Life cycle assessment of electricity from agricultural biogas

Die Seiten
von

arbi2.gif (2349 Byte)

Buchbestellungen

Life Cycle Assessment of the electricity production with agricultural biogas

W.Edelmann, U.Baier, H.Engeli, K.Schleiss

Additional data on downloads

Abstract

The aim the study "Life cycle assessment (LCA) of electricity produced by agricultural biogas" is to determine, whether electricity from farm biogas may be ecologically safe enough to fulfil the standards of the top label "naturemade star" for eco-electricity. The LCA was done with "Ecoindicator'99". Results of "Ecoindicator'95" and "UBP" are presented for comparisons. Details on the methodology of the new EI'99 are given.

In the study, different combinations of parameters were calculated: Two methods of construction were compared (i.e. digester construction material: cement or steel). Different substrates were used (pig or dairy manure, respectively a mixture of both, eventually combined with the addition of co-substrates). The amounts of the emissions were varied by different technical assumptions (reduction of ammonia emissions by improved manure handling; use of the biogas generated in the slurry tank by covering it with a plastic membrane in order to reduce CH₄ and N₂O-emissions; assumed higher values for the emission of methane from the storage of undigested manure (reference); use of different motor technologies for electricity generation). The calculation of sensitivities allowed an accurate discussion of the parameters important in the context of this LCA.

It can be shown that the production of electricity generated by agricultural biogas (even without any technical improvement) is environmentally safer than the electricity generated by fossil sources or a mixture of electricity such as generated conventionally in Europe. The label "naturemade star" allows actually at the most 3'875 EI'99-points as weighted by the so-called "Hierarchist". Here, technical solutions are needed to fulfil the conditions imposed: At the least, the manure storage tank must be covered by a membrane to take profit of the escaping methane and to reduce N₂O emissions, which could escape from an aerobic scum layer.

Biogas production was compared to ordinary storing of undigested manure, i.e. only surplus emissions caused by biogas production were taken into account. The amounts of CH₄, N₂O and NH₃-emissions may vary considerably. At the same time, these gaseous emissions show to have a very large influence on the result of the LCA (ammonia surplus emissions of the slurry while being stored and brought out to the fields count for over 50% of the environmental impact). The amounts of these emissions depend on factors such as: feeding diet of the animals, water use on the farm, kind of storage tank (covered/uncovered), type of soil, manure management etc.. Simultaneously, the bacteria producing the gases depend on a variety of abiotic factors such as (soil) climate, availability of oxygen and water, availability of organic matter etc.. Despite of these uncertainties, the following statement seem to be accurate:

	Modern agriculture, especially the management of animal manure, is very polluting and responsible for very large parts of the Swiss CH₄, N₂O and NH₃ emissions. Because having no contact to the soil while being stored, mineral nitrogen cannot be taken up and fixed by plants. It evaporates in the form of NH₃ and N₂O during storing already and especially while being brought out and immediately thereafter.
	While producing biogas, especially the emissions of ammonia are increased: anaerobic digestion increases the degree of degradation of the organic matter and hence the mineralisation of nitrogen. More ammonium-ions at simultaneously higher pH-levels cause higher concentrations of free ammonia and thus higher emissions - what shows to have a heavy impact on the result of the LCA. Without additional technical improvements, electricity form agricultural biogas is clearly not fit for the label "naturemade star".
	CH₄ and N₂O-emissions are reduced considerably, if covering the storing tank by a plastic foil in order to take profit of the biogas generated in the storage tank. This improvement (in comparison to the reference, i.e. ordinary storing of undigested manure) compensates for a large part of the surplus emissions of biogas production.
	Ammonia emissions may be reduced considerably by applying manure with accurate methods at good times (cool and humid weather, no wind, optimal growth stage of the plants, etc.). If a farmer changes its manure management, the savings may be up to 2-3 times larger than the total impact caused by biogas production. The ammonia emissions (dangerous to the environment) may be reduced specially, if the manure is not sprinkled (with a huge amount of tiny droplets and thus a huge surface for evaporation), but if applied directly to the soil by a trailhose or by a similar device.

A benefit was given for that part of the generator waste heat, which is used on the farm for room heating and warm water production: The environmental impact was calculated for producing the same amount of heat by a LowNO_x-burner (condensation) powered by natural gas. This impact was deduced from the impacts of biogas production. Table A shows the performance of some sensitivities including the benefits for the heat used on site.

Reference: Prod. of 1 TJ _el	Dim.	Eco-limit	MM	MM CH_4,N₂O opt.	MM opt. tot.	MM+Cos	MM+Cos CH₄, N₂O opt.	MM+Cos opt. tot.
EI'99, Hierarchist	Points	3'875	4'890	3'600	820	4'240	3'080	1'270

Tab. A: Surplus emissions caused by the production of 1 TJ electricity with agricultural biogas using the Standard co-generator (60 kW_el) in a 300m³ cement plant including the benefit for the heat used on site (extract of table 17)
MM: mixture of pig and dairy manure (50/50); Cos: co-substrate; CH₄ + N₂O opt.: using the biogas generated in the storage tank by installing a covering; Opt. tot.: Additional: improved manure management while bringing the manure out to the fields.

There are different reasons - which could not be quantified in this study - which suggest that agricultural biogas production performs even better than presented in the tables and figures (e.g. improvement of the manure quality, which is a third product besides of electricity and heat). Taking account of the arguments presented in the chapter "discussions", the performance of the biogas-electricity could be improved by several hundred EI'99-Pts. This would allow to reach the limit imposed by "naturemade star" also while using co-generators driven with 7-10% of fossil oil for ignition (these show higher environmental impacts than the other co-generators compared in this study).

It is recommended to award the label "naturemade star" for electricity from agricultural biogas if the following conditions are fulfilled:

	The storage tank is covered in order to reduce CH₄ and N₂O-emissions,
	The farmer brings out his manure as carefully as possible,
	The waste heat of co-generation is used for heating purposes as much as possible and
	The data of running the plant are recorded regularly for controlling purposes.

These data combined with the plant construction data will allow to make an estimation on the performance of a plant to be certified. The covering of the storage tank may not be necessary, if the farmer changes to a manure management, which is less polluting than sprinkling, simultaneously with the biogas plant construction. The savings of improved manure management will be larger than the additional impacts caused by biogas production.

back to the top

The full report is printed in German (93 pages). It may be ordered at Buchbestellungen.

Zugriffszähler

Life Cycle Assessment of the electricity production with agricultural biogas

Senden Sie E-Mail mit Fragen oder Kommentaren zu dieser Website an: info@arbi.ch Stand: 04. Juni 2004

Senden Sie E-Mail mit Fragen oder Kommentaren zu dieser Website an: info@arbi.ch
Stand: 04. Juni 2004