Tuesday, 14 February 2017

Chemical analysis of milk on a compact disc

We know how much food colouring has been added to a recipe by the intensity of the colour. Food colouring is made of dye molecules which absorb certain colours of light and scatter others. Measuring the concentration of food colouring is easy - all that is needed is a light emitting diode (LED) and a light sensor (link to laser cut detector). But how do we determine the concentration of molecules that are not coloured? One method that has recently gained a lot of attention is Raman spectroscopy. Put simply, a laser is used to excite the molecules, then a camera picks up a characteristic fingerprint derived from the way the molecules jiggle/vibrate due to thermal motion.

Michel Nieuwoudt and others in the Photon Factory have found that Raman spectroscopy can allow for the determination of all of the important components in milk such as protein, fat and health indicators. In a previous post I wrote about measuring the contaminant melamine in milk using gold coated Blu-ray discs which amplified the weak Raman signal to something detectable. This month we published a follow-up work that aimed to integrate milk analysis into a device that could be used in a dairy shed.

Giving milk
CC BY-NC 2.0 Giving Milk by Morton Just

The first challenge was finding a device into which to integrate the analysis. We wanted to make use of microfluidic technology, which uses techniques from the semiconductor industry to make very small fluid channels on the micron scale (your hair is about 100 microns across). Advantages include being able to pack many tests onto a single device and having very precise control over the liquid. Traditional microfluidic requires a lab full of pumps to operate, which is not conducive for analysis in a dairy shed, so we turned to the newer field of centrifugal microfluidics. Combining compact disc technology and microfluidics allows for standalone operation with pumping driven by the centrifugal force as the liquid pipetted into the centre of the disc is spun outwards through microfluidic channels.

CC BY 3.0 LabDisk for SAXS

To make this approach viable we have to use injection-moulded plastic discs to keep the costs down and to allow for large scale production of many discs that can be delivered to the farmer.

The problem with performing Raman analysis in a plastic device is that the plastic has a very strong Raman signal, which drowns out the weak Raman signal from the milk. In order to solve this problem we removed the plastic between the laser and the milk - holding the milk in an open channel using the capillary force. This is the same force that pulls water up the sides of a glass to form a meniscus.

Cross sections of the multilayer disc with the laser cut channel on the bottom, a layer of double sided tape in the middle and a top cover which leaves some of the channel open

The main contribution of the paper was working out how to use the centrifugal force to fill the channel without it overflowing by balancing the centrifugal force (controlled by the disc speed and distance from the centre of the disc) and the capillary force (controlled by the size of the channel). Something else I found quite cool was that the capillary force was enough to hold the liquid upside down in the channel, which means the detection could be done from underneath as in a traditional compact disc player. 

Diagram of the device: a) forces involved on the disc, b) liquid in a closed channel being pumped under rotation, c) open channel for spectroscopy, d) pressure due to the centrifugal pumping, e) balance between the capillary force and the pumping pressure.

We made use of this device to detect the contaminant melamine in milk using Raman spectroscopy and were able to detect down to the parts per million range (limit of detection (LOD) of 209 ppm). This is much more sensitive than infrared spectroscopy (LOD of 1300 ppm), but for higher sensitivities, the Blu-ray SERS surface would be needed (LOD of 70 ppb). 

The applications for this work go further than just milk analysis. This opens up all sorts of vibrational analysis such as infrared and Raman spectroscopy to the centrifugal platform which could provide disease diagnostics, water analysis and DNA detection using these advanced techniques. 

Monday, 5 December 2016

Gasification and carbon capture

In 2006, I became interested in gasification as a way of generating energy from biomass while storing atmospheric carbon in the ground. I thought I would explain some of the experiments I did and some of the interesting things I found out. 

What is gasification?

Gasification is the process of turning biomass, such as wood, into a fuel gas that an internal combustion engine can run on. Complete combustion of biomass produces water and carbon dioxide, but by restricting the amount of air allowed into the reactor you can produce an incompletely combusted gas made up of carbon monoxide, methane and hydrogen. This can then be piped into a normal spark ignition engine and used similarly to LPG. 

There are four steps in gasification: 
  1. Drying - The fuel is heated and water is removed from the biomass;
  2. Pyrolysis - The fuel heated without any oxygen breaks down and forms small volatile compounds (called tar or bio-oil) and solid charcoal;
  3. Combustion - The tar and charcoal are burnt in a small amount of oxygen from the air, generating heat for the entire process;
  4. Reduction - The amount of oxygen quickly runs out and the water and carbon dioxide are reacted on the hot charcoal surface to produce carbon monoxide and hydrogen.
Photo Credit: GEK

Some of the benefits of gasification, as opposed to combustion on an open fire, includes the increased fuel efficiency, as combustion is much more efficient and clean when using a gas instead of a solid fuel. The conversion of biomass to electricity using simple combustion requires steam turbines which are only economical on a large scale. The ability to power an engine that can drive a generator means it is also a low-cost method to generate small scale power. By adding the charcoal that is generated to the soil, the entire process can be carbon negative by trapping the CO2 the tree took in during its growth and locking it away in a stable form of carbon charcoal.

These types of gasifiers were heavily deployed (in over a million vehicles) in Europe during WWII when fossil fuels were in limited supply. My favourite photo from this time is a picture of a tank powered by a gasifier.

Photo credit

I built two different types of gasifiers - a gasifier stove and a downdraft gasifier, both of which I will outline below.

Gasifier stove

The first gasifier I built was a gasifier stove. The geometry of the gasifier is called a top-lit updraft gasifier (TLUD). This means the fuel is combusted from the top with the air moving up through the fuel. The diagram below shows the working principles. 

Photo credit
The fuel is lit from the top and air is supplied from the bottom. A flame front (migrating pyrolytic front) moves down through the fuel. The tar and water are pulled through the hot bed of coals, helping to break down some of the tar. Secondary air is then injected into the top of the reaction chamber which allows the fuel gas to burn cleanly. The stove was built from a computer supply box and used a forced draft from a computer fan which I powered on 12V DC. I mainly ran the stove on wood chips but I also used it to test the heat content of different fuels by heating water placed on top of the stove.

The stoves are not just a curiosity; thousands of them are being built and used in developing countries to improve the air quality for those who rely on solid fuels for cooking. The video below explains.

I also made use of the stove to study the combustion of algae during a summer research project working with Dr Rupert Craggs from National Institute of Water and Atmospheric Research (NIWA) in New Zealand. The algae were grown in open raceway ponds which used waste water to feed the algae.

Photo Credit: NIWA
I made use of a non-woven geotextile to dry the algae from the 98% water content down to 7-12 wt% which is suitable for combustion. The higher heating value for the algae was 23.06 MJ/kg compared with wood at 14-17 MJ/kg. The dried algae formed flakes which made for excellent fuel and allowed for combustion in the gasifier stove. One thing I didn't measure was the emissions, as the high nitrogen content would suggest a large amount of nitrous oxide could be generated. 

We published the results in a conference proceedings in 2010. https://www.waternz.org.nz/Article?Action=View&Article_id=786. In particular, we looked at the potential for algae to be carbonised to biochar to be a stable carbon sink.

Discovery model gasifier

The discovery model gasifier is a downdraft gasifier. This means the air is injected in the bottom and drawn down. The design of the gasifier is based on the Pacific class gasifier from a New Zealand company called Fluidyne. I scaled it down so that it could power a 660 cc engine at 1500 rpm outputting 3 kW of energy. This required a gas output of 9024 m3/hr of wood gas with a wood consumption of 4.19kg/hr. I initially had a 1kg hopper which allowed for a short test run of around 20 minutes. The design of the gasifier is really quite interesting and was designed to be built at a very low cost (a diagram of the gasifier is shown below). The fuel is loaded into the top and moves down as it is consumed. The fuel is dried and is broken down to tar and charcoal in the pyrolysis zone. Air is then injected through three nozzles and allows for combustion. A tube then comes up through the charcoal into the oxidation zone. Many gasifier designs make use of a metal throat that mechanically constricts the fuel. However, this throat can melt as the high temperatures are hot enough to melt steel. This gasifier makes use of the charcoal itself to act as the throat and the insulation allowing for low-cost materials to be used. As the carbon dioxide and water enter the tube the hot charcoal, in the absence of air, produces carbon monoxide and hydrogen.

The biggest advantage of a downdraft gasifier design is that all of the tar must go through the oxidation zone and then the reduction zone. This makes the fuel gas generated from these types of gasifiers very clean.

Here is picture inside the reactor with the constriction tube and the nozzles (the bolts are being stored there and are not used during operation). You can also see the diesel glow plug I used to start the gasifier in the top right corner.

The fuel I used was mainly wood chips or small wood rounds from the garden. The fuel gas then passed through a series of cleaning stages to prepare it for the engine. I used a blast tube to remove the large particles and some of the soot. A cyclone particle separator was made to remove the micron-sized soot particles. Cooling tubes were used to condense the water out of the gas and to generally reduce the temperature of the gas as well as to increase the density of the fuel gas. Finally, it went through a sawdust filter to remove any particles or tar that were missed in the previous stages. I later replaced the sawdust filter with a bag filter which could be cleaned and reused. 

The gasifier was designed for a large generator, which I didn't end up finishing, but I did some preliminary tests with a smaller generator. Here is an interview I did where I started up the gasifier and ran the engine.

I later increased the fuel hopper size using a propane tank and used fibreglass to insulate the fuel chamber so that fuel wouldn't get stuck in the hopper. Here is a video of the gasifier and the flare running using the air blower, showing a relatively clean flame.

One of the important aspects of making the gasifier work well (i.e. tar free) was to adjust the height of the reduction tube so that it was inside the oxidation zone and the grate height to allow the fuel to flow. Two good checks for a tar free operation was a blue flame (meaning no hydrocarbons in the fuel) and no hydrocarbons in the condensate from the fuel gas cooler. As you can see from the picture above I got close to correctly tuning the gasifier however Fluidyne's Andes class gasifier flaring shows a really excellently tuned gasifier with only carbon monoxide and hydrogen burning.

The microlab gasifier was built by Fluidyne in 2011 and is the same size as the discovery model gasifier but with two cyclones and is now being used for research at the University of Ulster.

Microlab gasifier
I have Doug Williams from Fluidyne to thank for showing me how to build and operate gasifiers. I also have to thank Peter Wilkinson from Wilkinson Transport Engineers, who allowed me to use his workshop and materials to build the gasifier. 

Continued interest

My PhD research is on combustion, global warming and reducing soot emissions from engines so this still interests me greatly.  Gasification of biomass is one of the key technologies for controlling the amount of carbon dioxide in the atmosphere. This is often referred to as bioenergy, with carbon capture and storage (BECCS). CO2 is captured by trees and the CO2 released during burning can be stored, making the process carbon negative.

Photo Credit: Drax Power
A second option is to burn some of the carbon to CO2 and to store the rest of the carbon as solid charcoal. This is called bioenergy-biochar systems (BEBCS). This does not sequester all of the carbon but as the charcoal is easier to handle and when added to the soil (referred to as biochar) can improve the holding of nutrients. This process is cheaper as the biochar can be sold to offset the cost.

Photo credit

I will probably be writing more about biochar in the future, but feel free to ask any questions about gasifiers.

Tuesday, 13 September 2016

Molecular tennis: Can nascent soot burn from the inside?

After arriving at the lab in Februrary I started working with Peter Grančič on collision studies of gas molecules with clusters of flat carbon molecules which resemble the very early soot particles (nascent soot) found in flames. So here is the paper.

I prepared some slides to explain the research.