Study on the Temperature Variation inside the Biodigester of Modified 2 m3

GGC 2047 Biogas Plant

Shankar Singh Dhami*, Sarbottam Pant**, Bhaskar Raja Maharjan** and Susan Shrestha**

*Author: Student of MSREE 066 at IOE, Pulchowk Campus, Nepal

Phone No: +977 9841614483, E-mail: dhamishankar37@gmail.com

** Co authors: Former IOE students

Abstract

As the gas yield from a biogas plant is proportional to the temperature of slurry inside biodigester, a constant higher temperature needs to be maintained inside the biodigester. This study was carried out to find the temperature distribution inside the biodigester of a kitchen waste and toilet attached GGC 2047 modified biogas plant. The research was carried out in a 2 m3 biogas plant at Banepa, Nepal during a period of two months (November-December, 2009). From the study, no significant variation of temperature inside the biodigester was observed. The heat generation due to the anaerobic digestion inside the biogas plant is negligible and the biodigester was always in thermal equilibrium with the surrounding earth. Thus, insulation of biodigester wall is worthless, unless any external source of heat is coupled with it.

Keywords: Kenneth Labs Formula, steady state, thermal mass, GGC 2047 Model

Nomenclature:

T m = Mean ground temperature or average solar temperature t the soil surface, oC A s = Annual temperature at soil surface (maximum air temperature - minimum air temperature), oC;T = time period (day or year or months); t = time of the year in days (current time in day); secX = Depth below the surface; mt 0 = phase constant, day of the year of the minimum surface temperature;α= thermal diffusivity of soil, in m2/day

Introduction

Biodigester is the most essential part of the biogas plant where anaerobic digestion of biodegradable materials takes place and biogas is produced. Fluctuations of temperature inside biodigester retard microbial activities and the biogas production decreases. Thus, a constant temperature needs to be maintained inside the biodigester for efficient biogas production.

Variation of temperature inside the biodigester takes place due to conduction of heat to and from the surrounding earth. Similarly heat is generated due to anaerobic digestion inside the biodigester. This, heat flow creates a combined effect of temperature fluctuation inside the biodigester.

Periodic variation of surrounding earth temperature with depth

Ground temperature is governed by a number of factors. The most important of which are: (1) Geometric, including latitude (L), altitude (β) and prevailing weather conditions; (2) Site characteristics, including surface conditions and surface temperature, landscaping, microclimate, and water table; and (3) thermal physical properties of the soil. The diurnal fluctuations appear largely at the soil surface and rapidly fade out with depth. Below 15 – 51 cm depending on soil type, thermal diffusivity (α), and moisture content - the soil temperature does not reflect daily changes at the surface. (M.Sc. Thesis, Pawan Basnyat).

Temperature changes with depth are determined by the amount of radiant energy that reaches the soil surface and by the thermal properties of the soil. The energy absorbed by the soil surface is disposed of in one or more of the following ways: (a) radiation to the atmosphere, (b) heating of the air above the soil by convection, (c) increasing the temperature of the surface soil, or (d) conduction to the deeper soil layers.

Kenneth Labs formula

The temperature of the ground is a function of the time of year and the depth below the surface. By Kenneth Labs formula, the temperature variation with the depth is given as

θ ( x ,t )=T m−A s e−X (π /αT)1 /2

cos [2 π /T {t−t 0−X /2(T / πα)1/2}]……. (8)

(Moustafa et al, 1980).

The “steady state” ground temperature may be assumed to occur below 1.8 – 3 m beneath ground surface. The ground water temperature at a depth of 9.1 – 18.3 m has generally been accepted as equivalent to stable “steady state” ground temperature which has in turn demonstrated to be roughly equivalent to annual average air temperature. (M.Sc. Thesis, Pawan Basnyat)

If we consider a time period of 365 days, the above formula would appear as below:

θ ( x ,t )=T m−A s e−X (π /α365)1/2

cos [2 π /365 {t−t 0−X /2(365/ πα)1/2}]……… …… (9)

(Moustafa et al, 1980)

Materials and Methods

Ten different thermometers (nine digital and one mercury) were used for measuring the temperature at different positions inside the biogas plant as shown in figure 1 below. The thermometers were calibrated for precision of ±1 0C. Position of different thermometers with reference to O in A-A plane is shown in the table below.

Figure1: Positions of thermometers

Among the ten thermometers, three thermometers were installed on the dome surface and next three thermometers were placed radially at a distance of 25 cm from the first three thermometers. Two thermometers coupled with metallic pole inserted inside slurry, are used for measuring the temperature of slurry. Another thermometer was used to measure the temperature of biogas produced inside the dome surface. The last thermometer (mercury) was used for measuring temperature of outlet slurry.

Table1: Position of thermometers relative to Point O.

Point Horizontal from top(cm)

Vertical distance (cm)

A 85 88B 61 68C 33 55A’ 102 70B’ 74 47C’ 41 31Tg 0 90T1 16 195T2 97 144

Then, Kenneth Labs Formula was used to calculate the temperature at different position of the dome surface. The measured temperatures were then compared with the calculated temperature for reliability of study. The various parameters used for Kenneth Labs Formula are:

T m =20.53 oC, A s =5.68 oC; T =365 day;

t =338 – 343 day;

t 0 =1;

α=0.0397 m2/day or 1.193 m2/month or 0.001656 m2/hour.

The annual temperature variation of the ground surface at different height is shown below in the graph.

0 50 100 150 200 250 300 350 40010

121416

182022

242628

Tc' (0.0354m)

Ta(0.5803m)

T(1m)

T(1.5 m)

T(2 m)

T (2.5 m)

days

Tem

pera

tue

(0C)

Figure2: Annual temperature profile at various depths with time period of one year.

Results and Discussion

The graph 3 and 4 shows the temperature readings of ten thermometers installed at different positions of biogas plant taken at different time of day.

0 4 8 12 16 20 248

101214161820

TgasT1T2Tatm

Hours of day

Tem

pera

ture

(oC)

Figure3: Temperature variations on 8/21

0 4 8 12 16 20 24101214161820

TcTc'TbHours of dayTe

mpe

ratu

re(o

C)

Figure4: Temperature variations on 8/21

The average maximum atmospheric temperature was 18 oC while the average minimum atmospheric temperature was 9.28 oC. However, the temperature shown by other thermometers were more or less constant and were within the temperature range of 14 oC to 16.9 oC except for TOS in which some variation is seen. This is due to the fact that, outlet hole of the digester is open to the atmosphere and the temperature variation in atmosphere causes the temperature of slurry to vary along with it. This fact showed that the temperature inside the digester remains in thermal equilibrium with its surrounding earth.

Similarly, same result was observed from figure 5 and 6 showing the average temperature readings of different thermometers taken for 13 days at different time periods.

0 4 8 12 16 20 248

10

12

14

16

18

20T1T2Tatm

Hours of day

Tem

pera

ture

(oC)

Figure5: Temperature variations for 13 days

0 4 8 12 16 20 248

10

12

14

16

18

20Average Temperature Variation for 13

Days TcTc'TbTb'Hours of day

Tem

pera

ture

(oC)

Figure6: Temperature variations for 13 days

00.20.40.60.8

11.21.41.61.8

2

Max

. te

mp

dif-

fere

nce

in

a da

y

Tgas T1Tb Tb'

Tc

Tc' Ta'Ta T2

19/8/2066

20/8/2066

21/8/2066

24/8/2066

Figure7: Max temperature difference in a day for different position of thermometers

Similarly, graph 7 depicts the maximum daily variation 1.9 oC for Tc

' i.e. at upper part of dome. As this part is near to the surface of earth temperature variation of atmosphere has direct effect on it. The temperature variation of other thermometers are within 0.5 0C. Considering the fact that the tolerance of the thermometer used was ±1 oC, it can be stated that the variation of temperature inside the biodigester is negligible.

338 339 340 341 34212

13

14

15

16

17Tc (0.278m)Tc' (0.0354m)Tb (0.3962m)Tb' (0.1777m)Ta(0.5803m)Ta'(0.38m)

Days

Tem

per

atu

e (o

C)

Figure8: Variation of temperature with depth (measured data)

338 339 340 341 34214

15

16

17

18Tc (0.278m)

Tc' (0.0354m)

Tb (0.3962m)

Tb' (0.1777m)

Ta(0.5803m)

Ta'(0.38m)

days

tem

pera

ture

(oC)

Figure9: Variation of temperature with depth (Kenneth Labs formula)

From the graph in figure 8 and 9 same pattern of temperature variation at the dome surface was observed in different height of digester for different days (day 338 to 343 i.e. 4th Dec. to 9th

Dec.) of the year. The value of temperature obtained from Kenneth Labs formula are somehow higher than the value actually measured. This, may be due to concrete dome surface and wet conditions of surrounding land.

From the graph 2 as the temperature of the surrounding earth changes throughout the year in a cyclic manner the temperature inside the biodigester also changes with it.

Thus, the temperature variation inside the biodigester is negligible and the biodigester always remains in thermal equilibrium with the surrounding earth. And hence, insulating outer walls of biodigester is worthless unless any external source of heat is coupled with the system.

Acknowledgement

The authors express their sincere gratitude to Dr. Tri Ratna Bajracharya, Director, and Center for Energy Studies and Mr. Mahesh Chandra Luitel, Assistant Campus Chief, Pulchowk Campus, for supervising their research work. The Authors are grateful to BSP-Nepal for providing financial support to the research work.

References

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