LOCALLY MADE BIOGAS

LOCALLY MADE BIOGAS 

Abstract

The current irrational use of fossil fuels and the impact of greenhouse gases on the environment are driving research into renewable energy production from organic resources and waste. The global energy demand is high, and most of this energy is produced from fossil resources. Recent studies report that anaerobic digestion is an efficient alternative technology that combines biofuel production with sustainable waste management, and various technological trends exist in the biogas industry that enhance the production and quality of biogas. Further investments in anaerobic digestion are expected to meet with increasing success due to the low cost of available feedstocks and the wide range of uses for biogas (i.e., for heating, electricity, and fuel). Biogas production is growing in the world energy market and offers an economical alternative for bioenergy production. The objective of this work is to provide an overview of biogas production from lignocellulosic waste, thus providing information toward crucial issues in the biogas economy.

Introduction

Biogas means a gas produced from biological materials. In fact, it is not one gas rather it is a mixture of gases such as CO₂, CO, H₂, H₂S, O₂. However, the proportion of methane within the biogas can vary between 50% and 80%, depending on whether some oxygen is able to enter at the beginning or during the process. If some oxygen is present, the bacteria will respire aerobically and will produce a gas with a higher proportion of carbon dioxide and a lower proportion of methane.

Biogas can be produced on a small scale in a biogas generator/digester, which can be made of simple materials.

The carbohydrate-containing materials are fed in, and a range of bacteria anaerobically ferment the carbohydrate into biogas. The remaining solids settle to the base of the digester and can be run off to be used as fertilizer for the land. These types of biogas generator are most commonly used in the developing world to satisfy the needs of a small family.

The optimum temperature for biogas production is between 32^oC and 35^oC. Temperatures above and below this optimum can result in less biogas being produced, which can be a problem in hotter and cooler countries.

Materials

It can be produced from any biological material yet it is preferred to use waste materials for its production such as

1.    plant materials,

2.     agricultural waste,

3.     kitchen waste,

4.     municipal waste,

5.    Manure (from cow dung) etc.

 

Materials to Be Excluded from Anaerobic Digesters

Materials that should be excluded as feedstock from anaerobic digesters include those containing compounds known to be toxic to anaerobic bacteria, poorly degradable material, and biomass containing significant concentrations of inorganic material. Poorly biodegradable materials require higher retention times, meaning they must spend more time in the anaerobic digester to be broken down and converted into biogas.
Inorganic materials, on the other hand, contain no carbon and cannot be converted into biogas. Materials such as sand bedding do not contribute to the biogas potential and may cause operational problems such as pipe clogging, premature equipment wear and volume reduction due to sludge accumulation. Also, the feedstock containing too much ammonium or sulfur should be avoided, because ammonium and sulfur inhibit anaerobic organisms.

Making of the biogas

Biogas digester can be started with the following materials.

a.    Two drums – One drum must be slightly smaller than the other so that the smaller one can be fitted into the larger one.

b.     Manure suited to creating the needed anaerobic bacteria culture.

c.     Input pipe and funnel for pouring in processed food waste.

d.     Output pipe for draining waste for fertilizer.

e.     3 uniseals.

f.      Valve for controlling gas out flow.

g.     Pipe for connecting outflow valve to burner Gas Burner

h.     Chicken wire & cable ties to make a cage for the biodigester.

i.       For cold climates and winter – black spray paint (if drums aren’t black) and/ or aluminum tape or other outdoor insulation materials.

STEP 1

Ensure that the drums fit tightly enough to prevent oxygen getting in and methane escaping, but not too snug so that the smaller drum doesn’t slide. The smaller drum should still slide down on its own due to its own weight so that there here is pressure from the smaller drum on the gas that is created.

STEP 2

Three holes need to be cut into the two drums. The solid’s input pipe and the gas outflow valve must be cut into the top of the smaller drum, while an output pipe for the leftover digested mass is needed at the bottom of the larger drum in order to drain the excess material for fertilizer. Cut holes suited to the size of the three holes and insert the appropriately sized uniseals. It will be necessary to seal these holes to ensure that gas doesn’t leak out and oxygen doesn’t get in and ruin the digestion process.

As a rule of thumb, your input pipe would be much larger than your outflow valve as the food going in would take up more volume and the smaller space for the outflow pipe will add to the pressure needed for effectively using the bio gas. Make sure that they are effectively sealed by putting the drum in some water. If it seeps through and into the drum, it’s not well sealed and a lot of gas will be lost. Lastly, the input pipe must have a sealed closure so that it is not left open after manure or vegetation matter has been added.

STEP 3

Create a cage for your drums that will prevent the top from popping off. Chicken wire shaped to give enough space for the small drum to rise would do the trick. Don’t drill holes into the larger drum to secure it though. That would allow oxygen in. Remember, the cage needs to allow access to the input and output pipes.

STEP 4

The manure needs to be diluted with water at a 1:1 ratio and stirred into a slurry like consistency. Then, pour it into the bottom of the larger drum carefully.

STEP 5

Insert smaller drum into larger drum and give the bacteria some time. After about three weeks to a month, adding in some cut grass is a good way to build up the system. Don’t add too much plant matter that is high in sugar such as grains, fruits and vegetables. First let the bacteria establish themselves and the correct pH level with the kind of plant matter that grazing animals very effectively turn into methane. After a while, any leftover vegetables can be added along with cut grass. Manure is needed from time to time to maintain the bacteria count.

STEP 6

Maintain the temperature. If you live in a cold climate, you may need to provide added insulation to your biogas digester system. The temperature needed by the bacteria ranges between 32 and 37 degrees Celsius or about 90 to 99 degrees Fahrenheit. If it drops below 15 degrees Celsius or 60 degrees Fahrenheit the bacteria will no longer be active.
A few tips for keeping the container insulated would be:

● Spray paint the container black for it to attract as much heat from the sun as possible.

● Bury the container in order for the earth to insulate the heat generated by the bacteria.

● Cover the container with aluminum tape or another insulation material that may be available to you that is suited to the outdoors.

STEP 7

Hook up your burner to the pipe you have attached to the outflow valve.

Biogas – Carbon Nitrogen Ratio

Examples Carbon(C) to Nitrogen(N) Ratios (25:1 is ideal)
Type of waste	C:N Ratio
Human sanitation waste	3:1
Pig waste	13:1
Food waste	15:1
Cattle manure	25:1
Grass	27:1
Brown tree leaves	47:1
Straw	87:1
Paper	150:1
Cardboard	560:1

Purification of Biogas

Biogas consists of methane (CH₄) and carbon-dioxide (CO₂) along with some trace gases such as water vapor, hydrogen sulphide (H₂S), nitrogen, hydrogen and oxygen.
Carbon dioxide and trace gases such as water vapor and H₂S must be removed before the biogas can be used because:

·         the hydrogen sulphide gas is corrosive

§  water vapor may cause corrosion when combined with H₂S on metal surfaces and reduce the heating value

Biogas purification technologies include;

·         Scrubbing

·         Chemical Absorption

·         Pressure Swing Adsorption

·         Membrane Purification

·         Cryogenic Separation

·         Biological Processes

Water Scrubbing

1.    Used to remove CO₂ and H₂S (more soluble in water than CH₄)

2.    The absorption process is purely physical.

3.    Usually biogas is pressurized and fed to the bottom of a packed column while water is fed on the top and so absorption process is operated counter-currently

4.    Water scrubbing can be used for selective removal of H₂S since H₂S is more soluble than CO₂ in water.

5.    Water which exits the column with absorbed CO₂ and/or H₂S can be regenerated and re-circulated back to the scrubber.

6.    Regeneration is accomplished by de-pressuring or by stripping with air in a similar column.

7.    Stripping with air is not recommended when high levels of H₂S are handled since water quickly becomes contaminated with S0which causes operational problems.

8.    When cheap water can be used, for example, outlet water from a sewage treatment plant, the most cost-efficient method is not to re-circulate the water

PEG Scrubbing

·         Polyethylene glycol (PEG) scrubbing relies on the same underlying mechanism as water scrubbing.

·         The big difference between water and PEG is that CO₂ and H₂S are more soluble in PEG which results in a lower solvent demand and reduced pumping.

·         Due to formation of elementary sulfur stripping the PEG is normally done with steam or inert gas rather than with air.

·         Removing H₂S beforehand is an alternative

Advantages/disadvantages

Advantages

·         No special chemicals required (except relatively inexpensive PEG) and removal of both CO2 and H2S.

·         Disadvantages

·         Requires a lot of water even with regeneration

·         Limitations on H2S removal, because the CO2decreases pH of the solution

·         Corrosion to the equipment caused by H2S.

·         Cost of water scrubbing;

Storage of Biomethane in Adsorbed Form

The natural gas or methane itself cannot be liquefied by simply increasing the pressure; it is also necessary to provide subcritical temperatures, and these two requirements make more expensive the transportation and storage of methane. Thus, adsorption on solid adsorbents allowed to expand storage capacity in comparison with compressed natural gas or pure methane at the same conditions of temperature and pressure. adsorption is an exothermic process, i.e. it loses energy to the environment; the reverse process, called desorption, is endothermic. The adsorption process is governed by the nature of solid adsorbents and forces distributed along the active surface and pores, so that the interactions involved are dependent on the adsorbent and adsorbate structures, crystal and pore size, purity of adsorbent and adsorbate and adsorbate size. The occurrence of adsorbate–adsorbate and adsorbate–adsorbent interactions are the main properties observed in adsorption systems, and the study of these properties is made by constructing sorption isotherms. These isotherms indicate whether the system under the conditions at the time of observation favors the adsorption or the adsorbate– adsorbate interactions are stronger than the force of attraction of the solid adsorbent. Methane storage is commonly accomplished through the use of microporous adsorbents depending on the temperature and pressure conditions employed. For the adsorption process at low pressures (approximately ambient pressure), microporous materials with a pore diameter above 7.6Å (diameter greater than two molecules of methane) are recommended.