The Methane Digester

Publicerad: 24 juli 2010

Amerikanska biogas rådet håller val för dess styrelse. Den self-nomination processen för alla ledamöter i god ställning började den 13 juli 2010. En nominering formulär finns på sidan medlemmar endast.  Alla nomineringar måste tas emot senast 26 juli 2010. En General Membership konferenssamtal är planerad till tisdagen 10 augusti 2010 östra samtidigt 3: 00 pm.Detta kommer att vara en möjlighet för styrelsen kandidater att presentera sig och svara på frågor.

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Checkbiotech Home Sign In | Register ViewEdit Report finds bioenergyproduction can expand acrossAfrica without displacing food Friday, July 23, 2010
Crops can be produced for bioenergy on a significant scale in west, eastern and southern Africa without doing damage to food production or natural habitats, according to a report produced by the Forum for Agricultural Research in Africa (FARA), Imperial College London, and CAMCO International.

The study was released today at the 5th African Agriculture Science Week in Burkina Faso.

“If approached with the proper policies and processes and with the inclusion of all the various stakeholders, bioenergy is not only compatible with food production; it can also greatly benefit agriculture in Africa,” said Dr. Rocio Diaz-Chavez, the report’s lead author and Research Fellow at Imperial College London. “Bioenergy production can bring investments in land, infrastructure, and human resources that could help unlock Africa’s latent potential and positively increase food production.”

The conclusions of the report, Mapping Food and Bioenergy in Africa, were drawn from a review of existing research and case studies of biofuel production and policy in six countries: Senegal, Mali, Tanzania, Kenya, Zambia, and Mozambique. Among the report’s findings is that there is enough land available to significantly increase the cultivation of crops, such as sugar cane, sorghum, and jatropha for biofuels without diminishing food production.

The case studies found that interest is growing across Africa in bioenergy to address both income and energy needs. For example, ethanol can be blended with fossil fuels to reduce dependence on expensive fuel imports that are a major drag on economic development. Ethanol also can be used in cooking stoves, reducing dependence on unhealthy and environmentally destructive charcoal and wood. In addition, using biodiesel to power electrical generators is of keen interest in many areas, given the challenges facing the power grid across Africa.

But as global demand escalates for biodiesel and ethanol, a key concern has emerged that a rush to expand production in Africa, particularly for export, could usurp land and resources needed food crops. But Diaz-Chavez said evidence uncovered in the FARA report finds “food versus bioenergy” should not be the choice, and that the more relevant discussion today is how to properly integrate bioenergy into agriculture production systems in different regions of Africa.

For example, the report concludes that sugar cane production for biofuels could be doubled in many areas “without reducing food production or destroying valuable habitats.” Furthermore, Diaz-Chavez said it’s clear that many African countries are sensitive to potential conflicts with food production and are pursuing policies that address this concern.

“There have been individual instances where bioenergy production has produced negative impacts, but that does not mean it is not possible to develop this sector in a sustainable manner,” she said.

For example, Mozambique has adopted a policy that designates only sugar cane and sweet sorghum for ethanol and jatropha and coconut for biodiesel. In South Africa, Parliament has decreed that maize can no longer be used for biofuel. Mali does not allow food crops to be used for biofuel production. Also, the report finds programs in Mali to increase production of jatropha, a shrub that produces seeds that can be converted into fuel, are benefiting local smallholder farmers “without compromising food production.”

The analysis reveals that the challenge today is not so much whether bioenergy production can co-exist with food production but rather how it can be scaled-up to help African countries realize their potential.

“There are the same challenges you see for any crops, which is if you don’t have adequate resources, you cannot boost production,” she said. For example, the report concludes that in the less developed countries of Africa, “it is possible to triple yields by using improved management practices, potentially freeing up more land for bioenergy production.”

Part of the study was devoted to identifying or “mapping” the wide array of groups in Africa who are getting involved in various aspects of bioenergy production. They include farmers groups, NGOs, industry and, in governments, agriculture, transport, energy, and environment ministries and agencies.

“We know there is intense interest in many sectors, but the problem is they are not necessarily communicating with one another,” said Dr. Monty Jones, Executive Director of FARA. “It is a situation that offers an opportunity for FARA, as an organization with partners in the public and private sector across the region, to provide a forum that brings all of these different interests together to form a cohesive and inclusive approach to bioenergy production.”

At a discussion of the report during the Science Week, a key issue that emerged was that bioenergy issues must be part of a broader conversation about how to meet the energy needs of African farmers.

“Energy is the key to modernizing agriculture in Africa,” said Dr. Ibrahim Togola, who heads an agriculture industry development group in Mail and is a professor at Mali’s Rural Polytechnical Institute. “We need to make our politicians understand that we will never have an agriculture revolution in Africa without having access to modern energy services,” he said.

Provided by Burness Communications

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The bacteria within a methane digester are solely responsible for both the break-down of organic matter and production of various gases. However just like  plants   these bacteria can only survive  within a certain range of conditions , where a mix of key variables affecting their growth are said to be ‘ideal’. These key factors include pressure , temperature and pH of the environment.

In the case of a methane digester the biogas we are aiming to produce is approximately 60-70% methane and so the bacteria we want to breed in the digester are  ‘methanogens’ or methane-forming bacteria. They will get energy by breaking down the organic matter and as waste gas they will emit carbon dioxide and methane gas as a mixture. Of course then we will be aiming to operate the digester at ‘optimal’ conditions for methane-forming bacteria.

Temperature Control

The key here which will have the greatest impact on digester design , is whether the methane-forming bacteria being produced in the digester are of the mesophilic or thermophilic type. The difference is at which temperature the bacteria operate at and the metabollic rate of each type.

Mesophilic bacteria are the most common methane-forming bacteria found in biogas digesters as they have a lower optimal temperature range at 30-35°C which means little or no heating of the digester is required depending on the climate. Mesophillic digesters account for 90% of biogas producing digesters simply for this reason , however the difficulty is in maintaining this constant temperature as below 30°C  acid-forming bacteria will grow and lead to digester failure. And so it is important when considering digester design to look at ways of insulating to reduce or eliminate the heat input required to maintain mesophillic conditions. A Mesophilic digester should be careful not to fall below 32°C as methane production will slow significantly and volatile acid formation can occur , 35°C is the preferred temperature for optimum methane production.

Thermophillic bacteria grow in the 50-60°C range anything higher than that are known as hyperthermophiles. The major advantage of operating a thermophilic digester as opposed to a mesophilic is that the metabollic rate of the bacteria is significantly higher. This means that the retention time is shorter and biogas is produced at a faster rate , at 50°C the retention time for waste is approximately 10 days as opposed to 20 days at 40°C. The problem is retaining 50°C requires significant heating and mixing to ensure the temperature profile is uniform across the digester  , if left unmixed small variations of temperature can occur in pockets that lead to the death of thermophilic bacteria. Thermophiles cannot tolerate fluctuations in temperature as well as mesophiles so variations should be kept within <1°C daily.

pH Control

Alkalinity is essential for proper pH control in a biogas digester as it serves as the buffer for any enzyme activity in the digester. acceptable levels for acid-forming bacteria are at pH 5.0 and below but any enzyme activity from methane bacteria will not occur at below pH 6.2. At start-up of a anaerobic digester pH will drop as volitile acids are produced in the sludge , then once the methane producing bacteria begin to consume these acids and produce alkalines the pH will stabilize . At around 5 days retention time the methane producing bacteria will rapidly consume these acids. A properly operated digester will fall within pH 6.8-7.2 , however the CO2 % of the biogas produced will have a huge effect on the pH of the system.

High alkalinity concentration has a stabilizing effect on digester , as dropping alkaline levels can serve as a marker for impending digester failure . The inability for methanogens to consume the voltile acids or produce methane and CO2 will lead to a rapid drop in pH which can be read first by dropping alkalinity concentration. Reasons for this could be presence of wastes that inhibit the methane producing bacteria or accumlation of acids due to incorrect temperature or other conditions.

Alkalinity is added to the system by introducing chemicals such as ammonium carbonate , sodium bicarbonate or sodium nitrate. The key is that any chemical added to a digester to control pH is done slowly, as bulk loading the digester can lead to unwanted conditions such as foaming , ammonium poisoning or too greater Oxidation Reduction Potential (ORP) which means the bacteria wont be able to produce methane as a reaction. If any excess alkalinity exists in the digester it can be neutralized by adding ferric cholride.

The key to pH control of a digester is knowing that pH is a measure of what has happened in the digester while alkalinity is a measurement of what is happening.

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A typical gas system comprises the digester cover, pressure and vacuum relief devices, water trap, flame trap, pressure regulator, gas meter, check valve, pressure gauges, waste gas burner and a gas holder.  Mixing of digesters by means of gas re-circulation requires a compressor. 

The digester is covered to contain odours, maintain temperature, keeps air out and to collect the gas.  Fixed covers are more usual than floating covers.  During normal operation, there is a space for gas collection between the cover and the liquid surface of the digester contents.  The cover of a digester has certain unique features that the operating staff must be aware of, for example, how the variation in pressure and the level inside the digester may affect the cover.  The biggest danger associated with the operation of fixed cover digesters occurs when the pressure relief device mounted on top of the digester fails or the sludge overflow line blocks and the liquid level in the digester continues to rise.  In such a situation, the excess gas pressure inside the digester can exceed the maximum design pressure and damage the cover or its mountings.  Fixed covers can also be damaged by excess negative pressure (vacuum) or if the rate of waste sludge withdrawal exceeds the feed rate or the vacuum relief device fails. 

The function of the pressure relief device is to allow pressure that exceeds a safe level to escape from the digester.  The manufacturer’s specifications should provide the following information to enable the operating staff to control the gas system safely.  (a) The system’s normal operating pressure (mm water gauge).  (b) The pressure at which the pressure relief device should operate.  (c) The rated gas flow capacity of the pipes. 

A gas system generally comprises three pressure relief devices; one mounted on the cover of the digester, one on the gas holder and one situated at the end of the gas line before the gas burner.  This device releases gas to the waste gas burner before the design gas pressure of the digester cover is reached. 

The vacuum relief device functions in the opposite manner to the pressure relief device and allows air to enter the digester in the event of the waste sludge being withdrawn too rapidly.  Air should not, under normal conditions, be allowed to enter the digester because a mixture of air and methane is potentially highly explosive. 

Gas leaving the digester is almost saturated with water vapour.  As the gas cools, the water vapour condenses causing problems.  The problem is more severe when digesters are heated.  To solve the problem it is essential to remove as much of the moisture as possible before the gas comes into contact with the gas system devices.  For this reason, water traps should be located as close to the digester as possible.  All piping should be sloped a minimum of 1% towards the water trap, which should be situated at a low point in the gas line. 

Flame traps are emergency devices installed in gas lines to prevent flames travelling back up the gas line (flashback) and reaching the digester.  The flame trap generally consists of a box filled with stone or a metal grid.  If a flame develops in the gas line, the temperature of the flame is reduced below the ignition point as it passes through the trap and the flame is extinguished. 

Pressure regulators are used when a lower pressure than the system operating pressure, is required for a specific device such as a boiler water heater or incinerator.  Regulators maintain a constant gas pressure at the point of use

The rate of gas production is probably the most sensitive process control indicator at the disposal of the operating staff.  Gas meters should be regularly serviced so as to give accurate and reliable gas readings.  Every effort should be made to have the gas meter calibrated every six months. 

Check valves (non-return valves) are installed in the gas line to allow gas flow in one direction only i.e. out of the digester. 

Gas pressure gauges indicate the pressure in the gas system and assist in locating any blockages in the line.  If a blockage occurs, a pressure reading downstream will register a lower pressure than that of a gauge upstream. 

The pressure relief device at the waste gas burner safely flares excess gas to the atmosphere and thus reduces the potential for a dangerous accumulation of gas in the system. 

Many anaerobic digestion waste treatment plants have a means of storing excess gas.  This can be in the form of either a floating roof on the digester or a separate gasholder. 

A mixture of biogas and air can be explosive.  Methane gas in concentrations of between 5% and 15% in air by volume is explosive.  Operating staff on waste treatment plants should ensure that no air is allowed to enter the digester or gasholder.  All piping and equipment must be sealed properly to prevent gas from escaping to the outside.  There must be no smoking and all electrical installations, including light switches, torches etc must be of the explosion-proof type, as the smallest spark could ignite escaped gases.

Michael Russell

Your Independent guide to Waste Treatments [http://waste-treatments.com]

Article Source: [http://EzineArticles.com/?Anaerobic-Digestion---Gas-Production&id=328579] Anaerobic Digestion – Gas Production

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Researchers around the world are looking at sheep and livestock diets and genetic engineering in an attempt to reduce the flatulance of our furry friends.

Its known that livestock contribute a great amount to global greenhouse emissions , around 10% for the large developed economies such as the U.S. But some smaller nations such as New Zealand  gas from livestock account for 48% of total emissions!

Its reassuring then that governments are  beginning to sink money into agricultural research , after all its the developing nations that rely so critically on agriculture and farming for their countries exports.

The scientists are analyzing the nutritional quality of livestock feeds and how they can economically create feed that will give comparible nutritional value for a lesser amount and produce less gas when digested. Testing of various ‘natural’ supplements have included garlic , cottonseed and even doses of chloroform to slow activity.

The core problem is how livestock like sheep , cattle and deer process their foods. It is digested with the aid of microbes known as ‘ruminants’. These ruminants are actually known in scientific terms as methanogens , they convert hydrogen gas made in a previous process into methane which the animal belches out into the atmosphere. The tests so far have showed no real long term results , it seems the sheep adapt quickly to supplements and methane generation rises back to previous levels. Special research facilities have even been constructed to measure sheep belches and farts over long periods , and apparently the animals love it so much they are willing to stay days at a time.

More eye-brow raising solutions include breeding sheep that have altered digestion systems so that less methane is emitted from the animal.

The break-through might not come for some time but this is just one piece of the puzzle that is global warming  and any efforts put forward in the right direction are worth commending.

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Gasification is another method of extracting useful energy from biomass and something i only recently begun to research into as i became interested after seeing a few homemade gasifier videos using wood chippings as the fuel on youtube.

A homemade gasifier produces gas in an entirely different mode of operation than digesters. The temperature alone is 700°C in most homemade gasifier units , which is considerbly higher than the 40°C encountered in digesters. At this high temperature the biomass is reacted with steam and oxygen streams which atomize the solid fuel, the result is known as synthesized gas or syngas for short, this syngas can then be used as a fuel . As a matter of fact this technology has been around for a long long time , with cars in the 1930s even fitted with rear mounted gasifiers that fed syngas into the combustion chamber.

Syngas can be further processed to produce hydrogen or methanol fuels. Its high combustion temperature is many times greater than regular petroleum based fuels , and anyone familiar with the carnot cycle from thermodynamics will remember  that the higher the combustion temperature the more efficient the thermodynamic cycle.

Nowadays  gasification technology is mainly used in the area of combine cycle heat-power systems (CHP)  , and is popular for rural locations where biomass is the most economical energy source and a waste product of agriculture. Recently gasifiers technology has been focusing on producing usable energy from plastic rich waste such as landfills. The same basic chemical reaction is taking place because plastics are essentially long chains of carbon atoms with sidegroups , except these sidegroups can produce a variety of toxic and polluting flue gas into the atmosphere that need to be captured by various gas scrubbing systems.

To the left is a basic project you can try that shows the fundamental concept of gasifying a biomass to produce syngas. The syngas itself is also combusted in this burner to produce heat , but at a much greater efficiency.

To the right is a more sophisticated unit built by victory gas works and shows just how much energy can be extracted from a small amount of biomass. Here he is powering a welder from the syngas generated in the homemade gasifier. Anyone who owns a welder knows those things draw alot of amps , impressive!

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Digester system that convert waste material into biogas are becoming more popular throughout the world. Farmers and rural communities now have a means to produce good quality fertilizer and natural biogas from waste materials like manure in a cheap renewable way.

However this biogas produced is roughly 60% methane and 29% Co2 with trace elements of Hydrogen Sulphide , and is not up to the quality of 99% pure methane natural gas if the owner was planning on selling this gas or using it as fuel gas for machinery. The corrosive nature of H2S will destroy any internals in machinery and corrode pipework.

The solution is to implement a biogas upgrading or purification system. Biogas upgrading is a series of processes where raw biogas is first cleaned from contaminant gases and then dried, so that what is left at the end of the process is 98%+ methane, natural fuel gas. Manufacturers that produce and sell biogas purification systems each have their own different processes and technology that they employ to produce the sale quality gas. A few of them are detailed below.

Water Washing Upgrading

This is the most common method of purifying biogas as shown above. Here raw biogas from the digester is first compressed and then fed into the scrubber vessel where passing water streams adsorb the soluble gas contaminants leaving near pure methane. The gas is then dried by dessicant in the drier columns and exit the system as 98%+ methane.

Pressure Swing Adsorption

Otherwise known as PSA , this purifcation method separates the Co2 , Nitrogen , Oxygen and Water from the raw biogas stream by adsorbing  gases at high pressure and desorbing them at low pressure as waste. The PSA system usually consists of 4 different adsorption columns working in sequence; Adsorption , depressurizing , desorption and repressurizing.

The raw biogas is compressed and fed into the bottom of the adsorption column where it is purified. during this time the remaining columns regenerate , such that there is always 1 adsorber column actively cleaning gas. PSA does not scrub hydrogen sulphide so this most be removed before it enters the compressor.

Polyglycol

Using polyglycol (Tradename Selexol)to purify biogas is similar to the water washing method with regeneration. Selexol can adsorb hydrogen sulphide , carbon dioxide and water. However the energy required to regenerate the solution after adsorbing H2S is high , so hydrogen sulphide is removed before the process.

Chemical Reaction

Raw biogas can be upgraded by various chemical reactions that remove the C02 and other contaminants from the gas stream. The chemicals such as Alkanolamines react at atmospheric pressure in an adsorption column with the Co2 and are regenerated afterwards with steam. The hydrogen sulphide must first be removed to avoid toxifying the chemicals.

Advantages and Disadvantages

Each plant type furfills its purpose of supplying high quality natural gas for grid injection. However depending on the site location, various environmental and economic factors might make selecting a certain type of system a more sensible choice. For areas where water is an expensive resource a more appropriate choice would be a PSA or Selexol system which regenerate the adsorbent , however this has to be offset against the heat input required in regeneration.

Another important factor to consider is the methane loss associated with each design. The methane loss is measured using gas analyzers and flowmeters at the suction and discharge sides of the plant. Most plants are guaranteed by manufacturers to acheive a maximum 2% methane loss. Some recent studies however have measured between 8-10% methane loss at PSA and Selexol plant sites , possibly due to leaks and poor maintenence. Chemical systems have even lower guaranteed losses since the chemicals selectively react with the Co2 in the gas stream instead of adsorbing.

Energy Demands

For a biogas upgrading plant the auxillary power required to drive the compressors, pumps etc is anywhere between 3-6% of the total energy output in the form of natural gas. The cost associated with upgrading biogas also decreases with larger plant size , a smallish plant of 100 metres cubed per hour will upgrade gas at more than twice the cost of a plant outputting 200 – 300 metres cubed per hour.

Conclusions

A Digestor is only the beginning of the process to convert biomass into useful high quality natural gas. A biogas purification system takes the raw biogas  at around 60% methane from the digester and through a special process outputs 98% methane for ether use as fuel gas or supplied to the grid. The four main upgrading processes are water washing , pressure swing adsorption,  polyglycol adsorption and chemical treatment. Water washing and PSA are the most predominantly used systems in the world today. Typical energy requirements for a biogas purification system are between 3-6% of the total methane output , with smaller plants cost more to run than larger ones. As digester systems become more common around the world and people begin to catch on to biogas as a renewable source of energy , no doubt we will see more of these systems become available and more innovative designs.

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A Methane Digester also known as a Biogas Digester or Anaerobic Digester is where waste organic matter is converted to Biogas. The gas made in a digester is usually about 70% pure methane and 30% Carbon dioxide with trace amounts of Hydrogen Sulphide , which in commercial digesters is scrubbed from the gas stream as Hydrogen Sulphide is highly corrosive and gives digesters the distinctive rotten egg smell. The bacteria responsible for the break-down of biomass into methane exist without oxygen , as such the process is known as Anaerobic Digestion.

A Methane Digester can come in many different sizes from small residential units the size of water tanks to the large underground commercial digesters , capable of upwards of 2000 m^3 an hour.

A Methane Digester can be designed for either batch production of biogas or continuous production. Batch digesters take raw biomass material and ferment it for a period of time known as the residency time. After methane production has begun to drop off the methane digester is opened,emptied and cleaned, and a new batch of digester material is loaded in. The advantage with batch systems is that they are simple to operate and any undigestable material that has been loaded can easily be removed at the end of production. The downside is that you are removing the bacteria culture after each clean and as such it takes time before the oxygen is removed from the digester and methane production meets previous levels. In biogas plants where batch type systems are used this sporadic production of methane is countered by used several digesters in parrallel. This way there is always a constant level of biogas being produced.

The other common type of methane digester is known the continous digester. In a continuous digester biogas production is maintained at a steady rate by adding small amounts of biomass daily. The advantage is that biogas supply is kept constant , which is desirable when producing gas for heating or power. In typical farm type digesters 0.5kg of pig manure is enough to keep producing approximately 20mins worth of 60% methane fuel gas. The disadvantage is that any insolubles will build up and other issues such as scum or caking  will reduce the digester efficiency over time and so operation of a continous loading type digester has to be managed more so than a fermenting batch type digester.

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Biogas is a niche market that has enjoyed booming growth over the past decade. It offers commercial natural gas producers a unique way to captilize on resources which all communities and locations have. It also offers rural communities and villages an opportunity to produce there own electricity with low maitenence.

I hope you find the information on this site useful for what ever interest you have in biogas systems.

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