Small scale energy optimization

Matching electrical supply with demand is an important aspect of reliably powering homes and industry; electric renewable sources make this issue more problematic.  Hybrid utilization of sustainable technologies when used optimally can help alleviate stress on the grid during peak usage.  This post will look at a study which uses computer analysis to maximize the return from hybrid systems, both in terms of finances, as well as CO₂ emissions.  The article is titled “A New Method for Energy Saving in a Micro Grid”, published in the journal “Sustainability”. The link as well as citation can be found at the bottom of the post.

Why is this important?

Using carbon free technology is the cleanest way to meet our electrical needs.  The generation of electricity from wind turbines and photovoltaic cells make carbon free energy generation possible.  The inherent indeterminacy with these technologies makes matching demand a challenging problem.

When electrical supply is insufficient to meet demand on the grid, blackouts and brownouts occur.  Due to the variability in renewable electric sources of energy, careful management must be used to ensure the correct amount of power is being generated.  Unlike combustion applications, renewable sources of electric energy are not varied by factors humans control.  When the wind is slow, or when the day is cloudy, electricity must be supplied by other means.  Besides the intermittency of sustainable sources, there is the issue with storage.  Unlike liquid, solid and gaseous fuels, electricity is not commonly stored.  Using hybrid multi-energy configurations, demand can be matched easier which would lead to stabilization in the electrical grid.  As we transition to more sustainable sources, the problems associated with instability will become much more pronounced, and smaller scale stabilization, will become more important.

Use of sustainable electric technologies on small scales is generally done with a single system.  The need for meeting peak power usage, requires large systems, which incur hefty capital costs when building a system to handle peak power requirements.  Very little attention has been paid to using multiple sources in conjunction with storage due to the inherent complexity.  Case studies have been analyzed where small scale hybrid technologies have been utilized, and they have shown to be more cost effective than standalone technologies.

As developed as most of the earth is, there is still a large portion of the world for which electricity is not accessible.  Besides regional poverty, another issue that makes connecting to the grid lies in the remote areas of many communities.  Using transmission lines connecting to existing power grids is highly unfeasible, so other solutions must be constructed.  As mentioned before, when designing a simple sustainable system (single power generating technology) the station must be built to withstand high demand, which in areas where cost is of the utmost importance, leads to difficulties.  Mitigating capital costs, as well as operational expenses must be done effectively.

The Experiment

What they did

Using a computer simulation to model a real building with large power usage, a model was analyzed; two separate software packages were used.  TRNSYS was used to model the thermodynamics of the various components of power generation, distribution (as process heat and electricity), and simulating building energy flow.  HOMER (which is widely used in research) was used to optimize the system configuration with a time varying approach.  It works with time steps of an hour, and for each step solves for the necessary power from each component.  Using considerations for cost of energy production, cost of investment, maintenance and fuel purchase, the software decides which conditions are optimal to minimize financial expenditure.  The building previously contained a large diesel boiler (400 kW) which was used for hot water as well as heating, as well as two large refrigeration units (140 kW each).  Using previous transient analysis of this specific building, the thermal and cooling loads of the system were identified for an entire year.

  

For the simulations, a multi-energy system was implemented to compare costs as well as CO₂ emissions to the current system.  The energy generation implemented in the simulation (for heat and electricity) included three items. A combined heat and power internal combustion engine was used, which was fueled by natural gas (100 kW, 150 kW and 200 kW options were explored) was utilized for electricity as well as heat.  Photovoltaic solar panels with peak power of 100 kW were also used for electricity.  A natural gas boiler was also used for heat generation. The systems were evaluated on a 20-year scenario so the factors such as capital cost could be considered more accurately than simply by using marginal cost.

Six different configurations for energy production were explored. The configurations had varying amount of power produced from the CHP station, as well as supplemental grid power.  Data was collected for each run pertaining to net present cost of operation, proportional quantity of electricity from each source, thermal load distribution, cost of energy, and CO₂ emissions.

For the analysis of energy storage, three systems were looked at.  Battery storage (Pb-Ac), flywheel, as well as compressed air (Caes). 

What they found

Energy production

The first important result is that every single configuration used in the model was more cost effective than the current situation. 

Part of the efficacy of these scenarios came from more efficiently meeting thermal load requirements.  Process heat from the CHP unit was almost sufficient to meet all of the requirements of the building.  Even in the configuration with the smallest CHP unit, there was less than 20% thermal load being generated from the boiler.  It was noted that the boiler was generally used when the need for electricity was so low that the CHP generator was not running.

On a per unit basis, the cost of energy reduced from  0.17 €/kWh (present configuration) to as low as  0.133 €/kWh, which was achieved with the largest CHP unit (200kw).  This situation is extremely dependent upon the price of natural gas.

Three of the scenarios were highly dependent on the price of electricity, two were highly dependent on the price of natural gas, and one was a more balanced situation.  It turned out the balanced scenario was less than 2% more expensive per kWh than the cheapest (which, as noted above, was highly dependent on natural gas prices). 

When looking at CO₂ production, all of the situations explored were significantly lower than the current configuration.  There were three main reasons for this.  Using natural gas instead of diesel fuel produces significantly less carbon dioxide per unit electricity produced.  Another key factor in reducing emissions was the solar panels which are generated carbon free energy.  Finally, using process heat from the CHP unit is much more efficient than blowing hot exhaust gasses directly through exhaust without utilizing some of the heat.

Energy storage

Due to the short lifespan of the batteries, the cost of energy they produce, operating costs were the most expensive for the batteries.  The flywheel had the highest capital investment, it has a very long lifespan, as well as a high conversion efficiency.  Compressed air was the least expensive to operate, however, it has a very low conversion efficiency.

The energy storage mechanism had the effect lowering demand on the grid during peak times.  This has a positive effect on main grid stability, as lowering demand during peak hours helps maintain a smaller gap between base and peak strain.

Thoughts

Although the energy storage helped take strain off the grid (by moving peak usage away from peak grid demand) it did not seem to be nearly as cost effective as creating the electricity on demand (although this was dependent on current natural gas prices).  The cost was about 30% higher, which becomes more economically feasible in situations where a significant price gap exists between peak and off peak electricity.  As intermittent power generation supplies a larger chunk of electrical production, the importance of reducing peak fluctuations will become of the utmost importance.  The inherent economic losses that are involved with electrical storage (both by capital investment as well as system inefficiencies) make it somewhat unattractive for individual micro grids to take on the costs.  It will be important to legislate such a difference in prices to make these technologies more viable.  

Although it is not new information, this study helps show the importance of using process heat.  By using natural gas (which burns hotter than diesel), the exhaust is able to be effectively utilized as heat.  Unfortunately mining for natural gas is highly destructive to local ecosystems and is known to contaminate groundwater heavily.  Although fracking is not done everywhere, it is a very common procedure which does not seem to be ending soon.   This seems highly applicable to rural communities without access to a main grid for a couple reasons.  Besides the obvious reasons about supplying electricity to areas where there is a lack, using a gaseous fuel as wells photovoltaic cells as a means to generate electricity areas can locally utilize available resources.  For most rural areas, even where natural gas would be difficult to obtain, biomass could be gasified and used in such a system.

It would be interesting to see the results of this study carried out using values for biogas. Even though it has a lower methane content, it would likely still be highly effective in a CHP system.  This could also be highly applicable to rural areas, which commonly have access to biomass.  Combining gasification of biomass in this approach would be a great way to mitigate costs as well as atmospheric carbon generation.  Even if biomass is used (with a near net zero carbon emission) it could be taken one step further.  Although this seems more applicable to larger scale operations, if waste treatment is processed locally to the generation, algal farming can be done as a means to process waste, and sequester carbon; for now, however, it may be best to expand the processes one step at a time.

What do you think?

Any comments or thoughts are appreciated!

Andrea Vallati, Stefano Grignaffini, and Marco Romagna. “A New Method to Energy Savings In a Micro Grid” Sustainability 7 (2015) 13904-13919. doi:10.3390/su71013904

http://www.mdpi.com/2071-1050/7/10/13904/pdf  (.pdf)

Viability of using 'loose' groundnut shells as biomass feed stock for gasification

This post will be looking at an experiment published in the "International Journal of Renewable Energy Development", volume 4 issue 2.  The article is titled "Gasification of 'Loose' Groundnut Shells in a Throatless Downdraft Gasifier."  In the study, edible nut shells were used in a gasification system to determine the viability of using such feed stock in their natural 'loose' form to create a syngas for heating and for use in internal combustion engines.  Citation and link to article are at the bottom of the article.

Background:
As mentioned in the previous posts, the ongoing effort towards sustainability will take a multi-faceted approach.  Not any one technology will "save" us from the crisis we face.  It is vital that all our resources are utilized fully, and managed in a way that has a minimal environmental impact.  Biomass is a sustainable, cost effective way to use resources, especially agricultural waste.

Biomass in general is one of the most utilized energy resources in the world (third after coal and oil).  About 14% of the world's energy usage is derived from biomass, and in developing countries it is an even more significant proportion(35%).  It is a cost effective way to produce renewable energy, while reducing net greenhouse emissions (as a substitute for fossil fuels).  Unlike other fuels, failure to utilize biomass releases greenhouse gasses, and contaminates groundwater aquifers (upon their natural decay).

Groundnut shells (such as peanut shells) are a woody fibrous material which is generally discarded after the nuts are separated.  They are commonly dumped on the sides of roads, or burned to reduce waste and make management easier.  Kenya (where this study was carried out) is the fourth largest worldwide producer of groundnuts, and given the rural and economic nature of the country, consolidating agricultural waste for use in large industrial systems is not feasible.

Gasification is a thermo-chemical process by which a carbon based fuel is processed into a gaseous form.  Using high temperatures, and limiting oxidization of the reaction, the material is mostly unburned, and it is transformed into a combustible gas, known as a "syngas" or "producer gas". There are multiple steps by which the process occurs, however, the specifics of the process are worthy of their own post, and not vital to understanding the study.  By using gasification instead of direct combustion, higher thermal efficiency can be achieved, smoke can be attenuated, and the gas can be used in internal combustion engines.  The resulting gasses produced can be much more easilly "scrubbed" of contaminants pre-combustion making the process much cleaner than direct combustion.

  There are many types of gasifiers (updraft, downdraft, fluidized bed etc.) with the main variations being how the gasifier is heated, where the fuel and oxidizers are fed, as well as how the fuel interacts with the flow of oxidizer. The study we will look into used a "throatless downdraft gasifier" for a few reasons.  Although throated gasifiers are able to more efficiently generate syngas, using loose biomass (such as groundnut shells) introduces a large concentration of undesirable volatiles (tar) into the final product.  These gases precipitate after combustion and form a "gunk" that makes them unsuitable for internal combustion applications. For this reason, a downdraft gasifier was used because it is known to minimize these contaminants.

Why is this important?
This study was carried out in Nigeria which is the fourth largest groundnut producer in the world.  Utilizing this resource effectively will help reduce needs for fossil fuels, increase agricultural value, as well as reducing pollution by smoke and greenhouse gas emissions.

Throated downdraft gasifiers (which most of the research has been done with) have flow problems with loose material (such as groundnut shells).  These issues affect the quality of the syngas negatively, thus loose material is generally pelletized to maintain uniformity within the gasification chamber.  This study utillized a throatless downdraft gasifier to determine the quality and quantity of the syngas produced from such a system, thus giving insight into larger scale usage.

The Experiment

What they did
A small 5 kW laboratory sized throated downdraft gasifier was used.  A gas cleanup unit (comprised of a series of filters) was placed downstream of the gasifier, and upstream of a sample port.  The producer gas was flared (burned) when it was not being sampled.  Air was used as the oxidizer; it was inputted by use of an air compressor.  Flow was controlled using a valve, and measured using a rotameter (flowmeter).  Thermocouples were used to measure temperatures in the pyrolysis zone as well as oxidation zone.

The gasifier was started using charcoal,oil and paper to start the combustion in the oxidation zone.  Once the zone was at sufficient temperature, and excreted gasses were opaque, shells were loaded into the gasifier.  Unlike commercial gasifiers which use a continuous loading of fuel, this setup was run in batches.

What they measured
Using a gas chromatagraph, the N₂, CO, CO₂, H₂ and CH₄ composition of the syngas was measured.  The performance of the gasifier measured using independent variables of gas flow rate, air to fuel ratio, equivalence ratio and groundnut shell consumption rate.

What they found

• Content of gas was very similar to gas produced from wood.
• Ash content was low, making the gasifier well suited for continuous use with minimal cleaning.
• Gas produced had low enough volatile content to be used directly in an internal combustion engine.
• Low density of the shells was not too low to be used well in the gasifier setup.
• Producer gas was on the high end of documented values for the downdraft gasifier systems.
• The ideal air flow in this experiment was 0.0071 m³s^-1  through the reactor.  Above and below that flow rate two things occurred.  First, the composition of combustible to gas (CO, H₂, CH₄) to non-combustible gas(CO₂, N₂)was lower.  The other effect of increasing or decreasing the flowrate past 0.00071 m³s^-1 was a decrease in the heating value of the gas produced.  So, not only was there less combustible gas produced, the gas produced was less effective.

Thoughts

There were a few things that struck me from this article besides the grammatical errors, ambiguous references to gas flow rate (weather produced or oxidizer) and references to incorrect figure numbers.

The ability to use a less dense material could have helped optimize conditions so that a very large portion of the shells was transformed into combustible gas. This could be part of the reason the ash content was noted to be so low, and the heating values were on the high end of the values that are seen for this type of system.  It seems plausible this type of gasification could be used well for similar loose, low density biomass material (which is not ideal in other style gasifiers) and would not be limited to groundnut shells.  

Regardless of environmental concerns, it would seem this type of utilization of waste biomass would be particularly useful for less developed oil importing countries.  Utilizing waste products effectively would cut down on import spending, as well as putting more money into the local economy would seems like a win-win.  Since these products would otherwise be burned without being utilized to create fuels or discarded on the side of a road or burried, there is a carbon neutral effect.   By using the shells to produce energy through gasification, the greenhouse gasses that would otherwise be released from the natural decay can be avoided, excess smoke is mitigated, while simultaneously producing energy as well as increasing agricultural value of the product.

It makes a lot of sense these professors carried out this study given the production rate of groundnuts in Kenya, and the rural nature.  It was noted that commonly the shells are discarded on the sides of the roads so it would be common for people on the roads to see them as waste.  Since agricultural "waste" would commonly be seen as such, publicizing this locally would be a great way to help bring the ideas of biomass conversion to a place where it is far from mainstream.  The environment as well as the economy would see positive effects.

What do you think?

Please mention any thoughts you have relating to this article or biomass gasification in general!


Citation
Kuhe, A and Aliyu, S.J. (2015) Gasification of 'Loose' Groundnut Shells in a Throatless Downdraft Gasifier. Journal of Renewable Energy Development, 4(2), 125-130.
http://dx.doi.org/10.14710/ijred.4.2.125-130

Optimal conditions for algae growth

In this post we will look at an article in the "Biofuel Research Journal" where researchers explore various factors that affect the growth of a specific strain of algae as well as the lipid content.  The article citation and a link to the article are at the bottom of this post.

A little background

Microalgae has great potential as a biofuel feedstock.  There are many reasons why this is the case but four stand out among the rest:

·         Microalgae has the ability to grow in places that are unsuitable for traditional agriculture, either due to climate or soil conditions.

·         Growing microalgae can be accomplished in a much smaller area than any other biomass.

·         The small footprint along with other factors make it highly suited for sequestering carbon dioxide.

·         Microalgae produce lipids instead of sugars, which makes the process of transforming them into a useable fuel much simpler. Instead of creating ethanol from sugars that must be mixed with petroleum products to be used in traditional vehicles, the lipids can be easily, and inexpensively, processed into a biodiesel. 

Why this study is important

If algae are to be successfully grown commercially for use as a biofuel feedstock, optimal growth conditions must be identified.  Compared with crude oil, producing green crude (as it is known) from algae is currently more expensive.  Minimizing expenses by increasing production efficiency will be key in making this a viable fuel source.

The Experiment

What they did

Based on previous research, the researchers determined that the Chlorella species of algae was the most suitable candidate for growth for biofuel conversion.  This was based on the growth rate in atmospheric conditions and the ability to be cultivated in open ponds.  

Four factors known to alter the growth rate and lipid content were identified, and used for independent variables.  These factors are light intensity, CO₂ concentration, nitrogen content (used NaNO₃ as a nitrogen source), as well as aeration rate.  Two values for each variable were used, with eight total variations.

Using the Taguchi method, the experimenters compared these independent variables without having to use separate experimental apparatuses to explore each variable.  This allowed the researchers to use eight experimental setups instead of 48, saving time and money.  A loss function, which calculates a signal/noise ratio, was used to qualify the validity of results.

An enclosed photobioreactor was used for each apparatus, with halogen bulbs as the light source.  A schematic can be viewed in the study (link at the bottom)

What they found

Effect on biomass productivity

Biomass productivity was almost four times more active in the most active system when compared with the least active.  Results showed the most important condition (within the parameters of experimental values) was nitrogen content, followed by CO₂ concentration, with light intensity being less significant.  It was found that aeration had no statistically significant effect on biomass production.

Effects on lipid production

The conditions used varied lipid content of the algae from 8.6% to 19.7%.  The most important factor influencing lipid concentration was nitrogen content, with aeration playing a smaller but significant role; light intensity as well as CO2 concentration were not found to be statistically significant.  The lipid content dropped appreciably with increased nitrogen within the bounds of the experiment.

My Thoughts

One part of the analysis that struck me was the complete separation of optimizing biomass and lipid content; both situations were analyzed independently of each other.  Given that the end goal is to process the algae using a transesterification reaction, the total quantity of lipids is an important metric to be evaluated.  Below this paragraph I have included a chart (Chart I, Figure I) which does so.  I multiplied the average biomass production by the average percentage of lipids for each condition.  Although experiments 2&3 account for the highest values for biomass production and lipid concentration respectively, this shows that experiment 6 would be the most productive for the end goal of creating the most biofuel.  

  

Experiment	Average biomass [mg/(L*day)]	Average lipids %	Average lipids [mg/(L*day)]
1	61	13.7	8.357
2	210.9	8.2	17.2938
3	85.9	19.7	16.9223
4	105.4	8.6	9.0644
5	54.3	16.2	8.7966
6	196.9	11.2	22.0528
7	67.2	12.3	8.2656
8	72.9	9.1	6.6339

While CO₂ concentration and light intensity alter the rate of production, they are statistically insignificant when looking at lipid concentration.  Conversely, while aeration affects lipid content, it is statistically insignificant when dealing with biomass production.  It follows that three of the four variables are significant in either the biomass production or lipid concentration, but not both.  With that information, it is clear which values are ideal for production.  Alternatively, increased NaNO₃ concentration was effective in generating more biomass, but reduced lipid content; there were competing effects, which makes looking at a single metric (total lipid mass) even more important.  This way the end goal can be directly compared to the inputs.

Experiment 6 had the most productive levels of all the variables except aeration.  It seems logical that experiment 6 would be even more productive if it used an aeration rate of 3.33 vvm with all the other variables kept the same.

The values used for light intensity were a bit interesting to me as well.  The highest light intensity used was 14.5 klux, which corresponds closely with a shaded area on a bright sunny day.  It seems that it would be more realistic to use a value that would be incurred with direct radiation on a sunny day.  My guess is that these values are based upon data previously available, that the light intensity of direct afternoon light is stifling to production.  This is backed up by the fact that the lower intensity situations actually corresponded to about a 17% increase in production.  Given the small number of data points, more investigation is needed to know if there is maximum production between the two values.

The fact that lower intensity light makes the microalgae thrive seems to be beneficial, at least for closed systems.  Instead of building flat ponds, it seems plausible that an array of angled photobioreactors could be used so that area is maximized while keeping light intensity to a reasonable level.

It would be interesting to see how the wavelength of the light used affected production as well.  This experiment was done with a halogen bulb,which is reasonably close in the spectrum of the sun, but nonetheless has a different spectrum.  It is possible that different wavelength spectrum contained in sunlight would affect the optimal intensity.

Another area that may warrant additional investigation is the idea of using these variables in a time varying approach. The authors cited a couple different studies concluding that certain conditions, such as low medium homogenization and nutrient deprivation, can cause the algae to produce a higher lipid content as a response to stress.  This effect comes at the cost of biomass production.  It would be interesting to see how this could be used with a time varying approach.  It seems plausible that conditions favorable to biomass productivity could initially be used in the algal lifecycle, by growing the culture to size, with the later part of the cycle becoming focused on maximizing lipid content.  The efficacy of such an approach would highly depend upon the rate at which nutrient deprivation affects lipid concentration.

Let's discuss this!

Please comment with any thoughts you have; wheather it is about the study, its implication, methodology or my comments, all comments are welcomed!

Article citation:  Aramal M.S., Loures C.C., Da Ros P.C.M., Machado S.A, Reis C.E.R., de Castro H.F., Silva M.V. Evaluation of the cultivation conditions of marine microalgae Chlorella sp. to be used as feedstock in ultrasound-assisted ethanolysis. Biofuel Research Journal 7 (2015) 288-294.  DOI: 10.18331/BRJ2015.2.3.7

http://www.biofueljournal.com/pdf_10392_777bedbbf44276cbf08016322ea2b158.html (.pdf)

Brief background on fossil fuels, biofuels, and greenhouse gasses.

What are fossil fuels?

Fossil fuels are carbon based geological deposits.  When organic matter is buried, decays, and is compressed for millions of years it transforms.  They can form crude oil, natural gas, or coal.   They are called fossil fuels because they are essentially made up of fossils of organic material that has transformed.  Instead of the carbon being released into the atmosphere, it has been captured in these forms.

So, what’s the problem with burning fossil fuels?

The main problems with burning fossil fuels for energy are threefold.

1.       Mining the fuels is “dirty”.  Simply put, oil spills; mining for coal involves ripping the tops off mountains, and “fracking” for natural gas is possibly the worst things we can do to contaminate our drinking water.  This is a very interesting topic, but we will not focus on it here.

2.       Burning fossil fuels (burning anything for that matter) releases greenhouse gasses into the environment.

3.       Fossil fuels are not pure substances and have many contaminants that also are spewed into the atmosphere (eg. sulfur in coal)

What are biofuels?

Simply put, biofuels are fuels that you make from living things, or their byproducts.  As an example, wood is among the simplest biofuels.  Since biofuels are grown, they have an inherent sustainability in their production.  In contrast to fossil fuels, bio fuels are replenishable, and can be grown well after fossil fuels are scarce.  Biofuels are traditionally broken down into three categories called first, second and third generation.

1) First generation biofuels are made from the sugars of edible plants.  Some of the most common examples are corn, beets and sugarcane.  The sugars or starches from these plants are fermented to produce enthanol.  Although the technology is there, turning edible crops into fuels can be a bit troublesome as a world where people are going hungry.  Given our finite resources (water, land, phosphorous etc.) it is not a socially responsible system.  Food crops are also not as energy dense as other crops that can be farmed, if the goal is simply to produce fuel.  

2) When using non-edible agricultural products such as wheat straw and corn husks for biofuels they are known as second generation biofuels.  Benefits include using parts of plants that cannot feed people as a source of fuel.  The extraction process is more expensive (financially and in terms of energy), however, they do not have the pitfalls of using food for fuel.  The sugars are not readilly available (they contain complex carbohydrates instead) as they are in first generation biofuels so they must undergo extensive treatment to "extract" a useful fuel.

3) Although algae could be looked at as a second generation biofuel, its efficacy is so promising that they are generally considered in a category of their own, called third generation biofuels.  Not only does algae grow fast, it contains a relatively large concentration of lipids (fats) and can be extracted with moderate ease.  A significant amount of the fats can be expeller extracted (like olive oil) and even more can be extracted using a recoverable solvent.  The algae extraction products known as "green crude" is very similar to crude oil chemically, thus can be turned into a biodiesel with moderate ease.  The main limitation for algae is phosphorous, which is a vital nutrient for growing the food we eat.  Although this is a problem for all biofuels, the rate at which algae can be grown, combined with the lipid content and ease of extraction make it such a strong candidate for fueling our transportation fleet.  Phosphorous limitations become more important with algae because it is seen as having such enormous potential, but unfortunately we need an enormous amount of phosphorous to utillize it.  Currently it is very energy intensive to recover the phosphorous from the algae waste; hopefully easier methods are discovered.

What are greenhouse gasses and why do they matter?

Greenhouse gasses act just like the glass in a greenhouse.  In a greenhouse the light enters, but when it reflects off the ground some of it gets trapped, thus leading to heating of the enclosed area.  Gasses are like other materials in the way that dense objects sink while less dense objects float (think of a balloon on water).  Greenhouse gasses such as carbon dioxide are denser than air, thus they sink near the surface. Being that they sink, greenhouse gasses accumulate near the surface.  They act as a lens and trap the heat close to the surface in the same way the glass in a greenhouse does.  There are many different greenhouse gasses which vary in how effective they are at trapping heat.

What causes these greenhouse gasses to accumulate? What does it have to do with fuels?

When burning hydrocarbon fuels (coal, oil, gasoline, diesel, natural gas etc.) the main products of combustion are water vapor and carbon dioxide.  Although carbon dioxide is not the strongest greenhouse gas (methane is about 21 times more effective), we release an extremely large amount of it into the atmosphere, which makes the effects large.  The most prominent source, is from combustion.  Combustion is how we obtain roughly ¾ of the electricity in this country, and almost all of the energy fueling cars and trucks.

It's great that biofuels are sustainable, but wouldn't burning them release greenhouse gasses as well?

This is an important question, and the answer helps illustrate the importance of biofuels.  As discussed earlier, combusting hydrocarbons releases carbon into the air, even if they are derived sustainably (so yes, even biofuels produce greenhouse emissions when burned).  The important distinction is that biofuels by their nature used carbon from the atmosphere to be produced, and the carbon would be released upon their decay.  Photosynthesis (which is the reaction that enables plants to grow) uses carbon dioxide as a reactant (ingredient), and releases oxygen as a product (a simplified view, but true nonetheless).  This process uses carbon in the air and uses it for growth.  If you have ever wondered where the “stuff” of a plant comes from, it’s the carbon from photosynthesis!  So, when burning a biofuel, it has a somewhat net neutral effect on the amount of carbon in the atmosphere.  It is a bit more complicated than that, as they need energy to be grown, transported and processed.  Ideally in the future all of our energy needs will be met using carbon-free sources, however, that is a long time away.  In the near future it is imperative  that we figure out ways of reducing our carbon emissions on the planet.