UNIVERSITY OF AUCKLAND
DEPARTMENT OF ENGINEERING

ENCI790 RESEARCH PROPOSAL:
“An evaluation and survey of methods available for converting biomass into fuels”

Candidate:
Surya Sappa

Supervisors:
Professor John Chen
Doctor Robert Kirkpatrick

ABSTRACT

Biofuel technologies have become more and more prominent in addressing the need for a continuous supply of fuels. Biofuels are fuels that are produced through the biological process of converting biomass/biowaste into a fuel source to reduce the negative impacts faced by fossil fuels

combustion (reduction in greenhouse gasses) and availability of supply.
Biofuels can be derived from a wide variety of biomass, and are categorised into four generations: 





First generation biofuels are made from sugars, starches, oil, and animal fats that are converted into fuel using already-known processes or technologies. These fuels include biodiesel, bioalcohols, ethanol, and biogases, like methane captured from landfill decomposition. Second generation biofuels are made from non-food crops or agricultural waste, especially ligno-cellulosic biomass like switch-grass, willow, or wood chips.
Third generation biofuels are made from algae or other quickly growing biomass sources.
Fourth generation biofuels are made from specially engineered plants or biomass that may have higher energy yields or lower barriers to cellulosic breakdown or are able to be grown on non-agricultural land or bodies of water.

However, criticism falls on this solution, with the biofuels being pursued to reduce climate change impacts from energy consumption being relatively ineffective. Analysis of the carbon lifecycle associated with biofuels articulates that carbon dioxide from the atmosphere is absorbed by the biomass feedstock to cultivate and grow. But the biofuel production process and burning of the biofuels depollutes the atmosphere with carbon dioxide as a by-product. While utilising biofuels contributes positively to society the resulting carbon dioxide and land use to cultivate the biomass can be detrimental to the environment due repollution of carbon emissions, deforestation (site-clearing) and energy consumption.
For these reasons, methods of biofuel production must be optimised so that their potential impacts on the surrounding environment are minimised. The following proposal pertains to a research project investigating the available or upcoming methods in converting biomass into biofuels which would advocate a better path forward for the biofuel industry. More specifically, the methods of conversion for second generation biofuels (bio-waste) will be examined.

Previous studies have suggested that greater emphasis has been placed on on 2nd-generation

biofuels produced from ligno-cellulosic feedstocks, due to limitations faced by 1st generation biofuels generated from food crops. However 2 nd generation biofuels have also come and will continue to face major constraints to be completely commercial deployed. The logistics of providing a competitive, all-year-round, supply of biomass feedstock to a commercial-scale plant is challenging, as is improving the

performance of the conversion process to reduce costs. The biochemical route utilised for lingo-cellulosic produced biofuels is less mature and has a greater cost reduction potential than the thermo-chemical process. However with the added risk, it is rewarded by being able to produce a wider range of synthetic that can be suited for land, aviation and marine applications.
The literature relating to the above areas of research provides some understanding of the different method of biofuel production. Existing studies are however limited in terms of their scope and in the quality of experimental data generated. The proposed research project will provide further insights to which method of biofuel generation

meets the agreed environmental, sustainability and

economic policy goals of today’s generation. .

TABLE OF CONTENTS
ABSTRACT ............................................................................................................................... 2
1

INTRODUCTION ............................................................................................................. 2

2

BACKGROUND & LITERATURE REVIEW ................................................................. 5
2.1

Dimensional Analysis ............................................... Error! Bookmark not defined.

2.2

Experimental methods ............................................... Error! Bookmark not defined.

2.3

Modelling .................................................................. Error! Bookmark not defined.

2.3.1

CFD Models ....................................................... Error! Bookmark not defined.

2.3.2

Integral Models .................................................. Error! Bookmark not defined.

2.4

State of the Art .......................................................... Error! Bookmark not defined.

2.4.1
2.4.2
3

Boundary Interaction ......................................... Error! Bookmark not defined.
Stratified Ambients ............................................ Error! Bookmark not defined.

RESEARCH OBJECTIVES & METHODOLOGY .......................................................... 8
3.1

Experimental Investigation of Inclined Dense Jets Impinging a Solid Boundary
Error! Bookmark not defined.

3.2

Integral Modelling ..................................................... Error! Bookmark not defined.

3.3 Experimental Investigation of Inclined Dense Jets in Stratified Ambients .......Error!
Bookmark not defined.
4

DATA COLLECTION .................................................... Error! Bookmark not defined.

5

RESOURCES & BUDGET ............................................................................................... 8

6

OUTPUTS........................................................................ Error! Bookmark not defined.

7

TIMETABLE ................................................................... Error! Bookmark not defined.

8

REFERENCES ................................................................................................................ 10

1

1

INTRODUCTION

With an ever growing population, the strain on natural resources such as fresh water, energy sources and land mass is becoming increasingly evident. These resources are slowly becoming a scarce commodity with many non-renewable sources of fresh water, fossil fuels and land being depleted.

At present it is estimated that almost 20% of the world's population is facing a lack of access to safe drinking water (Danoun, 2007). Moreover, the United Nation’s “World Water Development Report” suggests that more than 50% of countries will face water shortage issues by 2025 (United Nations,
2003). Traditional sources of freshwater such as surface water and ground water have proven to be unpredictable and unsustainable. Therefore the challenge that has been presented is to find new, creative and sustainable methods to address this problem. The development of desalination as a means of providing this necessary resource has become increasingly viable because of process improvements and associated cost reductions. Desalination is the process in which fresh water is extracted from salt water (typically seawater) for human consumption or irrigation purposes.

Desalination has typically proved popular in semi-arid areas such as the Middle-east where desalination technologies have historically been at the fore-front. With the development of more energy and cost efficient technologies, desalination plants have been utilised in areas such as
Australia, Europe and North America. Approximately 15000 desalination plants currently exist providing freshwater from salt and brackish water. While the benefits of ocean water desalination to society are significant, the resulting effluent which is discharged back into the ocean can be detrimental to the marine environment. The nature of the effluent depends on the desalination method used. Two methods have been utilised in large scale desalination plants; a distillation process called
Multi-Stage Flash (MSF) or a membrane process called Reverse Osmosis (RO). Effluent from RO plants are typically high concentration brine discharges. The elevated salt concentrations results in negative buoyancy with an approximate 3% density difference in comparison with its surrounding ambient (~1050 kg/m3 - 1065 kg/m3 compared to typical seawater density levels of 1025 kg/m3).
Similarly, MSF desalination produce discharges with increased salinity (~1035 kg/m3), but also significantly elevated temperatures in comparison with the ambient (approximately 5 to 15 0C above the ambient seawater level). The dense effluent that is discharged into the ocean is referred to as a negatively buoyant jet (NBJ). In addition to increased brine concentrations and temperature levels,
Danoun (2007) states that desalination effluent have been found to contain harmful metal sediments from the pre-treatment process and increased alkalinity levels. Many organisms can adapt to minor changes in their environment and may even tolerate extreme situations temporarily. However a continuous exposure to unfavourable conditions can be fatal. Organisms can be attracted or repelled

2

by the new environmental conditions causing a lasting change in species composition at the discharge site (Lattemann & Hopner, 2008). For these reasons, regulatory standards are set in relation to the minimum effluent dilution that must be achieved within a specified mixing zone.

The dilution achieved by these discharges will be influenced by the presence of various ambient environments. The present study will focus on the interaction of discharges impinging a solid boundary (e.g. seabed) and density stratified environments with linear profiles.

Density stratified environments are caused by temperature and/or salinity differences in the water column. Density differences are particularly noticeable during the spring to summer months where the upper layers are heated through increased solar radiation and thus less dense than the cooler lower layers. This type of environment can also be caused by an inflow of freshwater into the system which
‘flushes’ out the saline surface layer. As a result density discrepancies ensue between the surface layers and the saline bottom layer. Additionally, continued disposal of brine discharges can cause significant stratification at the discharge site. This effect can be especially prolific if discharges are released near the seabed where accumulation of the effluent introduces a density profile that varies locally. The occurrence of stratified environments in the vicinity of desalination discharge sites is unlikely as effluent is generally released near coastal locations. However with an increasing demand for larger scale plants where outfalls are positioned further offshore, ambient stratification becomes more prominent where their effect on effluent dilution is more relevant. Various studies have linked stratification to the occurrence of hypoxia within water columns as vertical mixing of dissolved oxygen is inhibited (Applebaum et al. 2005; Ritter & Montagna 1999; Ritter & Montagna 2001). In the same way, the mixing processes that dilute saline discharges may be hindered by this interface. Of more concern in stratified environments is that previous studies have suggested that under certain combinations of initial and ambient conditions, the flow collapses and mixing ceases. Seabed impingement of saline discharges also poses a serious threat to the mixing ability of such flows. The negative buoyancy of the effluent propagates the discharge downwards towards the seabed where it eventually spreads along the ocean floor. As a result, the dilution potential is appreciably diminished as the volume of ambient (clean) fluid available to be entrained is reduced and turbulent mixing processes collapse. Unless the concentration of these discharges has been diluted to regulatory standards, benthic communities such as seagrass beds can be directly affected by the high salinity and chemical residues (Lattemann & Hopner, 2008).

The desired outcome from this study is to qualitatively and quantitatively analyse the effects of these interfaces on NBJ’s. Using the information obtained, recommendations can be made so that design procedures can appropriately consider these effects. The following sections will introduce the topic of negatively buoyant jets as well as review the existing literature. The methodology that will be

3

employed to observe and analyse these flows has also been outlined. Finally the resources required to undertake this project and a predicted timeline of events over the next 3 years have been provided.

4

2

BACKGROUND & LITERATURE REVIEW

With demand for clean and environmentally friendly fuels being sought after, countries are urging the development and production of biofuels to replace the conventional fuels used.
Second generation biofuels derived from lignocellulosic feedstocks tackle the drawbacks posed by first generation biofuels ones concerning feedstock availability and competition with the food industries. The conversion of biomass to biofuels can be accomplished mostly through the biochemical and thermochemical process.
Thermochemical conversion is an ideal process selection as it is able to covert both food and non-food biomass into fuel via pyrolysis and gasification. Unfortunately, the presence of lignin in plant cell walls inhibits the breakdown of cell wall polysaccharides to simple sugars and the subsequent conversion of these sugars to usable fuel.( reference).

The application of pyrolysis and gasification of biomass are complex and dependent on numerous factors such as; the composition of lignocellulosic material ( amount of cellulose, hemicellulose and lignin) , operating conditions ( temperature and pressure) and presence of inorganic material which create a great variation in the efficiency among different biomass and process implemented.
2.1 Current methods
One of the popular methods of producing 2nd generation biofuels is the method of thermochemical conversion, which relies on well-established technologies such as Fischer-Tropsch synthesis and gasification (reference).

Gasification

In the gasification process, biomass is converted into a syngas by the partial oxidation of biomass at high temperatures [9]. Gasification occurs at a moderately high temperature and turns solid biomass into synthesis gas (combustible gas mixtures) through simultaneous reaction of exothermic oxidation and endothermic pyrolysis under limited oxygen supply
[2,10]. The resultant products of are synthetic gas (composed of CO, H2, CO2, CH4, H2O and
N2) and tar. (reference). The synthetic gas product can be used as an energy source to provide heat thru combustion synthesized to produce liquid fuels [2,13]. Figure 1 shows a

Thermochemical conversion of biomass to biofuels is a promising alternative route relying on well-established technologies including gasification and the Fischer–
Tropsch synthesis. The conjunction of these processes creates a pathway through which the production of biofuels is sustainable. However, the multiple interactions
5

between the processing steps greatly increase the difficulty in the accurate design of such processes. Detailed process modelling and optimization studies combined with process integration methods are necessary to demonstrate an effective way for the exploitation of these interactions. The aim of this work is to present and analyze the thermochemical conversion of biomass to second generation liquid biofuels as well as to indicate the emerging challenges and opportunities of the application of process integration on such processes towards innovative and sustainable solutions concerning climate concerns and energy security.
Turbulent Jets and Plumes are flow types closely associated with buoyant jets. By definition, a ‘jet’ refers to a flow dominated by the momentum initially imparted to it (M0), while a plume is buoyancy driven or a buoyancy dominated flow. This source of buoyancy may arise from density differences between the discharged and the ambient fluids. A buoyant jet, or a negatively buoyant jet (for the present study) is an initially momentum dominated flow with a non-zero initial buoyancy flux (Oliver,
2012). The behaviour of the discharge is governed by the imposed initial conditions:
Local mean flow properties = ∅( , , 0 , 0 , )

(1)

where ϕ is and unknown function, M0 is the initial specific momentum flux ( = the source volume flux (0 =

4

0 2 ); B0 is the initial specific buoyancy flux ( =

initial value of the modified acceleration term (′ 0 = (0 − )

1
);

2 2 ); Q0 is
4 0

2 ′ 0 ); g’0
4 0

g is the acceleration due to

gravity; uo is the initial discharge velocity; d is the discharge diameter; p0 and pa represent the initial fluid density and ambient density at the point of discharge and ν is the kinematic viscosity of the fluid.
A dimensional analysis of these parameters gives:

Local mean flow properties = ∅(

0 0 2
5
2

,

1

0 2

, 0 )

(2)

The first dimensionless parameter can be reduced to a densimetric Froude number (0 =

0
√g0 ′ .

).

This number defines the ratio of inertial to buoyant forces initially acting on the discharge. The second dimensionless parameter can be reduced to a discharge Reynolds number (0 =

0
),

a

dimensionless quantity representing the ratio of inertial forces to viscous forces. Equation 2 can therefore be written as:
Flow property = ∅(0 , 0 , 0 )

(3)

However it is widely established that the local mean parameters are independent of the Reynolds number for fully turbulent flows. It is important that this condition is satisfied as the mixing potential

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of laminar flows (low Re flows) is significantly lower. Thus the local mean parameters are only a function of the discharge angle and the densimetric Froude number.
Flow property = ∅(0 , 0 )

(4)

Using the initial fluxes and dimensional arguments, two important length scales used to characterise the properties of buoyant jet flows have previously been defined (Wright, 1984); the discharge length scale (ljQ) and the jet to plume length scale (ljp).

~

0

(5)

1

0 2
3

~

0 4

(6)

1

0 2

The discharge length scale (ljQ) represents the relative importance of the initial volume flux to momentum flux and defines the region where the discharge geometry influences the flow properties.
Therefore any analysis of the flow beyond this point can ignore the volume flux at the source. At some distance along its flow path, the contribution of buoyancy generated momentum flux (MB) surpasses the initial momentum flux, characterising a flow transition from a jet to a plume regime.
The distance at which this transition occurs can be represented by the jet to plume length scale (ljP). It is worth noting that combing these two length scales produces the densimetric Froude number:

~ 0

2.1

(7)

Potential future methods that hold promise

Researchers have traditionally obtained flow information through the use of point measurements and images (continuous and still). Point measurements can provide accurate dilution information however require the insertion of probes into the region(s) of interest (thereby affecting the flow). Conductivity probes have been most widely used allowing salinity levels (and therefore concentrations) to be calculated through the changes in electrical conductance. Modern day techniques predominantly involve the use of images to obtain flow information in a non-intrusive manner. Methods such as
Light Attenuation (LA), Laser Induced Fluorescence (LIF), Particle Tracking Velocimetry (PTV),

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3

RESEARCH OBJECTIVES & METHODOLOGY

To improve our understanding of the issues outlined at the end of section 2, the objective of this

research project is to evaluate and survey methods available for converting biomass into fuels. Comparisons will be made from evaluating the current available methods to future methods that may provide the best path forward for the biofuel industry. Specifically looking at the methods of production of 2nd generation biofuels from ligno-cellulosic feedstocks. 4

RESOURCES & BUDGET

The primary resources required to conduct this investigation are currently available for use in the engineering department. These include lab space, computers and access to journal articles at the
Engineering Library most of which are already available.

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