Gas production

Loading rates in digesters ,in terms of volatile solids ( VS ) added ,can vary from 0.7 to 5 Kg VS per cubic meter per day (1) .Pig slurry is reported to contain dry matter contents in the range of 3 to 12% ( 2 ) Higher loadings may be possible ,but this will depend on whether toxic build - up of volatile fatty acids occurs and on the water content of manure.
Commonly ,the biodegradable organic matter content ranges from 70% to > 95 % of the dry matter content . Substrates with dry matter organic contents less than 60% are rarely considered as valuable substrates worthwhile for anaerobic digestion (2).
The water content of slurries can change seasonally and may be influenced by different operational conditions ( dilution ,etc ).High water containing substrates not only unnecessarily increase the digester volume ,but also raise the heat input per m3 waste required ,resulting in unfavourable process economics. On the other hand ,high TS contents dramatically changes the fluid dynamics of substrates ,often causing process failure due to bad mixing behaviour ,solids sedimentation , clogging and scum layer formation .
Since animal wastes vary in terms of composition as well as solid content ,and since the collection system affects the nature of the waste ,each system needs to be evaluated separately before deciding on the type of digester operation ( 1 ).
The average volume of faeces and urine largely differ from one type of animal to another and mainly depend on their age and life weight .For comparison ,the general ' livestock unit '( LU ) is accepted widely .One LU represents a live weight of 500 Kg and equals to 1 cow , 6 fattening pigs or 250 laying hens.
According to the biogas yields ,one LU of pig produce in average 0.60 m3 biogas per LU . ( 2 ). Under good conditions ,the production of methane gas that can be obtained is about 15 m3 / year 100 kg of pig life weight ( about 25 m3 / year of biogas ) (5).If the digester is heated to mesophilic temperature the biogas yield can reach 350 m3 / ton living weight per year.

Main characteristics of the various feed stocks and their impact on the anaerobic digestion process.

The feedstocks for anaerobic digestion vary considerably in composition homogeneity, fluid dynamics and biodegradability.
We will base ourselves on standard values well accepted in the scientific community. This way the problem of the particular behaviour of individual waste types, that have to be analysed in laboratory, will be over passed.

Digester system

To produce biogas to feed our fuel cell an anaerobic digester system is needed .This one represents a major investment and designing effort.
To increase the biogas yields the digester needs to be heated up until mesophilic temperature ( 35 C ) is reached (1).
The energy required to increase the digester temperature can increase dramatically the operational costs of the anaerobic digestion ( a.d.) unless a cheap source of heat is available.
The use of waste heat from some other process may represent a solution for the problem .CHP schemes can provide the required source of energy while producing a high quality type of energy : Electricity.
This electricity can be used in the process ,in the farm works or even exported .If it is exported constitutes an income and aids to sink the investment .If the Fuel Cell system operates at full load continuously most probably the electricity will be exported during the night period.

Fuel cells produce waste heat that represents up to 40% of the low heating value of CH4

Based on these figures and on the average biogas yields the required power output from the fuel cell can be calculated.

For more accurate calculations on site experimental data and parameters are required :

- Volatile solids content of the pig slurry. - Effluent temperature and flow rate.
- Operating temperature of the digester.
- Seasonal ambient air temperature profile.
- Retention days .
- Insulation thickness.
- Thermal conductivity of insulation.
- The efficiency of heat transfer.
- The electricity demand profile of the a.d . system ( for mixing ,pumping and other auxiliary services )
- other

Hydraulic retention time (3)

The hydraulic retention time (HRT) describes the average time the substrate remains in a digester. It is defined by

HRT = liquid volume [m 3 *d] ./ daily flow [m 3] [ in days ].

Each case is a particular case so this kind of technical information is only available for specific sites .

With the aid of ' Chenmod 'software ( a set of spreadsheets kindly provided by Dr. Paul Harris ,lecturer in the Dept. of Agronomy and Farming systems ,University of Adelaide,Australia - http://www.roseworthy.adelaide.edu.au/~pharris/ ) a digester system can be simulated .

Initial assumptions

Due to the lack of technical and commercial information on fuel cells technology, we have to base our study on the scarce existing commercial information .Rather than having a real site where to proceed with our project we decided to simulate a case study using the available information .This way we could give a more accurate idea of the dimensions and evolution of costs involved on designing an integrated fuel cell system using biogas.
The phosphoric acid fuel cell ( PAFC ) was chosen for two main reasons : ( 1) it is commercially available with documented technical performance ,and (2) it rejects enough heat to warm up the bio digester to a temperature sufficiently high to maintain mesophilic bacterial life . The performance of the PAFC in this study is based on the ONSI PC25C fuel cell system ,which generates 200 KW of electric power.
In the ONSI system heat exchangers for heat recovery and delivery of the heated fluids are included in the overall fuel cell system and they deliver hot water streams at two different temperatures.
The chosen fuel cell has the following performance ( ONSI data )

Phosphoric Acid Fuel Cell

Electrical Efficiency : 40 % of the Low Heating Value (LHV)

Rejected Heat

121 C Hot water outlet : 1.1 Kwh per Kwe produced over 50% PAFC Rated capacity ( Given hot water inlet < 110 C )
60 C Hot water outlet : 0.684 Kwh per Kwe produced over 25% PAFC Rated capacity ( Given hot water inlet < 27 C )

The LHV of methane is equal to 35.7 Mj / Nm3 ( 6).

As only 40% is used by the FC to produce electricity we have to provide :

0.2 / 0.4 = 0.5 Mj/s

To generate 200 Kwe there is needed a CH4 flow rate of about :

0.5 Mj/s : 35.7 Mj/Nm3 = 1.4 E-2 Nm3 / s

Considering that pig slurry biogas has a methane content of about 70% to 80% of its total volume (7) ,the bio digester yield must range :

1.4 E-2 Nm3 / s : 0.7 = 0.02 Nm3 / s
1.4 E-2 Nm3 / s : 0.8 = 0.0175 Nm3 / s

Assuming that the Fuel Cell will be operated continuously at full power with an annual availability of 96 % ,the biogas consumption is of:

0.02 * 3600 * 24 * 365 * 0.96 = 605,491 Nm3 / year
0.0175 * 3600 * 24 * 365 * 0.96 = 529,805 Nm3 / year

As for a heated bio digester we can attain for each ton of pig living weight 350 m3 of biogas ,the pig farm has to have at least :

605,491 Nm3 / 350 Nm3 = 1,730 Ton .Pig life weight
529,805 Nm3 / 350 Nm3 = 1,514 Ton .Pig life weight

For a non heated reactor we would have :

605,491 Nm3 / 250 Nm3 = 2,422 Ton .Pig life weight
529,805 Nm3 / 250 Nm3 = 2,120 Ton .Pig life weight

Assuming that a fattening pig has a LU of 83 Kg ( 500 Kg / 6 ),as a rough indicator we can say that the above figures represent from 18,241 to 20,843 pigs for a heated reactor system and from 25,542 to 29,180 pigs for a non heated reactor system.

This may be considered a large scale pig farm.

More accurate values for gas yields using known data can be calculated through CHENMOD spreadsheets

Some conclusions about biodigester costs

The unitary cost per m3 of digester is constant.

Higher retention times mean larger digesters and also increased gas yields.

For a constant flow rate of pig slurry this means a proportional increase of the capital costs.

For an increased capital cost of the digester (larger digester ) there is also an increased gas yield (due to increased number of retention days ).

But the unitary cost of the gas produced increases with the digester increasing volume.However higher gas yields for the same digester decrease the unitary gas cost.

A compromise must exist between the digester volume and the gas yield so that the minimum cost can be found.

Accurate values for biogas costs can be calculated through CHENMOD spreadsheets for particular cases.

References

1 - ' Methane production from waste organic matter '- David A.Stafford,Dennis L. Hawkes,Rex Horton
2 - 'Feedstocks for anaerobic digestion'- Technical Paper - Steffen R.,O.and Braun,R - Institute of Agricultural Sciences Vienna.
3 - 'Process design of Agricultural Digesters ' - Technical Paper - Arthur Welliger - AD - Nett
4 - ' Environmental Aspects of Biogas Technology '- Technical Paper - Barbara Klinger , German Biogas Association
5 - ' Integrated Bio - Systems for Biogas recovery from pig slurry : Two examples of simplified plants in Italy ' - Technical Paper - Sergio Piccini,C. Fabbri,F.Verzelli - Centro de Ricerche Produzioni Animalli - CRPA
6 - ' Manual do Gestor de Energia ' - Portuguese Ministry of Energy and Industry Publication
7 - ' Anaerobic Digestion of Agro - Industrial Wastes : Information Networks - Technical Summary on Gas Treatment ' - Technical Paper - AD - Nett - Project FAIR - CT96-2083 ( DG12-SSMI)