P16487: Biochar Kiln Heat Recovery System

Preliminary Detailed Design

Table of Contents

Phase Summary

During this phase, prototyping was used to simulate engineering requirements pertaining to our projected design. Theoretical models were validated by conducting engineering analyses on prototype test results. This gave us the information we needed to determine detailed design specifications for our heat recovery system in order to meet heat and flow requirements.

Team Vision for Preliminary Detailed Design Phase

What we accomplished during this phase

Team Shared Vision for Phase 4

Test Plans & Simulation

Link to Dehydrator Prototype Build Plan

Link to Dehydrator Prototype Test Plan

Link to Water Pasteurizer Prototype Build/Test Plan

Prototyping, Engineering Analysis

Dehydrator Box Prototype

Testing & Results

1) To determine how much heat dissipates depending on distance from fire. 2) To determine if our mass flow rate calculations are accurate for our prototype. 3) To determine the relationship between temperature and mass flow rate. 4) To determine at different flow rates the time it takes to dry leaves.

From the raw data collected during testing, the max temperature inside the box was only 64.9C. According to our engineering requirements, we need a minimum value of 70C and ideally 90C. To achieve this requirement, the box needs to be located closer to the kiln.

Our experiment measured the reduction of moisture content of basil leaves over time. For our prototype, it took 15 minutes to evaporate one gram of water. Increasing flow rate for our actual dehydrator should increase the rate of evaporation.

Engineering Analysis

After the testing, an engineering analysis was conducted using the dimensions and temperatures of the experiment. The experimental volumetric flow rate was then compared to the calculated volumetric flow rate.

For this analysis, the dehydrator box was modeled as a pipe with a friction factor of .1. The density of ambient air is assumed to be 1.225 kg/m^3. The temperature of the ambient air is 11C, which was the ambient air temperature during our experiment. Air is treated as an ideal gas in fully laminar flow.

From this analysis, we can conclude that our method of measuring the volumetric flow rate during the experiment was not accurate. This is most likely due to the bag being too stiff and too small. However, even the calculations show an extremely small flow rate that will need to be increased for our actual dehydrator by altering the chimney and intake pipe dimensions.

Dehydrator Prototype Testing Conclusions

All the data can be found in this Excel File.

Water Pasteurizer Prototype

Testing & Results

Data shown for smallest three restrictor caps

This is a graph of the the temperature of output water (Blue dots) with constant known flow rate.

The Red line is a trend-line from Equation [3.a] which is used to determine how long it would take to heat water up to the desired temperature in this scenario.

To reach Temp = 70, time = 412min = 6.9hr

This graph shows data taken during our test when we closed off the outlet of the pipe. This was conducted to determine how long it would take to heat water in the pipe with no flow rate.

Red line shows the temperature of output water with closed pipe over a 10 min period.

Blue line is an extrapolated trend-line derived from Equation [3.b] and assumed linear. This equation is used to determine how long it would take to heat the water in the pipe to a desired temperature.

To reach a Temp = 70; Time = 31 min

Engineering Analysis

These four equations are used throughout our data analysis. Heat Recovery (Q):

The top two graphs are the heat recovered from the kiln calculated using Equation [1].


Heat Transfer Coefficient (U):

The bottom two graphs are the combined heat transfer rate which corresponds to respective Q values. Equation [2] is used.



Following the above flow chart we determined that water was exposed to the heat from the kiln for ~90 seconds.

Based on temperatures measured during high-flame periods, we anticipate the need to triple the heat exposure time.

Accounting for 60 seconds at desired temperature (70 oC), our total heat exposure time will need to be ~430 seconds, or 7.25 minutes.

Following the flow chart backwards, we can determine a necessary length of 5.37 meters.

Water Pasteurizer Prototype Testing Conclusions

Drawings, Schematics, Flow Charts, Simulations

Tea Leaf Dryer

From the prototype calculations, a sensitivity analysis was conducted on dimensions that we can alter to increase the flowrate. A sensitivity analysis was done for the height of the dehydrator, height of the chimney, length of the intake pipe, diameter of intake pipe, and diameter of chimney. The results are shown in the graphs below. Although not shown in the graphs, it is also important to note that the “diameter” of the dehydrator box has a significant effect on the flow rate. The flowrate increased dramatically just by scaling up the dehydrator box. From the sensitivity analysis, heights of the dehydrator and chimney have the greatest sensitivity to flow rate of the variables graphed.

Using the insight gained from the sensitivity analysis, variables were adjusted to achieve desirable flow rates. These values are highlighted in the chart above. A higher flow rate can also be achieved by minimizing the bend in the intake pipe.

Water Pasteurizer

The proposed heat exchanger design will be suspended by the support racks, which are located on either side of the kiln. At it’s lowest point, the heat exchanger will hang at half of the kiln height. The intention for this is to keep the heat exchanger as close to the flames as possible. As the level of biomass rises, the heat exchanger will shifted up to a higher level on the support rack to avoid interfering with the biochar.

The rack support structure will be similar to that used in the test validation model. The heat exchanger coil will be rigidly suspended from the pictured crossbar.

Cut and welding techniques will be used to manufacture the desired shape of the heat exchanger. The heat exchanger coil is intended to be manufactured out of mild steel.

Note: Length of heat exchanger coil was driven by flow rate used during the validation test. We anticipate a longer required length for optimal flow rate and analysis will be complete using known head pressure and minor/major head losses.

Engineering Requirements

Engineering Requirements

Bill of Material (BOM)

Prototype BOM pdf

The proposed cost for the full system build is well under the team's budget of $500. The section labeled as "Components Missing" details the estimated cost for the listed components. Actual costs for this section were not found because this section of the system has not been considered in detail yet.

It should be noted that this total cost does not yet meet our customer requirement keeping the full system less than $250.

Design and Flowcharts

Risk Assessment

Risk Assessment Phase 4

Phase 4 Action Items

Phase 4 Action Items

Plans for Next Phase

Team Shared Vision Phase V

Supplemental Review Material

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