Multidisciplinary Senior Design Conference Kate Gleason College of Engineering

Rochester Institute of Technology Rochester, New York 14623

Copyright © 2017 Rochester Institute of Technology

Project Number: P17487

KONTIKI KILN HEAT RECOVERY SYSTEM

Abigail Higgins Industrial Engineering

Leah Matczak Mechanical Engineering

Elisapeta Santoro Mechanical Engineering

Kelsey Thompson Mechanical Engineering

Thane Vollbrecht Mechanical Engineering

ABSTRACT The KonTiki kiln heat recovery system uses excess heat from an open flame to heat water in a reservoir for

showering purposes. The target temperature is 40°C, with an accepted target temperature of 35°C. The water

temperature should not exceed 49°C to avoid scalding hazards.[1] The system was tested using thermocouples and a

stopwatch and collecting data every second to estimate the amount of heat recovered within a given time frame. A

total of 6 trials were conducted, with 4 of the trials applying specifically to the design discussed in this paper. The

system was successful at heating water consistently, and improved in time after each trial. An instruction manual is

also provided to ensure consistency during burns and to address safety hazards that may arise from operating the

system.

NOMENCLATURE

Tf, Final water temperature

Ti, Initial water temperature

t, Total time to heat water

cp, Specific heat coefficient of water

ρ, Density of water

V, Volume of water heated

m, Mass of water heated

Q, Heat transferred

Q̇, Heat transfer rate

Ac, Coil surface area

lc, Coil pipe length

dc, Coil diameter

ℎ̅, Average heat convection

coefficient

INTRODUCTION The KonTiki kiln is a low-cost system developed by the Ithaka Institute and used to create biochar from

agricultural waste. The device, which is conical in shape, operates by consistently feeding burn material into an open

flame. The process reduces carbon emissions by pyrolysis, which is the decomposition of material using high

temperatures, thus making it a sustainable option for eliminating waste. Different forms of the biochar kiln currently

exist, with design modifications such as an added chimney for trapping gases, a pit design that eliminates the kiln

body material, and varying shapes and sizes. The goal of this project is to capture the excess heat for a useful

purpose in a rural environment, specifically a small community in Nepal. Ideally, the heat recovery system will be

low-cost, simple to operate, and successfully repurpose the heat.

The first iteration of this project (P16487) investigated water pasteurization and tea leaf drying. Water

pasteurization was inconsistent due to changing flow of the fluid during a single pass through the system, which

yielded a large range of water temperature values (between 35-100°C[2]). Tea leaf drying was successful, with a 2%

moisture content and 0.153kg produced during the experimental trial.[2] Following the recommendations from the

first iteration, this project aims to focus primarily on a consistent water heating process. Pasteurization was

discussed as an option, but could not be safely achieved on a low budget. Thus, the heat recovery system for this

project will heat water for showering purposes, allowing the next iteration to investigate water pasteurization once a

reliable system is in place.

METHODOLOGY Concept Selection:

Interviews were conducted with the customer and used the first iteration of this project as a primary reference.

Lists of customer and engineering requirements were created following these interviews and research. Since water

pasteurization had more variables and requirements that may not be met within the budget, general water heating for

showering was selected as the main objective. From this objective, the functions were branched out and summarized

in Fig. 1 below:

Figure 1: Functional decomposition; moving down the diagram answers “How” and moving up answers “Why”

“Collect heat” was the main function that was used for the brainstorming process. After developing a

morphological chart and running several Pugh analyses, the team decided to maintain the original coil design and

attach the coils to a single reservoir, which will create a natural convection loop to allow multiple passes for the

water. A schematic of the design is shown in Fig. 2 below:

39.4" [1 m]

45.5"

33.8"

0.5"

Reservoir

Platform

Kiln

Coils

T1

T2

Desired amount of water filled

Thermocouple

Figure 2: Schematic of the system and necessary parts

Proceedings of the Multi-Disciplinary Senior Design Conference Page 3

Copyright © 2017 Rochester Institute of Technology

Calculations:

To simplify the analysis of the system, the reservoir is treated as a control volume, with water passing through

the inlet and outlet. The following assumptions are made prior to the analysis:

• Conduction from the flame through the copper coils to the water is the primary heat transfer

• Minimal leakage occurs, so mass of water in the reservoir remains constant

• The water is heated right before boiling, thus steam effects are negligible to the water mass loss

• Density and specific heat coefficients of water are referenced values[2]

• Density change due to temperature is within 1% of the referenced value, thus this value is assumed to be

constant throughout the cycle

• Reservoir is covered at the top, resulting in minimal convection

transferred

cond,air

Tamb

ρwater

Vwater

mwater

Inlet Outlet

Exclude outlet

from control

volume

Figure 3: Control volume of the reservoir component

Additionally, Equations 1 and 2 describe the heat transfer from the pipe:

𝑄𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 = 𝑚𝑐𝑝(𝑇𝑓 − 𝑇𝑖) (1)

�̇�𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 = 𝑚𝑐𝑝(𝑇𝑓−𝑇𝑖)

𝑡 (2)

The testing process is outlined in Fig. 4 below. From this process, the team can obtain the parameters to use for

Equations 1 and 2:

Figure 4: Testing process

Set up kiln, heat recovery system, reservoir, and

platform

Place thermocouples and water in reservoir as shown in Fig. 2

• Record initial height of water then multiply by cross-sectional area to get volume of water

Place feedstock in kiln then ignite feedstock

Start timer and start collecting temperature data

on thermocouplesCommence burn

When burn is finished and quenching process occurs,

stir water, take final temperature reading, then

stop timer

An additional control volume analysis is applied to the heat recovery system itself to estimate the losses due to

ambient temperature on the performance. The system is modeled as a continuous hollow cylinder that is partially

insulated, as shown in Fig. 5 below:

transferred

pipe

lost

Figure 5: Control volume of uncoiled heat recovery system

The following assumptions are made prior to the analysis:

• Steady state, fully developed, laminar flow in the pipe, which has a small diameter of dc≈15.9mm

o This allows use of the Nusselt number Nu=3.66 for internal forced convection[2]

• No heat generated nor heat stored

• Biochar/fire temperature is assumed to be constant and uniform

• Inner surface temperature of the kiln is the same as the biochar/fire temperature

• The section of piping that is engulfed in the biochar/fire is assumed to be insulated and 13m in length[5],

resulting in minimal heat losses

• The section of piping that is not engulfed in the biochar/fire is assumed to experience the heat losses

• Treat pipe as a hollow cylinder

• Pipe experiences 1-dimensional flow in the radial direction

• Pipe surface temperature is equal to fire temperature, chosen to be 1027°C[4]

By conducting another control volume approach, the following derivation describes the expected heat that can

be delivered to the reservoir system:

�̇�𝑖𝑛 + �̇�𝑔𝑒𝑛𝑒𝑟𝑎𝑡𝑒𝑑 = �̇�𝑜𝑢𝑡 + �̇�𝑠𝑡𝑜𝑟𝑒𝑑

�̇�𝑝𝑖𝑝𝑒 = �̇�𝑙𝑜𝑠𝑡 + �̇�𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑 (5)

First, the convection coefficient for water is determined by Equation 6:

ℎ̅ =𝑁𝑢̅̅ ̅̅ 𝑘𝑓

𝐿 (6)

This yields a value of:

ℎ̅ =(3.66)(0.6𝑊/𝑚𝐾)

13.4𝑚= 0.164 𝑊/𝑚2𝐾

A heat circuit is applied to the pipe to yield Equation 7:

�̇�𝑝𝑖𝑝𝑒 =𝑇𝑓−𝑇𝑖

ln (𝑟2𝑟1

)

2𝜋𝐿𝑘𝑐+

1

ℎ𝜋𝑟22

(7)

Using Trial 6 data, an example calculation is shown below:

�̇�𝑝𝑖𝑝𝑒 =(1027−15.8)℃

ln (0.015875𝑚

0.0127𝑚 )

2𝜋(13.4𝑚)(401𝑊

𝑚𝐾 )+

1

(0.164𝑊/𝑚2𝐾)𝜋(0.015875𝑚)2

= 177.77𝑊 for a single pass in the system

Multiplying this value by the total time to heat the system yields the heat transferred into the pipe, Qpipe. This is

related to the efficiency of the system, as shown by Equation 8:

𝜂 =𝑄𝑡𝑟𝑎𝑛𝑠𝑓𝑒𝑟𝑟𝑒𝑑

𝑄𝑝𝑖𝑝𝑒×100 (8)

The results of these calculations did not balance out correctly, which suggests that convective losses might need

to be modeled more explicitly. Thus, the team relied primarily on Equations 1 and 2 to quantify the amount of

energy recovered in the system. Simulations were also conducted in ANSYS to validate the system results and are

discussed on the project website. [5]

Proceedings of the Multi-Disciplinary Senior Design Conference Page 5

Copyright © 2017 Rochester Institute of Technology

RESULTS AND DISCUSSION The physical system design is shown in Fig. 6 and is tested on a Garden KonTiki (1100) model. This model has

an upper diameter opening of 1.155m that tapers down to a 0.29m diameter.[6] A cylindrical heat shield is also

placed around the kiln for operator safety. The heat recovery system sits about 0.45m above the bottom of the kiln

and does not interfere with biochar production. The reservoir is placed on the platform and is close enough to the

kiln that the piping can be attached. The valves on the inlet and outlet are switched into the open position to allow

water flow. The final cost of the system is $144.59, which is below the customer requirement of <$150 to install.

Figure 6: Final system being tested

The team conducted 6 burn trials total, with 4 of the trials using the testing process outlined in Fig. 4. These 4

trials are summarized in Fig. 7 and Table 1. The temperature and time target values are assigned from the

engineering requirements:

Figure 7: Temperature data per trial

Table 1: Summary of main variables and final heat recovered

Trial Mass of

water [L]

Ambient

temperature

[°C]

Starting water

temperature

[°C]

Final water

temperature

[°C]

Time

[hours]

Total heat

recovered

[kJ]

Heat rate

[W]

3 75.7 12.22 14.9 32.7 1.05 5622.50 1561.591

4 46.56 13.89 15.8 23.9 1 1576.02 437.79

5 48.26 -6.67 5 35.8 1.12 7246.44 1537.89

6 53.83 11.11 15 42 0.633 6061.93 2660.66

Figure 8: Summary of heat recovered

Figure 9: Range of heat values based on experimental results

The variance in the heat recovered, as shown in Fig. 9, is primarily due to feedstock material and weather

conditions. Trial 4 had the lowest performance since the feedstock consisted of loose items (dried banana chips,

paper) and did not create proper biochar. Trial 6 had the fastest performance and was also conducted in the warmest

weather conditions (Tamb=11.11°C/52°F). After Trial 4, the biochar and feedstock process were standardized in an

operations manual to provide more consistency. Trial 5 started with the lowest water temperature due to snow being

added to the reservoir to meet the desired water level. During this trial, the desired ending temperature was still

achieved, thus proving the capability of the system for less ideal conditions.

0

1000

2000

3000

4000

5000

6000

7000

8000

3 4 5 6

Hea

t (k

J)

Trial

Heat Recovered per Trial

Target

Proceedings of the Multi-Disciplinary Senior Design Conference Page 7

Copyright © 2017 Rochester Institute of Technology

As shown in Fig. 7 above, the time target was largely overestimated since all trials experienced significant heat

recovery in roughly 60 minutes. This shows that the time target could be decreased for this system or that the

reservoir could be refilled multiple times during a burn.

CONCLUSIONS AND RECOMMENDATIONS The main goal of this project was met, as the desired water temperature was achieved and the burn process was

standardized by the end of testing. The requirements that were not met are summarized in Table 2 below:

Table 2: Summary of missed engineering requirements

Requirement Target

Value

Accepted

Value

Actual

Value Limitation

Time to heat

recovery system

<15

minutes

<25

minutes

>60

minutes

The coil system was not manipulated from the original

design. Copper was kept as the ideal material due to

its price and conductivity. Investigation of different

piping sizes was simulated, but not tested.

Product lifetime >2 years 5 years Unknown Testing for product lifetime was not completed, rather

researched to create recommendations.

Number of

components <5 7 20

Several components were purchased to create reliable

connections between systems. However, these parts

may not be manufactured overseas, and may be above

the budget.

Testing of multiple coil systems should be tested. The effects of diameter, number of coil rotations, material, or

depth placement within the kiln may improve the performance of the heating time, or provide a cheaper option for

users. Additionally, testing for pit designs with the same system should be investigated, as pit designs require less

components than actual kiln systems. The pit design requires the user to dig a hole and line the bottom with rocks,

then commence burn cycles as done previously. Eliminating components will allow for improved adaptability of the

design, especially in areas with less resources. One option is to find a way to build the system by placing the coils

into any open reservoir container without connectors, which were the bulk of the added components. This could be

achieved through siphoning, but different materials may be required. More information is available on the project

website.[5]

REFERENCES [1] American Society of Sanitary Engineering Scald Awareness Task Group. (2012). Retrieved from

http://www.asse-plumbing.org/WaterHeaterScaldHazards.pdf.

[2] Bossung, K., Derbyshire, K., Gustavesen, Z., Smith, C., Tran, P. (2016). Project Number: P16487 Biochar Kiln

Heat Recovery System. Retrieved from

http://edge.rit.edu/edge/P16487/public/Detailed%20Design%20Documents/P16487%20Technical%20Paper.pdf

[3] Bergman, T. L., Lavine, A., Incropera, F., DeWitt, D. (2011) Fundamentals of Heat and Mass Transfer. 7th ed.

Hoboken, NJ: John Wiley & Sons.

[4] Ithaka Institute. Kon-Tiki flame curtain pyrolysis. Retrieved from http://www.ithaka-institut.org/en/ct/101-Kon-

Tiki-flame-curtain-pyrolysis.

[5] P17487: KonTiki Kiln Heat Recovery System. (2017). Retrieved from

http://edge.rit.edu/edge/P17487/public/Home.

[6] Garden Kon-Tiki (1100) Mechanical Drawing. Retrieved from

https://edge.rit.edu/edge/P17487/public/Detailed%20Design%20Documents/CAD/kontiki_kiln.pdf.

ACKNOWLEDGMENTS

• Kathleen Draper and the Ithaka Institute (Customer)

• Dr. Michael Schrlau (Subject Matter Expert – Heat transfer)

• Dr. Robert Stevens (Subject Matter Expert – Heat transfer)