Energy Results

previous page

Finally, we fulfill the purpose of this little sub-project, which is to compare the energy needed to make bricks with and without recycled container glass as the grog.

Please recall that in contrast to other studies of glass as a raw material in brick manufacturing, we are using very coarse glass (12 mesh and finer), just as it is already being processed for the fiberglass industry. 
The 50 percent glass brick meeting ASTM C1272-05a, “Standard Specification for Heavy Vehicular Paving Brick,” was fired as follows:
90 minutes to 1850F
hold 20 minutes at 1850F

The minimum firing profile that produced a specification-meeting brick using 50 percent grog was
90 minutes to 2100F
30 minutes at 2100F

During brick production we have been monitoring energy use during each kiln firing.   The chart below shows the energy consumption in btu’s per pound for the glass and grog bricks:

  The blue line is the 50 percent glass bricks.  The purple line is the grog bricks.  The scale on the left is btu’s per pound of brick fired.

The kiln was then fired empty to the same two profiles.  This will show us how much energy  the bricks themselves used.

Here’s a graph of the energy consumed by the empty kiln, normalized for btu’s per pound of bricks when the kiln is full of bricks:

The purple line is for the profile with the grog bricks.  The blue line represents the profile for the glass bricks.  The scale is btu’s per pound of brick (when the kiln is full).

Subtracting these two graph, we get the graph of the actual energy that went into each brick, in btu’s per pound:

 The scale is btu’s per pound.  The blue line is the glass bricks, the purple line is the grog bricks.

One of the interesting things to observe is that as the glass bricks reach their final state of fusion, they are actually consuming more energy than the grog bricks.  This is because as the glass softens the mass conducts heat more efficiently, until it reaches maximum density.

Summarizing the graphs above, we have the following:

Grog bricks with kiln    3011 btu’s per pound
Glass bricks with kiln    2267 btu’s per pound
For a savings of 744 btu’s per pound, or 25 percent.

Subtracting the energy used by the empty kiln, which represents the heat loss through the sides of the kiln, we get:

Grog bricks        616 btu’s per pound
Glass bricks        478 btu’s per pound
For a savings of 138 btu’s per pound, or 22 percent.

The energy consumption of the bricks without the kiln is a more interesting figure because large-scale roller-hearth furnaces have economies of production that will make the wall losses per brick very small compared with these.

The brick manufacturing industry has been targeted by federal and state governments for decades as an industry that needs energy efficiency.  But bricks are a wonderful building material.  They are virtually permanent, sanitary, and structurally sound.  Bricks manufactured hundreds of years ago are still salvaged, cleaned, and used again.  When I was in Poland some years ago, I saw a Teutonic Knights’ castle for which the bricks were already old when they built the castle from older rubble in the 13th century.

Finally, this project begs the question of whether it’s worth using glass in brick manufacturing in monetary terms.

Comparing our actual energy invested in a grog brick of 616 btu’s per pound to the figures given by industry on earlier pages of this web site, it appears that industrial furnaces achieve about a 50 percent efficiency of total energy in vs. energy actually going into the brick.

A savings of 138 btu’s per pound of brick translates to 276 btu’s per pound of glass used to make the 50 percent glass brick, or 552,000 btu’s saved per ton of glass.  Assuming 50 percent efficiency for an industrial-scale brick manufacturing plant would result in about 1,100,000 btu’s saved per ton of glass used.  One cubic foot of natural gas contains about 1000 btu’s of energy.  So the savings would be 1100 cubic feet of natural gas.

This is somewhat of a disappointing result, since natural gas costs about $10 per thousand cubic feet for industrial users.  

So a case could be made that the value of glass in brick manufacturing would be $11 plus the cost of the grog currently being used.  in many cases, the source of glass may be much closer than the source of the grog, but this effect would be local and dependent upon transportation.

Other considerations to a manufacturer would be that firing at lower temperatures lengthens the life of the refractories and reduces maintenance costs, but we are in no position to estimate those costs.  And with California’s latest Greenhouse Gas reduction mandates, there may be other benefits to reducing energy consumption.  Finally, wouldn’t architects think it was sexy to call for 50 percent recycled glass bricks on a LEED job.

So our conclusion is that substituting recycled glass for ceramic grog saves about 25 percent of the energy it takes to make a brick.

More Energy Testing

I was in a case class in my MBA program many years ago.  I don’t remember the case, or the class, particularly, but I do remember answering a professor’s question about a financial decision “that’s a marginal difference.”  The professor replied “All decisions are made on the margin.”

I probably attributed more to his comment than it was worth, but it stuck with me.  In a sense, all decisions ARE made on the margin, if by the margin you mean the space where the information changes.

Say you’re lost in a forest of identical trees.  You wander about but you can’t differentiate between the trees and can’t orient yourself.  It’s only when you get a piece of outside information, e.g. the direction of the sun or the edge of the trees, that you can start making productive decisions.  On the margin.  

We tried making brick of the same thickness but with frogs and failed the strength test.  I was going to hang it up with the results from the previous tests, then realized that brick thickness itself is not a critical criterion for us.

We are trying to establish the energy that can be saved by making bricks with the same strength and absorption as standard brick mixes.  We are not actually using these as paving bricks (except in my yard), so maintaining a thickness established by custom is not pertinent.

Since the 1600 gram had passed the strength test by so much, we decided to make one more run at making a lighter brick.  The bricks made at 1850∞F with a 20 minute soak met the standards for Cold Water Absorption, and strength:

Dry weight    Absorption    Strength    Average Strength
1499               6.3%             2419               2410
1505.5             6.2%             2401

Although this set of two does not meet the statistical standard, it is apparent that the 20 minute soak also makes an adequate brick.  At an average breaking strength of 2410, the 20 minute soak bricks exceeded the strength standard by
    2410/1900 = 1.268:1
Flexure strength is proportional to the square of the depth, so it may be possible to make bricks exceeding the 1900 pounds strength standard with a thickness of
    1/(1.268)^.5 = .888
times the thickness of the first set of bricks.

The first bricks used 1600 grams of dry raw materials, so the new set will weigh
    .888*1600 = 1420 grams dry.

We also mentioned previously that the bricks appeared to not be fused completely in the middle.  This may meant that we can reduce the weight even more and get full fusion (and thus strength) in the middle.  So for the second test run, 1400 grams of dry material is used.  The standard mix for this run is:
∑ 700 grams 12 mesh glass
∑ 700 grams Redart Fireclay
∑ 231 grams water

A set of five bricks is made and tests as follows:

Dry weight    Absorption    Ave. Absorption    Strength    Ave Strength
1302.5             5.6%               5.74                1769              1914
1302                5.8%                                      1942    
1318.5             5.9%                                       1921    
1321                5.6%                                      1913    
1307                5.8%                                       2026    

So this set of bricks meets the ASTM standard for both Absorption and Strength.

On the other hand, the grog bricks we fired to 2100F did not truly meet the strength standard.  For a true comparison of energy, we had to make one more firing to 2125F.  Those bricks tested as follows:

Max Temperature    Dry weight    Absorption    Strength    Average strength
2125∞F                       1468              5.3%          1918            1979
                                   1478              5.3%          2040

The 2125∞F bricks meet both standards (albeit not statistically valid), so they will be used as the reference for energy used to make a brick.

Energy consumption

As before, energy savings will be developed in two ways: with and without the kiln wall losses.  As mentioned previously, the percentage of energy into the kiln was monitored through the 4-20 milliamp control signal.  Then a correlation was developed between the control signal and the actual energy input to the kiln.

Two bricks were made per kiln run.  Since our glass bricks are lighter, it no longer makes sense to calculate energy per pound.  Now all data are calculated as energy per brick. Energy usage, including both the kiln and the brick, looked like this: 

 Blue line is glass/clay brick, yellow line is grog/clay brick.  Left scale is btu’s per brick.

The kiln was then run empty, with the same profile, to determine the energy loss through the kiln walls.  The empty kiln, run at the same profiles as the glass/clay and grog/clay bricks, looked like this:

Blue line is empty kiln with glass/clay profile, yellow line is empty kiln with grog/clay profile. Left scale is btu’s per brick.
Subtracting the empty kiln from the full kiln results in the energy used to actually fire the brick:

 Blue line is actual heat into the glass/clay brick, yellow line is actual heat into the grog/clay brick. Left scale is btu’s per brick.

Again the two types of brick absorbed almost exactly the same amount of energy until the glass/clay brick reached its fusing range.  Then, as the glass softened and the brick mass became more conductive, the glass/clay brick actually used more energy then the grog/clay brick.  However, the grog/clay brick had to be fired to a higher temperature and for a longer time to reach full density.


 Energy to make glass/clay brick:
    Including kiln    7,717 btu’s per brick
    Without kiln losses    1,456 btu’s

Energy to make a grog/clay brick:
    Including kiln    11,085 btu’s
    Without kiln losses    2,382 btu’s

The percentage of energy saved:
    Including kiln    30.4%
    Brick only    38.9%

Now we’re talking!  By actually meeting the standard for the grog brick and reducing the thickness of the glass brick to the standard, we’re opened up the marginal difference to be something possibly really worthwhile.

Let’s see how worthwhile this might be.  According to my list, there are at least five brick plants in California.  The three we visited averaged about 200,000 tons of bricks produced per year.  That means about 1 million tons of bricks are made in California each year!

For verification of that figure, according to the U.S. Department of Commerce, production of common (building) and facing brick edged up 1.5% in 2003 to 8.6 billion units.  If the average brick weighs 3.5 pounds, that makes the weight of bricks produced in the United States
    8.6 x 109 bricks x 3.5 pounds per brick/ 2000 pounds per ton = 15,050,000 tons
For a U. S. population of 300 million;
    15,050,000 tons/ 300,000,000 people = .05 ton/person
for California population of 35 million
    .05 tons per person x 35,000,000 people = 1,750,000 tons of bricks.
The use of brick varies regionally, but the 1,000,000 tons for California makes sense.

It gets tricky making apples to apples comparisons, but the best estimate I’ve seen of the energy it takes to make a brick is the figure of 2180 btu’s per pound from a California Department of Energy report.  If using glass results in savings of 30 percent, as shown above, then the energy savings to California brick manufacturers would be:
    1,000,000 tons x 2000 pounds/ton x 2180 btu/pound x .3 = 1.3 trillion btu’s
    1.3 trillion btu’s / 1000 btu’s per cubic foot of natural gas = 1.3 billion cubic feet
at $10 per 1000 cubic foot
    1.3 billion x $10/1000 = $13 million.

This is still only $13 per ton, but reducing the costs to the industry by $13 million and reducing greenhouse gases by the amount of CO2 generated by burning 13 billion cubic feet sounds like a worthwhile thing to do.