biogas plant sizes and dimensions

7/23/2019 Biogas Plant Sizes and Dimensions

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Domestic biogas plants

Sizes and dimensions

Felix ter HeegdeJanuary 2010


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Biogas plants and dimensions

1 Plant size range.As each situation differs in terms of e.g. gas requirement or available feeding material, a unique lant si!ecould be calculated for each household. For larger dissemination rogrammes, ho"ever, this "ould beimractical. #herefore, most such rogrammes "or$ "ith a limited number of lant si!es that are exected to

cover the demand of %most& households. #he follo"ing examle sho"s ho" to develo a lants si!e range "iththe lo"est number of si!es covering a certain demand scoe.

1.1 Plant size parameters.Although si!e calculations can become very comlicated, for domestic alication the follo"ing arameterssuffice to arrive at a ractical lant si!e range

Parameter Explanation Values used'ung ( "aterratio

#heoretically, the dung ( "ater ratio deends on the total solids%#)& concentration of the dung, "hereby otimum fermentationresults are claimed at * to + #). #he #) of dung variesconsiderably, for livestoc$ in develoment countries #) values in

the 10 to 1- %cattle& and 1- to 20 %igs& range are reorted.

#he #) values suggest a dung ( "ater ratio of alittle under 1 1 for cattle dung and 1 2 for igdung. For ractical reasons. A 1 1 ratio has theadvantage that households can easily measure

the amount of required rocess "ater.)ecific gasroduction

#he secific gas roduction of dung deends on the tye andquality of dung.

For cattle, tyically 1 $g of dung fed to a digesterroduces about /0 litres of biogas er day.alues for other substrates "ill differ igs,oultry and human excreta tyically have higheryields.

inimum gasroduction

'eending on construction costs and gas demand attern, belo"a certain nominal gas roduction the investment becomesuninteresting for the household.

3ne cubic meter of biogas daily "ill render 2.- to4.- stove hours. #his could, deending on familysi!e, suffice for e.g. brea$fast and lunchrearation, and "ould then rovide ameaningful contribution.

Hydraulic5etention #ime

#he hydraulic retention time %H5#& is the eriod the dung("atermix fed to the installation remains in the lant. As the fermentationrocess "or$s better at higher ambient temeratures, installationsin "armer climates can "or$ "ith a shorter H5# and vice versa.As a longer H5# requires a larger digester volume, lants becomemore exensive to construct.

#yical H5#s for domestic %simle& biogas lantsare /0 to *0 days for "arm climates and -0 to +-days for temerate climates.

6as storagevolume

7iogas is generated more or less continuously, but consumtion inhouseholds tyically ta$es lace during 4 or / eriods during theday. #he generated gas needs to be stored in the installation.

For the gas storage volume, a fixed share of themaximum amount of daily generated gas, *0 ista$en

#he table summari!es the values of the main designarameters. As the aim is to develo a lant si!e range,the hydraulic retention time is alied as a minimum andmaximum value.

8alculation examles hereunder use arameter valuesfor a "arm climate

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1.2 Plant size range calculations

1.2.1 For the smallest plant size (size 1) in the range:

i. #he minimum dail su!strate "eeding%min sub fee1& is equal to the minimum required gasroduction for the smallest installation divided by the secific gas roduction min sub fee19 min gas prod1(spec gas prod.

3r min sub fee19 1.00 ( 0.0/0 9 2- :$g dung day;

ii. A feeding of dung of 2- :$g dung ( day; requires "ith a 11 dung to "ater ratio an equal amount of"ater. #he minimum "eeding%min fee1& to the lant thus arrives at -0 :ltr ( day;.

iii. For the situation in "hich the daily feeding corresonds "ith the minimum feeding amount for "hichthe lant "ill be designed, the hydraulic retention time is maximal %HRT max&. #he required digester #olume

%dig vol1& is equal to the hydraulic retention time multilied by the daily feeding dig vol19 HRTmaxx min fee1.

3r dig vol19 *0 x -0 9 4,000 :ltr;

iv. For the situation in "hich the daily feeding corresonds "ith the maximum feeding amount for "hichthe lant "ill be designed, the hydraulic retention time is minimal %HRTmin&. #he maximum "eeding%maxfee2&, then, equals the digester volume divided by the minimum hydraulic retention time max fee19 dig vol(HRTmin

3r max fee19 4,000 ( /0 9 +- :ltr(day;

v. A maximum feeding of +- :ltr(day;, "ith a dung ( "ater ratio of 11, then requires a maximumsu!strate "eeding%max sub fee1& of 4+.- :$g(day;.

vi #he maximum gas productionof this installation equals the maximum substrate feeding %max subfee1& multilied by the secific gas roduction max gas prod19 max sub fee1x spec gas prod.

3r max gas prod19 4+.- x 0.0/0 9 1.- :m4 biogas(day;

vii #he required gas storage #olumefor this lant then is *0 of the maximum daily gas roduction.

3r gas stor vol19 0.* x 1.- 9 0.< :m

4

;

viii #he resulting main dimensionsof lant si!e 1 then are$igester #olume: %&'' m%

as storage #olume: '.' m%

*otal plant #olume: %.' m%

1.2.2 For the second smallest plant size (size 2) in the range:i. For a range of lant si!es, the minimum daily feeding for the next si!e %min sub fee2& is equal to themaximum feeding for the next smallest installation %max sub fee1&. #he minimum feeding for lant si!e 2 thenequals +- :ltr(day; or, for the maximum substrate feeding %max fee2&, 4+.- :$g dung(day;.

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ii. #he minimum gas roduction for si!e 2, consequently, is equal to the maximum gas roduction of si!e1, and equal to the minimum feeding multilied by the secific gas roduction1 min gas prod29 min sub fee2xspec gas prod.

3r min gas prod29 4+.- x 0.0/0 9 1.- :m4

biogas(day;

ii.)imilar to 1.21. ii, for the situation in "hich the daily feeding corresonds "ith the minimum feeding amountfor "hich the lant "ill be designed, the hydraulic retention time is maximal %HRT max&. #he required digestervolume %dig vol2& is equal to the maximum hydraulic retention time multilied by the daily feeding dig vol29HRTmaxx min fee2.

3r dig vol29 *0 x +- 9 /,-00 :ltr;

iii. For the situation in "hich the daily feeding corresonds "ith the maximum feeding amount for "hichthe lant "ill be designed, the hydraulic retention time is minimal %HRTmin&. #he maximum feeding %max fee2&,

then, equals the digester volume divided by the minimum hydraulic retention time max fee29 dig vol2(HRTmin

3r max fee19 /,-00 ( /0 9 112,- :ltr(day;

#his corresonds "ith a maximum substrate feeding %max sub fee2& for this si!e of -*.2- :$g dung(day;

vi #he maximum gas productionof thisinstallation equals the maximum substrate feeding %maxsub fee2& multilied by the secific gas roduction maxgas prod29 max sub fee2x spec gas prod.

3r max gas prod29 -*.2- x 0.0/0 9 2.2- :m4 biogas(day;

vii #he required gas storage #olumefor this lantthen is *0 of the maximum daily gas roduction.

3r gas stor vol29 0.* x 2.2- 9 1.4- :m4;

viii #he resulting main dimensionsof lant si!e 2then are

$igester #olume: +&,' m

%

as storage #olume: 1.%, m%

*otal plant #olume: ,.-, m%

1.2.% For the next plant sizes:

=t "ill be clear that a reetition of the routine exlained above "ill roduce a range of non>overlaing lantsi!es. For domestic alications, a daily gas roduction significantly over -m 4"ill often not be consumed.Hence, larger installations "ould not be alicable for domestic use. ?sing arameter values from table 2, thetables belo" sho" the results for a "arm and a temerate climate.

2 Plant design

1 This is an approximation; in reality a similar amount of feeding will generate slightly more biogas

in a larger installation as the retention time is longer.

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A simle lant design similar to the designs in ietnam%@#&, 8ambodia %modified 'heenbandu& or #an!ania%modified 8amartec& is used for this examle.

=n this hemi>sherical design, the digester volume is thevolume under the lo"er slurry level %)&, and the gasstorage volume is the volume bet"een the lo"er andhigher slurry level %H)&.

For all lants "ith internal gas storage, the gas storagevolume in the lant is equal to the volume of thecomensation volume

2.1 *otal plant #olumeAs ic 1 sho"s, art of the dome volume, over the

higher slurry level, is not used by a "ell functioninginstallation. #he volume, often referred to as BdeadvolumeC is required, ho"ever, to accommodate thefloating layer on to of the slurry. =n addition, "hen gasroduction is less then nominal %cold seasons& or "hengas is slo"ly lea$ing, the higher slurry level can rise %uto overflo" level&. For that reason, the total lantvolume used for dimensioning should be higher thanthe lant si!e range volume results. For this examle,20 addition is allo"ed for this dead volume. Hence,ta$ing lant si!e 1 for the examle, the total lantvolume %digester D gas storage D dead volume& arrivesat 4.


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and V dome cap19 (*x 0.+ x %4 x 1.**

2 D 0.+2& 9 1.** :m4; .

#he digester volume, then, results in V dig19 Vdome1G Vdome cap1

Vdig19 /.*E G 1.** 9 4,02 :m4

; "hich is close to the design volume of 4.00 :m4

;. #he ) equals Rdome1minus a1 LSL19 1.40 G 0.+0 9 0.*0 :m;

2.% alculating gas storage #olume dimensions#he gas storage volume should be at least 0. of course, the height determines the dimensions of the comensation chamber.

For the ositioning of the overflo", there are t"o schools of thought1 #he overflo" should be ositioned under the bottom of the dome ie. #his "ill avoid slurry

reaching the bottom of the gas dome ie. )lurry can reach the bottom of the dome ie "henlants are lea$ing gas or "hen, for temerature reasons or other, the gas roduction issignificantly lo"er than the gas consumtion over a rolonged eriod of time.

2 #he overflo" should be ositioned higher than the bottom of the dome ie. #his allo"s a highermaximum ressure in the lant and ma$es the comensation chamber dimensions moreeconomic. )lurry entering the dome ie, then, is an indication of a mista$e in the construction of

the oeration of the installation, and should be remedied.=n this examle the overflo" is laced - cm under the to of the dome.

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Hence oh19 1.40 G - 9 1.2- :m;.

2., Pressure height chec 0.*0 9 0.*- :m;.

2.7 ompensation cham!er dimensions#he volume of the comensation chamber %V cc& shall be equal to the lants gas storage volume. =n case ofBsi!e 1C, then, V ccshall be 0.< :m4;. Follo"ing the earlier osition that the overflo" level should be lo"er thatthe to of the dome, the comensation chamber height%cch& is the difference bet"een the overflo" height %oh&and the higher slurry level %HSL& %9 comensationchamber floor level&.

For the examle si!e 1, the comensation chamberheight then is 1.2- G 0.E- 9 0./0 :m;.

Assuming a cylindrical comensation chamber, theradius of the comensation chamber %R cc& follo"sfromR cc9 %V cc( % x cch&&1(2.

3r R cc19 %0.

biogas plant sizes and dimensions

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