Telescoping HDPE Digester (THD), Popular Biodigesters & other valuable biogas INFO:

The information presented here is intended as a guide for choosing what digester is suitable for various or individual applications.

Links, references & sources are indicated so they can be accessed to obtain additional data needed.

This webpage attempts to provide most of the important information needed to study & understand the importance of biogas systems.

	Item	Fixed Dome: China Fixed Dome or CFD	Floating Dome: India Floating Cover or IFC	Tubular: Tubular Polyethylene Digester or TPD	Puxin	ARTI	HBS: Home Biogas System	THD: Telescoping HDPE Digester
1	Pressurized Gas	Variable	Constant	Variable	Constant	Constant	Variable	Constant
2	Construction Time	18 days	18 days	2 days	3 days	2 days	6 days	4 days
3	Skills Required	High	High	Medium	Medium	Medium	Medium	Medium
4	Training Required	3 mos	3 mos	1 day	1 wk	1 day	1 day	1 day
5	Need to be airtight	Imperative	Imperative	Imperative	Not- Imperative	Not- Imperative	Imperative	Not- Imperative
6	Maintenance	High	High	Medium	Low	Low	Low	Low
7	Cost	High	High	Low	High	High	Low	Low
8	Feedstock	Manure & organic wastes	Manure & organic wastes	Manure & organic wastes	Manure & organic wastes	Starchy Food Wastes	Manure & organic wastes	Manure & organic wastes
9	Minimum size	5m3	5m3	5m3	5m3	1m3	5m3	3m3
10	Operating Temperature	Warm & Cold	Warm	Warm	Warm & Cold	Warm	Warm	Warm
11	Agitation	No	No	No	No	No	Yes	Yes
12	Cleanable	No	Yes	No	Yes	Yes	Yes	Yes

Legend: Red Text = Poor Features & Blue Text = Good Features

Fixed Dome or China Fixed Dome (CFD) Digester)

Most well known digester design. Most widely used in China and countries with hot & cold climates.

1. Gas volume & rate of production is dependent on the type of feedstock, frequency of feeding & temperature.
2. Gas amount & pressure continuously varies. It is not efficient for biogas equipment like water heaters, lights & generators.
3. Digester construction requires a very high level of skilled labour.
4. The digester has to be constructed & sealed so that it is airtight – any crack will allow biogas to escape.
5. Failures are usually caused by cracks during construction, cracks that develop as cement cures and/or un-avoidable structural defects.
6. Cleaning the digester is difficult & dangerous.

Floating Dome Digester or India Floating Cover (IFC) Digester

1. Weight of the floating cover ensures that the gas produced is under constant pressure which gives this type of digester its main advantage.
2. Unfortunately this advantage is also its main disadvantage -- the sliding/telescoping mechanism requires specialized design & construction, is costly & requires constant maintenance.

http://www.biogassa.com/default.php?ipkCat=3&sid=3

Tubular or Tubular Polyethylene (TPD) Digester

1. Most in-expensive digester but also the least sturdy or durable.
2. Complete building instructions are available at:

http://www.adelaide.edu.au/biogas/poly_digester/aguilar/sld001.htm

http://www.fao.org/WAICENT/FAOINFO/AGRICULT/AGA/AGAP/FRG/Recycle/biodig/manual.htm

3. A video of a tubular digester being built is available at:

http://www.youtube.com/watch?v=3Sl0XEN5Bgo

Puxin Digester

A unique patented hydraulic digester sold commercially.

It claims eliminating the disadvantages & enhancing the advantages of traditional fixed & floating dome digester designs.

1. Its main feature is its ability to function as a hydraulic system.
2. Entire digester is flooded with water, with the water at the same level in the inlet, digester neck and outlet – this is responsible for creating the constant biogas pressure.

As the biogas is produced in the bottom of the digester, it rises upwards and is eventually caught in the dome. As the volume of gas increases, it starts to replace the water in a downward direction.

The resulting upward pressure of the replaced water ensures that the collected biogas in the dome is always under constant pressure (up to 8 bar).

3. Easily emptied & cleaned. Because it is so easy to clean, any type of organic material (that stays in digester as sediment like leaves, starw or grasses) can be used as feeder material.
4. For more inquiries about this diegester, please contact:

www.biogassa.com/default.php?ipkCat=3&sid=3 and/or

http://www.puxinbiogas.com/en/index.asp?a=41

ARTI Digester

1. A compact digester for producing biogas from food waste. It is ideal for urban or city use. It is not ideal for farm use & for use with animal manures.
2. Units are typically 1 m3 in size & can be built using a 750 liter plastic tank fitted inside a 1000 liter plastic tank. See picture below. (More detailed instructions are available for FREE at: http://www.arti-india.org/content/view/45/40/
3. Feed stock required: About 1 kl/day of starchy or carbohydrate-rich food wastes. Examples: breads, rice, vegetables, & fruits that have been chopped finely & allowed to ferment for a few days.
4. Video on ARTI Digester that also explain how digesters work: http://www.youtube.com/watch?v=BGSl72xZHNk

N O T E:

The Home Biogas System (HBS) & Telescoping HDPE Digester (THD) described below are recommended by this website.

They were developed by the website owner by combining, innovating & adopting excellent features of the popular digesters mentioned above.

The HBS & THD were also developed to become low-cost, simple & easy to build and efficient digesters.

Home Biogas System or HBS

1. This is a Do-It-Yourself (DIY) digester that uses commonly available materials that are in-expensive & available everywhere.
2. The building plans & operating instructions are available for FREE. Request for them at: biofuelswork@gmail.com
3. The main difference of this digester when compared to the Fixed Dome or CFD digester is a flat concrete slab in-lieu of a dome-type cover.

A flat concrete slab cover is easier & cheaper to build. By using a flat slab, problems created by the difficulty & complexity of building domes are avoided.

Successful projects are assured.

Telescoping HDPE Digester or THD

1. The use of concrete whether in dome-type or flat-slab cover is prone to having microscopic holes and/or developing cracks that result in gas leakage.
2. The Telescoping HDPE Digester or THD is under development to eliminate or minimize this problem. Prototypes are expected to be built soon.
3. In simple terms, the THD is a Floating Cover digester.

Its appearance will be similar to the India Floating Cover (IFC) digester shown below except that HDPE material will be used to construct the floating cover in lieu of sheet metal that is heavy, expensive & susceptible to corrosion.

As special tools & equipment are needed to form the HDPE floating cover, it will be factory-fabricated & mass-produced to ensure availability at high quality & low cost.

HDPE means High Density Polyethylene. It is a strong & durable liner material used for large biodigesters, landfills & other heavy duty applications.

http://home.btconnect.com/engindia/biogas.htm

Importance of Biogas

Biogas is a potent greenhouse gas (GHG).

Interest in capturing & collecting it, especially when it simply escapes into the atmosphere as a fugitive emission has been high because doing so:

· provides an in-expensive, abundant & eco-friendly LPG-like fuel;

· helps protect against Climate Change & Global Warming;

· controls pollution caused by indiscriminate waste disposal in rivers, canals, backyards, etc.; and,

· qualifies as a Clean Development Mechanism (CDM) project of the United Nations Framework Convention on Climate Change (UNFCCC) where carbon credits known as Certified Emission Reduction (CER) can be earned to defray project costs; provide subsidies & incentives; and, become financial rewards for doing something good.

Uses & applications for Biogas

When biogas is produced, especially in warm & tropical countries like the Philippines where ambient temperatures are ideal for year-round optimum biogas production, “more than what is needed” or excess biogas is usually produced. In small biogas systems, excess biogas is best shared (with neighbors) as shown below.

Here are descriptions of popular uses & applications of biogas:

1. Flaring: Using biogas as a fuel is not always the best alternative.

Biogas-fueled appliances, engines & equipment are expensive, high-maintenance, fast-depreciating, in-efficient & un-economical.

Several extremely large pigfarms in Chile opted to simply flare or burn their biogas. Their conscious efforts qualified for CDM benefits that earned CERs retroactively since 2001 although their projects were only registered at the UNFCCC on 02 Sept 2005.

As of 31 October 2006, the Pocillas & La Estrella; Peralillo and Corneche & Los Guindos farms have received 865,038, 446,517 & 406,829 CERs respectively for a total of 1,718,384 CERs. Since these farms are entitled to CERs until 2012 when the CDM program ends, they could receive another 1.7+ million CERs again. For reference CERs are valued at about €12/CER as of 31 August 2009.

To get more information, see: http://cdm.unfccc.int/Issuance/cers_iss.html

2. Cooking & Heating: Like flaring, using biogas for cooking & for heating purposes are most economical & least expensive. If the biogas is used instead of LPG or other petroleum-based fuels, the savings can be significant. If biogas is used in-lieu of firewood, the savings are small but contributions in the fight against Climate Change & Global Warming, deforestation & denudation are valuable.

See: http://www.youtube.com/watch?v=c5P-5XVOaQg

3. Generating Electricity: Making electricity from generators fueled with biogas is a very appealing & attractive but it may not be cost-effective.

Few people realize that generators produce huge amounts of excess electricity that is simply wasted because it cannot be saved or stored. Industrialized countries recognize this & allow excess power producers to “load to the grid.” This system allows electricity users to buy when they have a demand & sell back to the supplier when there is an excess.

Technically, large power producers situated in different locations generate electricity only when they can sell it to the grid or when there is a demand. It would be wasteful to generate electricity & nobody wants or needs it. In some locations like Kalayaan-Caliraya in the Philippines & Niagara Falls in the USA, huge amounts of excess electricity that cannot be turned-off are used to pump water into elevated reservoirs so it can be used later in a process called pumped-storage hydroelectricity.

http://richard-rowland-perkins.com/professional/renewable-tech-hydropower/

Excellent video on pumped storage hydroelectricity: http://www.youtube.com/watch?v=2EKnAyAwW6M

4. Compressing & Bottling: Biogas is usually 60% Methane (CH4), 35% Carbon Dioxide (CO2) & other elements.

By comparison, Natural Gas (NG) is usually 95% Methane (CH4). If CO2 & other elements in biogas can be removed, biogas becomes 90-95% CH4 & can be used just like Natural Gas. It can be added to existing Natural Gas (NG) supply lines and compressed into tanks for use as engine fuel.

Video of a dairy farm that clean, upgrade & compress their excess biogas into biomethane (90+% Methane) to use as fuel for their trucks. Interestingly, they could have used to make more electricity from their excess biogas but that would have been un-profitable if they could not use their excess electricity. Selling it to the grid most likely provides less income (in the form savings) compared to using biomethane as a vehicle fuel – that is expensive & becoming more expensive.

See: http://www.youtube.com/watch?v=lIkEm8JUOfY

The technologies to clean, upgrade & compress biogas are complex & expensive. They are not suitable for small applications.

However, as large biogas-to-elctricity systems especially in the Philippines where ambient temperatures are ideal for optimum biogas production have abundant excess biogas as shown in the bulging biodigester below, the website owner is developing solutions for this problem. Contact him at biofuelswork@gmail.com for more information.

5. Fuel for engines & vehicles: Engines with spark-ignition systems are commonly used for biogas-fueled applications.

Compression ignition engines like diesel engines are usually modified & fitted with spark plugs & ignition distributors for use as biogas fueled engines.

Note the addition of spark-plugs, high-tension wires and new manifold in this Isuzu diesel engine to convert it for biogas service.

For other questions or information, please email: biofuelswork@gmail.com
or visit our Home Page at: www.biofuelswork.com

Originally posted: 02 September 2009

Last updated: 03 September 2009