollars per
Million BTU's Useable Energy =
(Cost per unit of
fuel ($) x 1,000,000) ÷ (Energy Content Per Unit Of Fuel (BTU) x
Seasonal Heating Efficiency)
=($2.50 x
1,000,000) ÷ (392,000 x .6)
= $10.63
Therefore to
supply one million BTU's of heat to the house costs $10.63 when
this stove operates at 60% efficiency, burning corn at $2.50 per
bushel. The average older home requires approximately 100
million BTU's of useable energy per year. When you do the
calculations for your situation, keep in mind that the price
charged per bushel of corn may vary from the market price when
small quantities are purchased. Check the prices carefully
before doing these calculations.
Table 2. Heat Content and
Heating Efficiency of Various Fuels
Fuel Type |
Energy
Content
per Unit |
Seasonal
Heating Efficiency |
Shelled
Corn
|
7000
BTU/lb. (16,200 kJ/kg)
|
70% -
85%
|
56
lb./Bushel
|
392,000
BTU/56 Pound Bushel
|
|
48
lb./Bushel
|
336,000
BTU/48 Pound Bushel
|
|
Furnace
Oil
|
36,700
BTU/L (38,700 kJ/L)
|
70% -
85%
|
Propane
|
25,300
BTU/L (26,900 kJ/L)
|
70% -
85%
|
Natural
Gas
|
35,700
BTU/M3 (37,700 kJ/M3)
|
70% -
85%
|
Electricity-Resistance
|
3413
BTU/KWh (3600 kJ/kwh)
|
100%
|
Air
Source Heat Pump
|
C.O.P. =
2.75
|
275%
|
Water
Source H.P.
|
C.O.P. =
4.0
|
400%
|
Wood
|
8000
BTU/lb. (18,500 kJ/kg)
|
60%
|
Limitations of
Burning Corn for Heat
Possibly the
first and most important limitation of corn as a fuel is the
stove itself. If the stove uses augers to feed the corn into the
combustion chamber and fans to maintain combustion and move
heated air to the room then an electrical power interruption
will shut the stove down. Very simply with this style of stove,
no electrical power means no heat from your corn stove. Some
stoves require a manual reset after a power interruption, as a
safety feature.
Second, since
most house layouts do not allow the free movement of air through
the house, a centrally located stove will not heat the whole
house. If this is your case, size the stove to heat the room
where the stove is located. Oversizing the stove will result in
the room housing the stove becoming unbearably hot.
Stove Buying Criteria
When purchasing a
corn stove there are some questions which you should answer:
- (What is the
heat output of the stove? Do you know how much heat you
require to maintain the heated space at the desired
temperature?
- If you are
trying to heat your whole house with a stove or space
heater, does the house layout allow for the convective
movement of heat through the whole house? Most newer houses
are not built to allow convective air movement.
- What is the
size of the fuel hopper? Will it require filling on a daily,
weekly or biweekly schedule?
- What is the
seasonal heating efficiency of the corn stove?
- Does the
unit meet UL and CSA standards?
- Does the
unit have hot exposed surfaces which could cause burns to
skin?
- What type of
exhaust venting is required? Does it require a chimney with
a flue liner or can a combination flue/fresh air vent pipe
be used?
- Are you
prepared to clean out the clinker daily and clean the heat
exchanger of ash on a weekly basis?
- Will the
stove handle granular solid fuels other than shelled corn?
This is important in the event that the economics of burning
corn become unattractive or an alternative low cost pelleted
fuel becomes available.
- Will this
corn burning appliance be a primary heat source or act as a
supplementary heat source? Stoves with small fuel hoppers
will not keep a house warm for long periods of time,
unattended.
- How
will corn be stored for winter operation?
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