Self - Evaluation #3

After completing the third evaluation period for this blog project I feel that my performance has improved.  For the first time I feel that I have covered a wide range of topics within the waste to energy field, in a more interesting way.  I tried to tie together technologies in the waste to energy field with everyday experiences that people may have.  Including beer!  The one area I feel that my blog has suffered is in the publicizing area.  I have had minimal comments, especially from those other than the Prof. Schneider.  I do realize that most students have just enough time to write their own posts, let alone read others and comment.  I feel that I could have improved my blog by letting others know of my blog who are not in the class.  I have produced 15 posts this period and feel that a wide range of topics were covered in different ways.  I have slacked on commenting on others blogs, but have done well responding to comments I get on my blog.  The use of different media formats have been effective throughout my blog.

Overall I feel that this blog project has been effective in changing/improving my communication abilities.  I feel that I have made the transition from producing bland deficit model posts to more interesting and practical posts.  Although I would have never decided to create a blog on my own, I feel that this has made me a better communicator and I have enjoyed the experience.

Goodbye to school, this class, and this blog

If there are any avid readers out there that have followed my blog, you will know that this blog was done as an assignment for my Science Communication class.  If you do fall into the category presented by the first sentence of this post, then why haven't you posted any comments.  Have you agreed with everything that I have said, or is the content so boring that you couldn't even make up a question to ask, or at least a good job or way to go! Anyways...  Although I would have never decided to create a blog on my own I have enjoyed the experience.  If this is the first post you have read on my blog, then I will give a quick recap of what the purpose of my blog was throughout the semester.

From the title of the blog it is clear that the blog has focussed on waste to energy technologies.  I have covered anaerobic digestion, gasification, and algae to energy as my main three technologies to focus on.  I have also mentioned some off the wall ideas that show that there are others who share my interest in the waste to energy field (capturing energy from falling waste water in drain pipes, using dog poop to light a park, etc.).  My main goal of this blog was to introduce the major technologies that currently exist.  If the public understands that wastes can used to create energy then popularity of these facilities might grow here in the United States as they have done in Europe.  Any energy source is a good source in my book.  While I feel that fossil fuel energy is one of the most important energy sources, especially domestic fossil fuels, any energy will help.  A lot of the time these waste to energy technologies can even be incorporated into the fossil fuel industry to make their process more efficient.

Another goal of this blog was to show that you don't have to be an engineer or scientist, or be able to have access to large amounts of money to be able to understand and build a waste to energy process.  In particularly, anaerobic digestion has been used all over the world in high poverty area to create a cheap energy source for locals.  Often times these digesters are even sized for individual households, where family member operate their own personal digester.  I have covered issues that arise when certain processes are being implemented, and even covered what to do with the wastes of the waste to energy processes.  YouTube has been a great source foe information on this blog and if you are interested in building just about anything, dealing with waste to energy or not, check out YouTube for helpful do-it-yourself videos.  If anyone does come to my blog and like something they read or have comments or questions don't hesitate to leave a comment, I will get an email notifying me!

New Belgium Brewery and there waste treatment

Although the partnership between New Belgium Brewing and Solix Biofuels did not work out in the end, New Belgium is still committed to reducing their wastes as much as possible.  And if they can make some energy out of their wastes then they will surely do it.  The video below gives an overview of the breweries waste water treatment plant that utilized anaerobic digestion.  The biogas they produce during this process is then used to power their combined heat and power unit.  Their combined heat and power unit does just what the name implies.  The biogas is combusted in an internal combustion engine that then powers an electrical generator.  The heat that is produced by the combustion of the biogas is also used to heat certain processes throughout the brewery.  Whats the only thing that is better than waste to energy? Beer!

 

Wastes of the wastes, Part 3

So we have covered several wastes of anaerobic digestion, and one waste of gasification.  How about the other major waste of gasification, fly ash.  Why is it considered fly ash? Because it literally flies away with the gas stream.  Fly ash produced in the gasification process can be harmful to humans if inhaled.  In order to protect the public this fly ash must be collected and used in a productive way.  First after the gasification process has converted nearly all of the solid material to gases, some solids are left over, the fly ash.  Because the fly ash is so light in weight it travels out of the reactor with the gas stream.  In order to separate the ask from the gas stream a cyclone is used.  A cyclone is a device that changes the path of the flowing gas so that is is spun, like the motion of a tornado.  This high velocity path that the gas/fly ash stream takes cause there to be centrifugal forces acting on the heavier object.  Centrifugal force can be though of as the force that holds water in a bucket when the bucket is swung over your head.  The fly ash in this case has the most centrifugal force acting on it and travels to the outside of the cyclone.  Here the fly ash is allowed to exit the cyclone to be collected.

What is there to do with the fly ash once it has been collected?  One typical way of utilizing this fly ash is to use it as a component of concrete.   The fly ash when added to concrete can increase both the strength and durability of the concrete.  This is not just a recent technology either.  The ancient Romans used materials that contained volcanic ash to build a lot of their aqueducts and monuments.  One reason these structures are still standing is due the increased strength and durability added to the concrete by the volcanic ash present.

Waste of the wastes, Part 2

Another waste of anaerobic digestion is hydrogen sulfide gas.  Hydrogen sulfide is also produced as a waste product in all types of gasification, as long as sulfur is present in the feedstocks used in the the process.  Hydrogen sulfide is very toxic and can be deadly in the parts per million range.  In order to produce a safe product from digestion or gasification the hydrogen sulfide must be removed.  Luckily numerous technologies exists to get rid of this harmful gas.  

One simple technique is to pass the sour gas (gas that has hydrogen sulfide in it) over iron shavings.  Iron reacts very quickly with hydrogen sulfide and removes a majority of the hydrogen sulfide from the gas.  The exiting clean gas is then considered sweet gas.  This process is known as a non regenerative process because the iron that reacts with the hydrogen sulfide becomes spent or useless after a certain amount of hydrogen sulfide has come into contact with it.  Other technologies follow this same type of process but use a different scavenger.  The scavenger, iron shavings in this case literally grab the hydrogen sulfide out of the passing gas and convert it into a non toxic form.  Other scavengers include iron oxides, and zinc oxides.  

A regenerative process that is typically used in the natural gas industry is an amine gas treating unit.  This process uses a mixture of water and amines (to be discussed later) to absorb the hydrogen sulfide from the passing sour gas stream.  The sweet gas is allowed to continue upward and on with processing while the water/amine/hydrogen sulfide solution falls and is regenerated.  Regeneration occurs when the temperature is increased and the pressure is dropped.  This can be described with a simple bottle of soda analogy.  When a bottle of soda is allowed to warm up and the cap is suddenly taken off (decreasing the pressure inside the bottle) a bunch of fizz is produced.  This fizz is simple carbon dioxide escaping from the liquid phase to the gas phase.  So back to amines.  Amines are simple compound that resembles ammonia.  Ammonia is a compound that contains one nitrogen atom bonded to three hydrogen atoms.  An amine is produced when at least one of these hydrogens bonded to the nitrogen is replaced with a larger hydrocarbon chain.  Amines have the great property of  being slightly basic.  When the hydrogen sulfide comes into contact with the water a slight acid is produced.  And when a acid and base come into contact they react.  A simple acid base reaction that may ring a bell is heartburn.  Acid from your stomach travels up your esophagus causing irritation.  Once you feel this pain you take some Tums.  Tums make this feeling go away (hopefully), because Tums are made up of basic compounds that react with the acid and neutralize it, or turn it into water.

Wastes of the wastes, Part 1

We have covered several different types of wastes to energy through this blog, but what about the wastes that are produced from these processes.  For anaerobic digestion there are several main waste streams that much be handled correctly in order for the process to have minimal effects on the environment.  As mentioned before, there is a large amount of wet sludge that is left over from anaerobic digestion.  This wet sludge has had all of the volatile (vapor acting) molecules removed, which is the known as the biogas.  Although most of the solids in this process are digested and converted to methane and carbon dioxide there are some left over solids and water that make up the wet sludge.  Typically the wet sludge contains about 6% solids by weight.  Meaning that if the water and solids were to be separated from each other than the weight of the solids would be 6% of the total weight of the wet sludge.  While this wet sludge isn't particularly harmful or polluting it must be disposed of used in a certain way.  Most commonly the wet sludge is give to farmers so that they can apply this digested material to their crop land as fertilizer.  Remember cow manure, the feedstock to the anaerobic digester, is also a reliable and effective fertilizer used by farmers around the world.  There have been studies showing that not only does the application of wet sludge to field not stink as bad as fresh manure but that it is also better for the environment.  This is due to the fact that when fresh manure is applied to the fields the volatile compounds contained within the manure are released to the atmosphere. These compounds being methane, ammonia, and carbon dioxide. 

Another typical way of handling the wet sludge produced from anaerobic digestion is composting.  The wet sludge is dried and allowed to rest while the left over naturally occurring microorganisms continue to degrade the left over solids.  This compost can be used just as any other compost would be used.  The dried solids from the wet sludge have also been recycled and used as bedding for livestock.  The pictures below show the two different ways of applying both fresh manure and wet sludge to crop land.  

Liquid application of left over wet sludge from anaerobic digestion.

Fresh manure application.

Solix Biofuels Update

My last post was based on a article that was published in 2007, so I wanted to find some updates on the project and see where things were at here in 2011.  After searching both New Belgium's and Solix Biofuel's websites it appears that Solix has decided to implement its technolgy else where.  After reading a post on New Belgium's blog, investors thought that it would be a better investment to implement a demonstration plant near a coal bed methane plant where more carbon dioxide is available and on a more reliable basis.  The demonstational plant is known as the Coyote Gulch Plant and is located in Durango, CO.  The waste water streams from the coal bed methane plant are utilized as well as carbon dioxide from a amine gas scrubbing plant nearby.  The amine gas scrubbing plant cleans raw natural gas streams. Raw natural gas can contain large amount of carbon dioxide and must be removed from the natural gas.  Carbon dioxide is considered an acid gas and when mixed with water can produce carbonic acid.  After the carbon dioxide is absorbed by the amine solution in an amine scrubbing unit the amine solution that is rich in carbon dioxide can be regenerated by both heating and reducing the pressure of the amine solution.  This is analogous to opening a bottle of soda.  Carbon dioxide is used to carbonate the soda and as many of you know that when soda gets hot and you take the lid off large amounts of fizz is produced.  This is because the carbon dioxide in the soda wants to escape and does so when the cap is opened or when the pressure is dropped.  So overall an amine scrubbing plant takes the carbon dioxide from the natural gas stream and moves it to an amine solution stream, and finally the carbon dioxide is released from the amine solution.  Where the carbon dioxide can be utilized, like it is here at the Coyote Gulch Plant,  it is fed into the algae bio reactors and used by the algae to grow and produce oils.

Beer brewing wastes to energy, Part 2

So not only does the brewing process produce a large spent grain waste stream, but brewing also produces large amounts of carbon dioxide.  Carbon dioxide is a by product during the fermentation step of brewing as the yeast eat the sugars present in the wort and turn it into alcohol.  So how can this waste stream be used to generate energy, Solix Biofuels of Fort Collins, CO thinks that it has come up with a way.  Solix Biofuels is a new biofuels company that has partnered with Colorado State University to research and find a way to implement energy from algae.  What make this process so exciting is that algae only need sunlight and carbon dioxide to grow.  Sunlight is not hard to come by and neither is carbon dioxide when you are next door to New Belgium Brewery in Fort Collins.  Solix Biofuels is conveniently located next door to New Belgium Brewery, where almost 5,000 metric tons of carbon dioxide are produced a year from the brewing processes.  So with the carbon dioxide from New Belgium, Solix Biofuels has come up with a technology that will grow and harvest the lipid rich algae produced.  When algae grow they produce large amounts of lipids or oils for their overall size.  By injecting carbon dioxide into the a water algae mixture contained in a transparent bioreactor the algae can grow and produce oils.  After a certain period of time the algae can be harvested.  After harvesting the algae can be squeezed and pressed to extract all of the oils contained within each algae.  These oils can then be further refined into liquid bio-diesel.          

Beer brewing wastes to energy, Part 1

This last month me and my father brewed our third batch of beer.  The past two batches we brewed were extract beers.  This simply means that the malting, mashing, and sparging process had already been done for us, which makes it a whole lot easier on us just being beginners.  As many of you know who have went through the whole Coors tour and not just the short tour, malting is the process of germinating the grains used to brew the beer.  This is done in a temperature and humidity controlled environment.  Once the grain starts to germinate the process is stopped and then the malted grain is mashed.  Mashing can be thought of as making a giant batch of tea.  The malted grains are first lightly crushed and mixed with hot water.  The hot water extracts the nutrients and sugars from the grain making a sweet sugary "tea".  The grains contain many complex sugars and carbohydrates that much be broken down into smaller and smaller sugars so that the yeast used to brew the beer can utilize them.  Mashing is the process that breaks down these sugars. By just draining off the "tea" many of the sugars stay trapped in the grains, so sparging is utilized to get the remaining sugars out of the grains.  Sparging can be thought of as just rinsing the grains until mostly all of the sugars are extracted.  So after all of that processing you are left with wort, the brewing technical word for the "tea".  For many beginner home brewers all these steps can be skipped by brewing an extract beer.  The extract is just concentrated wort that looks about like molasses.  This extract is mixed with water and the then the beginner home brewer can continue on with the brewing process to make beer.  So this time my father and I decided to step it up and we tried brewing an all grain beer.  An all grain beer is one that is brewed by doing all of the steps talked about earlier, except for malting.  So after we mashed and sparged our malted grains we had nearly 20 pound of spent grain.  This twenty pounds of grain was used to brew just five gallons, can you imagine how much grain is used at Coors.  So what are we to do with all of this spent grain.  I got on the internet and did a Google search and found that people have done many creative things with their spent grains.  One that I thought was very interesting was making dog biscuits.  But anyways after reading about many different ways to use this spent grain, I got to thinking about anaerobic digestion and if energy could be extracted from the spent grains in this way.

Spent Grain Dog Treats!

So after another Google search I found an article in Biomass Magazine that talked about a Germany brewery that has done just this as an demonstration.  The article says that most breweries around the world do already have anaerobic digester to treat their wastewater streams, but this brewery has claimed to be the first to demonstrate that a digester for spent grains can be done.  One big obstacle that digesting the spent grains brings about is that in the past the spent grains have been sold to the agricultural industry as animal feed.  Bringing $9 to $18 a metric ton, digesting spent grain may not always be economical.  The brewery that has implemented this demonstrational anaerobic digestion process says that its been more of a marketing strategy that shows those around them that they can be self sustaining and not use fossil fuel energy.  Convincing the rest of the brewing industry to implement such a technology may be very difficult, but this might just make it possible for home breweries or craft breweries to lead the way!  

Landfill gas to energy statistics

Here are some quick statistics on the number of landfill gas to energy plants by state. California has 77 operational plants, Illinois has 32, Texas has 27, and Colorado has 1.  Granted we have a much smaller population than all of these other states, but with over 30 landfills in the state of Colorado there should be more of these plants in operation.  The only landfill gas to energy plant in operation is located at the Lowery Landfill in Aurora.  The gas gathering system consists of 218 wells and collects LFG from both the Lowery Landfill and the Denver Arapahoe Disposal Site (DADS).  Between the two landfills, Lowery being closed and DADS still in operation, they collect enough gas to produce 3.2 megawatts of electricity.  This is enough electricity to power nearly 3,000 homes.  In April of last year, a new LFG to energy plant broke ground at the Larimer County Landfill, located in Fort Collins.  This plant is expected to produce 1.6 megawatts of electricity.  Both the Lowery Landfill/DADS and Larimer County Landfill have plant sized below the average size of simlilar plants in the United States, which is 3.4 megawatts.  The largest LFG to energy  plant is located in Puente Hills, CA and produces 50 megawatts.  

How much methane does a landfill produce?

I have been doing a lot of research to try to determine how much methane or LFG an actual landfill can produce.  Many factors go into the production of the LFG and include, age of garbage, temperature, moisture level, and type of garbage.  In general the more organic the garbage the more methane and carbon dioxide that is produced.  Also, the warmer the better and the higher the moisture content the better. As for the age of the garbage there are wide ranges of time periods where many experts say the LFG production is at its peak.  Gas production normally starts around the 1 to 3 tears after the garbage has been covered, and after 5-10 years gas production is at its peak.  There are many different suggestions to when the landfill stops producing gas ranging from 10 - 40 years.  And many times the life of gas production varies depending on the conditions of the landfill and its contents.  Here is a quick video that show LFG being emitted from a bore hole drilled into a garbage layer.  Sure does seem like a lot of LFG being produced!

Landfill Gas, does this sound familiar

Making the shift from anaerobic digestion to other types of waste to energy technologies will be easier if we look into a process that is nearly the same.  This type of technology is known as landfill gas.  Landfill gas is produced in almost all landfills and many times the landfill gas that is produced is lost to the atmosphere.  Landfill gas is made up of 50% methane(CH4) and 50% carbon dioxide(CO2).  Sound familiar, this is the nearly the same composition of biogas.  Landfill gas (LFG) is also produced in the same way.  Microbes deep under the layers of trash in landfills eat away at the trash and transform the garbage into it most basic form, which happens to be both methane and carbon dioxide.  One major difference in this process is that while nothing special needs to be done for the LFG to be produced, such as buying a digester and then agitating and heating it (as in anaerobic digestion), but collecting the :LFG is more difficult.  In order for LFG to be captured the garbage is first covered with a layer of soil that both contains the garbage but also helps create an environment suitable for the naturally occurring microbes to begin their work.  Once the LFG is produced it slowly makes it way up through the layers of garbage until it is emitted to the atmosphere.  When the LFG is lost to the atmosphere, it is known as fugitive emissions.  Collecting the LFG to use in productive ways is accomplished by drilling wells into the garbage.  Lining these wells with PVC pipe gives the LFG a path to take to get to the surface.  Just like most things in this world, the LFG will take the path of least resistance and that is the PVC pipe.  An array of these wells are drilled and connected to a central LFG collection line.  This raw LFG also contains some other gases that are non desirable such as ammonia (NH3) and hydrogen sulfide (H2S).  Therefore, the LFG is treated to remove these gases.  Once the LFG has been cleaned up, it can be used just as biogas or even natural gas would be used.  Here is a diagram that may help you visualize what a LFG system looks like.

Back to the Future!

How long has the idea of using trash for energy been around?  At least since the the movie "Back to the Future" came out in 1985.  In "Back to the Future," Doc fills his time machine with garbage to power a trip back to the future.  Mr. Fusion, the name given to this garbage to energy generator, may have seemed like some science fiction idea that may never become reality, but it has.  A larger scale Mr, Fusion has been created and has proven to turn everyday garbage into energy.  This technology consists of a tank that contains plasma arcs that reach almost 6000 degrees Fahrenheit.  Simply put a plasma arc is a  lightning strike.  The high temperatures of the plasma arc cause anything within the tank to break apart into smaller and smaller particles until a gas is produced, that can be burnt just as natural gas is burnt, to create mechanical energy.  This mechanical energy can be used to power a car, or to make electricity.  So if this type of technology can convert any type of garbage into energy why did Doc pick through the trash and then pour a beer into Mr. Fusion?  I'm sure it was done that way just to make the scene more interesting and to grab the attention of the audience.  By the way pouring liquids into a fusion gasifier ( the technical name of this "lightning tank") would cause big problems. But isn't it great when seeming out of this world ideas in movies become reality.

  

DIY Anaerobic Digester

Whos more wasteful? -- or -- Who releases more waste energy streams to the environment? A Cow or a Human?

So after doing my last post I became curious of who is more wasteful releases more energy to the environment, a cow or a American citizen.  As in who throws away more  Garbage being the assumed largest waste stream for the human and manure being the largest waste stream for the cow (garbage being manure for the cow).  Off the top of their head I'm sure most people would say that humans are the most wasteful out of all species and release a lot of wasteful energy to the environment.  But after noticing that the average cow produces 14.4 pounds of manure per day and the average American citizen produces 4.4 pounds of garbage, I began to think that a cow may release more energy to the environment be more wasteful.  To the compare the two candidates I will calculate how much energy can be recovered from the manure and the garbage.  Garbage has an energy content of 10,000 BTU/lb totaling 44,000 BTU per person per day.  Manure has an average energy content of 8,500 BTU/lb totaling 122,000 BTU per cow per day. Using this simple procedure, an average cow essentially releases wastes (poops out) 277% more energy than an average American throws in the garbage can.  Now we will just need to determine the best way to utilize this wasted energy.

The cow is victorious in release the most energy to the environment being more wasteful.

 

Shifting from cow manure to garbage

Throughout my blog I have focused on anaerobic digestion of cow manure.  While this has been proven to be a valuable source of energy not everyone or every city is near a large population of cattle.  Take for instance here is Colorado, northeast Colorado has a large population of dairy farms and cattle feedlots, however, Denver and the other front range cities are farther away.  What do these large cities have instead of cows and there energy rich manure....TRASH.  According to the EPA, the average person produces 4.4 pounds of trash per day.  Multiply this by the population of Denver and the surrounding area of 2.5 million people and that totals 11 million pounds of trash per day.  The average energy content of wood/paper is typically 5-7,000 BTU/lb, while plastics typically have an energy content of 15-20,000 BTU/lb. Using this data we are able to assume an average energy content of garbage as 10,000 BTU/lb.  Multiply this by the 11 million pounds of trash produced in the Denver area to get 110 billion BTU's of energy.  That is enough energy to power almost 360,000 homes that use on average 300,000 BTU/day.  

How can this energy be collected and transfered into a useable form of energy?  First, the trash can be burned or incinerated to produce heat that is then used to produce steam and finally electricity.  Or the trash can be gasified to produce syngas that can be burned to produce energy.  Or how about just let natural decomposition of the trash take its course in landfills to produce landfill gas.  Landfill gas consists of methane and CO2, the two main gasses that make up biogas from anaerobic digestion of cow manure.  Of course there are efficiency losses in all of these processes, but why not use this energy, as it is just sitting there in the landfills waiting to be used.

Self-Evaluation #2

For the second blogging time frame I believe that I have improved my blogging.  I have posted 9 additional posts and received 12 comments.  I have commented back to others who have commented on my posts very frequently.  I have also followed several other blogs that are interesting to me and commented on their posts.  When I have came across articles or news that seems helpful to others and their blog topics I have found ways to comment on existing posts they have and let them know what I have found on their topic.  I have posted on a variety of different topics within the waste to energy field.  I have used all types of media including pictures, videos, and hyperlinks.  I have moved away from deficit model posts and presented some interesting posts.  I have brought up important issues and problems in the waste to energy field and gotten some good comments on these posts.  I have also connected this class with my senior design class to help me find and received recommomendationsAfter looking at my "Stats" I have received over 410 page views from viewers in 9 different countries.  I have commented on other blogs in the waste to energy field and received some recommendations from one of these blogs to make mine better.  The blog I received recommendations from was from a Renewable Energy Advisor working in India.  Some things I still need to work on include increasing my number or posts and incorporating other experts in the industry.  Overall I feel that I have improved my blog this period and deserve a high B to low A. 

Spring Break 4-Wheeler Jumping

Here is how me and some friends from back home spent Spring Break. YFZ 450 jumping the John Deere 280 skid steer used to build the jump.

Senior design and complications with anaerobic digesters: temperature

Not only is agitation of the sludge an important aspect of anaerobic digesters, temperature is even more important. The sharp temperature fluctuations here in Colorado cause issues with the living organisms.  The optimal temperature for these organisms is 85 F to 95 F.  This is much higher than the average temperature here in Colorado for more than 3/4 of the year.  Only parts of June, July, and August have these high enough temperatures.  To combat this issue our team has determined that a temperature control system must be used.  Using the high temperatures of the products from the gasifier we should be able to use heat integration.  Heat integration is the process of using heat generated in process units to heat other streams in the process.  This same idea can be used to cool certain streams that need to be cooled as well.  This heat integration can be achieved with a immersed piping coil that allows for heat transfer to flow from the hot gases from the gasifier to the sludge being digested.  Again, this adds more equipment to the once simple digester that has already had a mechanical agitator added. 

One way that digesters existing on their own can stay within this optimal temperature range is by burning some of the gas that hey produce. Several ways of heating digesters are available internal and external.  Internal processes leave the sludge in the tank, while external processes move the sludge outside of the tank.  The sludge can be pumped through a heat exchanger, where heat will be transferred to the sludge via a heating fluid such as water.  Types of heat exchangers that can be used include tube-in-tube and shell and tube exchangers.  A boiler uses some of the produced biogas to heat water that is used to heat the sludge.  In a tube-in-tube configuration the water flows in the annulus area counter current to the flow of sludge in the center tube.  The diagram below shows this in more detail.

Shell and tube heat exchangers are similar where the sludge flows in a bank of tubes while heated water flows over the tubes.  The diagram below shows this configuration.  Again a boiler uses produced biogas to heat the water.

A second way of controlling the temperature of a digester without moving and pumping the sludge is by pumping the hot water into tubes within the digester.  This system will not have as good of heat transfer but will keep the sludge from being pumped around.  I feel that this system would be the best due to the fact that pumping the sludge around can put severe stresses on the organisms and may cause them to go dormant or even kill them.  I am not sure of the pumps that would be used or if there are pumps that can be used that reduce this stress.

Senior design and complications with anaerobic digesters: agitation

I chose my topic of waste to energy at the beginning of the semester having a interest in the industry and technology. Later I was able to pick from four senior design topics for my Chemical and Biochemical major, one including a gasification of waste streams to energy.  One stipulation of the project was to integrate a biochemical process with the thermochemical process of gasification to obtain energy from the waste feed stocks.  My group has been looking at the possibility of implementing an anaerobic digester and gasifier to obtain energy from multiple waste feed stocks.  Feed stocks include farm wastes (cow manure), plastics, and woody biomass.  The process will have two feed prep areas where the biodegradables will be fed to the anaerobic digester and the non-biodegradables will be fed to the gasifier.  The left over sludge from the anaerobic digester will then be dried and added to the gasifier to obtain the full amount of energy possible.

After looking into these processes several issues have come up with anaerobic digesters.  Agitation is first.  The time that it takes for the feed stocks to be digested into CH4 and CO2 varies greatly from digester to digester.  One important aspect of this time is agitation.  If the sludge is agitated and well mixed the digestion takes place at a faster rate.  This complicates a simple anaerobic digester from a simple air tight vessel to a vessel with moving parts.  Mechanical agitators can be used to achieve this, but cost more money and expertise on operating the digester.  Because there are living organisms within this sludge, to much agitation can also be harmful to the organisms.  Walking this fine line of how much agitation is needed to speed the process up is making things more complicated.  We hope to find this information soon so that we can begin sizing and pricing the agitation that is needed.  Any suggestions from other processes that people know about in any industry would be greatly appreciated.