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ENVIRONMENTALLY-BENIGN CONVERSION OF BIOMASS

RESIDUES TO ELECTRICITY

A Thesis presented

by

Andrew Davies

to

The Department of Mechanical and Industrial Engineering

In partial fulfillment of graduation requirements for

Master of Science

in

Mechanical Engineering

Northeastern University

Boston, Massachusetts

May 2013

Copyright (©) 2013 by Andrew Davies

All rights reserved. Reproduction in whole or in part in any form requires the prior written

permission of Andrew Davies or designated representative.

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Abstract

As petroleum resources are finite, it is imperative to use them wisely in energy conversion

applications and, at the same time, develop alternative energy sources. Biomass is one of the

renewable energy sources that can be used to partially replace fossil fuels. Biomass-based fuels

can be produced domestically and can reduce dependency on fuel imports. Due to their abundant

supply, and given that to an appreciable extent they can be considered carbon-neutral, their use

for power generation is of technological interest. However, whereas biomasses can be directly

burned in furnaces, such a conventional direct combustion technique is ill-controlled and

typically produces considerable amounts of health-hazardous airborne compounds [1, 2]. Thus,

an alternative technology for biomass utilization is described herein to address increasing energy

needs in an environmentally-benign manner. More specifically, a multi-step process/device is

presented to accept granulated or pelletized biomass, and generate an easily-identifiable form of

energy as a final product. To achieve low emissions of products of incomplete combustion, the

biomass is gasified pyrolyticaly, mixed with air, ignited and, finally, burned in nominally pre-

mixed low-emission flames. Combustion is thus indirect, since the biomass is not directly

burned, instead its gaseous pyrolyzates are burned upon mixing with air. Thereby, combustion is

well-controlled and can be complete. A demonstration device has been constructed to convert the

internal energy of plastics into “clean” thermal energy and, eventually to electricity.

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Acknowledgments

I would like to thank my academic advisor, Dr. Yiannis A. Levendis, and the combustion lab

manager, Chuanwei Zhuo (Ph.D. candidate) for their help and assistance in developing the topic

for my thesis and collaboration with designing the overall pyrolysis system. Rasam Soheilian and

Saber Talebi Anaraki for their support on this project with both prototype construction and

engineering analysis, and Northeastern University machinist Jonathan Doughty for his expertise

and helpful advice on manufacturing parts and component-level system design. In addition, I

would like to acknowledge my employer GE Aviation, which made my education possible by

sponsoring time off from work to pursue my degree.

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Table of Contents Abstract ........................................................................................................................................... 2

Acknowledgments........................................................................................................................... 3

TABLE OF FIGURES .................................................................................................................... 6

CHAPTER 1 ................................................................................................................................... 8

Introduction ................................................................................................................................. 8

CHAPTER 2 ................................................................................................................................. 11

DDGS Background ................................................................................................................... 11

King Grass Background ............................................................................................................ 13

CHAPTER 3 ................................................................................................................................. 14

Experimental Apparatus............................................................................................................ 14

Experimental Procedure ............................................................................................................ 21

CHAPTER 4 ................................................................................................................................. 23

Results and Discussion ............................................................................................................. 23

Elimination of Tars and Waxes from the Effluent Gas............................................................. 29

Proof of Concept: Use of Biomass Pyrolyzate Gasses to Generate Electricity ........................ 32

CHAPTER 5 ................................................................................................................................. 33

Conclusions ............................................................................................................................... 33

Acknowledgments......................................................................................................................... 35

Works Cited .................................................................................................................................. 36

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SUPPLEMENTAL CALCULATIONS ........................................................................................ 39

APPENDIX 1: Required amount of biomass feedstock to operate the pyrolytic gasifier in a

self-sustaining mode, i.e., without external heating.................................................................. 39

APPENDIX 2: Calculation of Self-Sustaining Pyrolysis System Efficiency ............................ 45

APPENDIX 3: Thermocouple temperature measurement correction for radiation effects. ..... 47

APPENDIX 4: Economic Considerations for Large-Scale Operation ..................................... 49

APPENDIX 5: CAD Models and Drawings Generated for Assembly and Fabrication of

Components .............................................................................................................................. 54

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TABLE OF FIGURES

Figure 1: One gram samples of crushed corn-residue-based DDGS (left), and King Grass-base

biomass (right). ............................................................................................................................. 12

Figure 2: Proximate Analysis of King Grass and DDGS. ............................................................ 12

Figure 3: Laboratory-scale pyrolytic gasification apparatus, specifically designed and constructed

for the needs of this study. ............................................................................................................ 17

Figure 4: (a) a schematic of the feeding system; (b) a CAD model of the pyrolysis system; (c)

photograph of pyrolysis chamber with metal plates shown; (d) cross section of gasification

chamber; (e) Gas temperature gradient of gasification chamber. ................................................. 18

Figure 5: Perforated steel plates covered with stainless steel mesh (left) and without mesh (right).

....................................................................................................................................................... 19

Figure 6: Gage pressure holding capability of the furnace tube with time at a furnace set-point

temperature of 850°C. ................................................................................................................... 19

Figure 7: Results of feeding system calibration run for granulated DDGS and King Grass. ....... 22

Figure 8: Percentages of detected hydrocarbon species in King Grass pyrolyzate gases. ............ 23

Figure 9: Comparison of Pyrolyzate Gas Composition. ............................................................... 24

Figure 10: Photographs of nominally premixed flames burning gaseous pyrolyzates of (a) corn-

based DDGS biomass, and (b) King Grass biomass. ................................................................... 25

Figure 11: CO2/CO vs. Equivalence ratio for corn-based DDGS and King Grass biomasses. .... 27

Figure 12: Velocity profile of the volatiles inside the chamber.................................................... 28

Figure 13: Steam engine apparatus set up in close proximity to pyrolysis chamber outlet. ......... 30

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Figure 14: Flame generated by the steam engine burner (left) and still photograph of the steam

engine in operation (right). The coupled electricity generator powers the light bulb. .................. 32

Table of Figures Included in the Appendices

Figure A.1: Schematic Picture of Insulation Thickness and Length (units in inches). ................. 43

Figure A.2: Relation Between the Self-Sustaining Efficiency (ηs) and the Feeding Rate (g/min).

....................................................................................................................................................... 46

Figure A.3: Economic analysis of biomass pyrolysis as used for energy production. ................. 50

Figure A.4: Biomass electrical generation profitability as a function of feed rate. ...................... 51

Figure A.5: Percent share of total running cost for N2 carrier gas. ............................................... 52

Figure A.6: Complete 3-D model of the assembled system (also shown: the frame support

structure). ...................................................................................................................................... 54

Figure A.7: Exploded view (left) and cutaway (right) of pyrolysis chamber. .............................. 55

Figure A.8: 3-D mockup of the frame system constructed to support the pyrolysis chamber. .... 55

Figure A.9: 3-D model (above) and engineering drawing generated (below) for custom

fabrication of the feed hopper. ...................................................................................................... 55

Figure A.10: 3-D model (above) and engineering drawing generated (below) for custom

fabrication of the pyrolysis chamber............................................................................................. 55

Figure A.11: 3-D model and cutaway (above) and engineering drawing generated (below) for the

machining and assembly of the feeding box and bearing support. ............................................... 55

Figure A.12: 3-D model (above) and cutaway view (below) showing construction and assembly

of the feeding system. ................................................................................................................... 55

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CHAPTER 1

Introduction

The fact that fossil fuel resources are finite, led governments and industries worldwide to pursue

alternatives to these types of non-renewable resources. One of these alternative sources of energy

is biomass. There is a significant amount of energy stored in biomass; for instance, Danje [3]

determined the energy content of corn stover and corn cob to be 18.06 and 19.14 MJ/kg (high

heating value, HHV), respectively. US production of energy from biomass sources was

4.76×1012

MJ (4.411×1015

BTU), which amounts to approximately 5.8% of total US energy

production [4,5]. It has been projected that worldwide energy demand will rise by 53% by the

year 2035 with renewable energy sources being one of the fastest growing energy types at 2.8%

year-over-year growth rate [6]. With such rising energy demand, biofuels will play a more

important role and make up a greater percentage of worldwide energy production. Therefore, an

efficient method of converting biomass to usable energy in an environmentally benign manner is

essential to providing energy in the future to fuel the high growth rate of developing countries

and the proliferation of power-hungry electronic devices therein. Efforts have been made on

advanced technology converting the energy stored in biomass into gaseous fuels. Oxidative or

steam gasification (with oxygen present in the carrier gas) and pyrolytic gasification (with

oxygen absent in the carrier gas) are popular methods; highlights are given below.

Gasification of biomass prior to combustion offers several advantages [7], including: ease of

distribution in pipelines, continuous operation, better control of combustion, efficient

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combustion since the correct amount of air can be mixed for optimum combustion, clean

combustion since impurities are removed in the gasifier, high temperature combustion for

making glass or cement, increased heat transfer, and facilitation of chemical synthesis. There has

been a large amount of work on biomass gasification, and includes investigations on tar

elimination and product distribution by using fluidized-beds, implementing different gasifying

agents, changing equivalence ratios, etc. [8-15]. This investigation is focused on pyrolytic

gasification of biomass. There has been prior research on biomass pyrolysis. Zanzi et al. [16]

studied rapid high temperature pyrolysis (1073 K or 800 oC) of two types of biomass (wood and

agricultural residues) in a free-fall reactor that was heated by eight independent electric heaters.

Their experiments were done at heating rates that were also used for fluidized bed reactors.

They observed that rapid heating enhances devolatilization and forms less char than slow

heating. They reported that compared with coal, biomass produces more volatile fractions and

less char. They also reported that biomass pyrolyzes at lower temperatures than coal. Zanzi et al.

[16,17] also worked on rapid pyrolysis of agricultural residues at higher temperatures (800-1000

oC or 1073-1273). Residence time for particles of size 0.5 to 1 mm was reported to vary from 1.4

up to 1.7 seconds. They mentioned that in rapid pyrolysis this residence time was not sufficient

to volatilize all the fractions. They also reported that at these high temperatures the concentration

of CO2 decreased and the concentration of CO in the products increased with temperature in the

aforesaid range. Demirbas [18] mentioned that cracking gaseous hydrocarbons, by increasing

temperature, enhances the production of H2. Demirbas [19] also worked on slow pyrolysis

(having 10 K/s heating rate) of agricultural residues in the temperature range of 677-977oC. They

observed that the yield of char increased by increasing particle size; besides, higher lignin

content in the biomass lead to higher char yield. Chen et al. [20] studied the effects of different

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parameters on pyrolysis/gasification of biomass for gas production and reported that the gas

yield increased significantly by increasing the pyrolysis reaction temperature and/or the

residence time of the volatiles. One of the major observations in pyrolyzing biomass is that by

increasing the pyrolysis temperature, liquid and char yields decrease while gaseous products

increase [17,18].

There are differences between oxidative (or steam) gasification and pyrolytic gasification of

biomass, which consists of three principal components – cellulose, hemicellulose and lignin

[reference]- along with minor amounts of extractives. In the case of lignin, it has been reported

that its pyrolytic gasification yielded slightly lower amounts of char than in its oxidative

gasification, when lignin was gasified in a laminar entertained flow reactor at temperatures of

800 and 1000 oC with residence time of 1 second[21]. It was also reported that mass loss of

lignin during oxidative (or steam) gasification and pyrolytic gasification of lignin in TGA

(Thermogravimetric Analysis) is comparable; however, depending on the type of lignin, this

mass loss can be higher during pyrolytic gasification [21]. Thus, pyrolytic gasification may be

used to maximize the yield of gaseous products.

The purpose of this experimental study is to demonstrate that pyrolytic gasification of biomass

feedstocks can produce a high-energy-content gaseous fuel mix, which can be burned in an

environmentally-benign manner, similar to natural gas, for power generation, process heat and

other energy-related applications. A laboratory-scale system was designed and constructed to

pyrolytically gasify waste solid fuels, such as residual biomass, generate a combustible gaseous

fuel and demonstrate its conversion to heat and, eventually to electricity.

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CHAPTER 2

DDGS Background

DDGS is produced as a byproduct of the ethanol manufacturing process and is created when the

dry grind method of ethanol production is used. Dry grind involves crushing the corn to reduce

the dry particle size via high speed hammer mill. The dried corn is fermented and distilled into

ethanol, whereas the leftover byproducts (stillage) is converted into its constituents, DDGS, wet

distillers, dried grains, etc [22]. DDGS has a long shelf life, due to its low moisture content

(~10-13 wt%). The processing of corn yields 378 L of ethanol and 309 kg of DDGS, per metric

ton [23], and is primarily used as a low-cost high-protein feed for cattle along with other forms

of ethanol production byproducts [24]. Being generated in large volumes as a byproduct of

ethanol production and given its high specific internal energy or heating value (19.8 MJ/kg),

DDGS is an attractive option for biomass-based waste-to-energy generation. The biomass pellets

were crushed and sized on a #8 mesh (2.36 mm) sieve to collect the smaller particles that the

feeding system is optimized to process. A one gram sample is shown below in the left entry of

Figure 1. A breakdown of the composition of DDGS is shown in Figure 2.

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Figure 1: One gram samples of crushed corn-residue-based DDGS (left), and King Grass-base biomass (right).

Proximate Analysis Giant King Grass (% Dry basis) [26] DDGS (% Dry basis) [28]

Volatile Matter 76.3% 78.2%

Ash 4.2% 7.1%

Fixed Carbon 19.4% 14.7%

Total Sulfur 0.13% 0.4%

HHV 18.4 MJ/kg 19.8 MJ/kg

Figure 2: Proximate Analysis of King Grass and DDGS.

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King Grass Background

Giant King Grass is a rapidly growing biomass crop that has similar characteristics to sugar cane.

The King Grass is a proprietary crop produced by Viaspace Inc. that has been grown in

Southeast Asia and has recently been approved for planting and harvesting in the US [25, 26].

Giant King Grass grows extremely fast; it can reach a height of 13 feet in 190 days, which is the

optimum for biomass pellet production. In addition, the King Grass can be continually harvested

every 120 days to ensure a steady production rate. Yields of raw King Grass are 375 metric tons

per hectare with 70% moisture content when grown in tropical climates, which equates to 100

metric tons per hectare of dried king grass [25, 26]. The specific energy content of dry King

Grass is 18.4 MJ/kg, resulting in 1.84 x 106 MJ per hectare, enough energy to power 44

American homes for one year (1.84 x 106 MJ = 511.1 GWh per year) [4]. The composition of

King Grass as compared to DDGS is shown in Figure 2.

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CHAPTER 3

Experimental Apparatus

A laboratory-scale pyrolytic gasifier system has been designed and constructed to accept

granulated or pelletized feedstocks, and to thermally decompose (devolatilize) them via

pyrolysis. The generated gaseous pyrolyzates are then mixed with air and burned in a nominally

premixed burner. For demonstration purposes, the burner is also coupled with a miniature steam

generator and steam engine setup (Wilesco, Model D18) to generate electricity. The pyrolytic

gasifier is shown in Figure 3 and details are shown in Figure 4 Its major components are a

feeding system and a furnace or heating chamber. The feeding system incorporates a reservoir

with a hopper where pelletized feedstocks are stored, an electric motor and an auger-driven

feeding box. Pellets are gravity-fed from the reservoir through the hopper. The variable-speed

electric motor (Leeson Corp. model 985.613F, 8.7 N-m peak torque, 0 – 94 RPM, continuous

duty) drives the horizontally-oriented auger, which carries the pellets from the feeding box to the

vertical purge chamber leading to the furnace. The rotating auger uses a sealed bearing to

minimize leakage from the system and, thus maintain the pressure of the inert carrier gas.

The furnace is defined by a stainless steel tube, whose dimensions are designed to provide

sufficient room for the expansion of the biomass pyrolyzate gases. The furnace is heated by

electrical resistance elements (ATS, Series 3110), rated at 1.43 kW. The furnace is connected to

a proportional integral derivative (PID) loop temperature controller (ATS, Series XT16). This is

a feedback controller that computes an error value based on the difference between the measured

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variable and a desired set point and it tries to minimize such error by varying the inputs. This

allows for precise and reliable regulation of system temperature at any desired set point. The

speed of the electric motor that drives the feeding system is adjusted to obtain the desired feeding

rate of material. The direct-drive motor and screw-type auger system provide a linear

relationship between feeding rate and motor speed (RPM), thus allowing for precise control of

feeding rate. As experiment runs, the motor speed (and thusly the feedstock feeding rate) and the

carrier gas flow are adjusted as necessary to obtain a stable flame operation.

At several vertical locations in the tubular furnace, asymmetrically-perforated disks have been

inserted to intercept the falling biomass pellets and facilitate their gasification (Figure 5). Since

the biomass pellets were crushed to smaller entities (granules), an additional stainless steel mesh

was attached to each perforated plate. In this manner, the biomass granules devolatilize in the

radiation cavity of the furnace, instead of settling at its bottom. In addition to this feature, the gas

exit tube in the chamber is elevated from the bottom, to avoid being plugged by settling chars

and tars, and it is protected by a small conical roof to prevent impingement of remaining

material. The system is fitted with two relief valves, to avoid over-pressuring.

The most leak-prone connection in the system is that between flanges inserted between the

gasification and purging chambers. High-temperature gaskets (THERMA-PUR style 4122

corrugated metal gasket, manufactured by Garlock Sealing Technologies) have been used for

sealing the flanges. The gradient of pressure drop with time is shown in Figure 6. The flange

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bolts were tightened to 45.2 N-m (400 in-lb) and the set pressure was 68.9 kPa (10 psi). The

relief valves (rated at 234.4 kPa/34 psi) were tested at both STP and operating conditions 850 °C.

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Figure 3: Laboratory-scale pyrolytic gasification apparatus, specifically designed and constructed for the needs of this

study.

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Figure 4: (a) a schematic of the feeding system; (b) a CAD model of the pyrolysis system; (c) photograph of pyrolysis

chamber with metal plates shown; (d) cross section of gasification chamber; (e) Gas temperature gradient of gasification

chamber.

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Figure 5: Perforated steel plates covered with stainless steel mesh (left) and without mesh (right).

Figure 6: Gage pressure holding capability of the furnace tube with time at a furnace set-point temperature of 850°C.

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A typical axial gas temperature distribution in the furnace, as measured by a type-K

thermocouple (Omega) is shown in Figure 4e. In this case, the controller was set to maintain a

constant furnace wall temperature of 950 °C. Temperature measurements were taken at each of

the perforated disks in the chamber in order to gauge the uniformity of the heating. The

temperature gradient plateaued with distance in the bottom-half of the chamber, were the

maximum temperatures prevailed.

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Experimental Procedure

In each experiment, granulated biomass was loaded into the hopper, and the feeding system was

sealed. The carrier gas was nitrogen at a flow rate of 1 lpm. Nitrogen was used to purge the air

out of the system and ensure oxygen-free pyrolysis. Thereafter, the biomass granules were

introduced to the system at the desired feeding rate. A typical plot of the feeding mass of

granules with time is shown in Figure 7. The feeding system was detached from the pyrolysis

apparatus and was setup on a bench top where a calibration experiment was carried out using an

electronic mass balance, positioned to catch and measure the output. The hopper was loaded with

granulated biomass feedstocks and the drive motor was run at constant speed, with data readings

taken manually every 5 seconds. Results of three different runs are shown in Figure 7,

demonstrating good repeatability. A rather linear feed rate of approximately 1 g/min is apparent,

at this particular setting of the motor.

During the course of the experiment, the pyrolyzate gasses were then channeled to either the

miniature steam engine burner, a Bunsen burner, or captured via syringe to be analyzed in the

gas chromatograph. The steam engine was operated with its separate set of manual controls to

maintain sufficient speed to illuminate the light bulb, whilst (if used) the Bunsen burner air inlet

was adjusted to vary the air/fuel mixture to capture and record data.

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Figure 7: Results of feeding system calibration run for granulated DDGS and King Grass.

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CHAPTER 4

Results and Discussion

The effluent gases from pyrolytic gasification of the King Grass and the corn-based DDGS

biomass were analyzed for composition and results are tabulated in Figure 8 and graphically in

Figure 9; both N2 and CO2 is used as the inert gas carrier, at a flow rate of 1 lpm. The mass flow

rate of granulated biomass in all experiments was set at 1 g/min. To obtain the chemical

decomposition of the gases, gas chromatography was conducted with a Hewlett Packard gas

chromatograph (model 6890), coupled to flame ionization and conductivity detectors (GC-

FID/CD).

Figure 8: Percentages of detected hydrocarbon species in King Grass pyrolyzate gases.

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Figure 9: Comparison of Pyrolyzate Gas Composition.

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During the combustion experiments, corn-based DDGS and King Grass biomasses were fed to

the pyrolytic gasifier at steady-state steady-flow conditions, and the ensuing pyrolyzates were

channeled to a Bunsen burner. Upon mixing with air therein, the charge was ignited and

nominally-premixed flames were obtained. The resulting flames from both biomasses were blue

in the center with pale orange surrounds, see Figure 10. It is worth mentioning here that

variations in the flow rate of nitrogen did not have any significant effect on the stability or

characteristics of the flames.

Figure 10: Photographs of nominally premixed flames burning gaseous pyrolyzates of (a) corn-based DDGS biomass, and

(b) King Grass biomass.

(a) (b)

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The equilibrium code Stanjan [31] was run under the constant enthalpy and constant pressure

conditions, and the inputs were the mole fractions of the experimentally-detected gaseous

pyrolyzates, listed in Figure 8. Different amounts of air inputs to the Bunsen Burner result in

different equivalence ratios in the nominally-premixed flame and, consequently, result in

different amounts of CO and CO2 in the products. These theoretical predictions of the CO2/CO

ratios in the flame are plotted in Figure 11, against the input equivalence ratios. The effluents of

the flame were also experimentally assessed for CO and CO2 concentrations by direct sampling

using an aspirated probe and channeling the sampled gas to an on-line California Instruments,

model 200 infrared gas analyzer. These experimental measurements of the effluent CO2/CO

ratios were superimposed to the predicted CO/CO2 ratios shown in Figure 11. Results suggest

that both biomass flames burned under mildly-rich conditions at an equivalence ratio (defined as

φ = (mfuel/mair)actual / (mfuel/mair)stoichiometric) in a range of 1.05 < φ < 1.1. It should be mentioned

that this range is approximate, as numerical predictions cannot be compared directly with the

experimental results since the effects of heat loss and chemical kinetics are not considered in the

chemical equilibrium Stanjan code.

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Figure 11: CO2/CO vs. Equivalence ratio for corn-based DDGS and King Grass biomasses.

0

10

20

30

40

50

60

70

80

90

100

1.05 1.1 1.15 1.2 1.25 1.3

CO

2/C

O

Equivalence Ratio

DDGS Biomass

950 C

King Grass Biomass

950 C

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Figure 12: Velocity profile of the volatiles inside the chamber.

As mentioned before, Chen et al. [20] showed that by increasing the residence time of the

volatiles in a furnace, from 1.3 to 10 s, the gas production from biomass increases. In this work

the residence time in the heated zone of the furnace assessed based on modeling results using the

Fluent-Ansys computational code. Based on computed velocity profiles in the furnace, illustrated

in Figure 12, the residence times of biomass volatiles therein were estimated to be in the range of

10-20 s, which according to the aforementioned reporting should have promoted effective

gasification.

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Elimination of Tars and Waxes from the Effluent Gas

Initial experiments revealed that the King Grass biomass generated a lot of tars in the lines and

burner, which were troublesome as they often resulted in plugging the flow of gas. The initial

operational setup of the experimental apparatus featured approximately 2 m length of 6.35 mm

ID tubing, connected between the exit of the pyrolytic gasifier chamber and the inlet of the

Bunsen burner. During the experimental runs it was noticed that an excess amount of tars and

waxes were building up on the inlet of the Bunsen burner, thus restricting the flow of the

pyrolyzate gas to be combusted. Since the opening on the Bunsen burner is very small (1 mm),

only a minor amount of blockage is necessary to completely restrict the flow of gas and thusly

stop the experiment. Due to the long length of un-insulated tube that carries the furnace effluent,

the pyrolyzates were cooled and some tars and waxes that were present in the gas-phase at the

exit of the hot furnace would condense in the tube and form a solid buildup. This was confirmed

by a disassembly of the system and an inspection of the tubes for blockage.

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Figure 13: Steam engine apparatus set up in close proximity to pyrolysis chamber outlet.

To alleviate this problem, a literature search was performed to identify the temperature at which

tars and waxes condense out of the biomass pyrolyzates. Mansur et al. [29] noted that amount of

tars present in the effluent gas (by mass) started to decrease above 200 °C and dramatically

decreased above 250 °C during a study on the effects of hydrothermal temperature DDGS

product yield. Meng et al. [30] observed that during the pyrolysis of DDGS, olive residue, and

other forms of biomass it was necessary to heat the gas temperature to 150 °C to avoid the

condensation of gas in the line when attempting to sample the products in a spectrometer. Based

on these results, new experiments were conducted where the tubing connecting the pyrolytic

gasifier and the burner was shortened to only 20 cm and it was wrapped the in an thermal tape

Model Steam

Engine

Electronic Thermal

Insulation

Pyrolysis

Chamber

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(Thermolyne, BW0 series) to heat the pyrolyzate gas, which was then covered by a few mm-

thick ceramic fiber insulation. The thermal tape was electrically powered and connected to a

Variac controller, set to maintain 200 °C. A photograph of the apparatus as set up in this

condition is shown in Figure 13.

The pyrolysis system was operated normally while feeding King Grass and a thermocouple was

inserted in the insulation to obtain the approximate gas temperature. King Grass was chosen for

this run due to its high buildup of tars and waxes that clogged up the system and only enabled a

sustained flame for a brief time (ca. 5 seconds). With the thermal tape maintaining a constant

200 °C gas temperature, the flame as produced by the burner was consistent and reliable.

The King Grass fueled flame was sufficiently steady to operate the steam engine continuously

and generate a small amount of power. Upon inspection after shutdown, the burner and gas

supply tube was completely free of any buildup of tars and/or waxes. Maintaining the pyrolyzate

gas above 200 °C showed a drastic improvement in the length of time a flame could be

sustained, which is driven by the reduced blockages as a result of less tar and wax buildup.

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Proof of Concept: Use of Biomass Pyrolyzate Gasses to Generate

Electricity

A technical goal of this project has been to produce a flame with sufficient energy to run the

miniature steam engine and produce DC electricity sufficient to run a light bulb, shown in Figure

14. The operation of the miniature engine should be akin to that obtained burning natural gas.

As mentioned before, a biomass mass feeding rate of 1 g/min generated a flame that was more

than sufficient to produce steam and run the miniature steam engine at a high speed. The steam

engine system was able to sustain a boiler pressure of 1 bar and operate consistently at 1800

RPM for duration of an experiment, typically 20 minutes. This operational speed was sufficient

to use the on board generator to produce a small electric current to illuminate the miniature light

bulb. This demonstrated that the current design of the pyrolytic gasifier can be used to produce

gaseous fuels with high energy content and, in turn, can reliably generate useful work in the form

of electricity. Supplemental calculations for this section are found in Appendix 1.

Figure 14: Flame generated by the steam engine burner (left) and still photograph of the steam

engine in operation (right). The coupled electricity generator powers the light bulb.

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CHAPTER 5

Conclusions

Based on conceptual ideas and a literature review of similar concepts, a multi-step process has

been developed to pyrolyze biomass-based feedstocks and use the pyrolyzate gasses to generate

a clean-burning energy source. A laboratory scale system was designed, fabricated, constructed,

and successfully produced flammable pyrolyzate gas. The laboratory-scale system was then

integrated with a model steam engine and dynamometer and the pyrolyzates were combusted to

heat the boiler, which in turn produces steam to run the steam engine and generate electricity to

illuminate a light bulb. Throughout the course of development, some modifications to the overall

design were made to reduce the amount of oils and waxes in the effluent gas, increase system

tolerance to ash/carbon buildup in the pyrolysis chamber, and feed the biomass more effectively.

The overall result of the experimentation revealed that biomass-based feedstocks can be easily

turned into a clean burning, light hydrocarbon-based fuel via fast pyrolysis. This pyrolysis

process results in the creation of a flammable gas that can be combusted in a pre-mixed manner,

which can be very well controlled and adjusted to obtain the desired equivalence ratio and

minimize the production of soot. Gas chromatography analysis revealed the composition of the

pyrolyzate mixture to be mostly ethylene and methane which results in an extremely clean-

burning flame.

Future work will expand on maintaining pyrolyzate gas temperature above 200 °C to minimize

tar/oil production, improvements in pyrolysis chamber design to enable collection of ash build-

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up, and feeding system improvements to reduce jams/clogs and increase tolerance to biomass

feedstock size variation. Design and construction of a pilot-scale biomass pyrolysis system

(~100 kg/hr) with a self-sustaining pyrolyzer is the next logical step in this course of research.

To facilitate the modeling and baseline engineering analyses on the scaling and capacity of a

large-scale system, a simple economic analysis has been conducted to ascertain the profitability

given some basic assumptions of inputs and running costs. Based on the initial predictions, a

feed rate in excess of 2700 kg/hr will be necessary to generate enough electricity to turn a profit,

based on electricity rates at the time. Utilizing a scheme to recirculate the CO2 gas that is

generated in the products of combustion from this electric power generation will greatly reduce

running costs and increase profitability (details found in Appendix 4). Maintaining an accurate

economic model is essential, as construction and operation of a pilot-scale facility requires

considerable investment which traditionally comes from small investor-sourced venture capital

funds. Obtaining the necessary venture capital investment must show the system can be operated

to turn a profit and the technology is mature enough to perform this task reliably; thus the need to

continue to refine the engineering design of the system and process, as well as the economic and

financial model are important undertakings to pursue as future work.

Page | 35

Acknowledgments

The authors would like to thank Massachusetts Clean Energy Center (MassCEC) for financial

support through the MassCEC Catalyst Award program, the Garlock Sealing Technologies for

providing gaskets, the Viaspace Company for providing biomass samples, John Doughty and the

Northeastern Student Machine Shop for their help and advice on fabrication, and Mr. Saber

Talebi Anaraki for his technical assistance in the design and construction of the apparatus.

Page | 36

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Page | 39

SUPPLEMENTAL CALCULATIONS

APPENDIX 1: Required amount of biomass feedstock to operate the

pyrolytic gasifier in a self-sustaining mode, i.e., without external

heating

Applying the energy balance for the system determines the amount of biomass required. For

example, the input of DDGS pellets required, so that they can provide sufficient energy to heat

the system from room temperature to 800˚C. In the derivation below, feed energy (EFeed)

corresponds to the mass of gaseous fuel blend required to heat the pyrolysis chamber to 800˚C

multiplied by its energy content. Pyrolysis energy (EPyrolysis) is the amount of energy required to

gasify the pellets. Heat Loss energy (ELoss) is the heat loss from the system to the ambient and all

other unforeseen losses. EOut is the output energy required for the system to be self-sustaining,

i.e. to produce an equivalent amount of energy as input to the system as the electric heater does

in the current setup (1430 W). The energy balance equation is shown in equation (1).

Equation (1)

Calculation of

The energy required for pyrolysis of DDGS is not a known quantity; due to DDGS being plant

based, the pyrolysis energy of cellulose is used as a surrogate, which is 584 kJ/kg at 355˚C [27].

Page | 40

Our laboratory system runs at a higher temperature to minimize production of tar and wax. At

the higher temperature of 800˚C the corresponding energy requirement per unit mass of

feedstock is calculated using Equation (2) by taking the sum of pyrolysis energy at 355˚C and the

energy to raise the biomass pyrolyzate temperature from 355˚C to a higher temperature, say

800˚C.

Equation (2)

The cp in this calculation is used for ethylene gas; due to the nature of the pyrolyzate

composition, ethylene is used as a surrogate for these purposes due to its dominance in the

pyrolyzate gasses. Performing the interpolation to calculate specific heat of ethylene at T=800 ˚C

is shown in equations (3 – 5).

Equation (3)

Equation (4)

Using the calculated CP from equation (4) and substituting into equation (1) yields the following

pyrolysis energy and the corresponding rate equation:

( )

Equation (5)

Equation (6)

Page | 41

Calculation of

Two primary sources of losses in the system is heat transfer from the pyrolysis chamber to

ambient through the insulation, and energy required to raise the temperature of the Nitrogen

carrier gas.

Equation (7)

The conductive heat loss is calculated using the relationship in equation (7).

Equation (8)

The geometry of the heater and insulation is evaluated via Figure A.1 and equations (9 – 10),

where A is the outer surface area of the furnace and Thk represents the thickness of the

insulation.

- ( ) Equation (9)

( ) ( ) Equation (10)

Equation (11)

Substituting equations (9 – 11) into equation (8) yields the following result, with k = 0.05 W/mK

for calcium silicate for the insulation around the furnace.

Page | 42

(

) ( )( )( )

( ) (

) Equation (12)

The relationship to solve for energy required to heat up the Nitrogen carrier gas in the pyrolyzer

is calculated in equation (13).

Equation (13)

Substituting in known values for specific heat, gas density, flow rate, and temperature yields the

following (equation 14):

(

) ( ) (

)

( )( )

(

)

Equivalently, (

) Equation (14)

The value of is several orders of magnitude less than and thusly is determined to be

negligible. The following relationship is then assumed for (from equation 7):

Equation (15)

Page | 43

Figure A.1: Schematic Picture of Insulation Thickness and Length (units in inches).

Multiplying the total heat loss by a factor of 2 to consider all unpredicted and other unknown

losses yields the following value:

Equation (16)

Calculation of

EIn is the total energy input to the system from biomass feedstocks and is calculated with the

relationship below.

Equation (17)

Equation (18)

Setting up the expression for to solve for

Page | 44

To solve for the minimum mass flow rate of biomass to maintain the self-sustaining reaction, the

output of the system must be equal to the amount of energy supplied by the furnace. Using EOut

as the energy produced by the electric furnace (rated at 1430 W, ATS series 3110), the overall

energy balance expression is shown below.

Equation (19)

To calculate the overall heat loss in the system, values from equations (6, 16, 18) are used with

the thermal balance equation (1) to perform the following calculation:

Equation (20)

Equation (21)

Equation (22)

This calculation shows that 5.5 g/min of biomass feedstocks are required to heat this laboratory-

scale pyrolyzer to 800˚C. This amount of biomass mass flow rate (5.5 g/min) is projected to

generate enough energy to sustain its own operation, i.e., it will be energy self-sufficient.

Consequentially, it will take biomass mass feed rates higher than 5 g/min to start generating

gaseous fuel for net power generation, i.e., for external applications.

Page | 45

APPENDIX 2: Calculation of Self-Sustaining Pyrolysis System

Efficiency

The system efficiency is defined as the ratio between the net energy output and the overall

energy input.

Equation (23)

Wherein Eout is the energy output of the proposed self-sustaining waste-to-energy process,

EFurnace is the energy input to the system from the electric furnace to heat and pyrolyze the

feedstocks, and EIn is the total energy input to the system via the biomass feedstocks.

Equation (24)

Equation (25)

Taking the known values of equations (24 – 25) that were calculated previously (equations 18,

21) and substituting into the efficiency equation (23):

(

)

Equation (26)

Collecting terms and solving for ηS:

Equation (27)

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As shown in Equation (27), for the apparatus used in this study, there is a relation between the

self-sustaining efficiency, ηs, and the mass feeding rate, , as shown in Figure A.2. Due to the

high energy content of the feedstocks, the low thermal losses, and extremely low pyrolysis

energy, the total energy output of the system scales up quickly and the ideal efficiency can be as

high as 92%, at high feed rates for an industrial scale (10 g/min or higher).

Figure A.2: Relation Between the Self-Sustaining Efficiency (ηs) and the Feeding Rate (g/min).

Page | 47

APPENDIX 3: Thermocouple temperature measurement correction

for radiation effects.

The temperature at different elevations of the pyrolysis chamber was measured with a bare

thermocouple. These readings need to be corrected to exclude the furnace wall radiation effects.

This correction is based on an unsteady state energy balance on the thermocouple bead.

Equation (29)

It is assumed that the bead of the thermocouple is in thermal equilibrium with its surroundings

(steady state). A k-type OMEGA thermocouple was used for these measurements and the

diameter of the bead was measured to be 500 µm. By considering steady state conditions,

equation (29) equation changes to below:

Equation (30)

Defining the convection and radiation terms:

( ) Equation (31)

(

) Equation (32)

Page | 48

Where hconv is the convection heat transfer coefficient, Ab is the area of the bead and Tgas and Tb

are temperatures of the gas and bead respectively. Heat transfer coefficient h can be determined

from following equations (33 – 35) by considering the bead to be a spherical object.

(

)

Equation (33)

( )

Equation (34)

The Nusselt number calculated is valid for RaD < 1011

and Pr > 0.7.

Finally, equation (31) becomes:

( ) (

) Equation (35)

It needs to be mentioned that for obtaining convection coefficient we need to have the thermal

conductivity of the gas. For calculating thermal conductivity there are relations that can be used

and since these relations are based on gas temperature, it is needed to first assume a number for

gas temperature and then by trial and error zero the equation (35).

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APPENDIX 4: Economic Considerations for Large-Scale Operation

As an exploratory exercise, a number of calculations have been conducted to approximate the

losses and determine the potential net energy and financial gains/losses associated with scaling

up the aforementioned pyrolysis process to an industrial scale. Making some general

assumptions on running costs, system uptime, and incorporating data from this research, a basic

economic model is constructed to ascertain the profitability of pyrolyzing giant king grass and

using the resulting pyrolyzate gasses to generate electricity. Figure A.3 is an overall spreadsheet

used to perform these basic calculations.

Page | 50

Figure A.3: Economic analysis of biomass pyrolysis as used for energy production.

Biomass Fed kg/hour 5000

Carrier gas required L per kg of biomass 250

Carrier Gas Feed [Nitrogen] L/hour 1250000

Hours/Shift Hours 8

Shifts/Day Quantity 2

Uptime 75% 0.75

Hours/Day Hours 12

Maintenance Rate $/hour 50

Daily Maintenance Cost $/day 600

Labor Rate $/hour 50

Daily Labor Cost $/day 600

Daily Biomass Feed kg/day 60000

Daily Carrier Gas Use L/day 15000000

Energy Content King Grass kJ/kg 18400

Pyrolysis Energy King Grass kJ/kg (Per appendix 2 calculations) 1221

Energy Input kJ/day 1104000000

Energy used to Pyrolyze kJ/day (10% of total input) 73260000

Energy Available for Recovery kJ/day 1030740000

Wholesale Energy Price New England, (http://www.eia.gov/electricity/wholesale/), $/kWh 0.056

Recovered Energy Efficiency Factor 40%, typical rankine cycle power plant 0.4

Recovered Energy kJ/day 412296000

Recovered Energy kWh/day 114526.6667

Recovered Energy Value Per Day $/day 6413.493333

Cost of Carrier Gas $/L 0.00028

Daily Cost of Carrier Gas $/day 4200

Page | 51

Varying the feed rate of this economic model yields the following relationship between feeding

rate and profitability (Figure A.4).

Figure A.4: Biomass electrical generation profitability as a function of feed rate.

The income per kg of biomass is constant, due to the fact that each kg of biomass contains the

same amount of energy and thus generates the same amount of electricity. Whereas, the

expenditures roll up items that are remain the same per kg of feedstock (carrier gas) and constant

regardless of feed rate (labor and maintenance costs). Thus, the expenditures experience

economies of scale effect as feed rate increases and eventually become less then the income,

which would indicate the generation plant is making a profit. For the current setup and

assumptions, at feed rates of approximately 2700 kg/hr and higher, the energy conversion will be

a profitable endeavor.

Page | 52

One of the main assumptions used herein for this economic model is use of bottled N2 as a

carrier gas to facilitate the pyrolysis process due to its inertness and ready availability in a

laboratory setting. Bottled gas becomes prohibitively expensive when used on an industrial

scale, given the proposed biomass feed rates and the ratio of N2 needed to maintain pyrolysis as

experimentally determined in the laboratory. A plot of the fraction of total running cost that is

used to procure the carrier gas is shown in Figure A.5.

Figure A.5: Percent share of total running cost for N2 carrier gas.

The cost share of carrier gas used increase rapidly as feed rates increase, as measured against the

total expenditures, due to the fact that manpower and maintenance costs remain the same while

usage of carrier gas increases. One of the ideas put forth in Chapter 4 is the feedback of

combustion product gasses (CO2) to replace the N2 carrier gas. When biomass pyrolyzates are

Page | 53

used in combustion to generate electricity, regardless of its use in an internal combustion engine

or boiler to generate steam, the product of combustion will be mostly CO2. As tested in the lab,

the use of CO2 as a replacement for N2 had no discernible effect on overall pyrolyzate gas

composition. The use of these products of combustion gasses eliminate the need to purchase and

store costly bottled N2 gas on-site, which will greatly improve the overall profitability and

decrease running costs. It should be noted that the products of combustion gasses will need to be

treated and filtered for items such as any particulate matters present in the gas stream and utilize

a condenser to eliminate water vapor from entering the pyrolysis system.

Page | 54

APPENDIX 5: CAD Models and Drawings Generated for Assembly

and Fabrication of Components

Figure A.6: Complete 3-D model of the assembled system (also shown: the frame support structure).

Page | 55

;

Figure A.7: Exploded view (left) and cutaway (right) of pyrolysis chamber.

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Figure A.8: 3-D mockup of the frame system constructed to support the pyrolysis chamber.

Page | 57

Figure A.9: 3-D model (above) and engineering drawing generated (below) for custom fabrication of the feed hopper.

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Figure A.10: 3-D model (above) and engineering drawing generated (below) for custom fabrication of the pyrolysis chamber.

Page | 59

Figure A.11: 3-D model and cutaway (above) and engineering drawing generated (below) for the machining and assembly of

the feeding box and bearing support.

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Figure A.12: 3-D model (above) and cutaway view (below) showing construction and assembly of the

feeding system.