Difference between revisions of "Fixed-dome Biogas Plants"

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►[[Biogas|Back to Biogas Portal]]
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►[[Biogas|Back to Biogas Portal]]<br/>
  
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= Overview =
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= Overview<br/> =
  
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There are different types of small scale biogas digesters. Some of the biogas digesters are summarized below:
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There are different types of small scale biogas digesters. <u>Some of the biogas digesters are summarized below:</u>
  
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= Fixed-dome Plants<br/> =
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*'''Fixed-dome Plants'''
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*[[Floating Drum Biogas Plants|Floating-drum Plants]]
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*[[Low-Cost Polyethylene Tube Digester|Low-Cost Polyethylene Tube Digester]]
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*[[Balloon Digester for Biogas Plants|Balloon Plants]]
  
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A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on top of the digester. When gas production starts, the slurry is displaced into the compensation tank. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank.
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<br/>The table below gives a first comparison of the different types.<br/>
  
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The costs of a fixed-dome <span data-scaytid="37" data-scayt_word="biogas">biogas</span> plant are relatively low. It is simple as no moving parts exist. There are also no rusting steel parts and hence a long life of the plant (20 years or more) can be expected. The plant is constructed underground, protecting it from physical damage and saving space. While the underground digester is protected from low temperatures at night and during cold seasons, sunshine and warm seasons take longer to heat up the digester. No day/night fluctuations of temperature in the digester positively influence the bacteriological processes.
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{| cellspacing="0" cellpadding="0" border="0" style="width:100%"
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|-
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! scope="col" style="width: 126px" | <p style="text-align: center">&nbsp;Factors</p>
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! scope="col" style="width: 126px" | <p style="text-align: center">Fixed dome</p>
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! scope="col" style="width: 126px" | <p style="text-align: center">Floating drum</p>
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! scope="col" style="width: 126px" | <p style="text-align: center">Tubular design</p>
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! scope="col" style="width: 126px" | <p style="text-align: center">Plastic containers</p>
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|-
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| style="width:126px" | <p style="text-align: center">Gas storage</p>
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| style="width:126px" | <p style="text-align: center">Internal Gas storage up to 20 m³ (large)</p>
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| style="width:126px" | <p style="text-align: center">Internal Gas storage drum size (small)</p>
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| style="width:126px" | <p style="text-align: center">Internal eventually external plastic bags</p>
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| style="width:126px" | <p style="text-align: center">Internal Gas storage drum sizes (small)</p>
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|-
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| style="width:126px" | <p style="text-align: center">Gas pressure</p>
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| style="width:126px" | <p style="text-align: center">Between 60 and 120 mbar</p>
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| style="width:126px" | <p style="text-align: center">Upto 20 mbar</p>
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| style="width:126px" | <p style="text-align: center">Low, around 2 mbar</p>
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| style="width:126px" | <p style="text-align: center">Low around 2mbar</p>
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|-
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| style="width:126px" | <p style="text-align: center">Skills of contractor</p>
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| style="width:126px" | <p style="text-align: center">High; masonry, plumbing</p>
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| style="width:126px" | <p style="text-align: center">High; masonry, plumbing, welding</p>
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| style="width:126px" | <p style="text-align: center">Medium; plumbing</p>
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| style="width:126px" | <p style="text-align: center">Low; plumbing</p>
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|-
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| style="width:126px" | <p style="text-align: center">Availability of Material</p>
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| style="width:126px" | <p style="text-align: center">yes</p>
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| style="width:126px" | <p style="text-align: center">yes</p>
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| style="width:126px" | <p style="text-align: center">yes</p>
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| style="width:126px" | <p style="text-align: center">yes</p>
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|-
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| style="width:126px" | <p style="text-align: center">Durability</p>
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| style="width:126px" | <p style="text-align: center">Very high >20 years</p>
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| style="width:126px" | <p style="text-align: center">High; drum is weakness</p>
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| style="width:126px" | <p style="text-align: center">Medium; Depending on chosen liner</p>
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| style="width:126px" | <p style="text-align: center">Medium</p><p style="text-align: center">&nbsp;</p>
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|-
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| style="width:126px" | <p style="text-align: center">Agitation</p>
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| style="width:126px" | <p style="text-align: center">Self agitated by Biogas pressure</p>
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| style="width:126px" | <p style="text-align: center">Manual steering</p>
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| style="width:126px" | <p style="text-align: center">Not possible; plug flow type</p>
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| style="width:126px" | <p style="text-align: center">Evtl Manual steering</p>
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|-
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| style="width:126px" | <p style="text-align: center">Sizing</p>
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| style="width:126px" | <p style="text-align: center">6 to 124 m³ digester vol</p>
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| style="width:126px" | <p style="text-align: center">Up to 20 m³</p>
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| style="width:126px" | <p style="text-align: center">Combination possible</p>
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| style="width:126px" | <p style="text-align: center">Up to 6 m³ digester vol<br/></p>
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|-
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| style="width:126px" | <p style="text-align: center">Methane emission</p>
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| style="width:126px" | <p style="text-align: center">High</p>
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| style="width:126px" | <p style="text-align: center">Medium</p>
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| style="width:126px" | <p style="text-align: center">Low</p>
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| style="width:126px" | <p style="text-align: center">Medium<br/></p>
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|}
  
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The construction of fixed dome plants is labor-intensive, thus creating local employment. Fixed-dome plants are not easy to build. They should only be built where construction can be supervised by experienced <span data-scaytid="40" data-scayt_word="biogas">biogas</span> technicians. Otherwise plants may not be gas-tight (porosity and cracks).
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<br/>
  
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<br/>
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= Fixed-dome Plants<br/> =
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A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on top of the digester. When gas production starts, the slurry is displaced into the compensation tank. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank. The costs of a fixed-dome <span data-scaytid="37" data-scayt_word="biogas">biogas</span> plant are relatively low. It is simple as no moving parts exist. There are also no rusting steel parts and hence a long life of the plant (20 years or more) can be expected. The plant is constructed underground, protecting it from physical damage and saving space. While the underground digester is protected from low temperatures at night and during cold seasons, sunshine and warm seasons take longer to heat up the digester. No day/night fluctuations of temperature in the digester positively influence the bacteriological processes. The construction of fixed dome plants is labor-intensive, thus creating local employment. Fixed-dome plants are not easy to build. They should only be built where construction can be supervised by experienced biogas technicians. Otherwise plants may not be gas-tight (porosity and cracks).<br/>
  
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The basic elements of a fixed dome plant (here the '''<span data-scaytid="44" data-scayt_word="Nicarao">Nicarao</span> Design''') are shown in the figure below.
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<br/>The basic elements of a fixed dome plant (here the Nicarao Design) are shown in the figure below.
  
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{| class="FCK__ShowTableBorders" border="0" cellpadding="0" cellspacing="0" align="center" style="width: 600px;"
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{| cellspacing="0" cellpadding="0" border="0" align="center" class="FCK__ShowTableBorders" style="width: 100%"
 
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| [[File:Nicarao biogas.gif|300px|thumb|Fixed dome plant Nicarao design<ref name="TBW">TBW</ref>''': 1. Mixing tank with inlet pipe and sand trap. 2. Digester. 3. Compensation and removal tank. 4. Gasholder. 5. Gaspipe. 6. Entry hatch, with gastight seal. 7. Accumulation of thick sludge. 8. Outlet pipe. 9. Reference level. 10. Supernatant scum, broken up by varying level.''']]
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| style="width: 388px" | [[File:Nicarao biogas.gif|thumb|left|429px|Fixed dome plant Nicarao design: 1.Mixing tank with inlet pipe and sand trap. 2.Digester. 3.Compensation and removal tank. 4.Gasholder. 5.Gaspipe. 6.Entry hatch, with gastight seal. 7.Accumulation of thick sludge. 8.Outlet pipe. 9.Reference level. 10.Supernatant scum, broken up by varying level.]]
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|[[File:Fixdomscheme.gif|300px|thumb|picture<ref name="OEKOTOP">OEKOTOP</ref>:Basic function of a fixed-dome biogas plant, 1 Mixing pit, 2 Digester, 3 Gasholder, 4 Displacement pit, 5 Gas pipe]]
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| style="width: 376px" | [[File:Fixdomscheme.gif|thumb|right|263px|Basic function of a fixed-dome biogas plant: 1.Mixing pit, 2.Digester, 3.Gasholder, 4.Displacement pit, 5.Gas pipe]]
 
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|}
  
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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<br/>
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
  
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== <span style="font-size: 17px;">Function</span> ==
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== Function ==
  
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A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable, rigid gas-holder and a displacement pit, also named 'compensation tank'. The gas is stored in the upper part of the digester. When gas production commences, the slurry is displaced into the compensating tank. Gas pressure increases with the volume of gas stored, i.e. with the height difference between the two slurry levels. If there is little gas in the gas-holder, the gas pressure is low.
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A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable, rigid gas-holder and a displacement pit, also named 'compensation tank'. The gas is stored in the upper part of the digester. When gas production commences, the slurry is displaced into the compensating tank. Gas pressure increases with the volume of gas stored, i.e. with the height difference between the two slurry levels. If there is little gas in the gas-holder, the gas pressure is low.<br/>
  
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
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<br/>
  
 
== Diges​ter ==
 
== Diges​ter ==
  
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{| class="FCK__ShowTableBorders" border="0" cellpadding="10" align="right" style="width: 248px;"
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[[File:Biogastunesia01.jpg|thumb|right|261px|Fixed-dome plant in Tunesia. The final layers of the masonry structure are being fixed.]] The digesters of fixed-dome plants are usually masonry structures, structures of cement and <span data-scaytid="52" data-scayt_word="ferro-cement">ferro-cement</span> exist. Main parameters for the choice of material are:<br/>
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| style="width: 220px;" |
 
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<small class="IMGLEGEND">'''[[File:Biogastunesia01.jpg|200px|RTENOTITLE]]'''</small>
 
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<small class="IMGLEGEND">Fixed-dome plant in <span data-scaytid="102" data-scayt_word="Tunesia">Tunesia</span>. The final layers of the masonry structure are being fixed.</small><ref name="gtz/GATE">gtz/GATE</ref>
 
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|}
 
  
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The digesters of fixed-dome plants are usually masonry structures, structures of cement and <span data-scaytid="52" data-scayt_word="ferro-cement">ferro-cement</span> exist. Main parameters for the choice of material are:
 
 
*Technical suitability (stability, gas- and liquid tightness);
 
*Technical suitability (stability, gas- and liquid tightness);
 
*cost-effectiveness;
 
*cost-effectiveness;
 
*availability in the region and transport costs;
 
*availability in the region and transport costs;
 
*availability of local skills for working with the particular building material.
 
*availability of local skills for working with the particular building material.
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<div style="clear:both"></div>
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Fixed dome plants produce just as much gas as floating-drum plants, ''if they are gas-tight''. However, utilization of the gas is less effective as the gas pressure fluctuates substantially. Burners and other simple appliances cannot be set in an optimal way. If the gas is required at constant pressure (e.g., for engines), a gas pressure regulator or a floating gas-holder is necessary.
  
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<br/>Fixed dome plants produce just as much gas as floating-drum plants, ''if they are gas-tight''. However, utilization of the gas is less effective as the gas pressure fluctuates substantially. Burners and other simple appliances cannot be set in an optimal way. If the gas is required at constant pressure (e.g., for engines), a gas pressure regulator or a floating gas-holder is necessary.
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<br/>[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
  
 
<br/>
 
<br/>
  
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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== Gas-Holder ==
  
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<br/>
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The top part of a fixed-dome plant (the gas space) must be gas-tight. Concrete, masonry and cement rendering are not gas-tight. The gas space must therefore be painted with a [[Plasters and Coats for Biogas Digesters and Gas-holder|gas-tight layer]] (e.g. 'Water-proofer', Latex or synthetic paints). A possibility to reduce the risk of cracking of the gas-holder consists in the construction of a [[Parts of a Biogas Plant#Weak Ring|weak-ring]] in the masonry of the digester. This "ring" is a flexible joint between the lower (water-proof) and the upper (gas-proof) part of the hemispherical structure. It prevents cracks that develop due to the hydrostatic pressure in the lower parts to move into the upper parts of the gas-holder.<br/>
  
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== <span style="font-size: 19px; line-height: 23.80000114440918px;">Gas-Holder</span> ==
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[[Types of Gasholders for Biogas Plants#Fixed-dome Gasholders|►Read more.....]]
  
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The top part of a fixed-dome plant (the gas space) must be gas-tight. Concrete, masonry and cement rendering are not gas-tight. The gas space must therefore be painted with a [[Plasters_and_coats_for_digesters_and_gas-holder|gas-tight layer]] (e.g. 'Water-proofer', Latex or synthetic paints). A possibility to reduce the risk of cracking of the gas-holder consists in the construction of a [[Parts_of_a_Biogas_Plant#Weak_Ring|weak-ring]] in the masonry of the digester. This "ring" is a flexible joint between the lower (water-proof) and the upper (gas-proof) part of the hemispherical structure. It prevents cracks that develop due to the hydrostatic pressure in the lower parts to move into the upper parts of the gas-holder.
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<br/>[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
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<br/>
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
 
  
 
<br/>
 
<br/>
  
 
== Types of Fixed-dome Plants<br/> ==
 
== Types of Fixed-dome Plants<br/> ==
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[[File:Camartecfixdome.gif|300px|thumb|<small class="IMGLEGEND">'''Fixed dome plant <span data-scaytid="111" data-scayt_word="CAMARTEC">CAMARTEC</span> design'''</small><ref name="TBW">TBW</ref>]]
 
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*'''Chinese fixed-dome plant''' is the archetype of all fixed dome plants. Several million have been constructed in [[Biogas Technology in China|China]]. The digester consists of a cylinder with round bottom and top.
 
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*'''<span data-scaytid="103" data-scayt_word="Janata">Janata</span> model '''was the first fixed-dome design in [[Biogas Technology in India|India]], as a response to the Chinese fixed dome plant. It is not constructed anymore. The mode of construction lead to cracks in the gasholder - very few of these plant had been gas-tight.
 
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*'''<span data-scaytid="104" data-scayt_word="Deenbandhu">Deenbandhu</span>,''' the successor of the <span data-scaytid="105" data-scayt_word="Janata">Janata</span> plant in India, with improved design, was more crack-proof and consumed less building material than the <span data-scaytid="106" data-scayt_word="Janata">Janata</span> plant. with a hemisphere digester
 
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*'''<span data-scaytid="109" data-scayt_word="CAMARTEC">CAMARTEC</span> model''' has a simplified structure of a hemispherical dome shell based on a rigid foundation ring only and a calculated joint of fraction, the so-called weak / strong ring. It was developed in the late <span data-scaytid="110" data-scayt_word="80s">80s</span> in [[Biogas Technology in Tanzania|Tanzania]].
 
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<br clear=all>
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
 
  
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== Climate and Size<br/> ==
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*'''Chinese fixed-dome plant''' is the archetype of all fixed dome plants. Several million have been constructed in [[Biogas Technology in China|China]]. The digester consists of a cylinder with round bottom and top.<br/>
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*'''<span data-scayt_word="Janata" data-scaytid="103">Janata</span> model '''was the first fixed-dome design in [[Biogas Technology in India|India]], as a response to the Chinese fixed dome plant. It is not constructed anymore. The mode of construction lead to cracks in the gasholder - very few of these plant had been gas-tight.
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Fixed-dome plants must be covered with earth up to the top of the gas-filled space to counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the option for internal heating makes them suitable for colder climates. Due to economic parameters, the recommended minimum size of a fixed-dome plant is 5 <span data-scaytid="112" data-scayt_word="m3">m<sup>3</sup></span>. Digester volumes up to 200 <span data-scaytid="221" data-scayt_word="m3">m<sup>3</sup></span> are known and possible.
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*'''<span data-scayt_word="Deenbandhu" data-scaytid="104">Deenbandhu</span>,''' the successor of the <span data-scayt_word="Janata" data-scaytid="105">Janata</span> plant in India, with improved design, was more crack-proof and consumed less building material than the <span data-scayt_word="Janata" data-scaytid="106">Janata</span> plant. with a hemisphere digester
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*'''<span data-scayt_word="CAMARTEC" data-scaytid="109">CAMARTEC</span> model''' has a simplified structure of a hemispherical dome shell based on a rigid foundation ring only and a calculated joint of fraction, the so-called weak / strong ring. It was developed in the late <span data-scayt_word="80s" data-scaytid="110">80s</span> in [[Biogas Technology in Tanzania|Tanzania]].
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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*'''AKUT fixed dome plant''' is an improvement of the above mentioned Nicaragua design. Digester volumes ranges from 8 to 124 m³ with gas storages from 2 to 19,4 m³; the gas production can reach 60 m³/d. The units from 32 m³ onwards are often used for small scale productive use including electricity generation. It has a cylindrical base with a spheric top. The expansion chamber acts as overpressure outlet.
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*'''AKUT Maendaleo''' (kisuaheli "progress") adds a gas storage ballon to collect access gas from the digestion chamber. This can be used for converted Diesel generators.
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== Summary ==
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[[File:Camartecfixdome.gif|thumb|left|300px|Camartec Fixed-dome]]<div style="clear:both"></div>
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<br/>[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
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'''Advantages:''' Low initial costs and long useful life-span; no moving or rusting parts involved; basic design is compact, saves space and is well insulated; construction creates local employment.
 
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Advantages are the relatively low construction costs, the absence of moving parts and rusting steel parts. If well constructed, fixed dome plants have a long life span. The underground construction saves space and protects the digester from temperature changes. The construction provides opportunities for skilled local employment.
 
  
 
<br/>
 
<br/>
  
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'''Disadvantages:''' Masonry gas-holders require special <span data-scaytid="120" data-scayt_word="sealants">sealants</span> and high technical skills for gas-tight construction; gas leaks occur quite frequently; fluctuating gas pressure complicates gas utilization; amount of gas produced is not immediately visible, plant operation not readily understandable; fixed dome plants need exact planning of levels; excavation can be difficult and expensive in bedrock.
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== Climate and Size<br/> ==
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Disadvantages are mainly the frequent problems with the gas-tightness of the brickwork gas holder (a small crack in the upper brickwork can cause heavy losses of <span data-scaytid="117" data-scayt_word="biogas">biogas</span>). Fixed-dome plants are, therefore, recommended only where construction can be supervised by experienced <span data-scaytid="118" data-scayt_word="biogas">biogas</span> technicians. The gas pressure fluctuates substantially depending on the volume of the stored g''as. Even though the underground construction buffers temperature extremes, digester temperatures are generally low.''
 
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''Fixed dome plants can be recommended only where construction can be supervised by experienced <span data-scaytid="119" data-scayt_word="biogas">biogas</span> technicians.''<br/>
 
  
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'''''Variations:''' Some companies are now looking into small <span data-scaytid="124" data-scayt_word="pre-fab">pre-fab</span> fixed dome plants made of <span data-scaytid="125" data-scayt_word="fibreglass">fibreglass</span> which appears to be a low cost alternative'' to construction intensive <span data-scaytid="126" data-scayt_word="masoned">masoned</span> plants. A custom made plant can be produced in 2 days and -after transport- installed in less than 1 day!
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Fixed-dome plants must be covered with earth up to the top of the gas-filled space to counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the option for internal heating makes them suitable for colder climates. Due to economic parameters, the recommended minimum size of a fixed-dome plant is 5 <span data-scaytid="112" data-scayt_word="m3">m<sup>3</sup></span>. Digester volumes up to 200 <span data-scaytid="221" data-scayt_word="m3">m<sup>3</sup></span> are known and possible.<br/>
  
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]<br/>
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= Floating-drum Plants<br/> =
 
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[[File:Floating drum mauretania.jpg|333px|thumb|right|Floating drum plant in Mauretania<ref>gtz/GATE</ref>]]Floating-drum plants consist of an underground digester and a moving gas-holder. The gas-holder floats either directly on the fermentation slurry or in a water jacket of its own. The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored. The gas drum is prevented from tilting by a guiding frame. If the drum floats in a water jacket, it cannot get stuck, even in substrate with high solid content.
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
 
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== The Drum<br/> ==
 
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In the past, floating-drum plants were mainly built in India. A floating-drum plant consists of a cylindrical or dome-shaped digester and a moving, floating gas-holder, or drum. The gas-holder floats either directly in the fermenting slurry or in a separate water jacket. The drum in which the biogas collects has an internal and/or external guide frame that provides stability and keeps the drum upright. If biogas is produced, the drum moves up, if gas is consumed, the gas-holder sinks back.
 
  
 
<br/>
 
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[[Fixed-dome Biogas Plants#toc|►Go to Top]]
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== Summary ==
  
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== Size ==
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'''Advantages:''' Low initial costs and long useful life-span; no moving or rusting parts involved; basic design is compact, saves space and is well insulated; construction creates local employment. Advantages are the relatively low construction costs, the absence of moving parts and rusting steel parts. If well constructed, fixed dome plants have a long life span. The underground construction saves space and protects the digester from temperature changes. The construction provides opportunities for skilled local employment.<br/>'''Disadvantages:''' Masonry gas-holders require special <span data-scayt_word="sealants" data-scaytid="120">sealants</span> and high technical skills for gas-tight construction; gas leaks occur quite frequently; fluctuating gas pressure complicates gas utilization; amount of gas produced is not immediately visible, plant operation not readily understandable; fixed dome plants need exact planning of levels; excavation can be difficult and expensive in bedrock. Disadvantages are mainly the frequent problems with the gas-tightness of the brickwork gas holder (a small crack in the upper brickwork can cause heavy losses of <span data-scayt_word="biogas" data-scaytid="117">biogas</span>). Fixed-dome plants are, therefore, recommended only where construction can be supervised by experienced <span data-scayt_word="biogas" data-scaytid="118">biogas</span> technicians. The gas pressure fluctuates substantially depending on the volume of the stored g''as. Even though the underground construction buffers temperature extremes, digester temperatures are generally low.'' ''Fixed dome plants can be recommended only where construction can be supervised by experienced <span data-scayt_word="biogas" data-scaytid="119">biogas</span> technicians.''
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[[File:Floatdrumschemewaterjack s.gif|300px|right|thumb|Water-jacket plant with external guide frame: 1 Mixing pit, 11 Fill pipe, 2 Digester, 3 Gasholder, 31 Guide frame, 4 Slurry store, 5 Gas pipe<ref>Sasse, 1984</ref>]]
 
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Floating-drum plants are used chiefly for digesting animal and human feces on a continuous-feed mode of operation, i.e. with daily input. They are used most frequently by small- to middle-sized farms (digester size: <span data-scaytid="132" data-scayt_word="5-15m3">5-15m<sup>3</sup></span>) or in institutions and larger agro-industrial estates (digester size: <span data-scaytid="252" data-scayt_word="20-100m3">20-100m<sup>3</sup></span>)'''''.'''''
 
  
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'''Disadvantages''': The steel drum is relatively expensive and maintenance-intensive. Removing rust and painting has to be carried out regularly. The life-time of the drum is short (up to 15 years; in tropical coastal regions about five years). If fibrous substrates are used, the gas-holder shows a tendency to get "stuck" in the resultant floating scum.
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''A specific environmental disadvantage is methane emission from the expansion chamber. ''<br/>'''''Variations:''' Some companies are now looking into small <span data-scayt_word="pre-fab" data-scaytid="124">pre-fab</span> fixed dome plants made of <span data-scayt_word="fibreglass" data-scaytid="125">fibreglass</span> which appears to be a low cost alternative'' to construction intensive <span data-scayt_word="masoned" data-scaytid="126">masoned</span> plants. A custom made plant can be produced in 2 days and -after transport- installed in less than 1 day!
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[[#toc|►Go to Top]]<br/>
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== Water-jacket Floating-drum Plants<br/> ==
 
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Water-jacket plants are universally applicable and easy to maintain. The drum cannot get stuck in a scum layer, even if the substrate has a high solids content. Water-jacket plants are characterized by a long useful life and a more aesthetic appearance (no dirty gas-holder). Due to their superior sealing of the substrate (hygiene!), they are recommended for use in the fermentation of night soil. The extra cost of the masonry water jacket is relatively modest.
 
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== Material of Digester and Drum<br/> ==
 
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The digester is usually made of brick, concrete or quarry-stone masonry with plaster. The gas drum normally consists of 2.5 mm steel sheets for the sides and 2 mm sheets for the top. It has welded-in braces which break up surface scum when the drum rotates. The drum must be protected against corrosion. Suitable coating products are oil paints, synthetic paints and bitumen paints. Correct priming is important. There must be at least two preliminary coats and one topcoat. Coatings of used oil are cheap. They must be renewed monthly. Plastic sheeting stuck to bitumen sealant has not given good results. In coastal regions, repainting is necessary at least once a year, and in dry uplands at least every other year. Gas production will be higher if the drum is painted black or red rather than blue or white, because the digester temperature is increased by solar radiation. Gas drums made of 2 cm wire-mesh-reinforced concrete or fiber-cement must receive a gas-tight internal coating. The gas drum should have a slightly sloping roof, otherwise rainwater will be trapped on it, leading to rust damage. An excessively steep-pitched roof is unnecessarily expensive and the gas in the tip cannot be used because when the drum is resting on the bottom, the gas is no longer under pressure.
 
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Floating-drums made of glass-fiber reinforced plastic and high-density polyethylene have been used successfully, but the construction costs are higher compared to using steel. Floating-drums made of wire-mesh-reinforced concrete are liable to hairline cracking and are intrinsically porous. They require a gas-tight, elastic internal coating. PVC drums are unsuitable because they are not resistant to UV.
 
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== Guide Frame<br/> ==
 
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The side wall of the gas drum should be just as high as the wall above the support ledge. The floating-drum must not touch the outer walls. It must not tilt, otherwise the coating will be damaged or it will get stuck. For this reason, a floating-drum always requires a guide. This guide frame must be designed in a way that allows the gas drum to be removed for repair. The drum can only be removed if air can flow into it, either by opening the gas outlet or by emptying the water jacket.
 
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The floating gas drum can be replaced by a balloon above the digester. This reduces construction costs but in practice problems always arise with the attachment of the balloon to the digester and with the high susceptibility to physical damage.
 
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== Types of Floating-drum Plants<br/> ==
 
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<u>There are different types of floating-drum plants:</u>
 
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*'''<span data-scaytid="136" data-scayt_word="KVIC">KVIC</span> model''' with a cylindrical digester, the oldest and most widespread floating drum <span data-scaytid="135" data-scayt_word="biogas">biogas</span> plant from India.
 
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*'''<span data-scaytid="138" data-scayt_word="Pragati">Pragati</span> model''' with a hemisphere digester
 
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*'''<span data-scaytid="139" data-scayt_word="Ganesh">Ganesh</span> model''' made of angular steel and plastic foil
 
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*floating-drum plant made of pre-fabricated reinforced concrete compound units
 
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*floating-drum plant made of <span data-scaytid="140" data-scayt_word="fibre-glass">fibre-glass</span> reinforced polyester
 
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*low cost floating-drum plants made of plastic water containers or fiberglass drums: [[Biogas Plant by ARTI|ARTI Biogas plants]]
 
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*'''<span data-scaytid="144" data-scayt_word="BORDA">BORDA</span> model: '''The <span data-scaytid="145" data-scayt_word="BORDA-plant">BORDA-plant</span> combines the static advantages of hemispherical digester with the process-stability of the floating-drum and the longer life span of a water jacket plant.
 
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== Summary<br/> ==
 
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'''Advantages: '''Advantages are the simple, easily understood operation - the volume of stored gas is directly visible. The gas pressure is constant, determined by the weight of the gas holder. The construction is relatively easy, construction mistakes do not lead to major problems in operation and gas yield.
 
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'''Disadvantages: '''Disadvantages are high material costs of the steel drum, the susceptibility of steel parts to corrosion. Because of this, floating drum plants have a shorter life span than fixed-dome plants and regular maintenance costs for the painting of the drum.
 
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= Low-Cost <span data-scaytid="146" data-scayt_word="Polyethylen">Polyethylen</span> Tube Digester<br/> =
 
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== Digester ==
 
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In the case of the Low-Cost Polyethylene Tube Digester model which is applied in [[Low-Cost Polyethylene Tube Digesters in Bolivia|Bolivia]] (Peru, Ecuador, Colombia, Centro America and Mexico), the tubular polyethylene film (two coats of 300 microns) is <span data-scaytid="147" data-scayt_word="bended">bended</span> at each end around a 6 inch PVC drainpipe and is wound with rubber strap of recycled tire-tubes. With this system a hermetic isolated tank is obtained (figure <span data-scaytid="148" data-scayt_word="td1">td1</span>).
 
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[[File:In Outlet winding Tube Digester.jpg|thumb|left|300px|Picture td2: Winding In-/Outlet Tube Digester<ref name="GTZ / EnDev Bolivia">GTZ / EnDev Bolivia</ref>]]
 
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<br/>[[File:Low Cost Polyethylene Tube Digester Scheme.jpg|thumb|center|500px|Low Cost Polyethylene Tube Digester Scheme.jpg|alt=Low Cost Polyethylene Tube Digester Scheme.jpg]]
 
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<span style="font-size: 11px; text-align: center;">Figure <span data-scaytid="492" data-scayt_word="td1">td1</span>: Scheme of Low-cost Polyethylene Tube Digester. </span><ref name="GTZ / EnDev Bolivia">GTZ / EnDev Bolivia</ref>
 
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''One of the 6" PVC drainpipes serves as inlet and the other one as the outlet of the slurry. In the tube digester finally, a hydraulic level is set up by itself, so that as much quantity of added prime matter (the mix of dung and water) as quantity of fertilizer leave by the outlet.<br/>Because the tubular polyethylene is flexible, it is necessary to construct a "cradle" which will accommodate the reaction tank, so that a trench is excavated (picture <span data-scaytid="153" data-scayt_word="td3">td3</span>).''
 
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= Criteria for the Selection of a Type and Design =
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== Gasholder and Gas Storage Reservoir<br/> ==
 
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The capacity of the gasholder corresponds to 1/4 of the total capacity of the reaction tube (figure <span data-scaytid="155" data-scayt_word="td1">td1</span>).
 
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To overcome the problem of low gas flow rates, two 200 microns tubular polyethylene reservoirs are installed close to the kitchen, which gives a 1,3 m³ additional gas storage (picture <span data-scaytid="157" data-scayt_word="td4">td4</span>).
 
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[[File:Gas Storage Reservoir double inside.jpg|border|center|350px|Gas Storage Reservoir double inside.jpg|alt=Gas Storage Reservoir double inside.jpg]]
 
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[[File:Biodigestor familiar bolivia altiplano.jpg|thumb|center|180px|Biodigestor familiar bolivia altiplano.jpg]]
 
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<span style="line-height: 1.5em; font-size: 11px; text-align: center;">Picture <span data-scaytid="39" data-scayt_word="td4">td4</span>: Gas Storage Reservoir</span><ref name="GTZ / EnDev Bolivia">GTZ / EnDev Bolivia</ref>
 
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<span style="line-height: 1.5em; font-size: 11px; text-align: center;">Picture <span data-scaytid="60" data-scayt_word="td3">td3</span>: Tube Digester </span><span data-scaytid="59" data-scayt_word="Altiplano" style="line-height: 1.5em;"><span style="font-size: 11px; text-align: center;">Altiplan</span><span style="font-size: 11px; text-align: center;">o</span></span><span style="line-height: 1.5em; font-size: 11px; text-align: center;">of Bolivia</span><ref name="GTZ / EnDev Bolivia">GTZ / EnDev Bolivia</ref>
 
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<br/>To contrast these simple <span data-scaytid="351" data-scayt_word="biogas">biogas</span> plants, figure 2 gives an impression about dimensions of industrial plants which are, for example, built in Europe.
 
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[[File:Indplant.gif|605px|RTENOTITLE]]
 
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<small class="IMGLEGEND">Fig. 2: Industrial <span data-scaytid="353" data-scayt_word="biogas">biogas</span> plant with utilization of domestic organic wastes</small><ref name="TBW">TBW</ref>
 
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== Experience with <span data-scaytid="356" data-scayt_word="Plythylene">Plythylene</span> <span data-scaytid="354" data-scayt_word="Biogas">Biogas</span> Digester (<span data-scaytid="358" data-scayt_word="PBD">PBD</span>)<br/> ==
 
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*[[Experience with Polyethylene Biogas Digester (PBD)|Experience with Polyethylene Biogas Digester (PBD) from some countries (click HERE)]]
 
  
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<br/>
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A biogas plant should serve the owner of the plant. There it must be in any case financially viable, the benefits have to exceed the investment. In addition it has to be user friendly. easy to be fed and to be operated.
  
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== Variation <span data-scaytid="363" data-scayt_word="Tyre">Tyre</span> Tube Household Digester<br/> ==
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A sufficient gas storage is mandatory; a capacity of 10 hours is for many cases acceptable. In a domestic set up Biogas will not be used for 10 hours during the night. A small gas storage will lead to methane emissions and to monetary losses.
  
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*[https://energypedia.info/TyreTube Biogas Plant Variation tyre tube household digester]
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A gas presure of 10 mbar at least seem to be necessary to operate a burner. On ballon storages this load of 10 kg/m² surface have to applied to get enough pressure.
  
 
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= <span style="font-size: 17px;">Balloon Plants</span> =
 
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A balloon plant consists of a heat-sealed plastic or rubber bag (balloon), combining digester and gas-holder. The gas is stored in the upper part of the balloon. The inlet and outlet are attached directly to the skin of the balloon. Gas pressure can be increased by placing weights on the balloon. If the gas pressure exceeds a limit that the balloon can withstand, it may damage the skin. Therefore, safety valves are required. If higher gas pressures are needed, a gas pump is required. Since the material has to be weather- and UV resistant, specially stabilized, reinforced plastic or synthetic <span data-scaytid="10" data-scayt_word="caoutchouc">caoutchouc</span> is given preference. Other materials which have been used successfully include <span data-scaytid="11" data-scayt_word="RMP">RMP</span> (red mud plastic), <span data-scaytid="12" data-scayt_word="Trevira">Trevira</span> and butyl. The useful life-span does usually not exceed 2-5 years.
 
  
 
<br/>
 
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'''Advantages''': Standardized prefabrication at low cost, low construction sophistication, ease of transportation, shallow installation suitable for use in areas with a high groundwater table; high digester temperatures in warm climates; uncomplicated cleaning, emptying and maintenance; difficult substrates like water hyacinths can be used.
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= Further Information =
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Balloon <span data-scaytid="15" data-scayt_word="biogas">biogas</span> plants are recommended, if local repair is or can be made possible and the cost advantage is substantial.
 
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'''Disadvantages''': Low gas pressure may require gas pumps; scum cannot be removed during operation; the plastic balloon has a relatively short useful life-span and is susceptible to mechanical damage and usually not available locally. In addition, local craftsmen are rarely in a position to repair a damaged balloon. There is only little scope for the creation of local employment and, therefore, limited self-help potential.
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*[[Portal:Biogas|Biogas Portal on energypedia]]
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*[[:File:Insulation of Fixed-Dome Biogas Digesters in China.pdf|Insulation of Fixed-Dome Biogas Digesters in China.pdf]]
  
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'''Variations:''' A variation of the balloon plant is the channel-type digester, which is usually covered with plastic sheeting and a sunshade (fig.1-E). Balloon plants can be recommended wherever the balloon skin is not likely to be damaged and where the temperature is even and high.
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[[File:Foilplant.jpg|348px|RTENOTITLE]] <small class="IMGLEGEND">Photo<ref name="Henning"> Henning</ref>:Small "foil-plant" (Ivory Coast)</small>
 
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[[File:Plant-types s.gif|500px|Plant-types s.gif|alt=Plant-types s.gif]] <small class="IMGLEGEND">Fig. 1<ref name="Biogas Plants, L. Sasse, GATE, 1988, p. 14">Biogas Plants, L. Sasse, GATE, 1988, p. 14</ref>: Simple <span data-scaytid="17" data-scayt_word="biogas">biogas</span> plants. Floating-drum plant (A), fixed-dome plant (B), fixed-dome plant with separate gas holder (C), balloon plant (D), channel-type digester with plastic sheeting and sunshade (E).</small>
 
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== Horizontal Plants<br/> ==
 
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Horizontal <span data-scaytid="20" data-scayt_word="biogas">biogas</span> plants are usually chosen when shallow installation is called for (groundwater, rock). They are made of masonry or concrete.
 
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'''Advantages: '''Shallow construction despite large slurry space.
 
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'''Disadvantages''': Problems with gas-space leakage, difficult elimination of scum.
 
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<blockquote></blockquote>
 
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== Earth-pit Plants<br/> ==
 
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Masonry digesters are not necessary in stable soil (e.g. <span data-scaytid="22" data-scayt_word="laterite">laterite</span>). It is sufficient to line the pit with a thin layer of cement (wire-mesh fixed to the pit wall and plastered) in order to prevent seepage. The edge of the pit is reinforced with a ring of masonry that also serves as anchorage for the gas-holder. The gas-holder can be made of metal or plastic sheeting. If plastic sheeting is used, it must be attached to a quadratic wooden frame that extends down into the slurry and is anchored in place to counter its buoyancy. The requisite gas pressure is achieved by placing weights on the gas-holder. An overflow point in the peripheral wall serves as the slurry outlet'''.'''
 
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'''Advantages''': Low cost of installation (as little as 20% of a floating-drum plant); high potential for self help approaches.
 
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'''Disadvantages''': Short useful life; serviceable only in suitable, impermeable types of soil.
 
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Earth-pit plants can only be recommended for installation in impermeable soil located above the groundwater table. Their construction is particularly inexpensive in connection with plastic sheet gas-holders.
 
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== <span data-scaytid="23" data-scayt_word="Ferrocement">Ferrocement</span> Plants<br/> ==
 
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The <span data-scaytid="24" data-scayt_word="ferro-cement">ferro-cement</span> type of construction can be applied either as a self-supporting shell or an earth-pit lining. The vessel is usually cylindrical. Very small plants (Volume under 6 <span data-scaytid="25" data-scayt_word="m3">m<sup>3</sup></span>) can be prefabricated. As in the case of a fixed-dome plant, the <span data-scaytid="85" data-scayt_word="ferrocement">ferrocement</span> gasholder requires special sealing measures (proven reliability with cemented-on <span data-scaytid="86" data-scayt_word="aluminium">aluminium</span> foil).
 
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'''''Advantages''''': Low cost of construction, especially in comparison with potentially high cost of masonry for alternative plants; mass production possible; low material input.
 
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'''Disadvantages''': Substantial consumption of essentially good-quality cement; workmanship must meet high quality standards; uses substantial amounts of expensive wire mesh; construction technique not yet adequately time-tested; special sealing measures for the gas-holder are necessary. Ferro-cement <span data-scaytid="29" data-scayt_word="biogas">biogas</span> plants are only recommended in cases where special <span data-scaytid="30" data-scayt_word="ferro-cement">ferro-cement</span> know-how is available. <!-- Navigation Footer -->
 
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= References =
 
= References =
  
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<references /><br/>
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This article is a continuation of the article, [[Types of Biogas Digesters and Plants|Types of Biogas Digesters and Plants]]. For questions regarding the article, please contact the authors of this article as well as those of the article, [[Types of Biogas Digesters and Plants|Types of Biogas Digesters and Plants]].
  
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[[Category:Biofuel]]
 
 
[[Category:Biogas]]
 
[[Category:Biogas]]
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[[Category:Bioenergy]]

Latest revision as of 09:07, 7 January 2016

►Back to Biogas Portal

Overview

There are different types of small scale biogas digesters. Some of the biogas digesters are summarized below:


The table below gives a first comparison of the different types.

 Factors

Fixed dome

Floating drum

Tubular design

Plastic containers

Gas storage

Internal Gas storage up to 20 m³ (large)

Internal Gas storage drum size (small)

Internal eventually external plastic bags

Internal Gas storage drum sizes (small)

Gas pressure

Between 60 and 120 mbar

Upto 20 mbar

Low, around 2 mbar

Low around 2mbar

Skills of contractor

High; masonry, plumbing

High; masonry, plumbing, welding

Medium; plumbing

Low; plumbing

Availability of Material

yes

yes

yes

yes

Durability

Very high >20 years

High; drum is weakness

Medium; Depending on chosen liner

Medium

 

Agitation

Self agitated by Biogas pressure

Manual steering

Not possible; plug flow type

Evtl Manual steering

Sizing

6 to 124 m³ digester vol

Up to 20 m³

Combination possible

Up to 6 m³ digester vol

Methane emission

High

Medium

Low

Medium


Fixed-dome Plants

A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on top of the digester. When gas production starts, the slurry is displaced into the compensation tank. Gas pressure increases with the volume of gas stored and the height difference between the slurry level in the digester and the slurry level in the compensation tank. The costs of a fixed-dome biogas plant are relatively low. It is simple as no moving parts exist. There are also no rusting steel parts and hence a long life of the plant (20 years or more) can be expected. The plant is constructed underground, protecting it from physical damage and saving space. While the underground digester is protected from low temperatures at night and during cold seasons, sunshine and warm seasons take longer to heat up the digester. No day/night fluctuations of temperature in the digester positively influence the bacteriological processes. The construction of fixed dome plants is labor-intensive, thus creating local employment. Fixed-dome plants are not easy to build. They should only be built where construction can be supervised by experienced biogas technicians. Otherwise plants may not be gas-tight (porosity and cracks).


The basic elements of a fixed dome plant (here the Nicarao Design) are shown in the figure below.

Fixed dome plant Nicarao design: 1.Mixing tank with inlet pipe and sand trap. 2.Digester. 3.Compensation and removal tank. 4.Gasholder. 5.Gaspipe. 6.Entry hatch, with gastight seal. 7.Accumulation of thick sludge. 8.Outlet pipe. 9.Reference level. 10.Supernatant scum, broken up by varying level.
Basic function of a fixed-dome biogas plant: 1.Mixing pit, 2.Digester, 3.Gasholder, 4.Displacement pit, 5.Gas pipe


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Function

A fixed-dome plant comprises of a closed, dome-shaped digester with an immovable, rigid gas-holder and a displacement pit, also named 'compensation tank'. The gas is stored in the upper part of the digester. When gas production commences, the slurry is displaced into the compensating tank. Gas pressure increases with the volume of gas stored, i.e. with the height difference between the two slurry levels. If there is little gas in the gas-holder, the gas pressure is low.


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Diges​ter

Fixed-dome plant in Tunesia. The final layers of the masonry structure are being fixed.

The digesters of fixed-dome plants are usually masonry structures, structures of cement and ferro-cement exist. Main parameters for the choice of material are:

  • Technical suitability (stability, gas- and liquid tightness);
  • cost-effectiveness;
  • availability in the region and transport costs;
  • availability of local skills for working with the particular building material.

Fixed dome plants produce just as much gas as floating-drum plants, if they are gas-tight. However, utilization of the gas is less effective as the gas pressure fluctuates substantially. Burners and other simple appliances cannot be set in an optimal way. If the gas is required at constant pressure (e.g., for engines), a gas pressure regulator or a floating gas-holder is necessary.


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Gas-Holder

The top part of a fixed-dome plant (the gas space) must be gas-tight. Concrete, masonry and cement rendering are not gas-tight. The gas space must therefore be painted with a gas-tight layer (e.g. 'Water-proofer', Latex or synthetic paints). A possibility to reduce the risk of cracking of the gas-holder consists in the construction of a weak-ring in the masonry of the digester. This "ring" is a flexible joint between the lower (water-proof) and the upper (gas-proof) part of the hemispherical structure. It prevents cracks that develop due to the hydrostatic pressure in the lower parts to move into the upper parts of the gas-holder.

►Read more.....


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Types of Fixed-dome Plants

  • Chinese fixed-dome plant is the archetype of all fixed dome plants. Several million have been constructed in China. The digester consists of a cylinder with round bottom and top.
  • Janata model was the first fixed-dome design in India, as a response to the Chinese fixed dome plant. It is not constructed anymore. The mode of construction lead to cracks in the gasholder - very few of these plant had been gas-tight.
  • Deenbandhu, the successor of the Janata plant in India, with improved design, was more crack-proof and consumed less building material than the Janata plant. with a hemisphere digester
  • CAMARTEC model has a simplified structure of a hemispherical dome shell based on a rigid foundation ring only and a calculated joint of fraction, the so-called weak / strong ring. It was developed in the late 80s in Tanzania.
  • AKUT fixed dome plant is an improvement of the above mentioned Nicaragua design. Digester volumes ranges from 8 to 124 m³ with gas storages from 2 to 19,4 m³; the gas production can reach 60 m³/d. The units from 32 m³ onwards are often used for small scale productive use including electricity generation. It has a cylindrical base with a spheric top. The expansion chamber acts as overpressure outlet.
  • AKUT Maendaleo (kisuaheli "progress") adds a gas storage ballon to collect access gas from the digestion chamber. This can be used for converted Diesel generators.
Camartec Fixed-dome


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Climate and Size

Fixed-dome plants must be covered with earth up to the top of the gas-filled space to counteract the internal pressure (up to 0,15 bar). The earth cover insulation and the option for internal heating makes them suitable for colder climates. Due to economic parameters, the recommended minimum size of a fixed-dome plant is 5 m3. Digester volumes up to 200 m3 are known and possible.


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Summary

Advantages: Low initial costs and long useful life-span; no moving or rusting parts involved; basic design is compact, saves space and is well insulated; construction creates local employment. Advantages are the relatively low construction costs, the absence of moving parts and rusting steel parts. If well constructed, fixed dome plants have a long life span. The underground construction saves space and protects the digester from temperature changes. The construction provides opportunities for skilled local employment.
Disadvantages: Masonry gas-holders require special sealants and high technical skills for gas-tight construction; gas leaks occur quite frequently; fluctuating gas pressure complicates gas utilization; amount of gas produced is not immediately visible, plant operation not readily understandable; fixed dome plants need exact planning of levels; excavation can be difficult and expensive in bedrock. Disadvantages are mainly the frequent problems with the gas-tightness of the brickwork gas holder (a small crack in the upper brickwork can cause heavy losses of biogas). Fixed-dome plants are, therefore, recommended only where construction can be supervised by experienced biogas technicians. The gas pressure fluctuates substantially depending on the volume of the stored gas. Even though the underground construction buffers temperature extremes, digester temperatures are generally low. Fixed dome plants can be recommended only where construction can be supervised by experienced biogas technicians.

A specific environmental disadvantage is methane emission from the expansion chamber.
Variations: Some companies are now looking into small pre-fab fixed dome plants made of fibreglass which appears to be a low cost alternative to construction intensive masoned plants. A custom made plant can be produced in 2 days and -after transport- installed in less than 1 day!

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Criteria for the Selection of a Type and Design

A biogas plant should serve the owner of the plant. There it must be in any case financially viable, the benefits have to exceed the investment. In addition it has to be user friendly. easy to be fed and to be operated.

A sufficient gas storage is mandatory; a capacity of 10 hours is for many cases acceptable. In a domestic set up Biogas will not be used for 10 hours during the night. A small gas storage will lead to methane emissions and to monetary losses.

A gas presure of 10 mbar at least seem to be necessary to operate a burner. On ballon storages this load of 10 kg/m² surface have to applied to get enough pressure.


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Further Information



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References

This article is a continuation of the article, Types of Biogas Digesters and Plants. For questions regarding the article, please contact the authors of this article as well as those of the article, Types of Biogas Digesters and Plants.



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