Energy in Making Bricks: A Comparison between

Clay Bricks and Flyash Bricks

by Henry Liu, 7/14/07

1. Introduction

An attempt is made here to evaluate the energy needed for manufacturing flyash bricks using the process developed by Freight Pipeline Company (FPC), and then to compare the result with the energy needed for manufacturing clay bricks (more specifically, the fired clay bricks). The cost savings due to lower energy needed for manufacturing fly ash bricks than clay bricks are also assessed. This paper shows that more than 5 cents in energy cost can be saved in manufacturing each brick if flyash instead of clay is used.

2. Process Difference and Energy Use

(a) Flyash Brick

The FPC process for manufacturing flyash bricks utilizes the cementitious (self-cementing) property of Class C flyash. In this process, flyash is first mixed with a small amount of water (approximately 12% based on the weight of the mixture). Then, the mixture is fed into a mold and compacted at room temperature to a pressure in the neighborhood of 2,000 psi (pound per square inch). The compacted brick is then removed from the mold and cured in a wet room either saturated with vapor or steam. Steam curing is done at approximately 160 ^oF over a period of 8 to 24 hours. On the other hand, when curing is done by using vapor or mist, it is done for approximately one week at room temperature. The process diagram for manufacturing flyash bricks is given in Figure 1. In what follows, the energy consumed in manufacturing flyash bricks during each step is estimated:

Mixing – The energy used for mixing flyash with water can be estimated from using actual data on mixing collected in our laboratory. When we make flyash bricks in our laboratory at FPC, we used a KitchenAid Mixer rated at ¾ hp (horsepower). This mixer can, at a minimum, mix at least 8.4 kg (18.5 lbs) of flyash with water, which makes 4 bricks. Adequate mixing for each batch requires approximately 3 minutes. This means the electrical energy used in three minutes to mix a batch of flyash enough to make 4 bricks is (3/4) x 3 = 9/4 hp-min = 95.4 Btu. For each brick, the electrical energy used in mixing is thus approximately 24 Btu. When larger mixers are used in mass production of fly ash bricks commercially, the mixing will be more efficient, and the electrical energy consumption will be less than 24 Btu per brick. Therefore, 24 Btu/brick for mixing is a conservative assumption used herein.

Mixing Flyash with Water

Compaction in Mold

Curing

Figure 1. Flyash brick manufacturing process

Compaction – The energy for compacting flyash brick is the electrical energy used for powering the compaction machine (hydraulic press) that compacts the flyash-water mixture in the mold to make bricks. This energy can be estimated from the compaction energy as follows:

From our test results of flyash brick compaction, it is known that the total work done by compaction, calculated from integrating the compaction force with the compaction displacement, is approximately 0.86 Btu. Assuming that the hydraulic press used in compaction is only 50% efficient, then the total electrical energy consumed by the hydraulic press for compacting each flyash brick will be approximately 1.7 or 2 Btu.

Curing – Assuming steam curing for 24 hours at 160 ^oF, the temperature of the bricks must be raised from room temperature (73 ^oF) to 160 ^oF, or an increase of 87 ^oF. Knowing that the heat capacity of bricks is approximately 30 Btu/ft³/^oF, and the volume of each brick of 8”x 4”x 2.25” size is 72 in³ or 0.0417 ft³, the energy (heat) needed to raise the temperature of each brick by 1^oF is 1.25 Btu. Thus, to raise the temperature of each flyash brick by 87 ^oF (from 73 to 160 ^oF), the total energy (heat) needed is 1.25 x 87 = 109 Btu. Assuming the curing system to be only 50% efficient, the energy consumed for curing each flyash brick is approximately 218 Btu.

From the foregoing, the energy needed for mixing, compaction and curing each flyash brick is, respectively, 24 Btu, 2 Btu, and 218 Btu. The total is approximately 244 Btu.

The foregoing result shows that most of the energy used in manufacturing flyash brick is in curing (218 Btu/brick), next in line is mixing (24 Btu/brick), and the least energy intensive is compaction (only 2 Btu/brick). Some folks may think that using high pressure to compact will result in high energy consumption, which is a misconception. Because the high pressure used in compaction is needed only over a short distance (approximately 2 inches), very little work is done over such distance. The fact that the compacted brick has essentially the same temperature as that of the flyash-water mixture before compaction is evidence that compaction uses very little energy. Had it used far more energy, the compacted product would have developed a noticeably higher temperature than that of the pre-compacted material.

(b) Clay Brick

The process of manufacturing fired clay bricks is illustrated in Figure 2 below:

It involves the following essential steps:

Raw material (clay) preparation (conditioning) – Unlike flyash which is stored in silos, dry, pure, in powdered form, and needs no preparation (conditioning) before it can be used to make bricks, the clay for making clay bricks comes from outdoor pits and hence does not have the same consistency in properties, in terms of moisture, particle size, and purity. Therefore, before clay can be compacted or extruded into bricks, it requires some preparation including drying (when the raw materials is too wet, as when mined after rain), screening or sifting (to get rid of large clumps and rocks), and sometimes crushing. The amount of energy used in raw materials preparation varies with the conditions of the mined clay; it is hard to assess but is not negligible. This is an extra consumption of energy and cost not encountered in using fly ash to make bricks.

Clay Conditioning

Compaction

(or Extrusion)

Drying

Kiln

Firing

Cooling

Figure 2. Clay brick manufacturing process

Compaction or Extrusion – The prepared (conditioned) clay is then either compacted or extruded into bricks. This step is identical to compaction of fly ash to make bricks, and hence is expected to use the same amount of energy as for making flyash bricks – approximately 2 Btu per brick of 8”x 4”x 2.25” size.

Drying – The newly compacted or extruded adobes, also called “green bricks”, are first dried at a temperature of approximately 300 ^oF, before fired at much higher temperature in kilns. Without this drying process, the bricks fired in kilns will crack and deform. This drying process consumes energy (heat) in two ways: (1) due to heating of the material from room temperature (assumed to be 73 ^oF) to 300 ^oF, and (2) due to evaporation or loss of moisture.

As analyzed for flyash bricks, the energy required to raise the brick temperature by 1 ^oF without change in moisture is 1.25 Btu per brick of 8”x 4”x 2.25” size. Thus, to increase the temperature from 73 ^oF to 300 ^oF without change of moisture will require an energy of (300-73) x 1.25 = 284 Btu/brick. On the other hand, if we assume that the moisture removed from each brick is equivalent to 10% of the weight of the brick, with each brick weighing 4.5 lbs the moisture removed is 0.45 lb. Since the latent heat for evaporation is 970 Btu per pound of water, the heat required for drying each brick is 0.45 x 970 = 437 Btu. Thus, the total energy required to dry the adobe at 300 ^oF is 284 + 437 = 721 Btu.

Kiln Firing – The dried adobes at 300 ^oF are then to be heated in kilns to 2100 ^oF. The heat needed for such temperature rise for each brick is then (2100 – 300) x 1.25 = 2250 Btu.

From the above, the total heat needed for drying and firing each clay brick from room temperature to 2100 ^oF is 721 + 2250 = 2971 Btu. Assuming that the heating/firing process is 50% efficient, the thermal energy needed for drying and firing each clay brick is 5942 Btu.

Cooling – The clay bricks heated to 2100 ^oF and kept at such temperature for 8 hours or more must be cooled down to near room temperature before the bricks can be handled by human. The cooling, to be accomplished quickly, requires circulation of outdoor air through the kiln to cool the bricks. This again requires energy. However, the energy for cooling is not assessed here due to its smallness as compared to the energy required for heating and drying the adobes, and due to the difficulty in assessing this cooling energy analytically.

Therefore, if we neglect the relatively small amounts of energy used in the preparation of clay and in cooling the fired clay bricks, the total energy consumed for manufacturing each clay brick is approximately (2 + 5942) = 5944 Btu.

Note that the above analyses of energy used in manufacturing flyash bricks and clay bricks include only the energy consumed at the brick factory. They do not include the energy used in transporting clay and fly ash to the brick factory, or the energy needed for mining clay. Transportation energy is highly depending on the distance and the mode (rail, truck, barge, or conveying) of transportation. It is about the same for clay as for flyash for the same distance and mode. In contrast, the energy used in mining pertains to clay only, not flyash coming from the silos of power plants. These are additional factors that should be considered when comparing the energy and the cost of manufacturing flyash bricks versus clay bricks, especially for individual brick factories. They are to be ignored in this general (generic) study so that they will not obscure matters.

3. Comparing Flyash Brick with Clay Brick

(a) Energy Use

From the foregoing assessment, the energy consumed in manufacturing bricks of 8”x 4”x 2.25” size is 244 Btu for each flyash brick, and 5944 Btu for each clay brick. They differ by 24 times. This shows the great savings in energy that can be accomplished by manufacturing flyash bricks instead of clay bricks. For each brick, the saving in energy will be approximately 5944 Btu minus 244 Btu, which is 5700 Btu.

(b) Energy Cost

Now that the energy consumed in manufacturing clay bricks and flyash bricks have been determined, the result can be used to estimate the energy cost (when electricity is used) or fuel cost (when fossil fuel is used) for the two cases. They are estimated as follows:

Flyash Brick – The energy used in manufacturing flyash brick in both mixing and compaction are electrical energy, and in curing can be either electrical or other forms such as natural gas when natural gas is available, or the waste heat from steam if the flyash brick factory is located at a coal-fired power plant. The most conservative assumption is that the heat needed for curing also comes from electricity. In this case, all the energy needed for manufacturing flyash bricks will be supplied by electric power. Because 244 Btu is needed, it is equivalent to 0.0715 kwh. In Missouri, electrical energy cost is approximately 6 cents per kwh. Therefore, the energy cost in manufacturing flyash bricks in Missouri will be approximately 6 x 0.0715 = 0.43 cent or $0.0043 per brick, which is very low. This means the flyash bricks are cost-effective in terms of energy use.

Clay Brick – Assume that most of the energy used in manufacturing clay bricks is derived from burning natural gas, which is the most common way of heating kilns in the United States. The price of natural gas in Missouri is approximately $1.00 per therm (10⁵ Btu), which is equivalent to 1 cent per thousand Btu. Since clay brick manufacturing requires the use of approximately 5944 Btu per brick, the energy cost is approximately (5944/1000) x 1 = 5.94 or approximately 6 cents per brick.

4. Comparing with EPA Data

In a 2003 EPA report¹, the energy used for producing each ton of clay brick, in MBtu (million Btu), is given as: 2.0 (electricity), 3.0 (natural gas), 0.1 (diesel), and 5.1 (total). How do these values in the EPA report compare with the foregoing analysis for clay bricks?

In the foregoing analysis, we found that manufacturing each clay brick requires 5944 Btu of energy approximately, mostly from natural gas used for firing bricks in kilns. Assume that each brick weighs 4.5 lbs, each ton of bricks consists of 444 bricks. Therefore, the energy required for manufacturing one ton of bricks is 444 x 5944 = 2,640,000 Btu or 2.64 MBtu. This is 12% less than the 3.0 MBtu cited in the EPA report for the natural gas used in manufacturing each ton of clay bricks. The main reason for this difference is that while the current analysis includes only the energy consumed in the combustion of natural gas, the EPA figure also includes pre-combustion energy (i.e., the energy used in producing and transporting the natural gas before it is combusted at the clay brick factory). When the pre-combustion energy is excluded, the EPA report gives the amount of energy consumed from burning natural gas at brick factories as 2.67 MBtu, which is only 1% different from what is found in the current analysis.

The only major difference between the EPA report and the current analysis is the amount of electrical energy used in clay brick manufacturing. In a clay brick factory, electricity is needed for a number of purposes such as lighting the facilities and offices, running the various equipment including extruders (or compactors), overhead cranes, conveyors, robotic equipment, air conditioners (in summer in the offices), and various control equipment . For simplicity, such consumptions of electricity have not been included in the current analysis. Had they been included, the difference between manufacturing a clay brick and a flyash brick would have been greater than the 5700 Btu calculated before.

5. Conclusion

The foregoing analysis shows that manufacturing flyash bricks uses only about 1/24 (one-twenty-fourth) or 5% of the energy used in manufacturing clay bricks. By manufacturing flyash bricks instead of clay bricks, at least 5700 Btu of energy can be saved for each brick, and at least 2.5 MBtu of energy can be saved for each ton of bricks.

While the energy cost for manufacturing flyash bricks is less than 0.5 cent per brick, for manufacturing clay bricks the energy cost is about 6 cents per brick. This shows that more than 5 cents per brick can be saved (from energy cost alone) by manufacturing flyash bricks instead of manufacturing fired clay bricks.

Other cost savings also can be derived from capital costs, resulting from not using kilns and no need for emission control equipment. Reduction in social costs (costs to the public instead of the manufacturer) and environmental benefits of using flyash bricks instead of clay bricks will be treated in a separate document. Finally, because fly ash brick factories do not use fossil fuel and hence have zero emission, getting permit from government to build the factory should be must faster and easier.

For comments or questions about this document, please contact Henry Liu (Phone: 573-442-0080; E-mail: fpc_liuh@yahoo.com ).