Materials 4D03 Corrosion Report12/04/14

Biomass Power Heat Exchanger

Process-Based Solution

Submitted by:Jesse Carreau, 1219430

Carlos Orellana, 1213642Eryk Taylor, 1220016

Patrick Fondevilla, 1213786

Submitted to:Dr. Noël

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1.0 INTRODUCTION

1.1 Problem StatementA biomass power heat exchanger has severely corroded after just six years of

operation and must be replaced. An investigation will be completed in efforts to forge a

process-based solution to understand why and how this happened and to significantly

reduce the amount of corrosion of a new heat exchanger.

1.2 BackgroundA heat exchanger is a device of varying size used frequently in industry as well as in

households. A heat exchanger can be found in anything from a fridge to a plane engine.

These practical machines are so widely used because it saves money by utilizing as much

heat as possible. They also transfer heat without interfering with a reaction or a process

because there is no transfer of fluid. Instead, there is a bypass of two fluids whose heat

will be changed by the other. In retrospect, a heat exchanger involves the transfer of heat

by introducing and running a contained fluid through some sort of network where another

fluid is present resulting in a desired change of temperature of both fluids. With respect to

this investigation, the heat exchanger involves the burning of biomass, which is just

recycled and waste wood [1].

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Figure 1: Macroscopic inspection of the corroded pipes.

As it can be seen in Figure 1, air is driven through the inside of the above pipes,

under which the biomass is burned and the combustion gases are sent through a chimney

flue to contact the pipes. The material of the pipes and majority of the heat exchanger is

ASTM 513 MT-1010 carbon steel, which is a regular 1010 carbon steel mainly consisting

of iron, with the addition of manganese, phosphorus, sulphur and of course carbon. The

combustion gases within the chimney flue consist of CO2, H2O, O2, SOx, NOx and will be

considered throughout the investigation. As it can be seen in Figure 1, the corrosion at

the very beginning (within the first meter) is much worse than the rest and this will be the

focus point of the early investigation.

2.0 FORMS OF CORROSION

Many factors contribute to the corrosion of this carbon steel pipe. Since wood waste

materials are being burned to generate heat energy, several gases are released due to the

combustion reaction that is taking place. Specifically, CO2, H2O, O2, SOx, and NOx are

the main gases that are released. In general, the O2 can cause serious corrosion damage by

attaching to the walls of the pipe and forming oxides. Also, the H2O molecules can attach

to the pipes and cause further corrosion. Wear-related corrosion also contributes to the

depletion of the tube wall thickness. All metals contain some degree of surface defects

and can also contain pits. As the air that flows through the pipe in a laminar fashion flows

over these defects/pits, turbulent flow develops and causes further pitting and corrosion.

This is shown in Figure 2 below.

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Figure 2: Development of turbulent flow, causing further corrosion.

These

corrosion forms, however, would contribute to the corrosion along the entire 4 meters of

the pipes. As one can see in Figure 3 above, the most significant corrosion occurred

within the first meter of the pipes and the final 3 meters were only depleted by

approximately 0.25 millimeters. Therefore, since the corrosion forms aforementioned

only contribute to the uniform corrosion of the pipes, a different form of corrosion must

have caused such significant damage in the first meter of the pipes. The surrounding

temperature is known to be 170°C and the inlet temperature of the combustion air is

known to be 37°C. As the flue gas rises over the pipes, the large temperature gradient

between the air in the pipes and the surrounding air produces condensation on the outside

of the pipe. The moisture is removed from the flue gas since it is cooling down when it

interacts with the surface of the pipes and this moisture is left on the surface of the pipes

in the form of condensation. As the air moves through the pipes and is heating up, a

smaller temperature gradient will exist between the air in the pipe and the flue gas and as

a result less condensation occurs in the rest of the pipe. As seen in Figure 3, the most

significant corrosion damage happens in the first meter of the pipes after which the air is

heated to the point where there is a smaller temperature gradient. This smaller

temperature gradient produces much condensation, leading to reduced corrosion damage

in the final 3 meters of the pipes.

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Figure 3: Tube Wall Thickness vs. Distance from Tube Sheet

In Figure 1 above, one can see how the corrosion took place on the bottom half of the

pipes, and not so much on the top half. This justifies why the likely mode of corrosion

was due to the condensation. The moisture would drip down the pipe due to gravity,

leading to corrosion on the bottom half of the pipe. The triangle surrounding the data

point in Figure 3 indicates the point at which the most significant corrosion damage

ends, which is precisely at the first meter mark of the pipes. After this point, more

uniform corrosion is present that is of much less significance. If the temperature at

precisely the first meter of the pipes can be calculated, then a plausible solution can be

implemented by pre-heating the air before entering the biomass power heat exchanger

pipes to that of the temperature at the first meter of the pipes. This will lead to much less

significant corrosion damage due to condensation and will extend the life of the pipes

greatly. The calculations and pre-heating method will be discussed in the following

section.

3.0 SOLUTION

In order to solve the corrosion in the carbon steel pipe, the condensation formed from

the contact of the hotter flue gas with the cooler air in the pipes must be prevented or

reduced. Completely preventing the formation of condensation is not feasible because hot

air must heat up the cold air for the heat exchange to work, therefore condensation will

always occur in the pipes. To account for this, the amount of condensation will be

reduced instead, and this process-based solution will reduce the amount of corrosion

forming on the outside of the pipe.

As discussed before, a significant amount of corrosion formed in the first meter of

pipe, so by calculating the temperature of the air at this point, the temperature that the

cooler air should be increased to in the solution will be known. A few assumptions were

made before the temperature was calculated. The velocity of air was assumed to be 20m/s

[2]. Using the range of diameters for ASTM 513 from ASTM International, the inner and

outer diameters were assumed to be 0.15m and 0.2m respectively [3]. Through

calculations shown after, it was determined that the length of the pipe in the heat

exchanger should be approximately 6.48m. Finally, the properties for ASTM 513 MT-

1010 Carbon Steel were assumed to be similar to AISI 1010 Carbon Steel [4].

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To reduce condensation, the temperature of the cooler air will be increased so that the

difference in temperature of the flue gas and pipe air will be decreased. A T6 aluminum

alloy concentric pipe will be installed at the inlet of the carbon steel pipe. T6 aluminum is

a cheap metal, priced at approximately $6/cm from a metal supplier, Metal Supermarkets.

It is also an effective metal because it has a high thermal conductivity (177 W/m*K) [2],

which measures how easily heat can travel through material.

In Figure 4, the lighter region represents the T6 aluminum alloy concentric pipe and

the darker area is the carbon steel pipe. Steam at 100ºC will flow through the concentric

pipe and heat up the cooler air flowing through the carbon steel pipe and heat it from

37ºC to approximately 90ºC. The steam flow will run parallel to the cooler air flow. By

decreasing the temperature gradient of the flue gas and the pipe, the amount of corrosion

formed in the pipe should decrease.

4.0 CONCLUSION

It was determined that in order to reduce as much oxidation as possible, attention

was focused on the corrosion being caused by the precipitation. In order to do this the air

must be pre-heated to decrease the temperature difference. Therefore the suggested

solution is to install an inexpensive heat exchanger to warm the air entering the biomass

heat exchanger and this will decrease the amount of corrosion present.

5.0 REFERENCES

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Figure 4: Concentric pipe, parallel flow [3]

[1] Woodford, C. (2009) Heat exchangers. Retrieved on Nov. 29 from http://www.explainthatstuff.com/how-heat-exchangers-work.html.

[2] Fluid Velocities in Pipes. (n.d.). Retrieved November 30, 2014, from

http://www.engineeringtoolbox.com/fluid-velocities-pipes-d_1885.html

[3] Standard Specification for Electric Resistance Welded Carbon and Alloy Steel Mechanical Tubing. (n.d.). Retrieved November 30, 2014, from http://www.astm.org/Standards/A513.htm

[4] Incropera, F. (2007). Ch 11 Heat Exchangers, Appendix A. In Fundamentals of Heat and Mass Transfer (6th ed.). Hoboken, NJ: John Wiley.

6.0 Appendix

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