Solar, wind, water, geothermal and biomass are sources of producing renewable energy. Production of energy from sun, wind, water and geothermal need costly equipment. Even the smaller scale models of solar and wind energy for a medium sized home costs more than 1000 US Dollars. Individuals can not think of energy from water and geothermal as the required infrastructure will cost the heaven. They also require regular maintenance of moving parts, batteries and other things. Renewable energy from biomass is the cheapest form of all sources mentioned above, which require minimum initial investment and practically no maintenance at all. The biomass fuels like vegetation, crop residues, cattle waste, kitchen waste and aquatic plants are abundant in nature and also easily available throughout the year. A medium sized biomass digester can be built with about 200 US Dollars.
Here, I have experimented with producing energy from anaerobic digestion of weeds.
we had a very good rainy season this year in our area and everyone is happy. The rains also helped weeds to spread and grow at a faster phase covering open areas. road sides, home gardens and all ! There are weeds, weeds, weeds everywhere...
Few of our neighbors hired some help to weed out their patch of land. Some threw them out and few heaped them to make compost. Also, some farmers used chemical weed killers / herbicides to completely destroy all those weeds from their agricultural land. And me, left all those weeds in our home garden to make an experiment on making energy from them.
This instructable is aimed at the common man with simple to follow steps explaining how we can efficiently extract energy from all those unwanted weeds and use the remains as compost.
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Step 1: How Can We Extract Energy From Weeds ?
How can we extract energy from weeds ?
Energy from weeds can be extracted by anaerobic digestion process which breaks down the weeds and produce methane. Methane is an odorless & colorless gas and combustible in nature. So it can be used as energy for lighting, cooking, heating, fuel for vehicles and for running generators to produce electricity.
How it Works...
The biogas process happens in three simple steps namely Hydrolysis, Acidogenesis and Methanogenesis. Different groups of bacteria are responsible for each step to turn weeds into energy.
During hydrolysis, protein, carbohydrate and fat polymers present in the weeds are broken down to small molecules by different specialized bacteria. They produce a number of specific enzymes that accelerate the decomposition of the weeds. Lignin, an important plant component, cannot be decomposed under anaerobic conditions at all. In plant tissue cellulose and hemicellulose function as supporting material in the cell walls. These are tightly packed in lignin and are therefore difficult for bacteria to get at. So, by lightly pulverizing the weeds, we make it easy for the bacteria to break down the weeds.
Acidogenesis, also known as Fermentation
In acidogenesis or fermentation process, the broken down weeds are converted into acetic acid, carbon dioxide, hydrogen and volatile fatty acids.
The third and last step of biogas production is done by the methanogenic bacteria, which are divided into two groups. One group converts acetic acid into methane and the other from carbon dioxide and hydrogen. Degradation of acetic acid is responsible for about 70% of the methane produced, and the remaining 30% comes from carbon dioxide and hydrogen. These processes are finely balanced and inhibition of one will also lead to inhibition of the other.
Earlier I have posted an instructable on constructing a medium sized anaerobic Biogas digester using only a hacksaw blade and a knife as tools, which you can see at this link here :
Here, I have used the same biogas digester to make this experiment / study using only weeds as feed material to produce biogas.
I have started feeding the biogas digester with pulverized weeds since last week of September 2014 and still continuing with it. In the mean time, I have also collected the data of how much biogas can be produced and how much energy can be derived from weeds.
The subsequent steps will explain how simple it is to extract energy from weeds.
Step 2: Collect Weeds
The capacity of the digester in the biogas plant I have built earlier is 750 liters. Leaving about 50 liters for free board, we can consider the net capacity as 700 liters. Considering 30 to 35 days of retention time, I can feed about 2.5 to 3.5 kilogram of weeds per day in this plant. For my experiment with weeds, I choose to feed about 2.5 kilograms of freshly collected weeds mixed with enough water to make it to about 20 liters of diluted feed material. The mixture will also have a retention time of about 35 days in the digester. I will also get about 20 liters of composted slurry everyday, which can be diluted and applied as compost to plants in the garden
I have experimented with all type of weeds from the home garden and surrounding areas. They include weedy plants, weed creepers, leaves of trees, grass and whatever available nearby. Due to rains the ground was very soft and I could pull out most of the weeds with roots. About 4 kilograms of these freshly collected weeds will give 2.5 kilograms of feed material after trimming out the roots and removing the woody parts. Otherwise, you can just collect the weed clippings without the roots and use them as they are. You can also collect three to four days requirement in advance and keep them separately.
Step 3: Remove Roots and Chop Weeds
In the pictures above you can see that I have trimmed away unwanted roots and stems from the weeds. Then using a garden shear, I have also chopped down the weeds to manageable size.
To make an assessment of how much weeds make a kilogram, I have weighed about one kilogram of chopped up weeds in a bag. The red basket in the picture here contains about two and a half bags, that is, 2.5 kilograms of chopped weeds ready to be pulverized.
I feel it is not necessary to weigh the weeds again and again every day as the basket-full of chopped up weeds will make the daily requirement.
Step 4: Pulverize the Chopped Weeds
I have observed that feeding the chopped weeds directly into the plant did not digest the feed material fully. On closer examination of the slurry, I could find some partially digested chopped weeds drained out with the slurry. So, it is better to pulverize the weeds before feeding the plant.
using a spare metal jar of a mixer grinder, the weeds were ground with little amount of water in small batches. I have recycled the water collected from the kitchen which was used for rinsing rice and lentils and used it for dilution. The pulverized weeds were then collected in a small bucket and transferred to another larger one near the biogas digester. I have also reused the water used for cleaning the metal jar after use, for diluting the pulverized weeds.
For pulverizing about 2.5 kilograms of chopped weeds in small batches, I use a 550 watt mixer / grinder. It takes less than 10 batches and about 15 to 20 seconds per batch. The total running time used is between 3 to 4 minutes per day for which the electricity charges comes to less than a Dollar per month.
A word of caution here... Do not use chlorinated water. Also do not use soapy water used for cleaning the utensils. We use bore-well water at home which do not contain chlorine or any other chemical.
Step 5: Dilute With Water and Feed the Biogas Digester
I have used a 20 liter capacity bucket to transfer and dilute the pulverized weeds near the feeding point. More water is added up to the brim of the bucket to make to 20 liters. The diluted crushed mixture is then fed into the biogas digester through the feed pipe. This is a repetitive process and should be done everyday to get uninterrupted production of gas.
There may be some soil particles collected along with the weeds. These particles may get deposited at the bottom of the diluted mixture. So, do not empty the bucket containing pulverized weeds in to the feed pipe and discard the soil deposit away.
Step 6: A Study on Production of Energy
How much gas is produced per day ?
Calculating energy produced by this way may seem to be not in accord with the procedures followed by scientists in a controlled manner in laboratories. But I do not have that sort of facilities available with me. Moreover, I am using this gas for cooking purpose only, so I have compared it with normal LPG consumption at our home.
In terms of cubic meters
with 2.5 kilogram of pulverized weeds, let us see how much cubic capacity of gas is produced:
Inner diameter of gas holder = 80 centimeters (0.80 meters) (Radius is 0.40 meters)
Average height raise due to gas production = 45 centimeters (0.45 meters)
Cubic content of gas produced : 3.1428 X (0.40 X 0.40) X 0.45 = 0.226 cubic meters.
The above quantity also contains impurities like Carbon Dioxide, Nitrogen and Hydrogen Sulfide.
Liquefied Petroleum Gas (LPG) versus Biogas
Another test to find out the actual efficiency of the biogas is to compare with Liquefied Petroleum Gas (LPG) normally used in our household.
In our home, a full LPG cylinder containing nearly 15 kilograms of gas normally lasts for about 30 days. So, our consumption works out to about 500 grams of LPG per day.
As you can see in the pictures above, I have placed both the systems (one using LPG and another with Biogas) side by side. On 13 October 2014 we installed a new LPG Cylinder and started using both LPG and Biogas simultaneously. The LPG cylinder was fully emptied on 24 November 2014 and replaced with a new one.
The LPG cylinder which normally lasts for only 30 days has now been utilized for 41 days in conjunction with Biogas. At the rate of 500 grams of LPG per day, the additional 11 days X 500 grams = 5500 grams of equivalent LPG has been supplemented by Biogas.
Production of Biogas per day from weeds equivalent to LPG = 5500 grams /41 days = 134.15 grams
So, 0.226 cubic meter of biogas produced everyday ( mixed with Carbon Dioxide, Nitrogen and Hydrogen Sulfide), gave us 134.15 grams of equivalent calorific value of energy in comparison to LPG.
Step 7: Output Slurry
Whenever the biogas digester is fed with pulverized weeds, the equal volume of digested slurry is drained out through the outlet pipe. Here also, I have used a 20 liters bucket to collect the slurry, which can be used as compost in the garden.
Step 8: PH of Slurry
I have tested the pH of the output slurry with the help of pH test strips. The pH was measured between 7 and 8 in comparison to the Test chart provided with the kit. The slurry is slightly alkaline due to the presence of Ammonia produced by the weeds. I think this slurry is well suited as compost for our garden's red soil which is little bit acidic in nature.
Step 9: Maintenance of the System
The anaerobic digester needs very little maintenance. However the following steps will help in uninterrupted gas production.
The anaerobic process creates heat and some water vapor also gets mixed with the biogas. This water vapor will condense and may get trapped in gas pipe. To prevent this, keep the outlet valve closed at the gas holder tank after using the gas. This will help in most of the condensed water to fall back into the tank. Disconnect the gas pipe once in a week and drain out any condensed water from the pipe line.
Secondly, the slurry in the tank needs to be agitated to prevent the formation of surface crust. I do this after every feed using a vented ram through the feed pipe.
Step 10: Bottom Line
It takes less than an hour for me to collect, clean,chop, pulverize and feed the weeds in the anaerobic digester. It requires less than 5 minutes of mixture-grinder time to pulverize 2.5 kilograms of weeds in small batches, which is more than compensated by the output biogas and digested slurry as compost.
Advantages of the system
- Our home garden gets a regular weeding operation
- The weeds are utilized to extract bio-fuel
- The digested slurry is recycled as compost for the plants
- The bio-fuel meets 25 % of our cooking gas requirement
- Does not involve any running costs other than few minutes of electricity for pulverizing the weeds
- It is also eco-friendly
Some Do's and Don'ts
- Feed the Biogas digester regularly
- Do not over feed. Keep the excess weeds for tomorrow
- Do not collect weeds from areas where pesticides / herbicides were used
- Do not use chlorinated water. We use bore-well water for all our needs.
- Recycle water wherever possible.
- Do not use soapy water used for cleaning kitchen utensils.
It is almost three months since I started using weeds to produce energy. The Biogas digester is stabilized and producing regular supply of gas as well as composted slurry everyday. Every four months I save one full LPG cylinder which is being supplemented by biogas which is almost free of cost. As my biogas digester can support more load, I am planning to gradually increase the input from 2.5 kilograms to about 3.5 kilograms.
I hope I have explained everything in easy to follow simple steps here. Your inputs, comments and suggestions are most welcome.
Third Prize in the
MAKE ENERGY: A US-Mexico Innovation Challenge