Life-cycle-assessment of industrial scale biogas plants

Posted by Intissar LAZHAR
Intissar LAZHAR
Full LCA available on the web
Publication year: 
Food, Plants
Biogas plants
Quality and sources
Is the study a: 
Detailed LCA
Was a critical review performed?: 
Is the study compliant with ISO 14044?: 
Sponsor name(s): 
Prof. Dr. Wolfgang Lücke
Sponsor name(s): 
Prof. Dr. Michael Nelle
Sponsor type: 
Joachim Kilian Hartmann
Practitioner(s) type: 
Institut/Technical research center
Functional unit: 
The functional unit is one terajoule electricity fed into the public electricity networ
Goal and scope of the summary: 
The aim of this study is to acquire an answer to the question regarding the ecological effects of electricity generation from biogas generated by industrial scale biogas plants. The object under investigation will be a hypothetical biogas plant with an installed electric power of 1.0 MW, fed by biomass from energy crops and manure in accordance with the rules of the [EEG2004]. Mass and energy balances resulting from a life-cycle-assessment will determine the ecological effects. Data for the mass and energy balances will be taken from measured data from existing biogas plants, calculation from similar objects and estimations where no available data exists. The object under investigation is the biogas plant itself and up- and downstream processes related to the power plant. The scope of the data collection will be determined and adjusted within the LCA. Furthermore, all single unit processes will be defined in the life-cycle-assessment. The upstream process of the biogas plant and the production of renewable energy crops will both be analysed in detail by considering all mass and energy flows going into this process, emissions from the area under cultivation, and the effects on biodiversity from land use. The ecological effects produced by the use of specially produced energy crops have yet to be considered as an objective of scientific studies. Moreover, the nutrient recycling process of the biogas slurry as an organic fertiliser as defined by [§1 (2) DÜNGMG] and the effects on the ecological burden of the crop production have not been previously explored. A closed nutrient loop is only possible, if no nutrients are lost via evaporation or leaching. In this way crop production can be accomplished without the need of additional mineral fertilisers. The amount of nutrients put back into the fields could have a tremendous influence on the LCA results. Part of this study will examine to what extent this method of crop production can be realized. The ecological effects produced by the utilisation of organic waste as input to the biogas process will also be assessed in this study. Even if the amount of useable organic waste is comparatively small in regards to the possible amount of energy crops, additional waste matter can be acquired regionally. The calculation of the organic waste does not take into account its ecological effects as they are solely related to the main product, and therefore the waste is calculated without any ecological burden. The processes needed for the activation of this waste e.g. transport and treatment will only be taken into account. Assessing this information permits an ecological consideration of different inputs of the biogas process. The biogas plant is the procedural core of electricity generation in the biogas process. From related studies, it can be assumed that only small parts of the overall ecological effects are related to the plant [EDELMANN ET AL.2001]. Results from this existing study will be assessed in this investigation and adopted to industrial scale biogas plants. Process engineering, inputs, goal definition and transport will be researched in detail regarding industrial scale biogas plants. The transport efforts of in- and outputs of the biogas plant will be analysed, with special focus on their involved role in relation to the overall ecological effect. Additionally, the effects of some biogas slurry treatments will be investigated. From these results, extrapolations on the influence of the size of the biogas plant and the inputs and outputs of the transport efforts will be derived. An analysis will be carried out in order to monitor the acidification and eutrophication from biogas slurry after its distribution in fields. In order to do this, data from literature and field trials will be taken as a base for estimations. Approximations can be given for some possible scenarios due to the large variety of existing influences on emissions from biogas slurry (e.g. inputs, speed of wind, soil type). The possibility of biogas manure treatment and its influence on the emission scenario will also be analysed. When assigning the results of this study to further or more detailed studies, the scientific background of these estimations will be shown explicitly. Given that the biogas process generates electric and thermal energy, it is important to primarily understand the relation between energy used in the power plant itself and the production of the inputs and secondly the energy fed into the grid. This cumulative energy demand will be calculated for the system under investigation with the inclusion of all stages of the up- and downstream unit process and will then be related to the energy produced. This will provide clear information concerning the energetic reasonability of the electricity generation from biogas. The aim of this study is to obtain an overview of the ecological effects of electricity generation from biogas via industrial scale biogas plants. Suggestions for ecological improvements of the industrial scale biogas process will be made in light of the results gathered
In the standard scenario 87.6% of all ecological effects are caused by the agricultural energy crop production. In the scenario considering no direct use of land 67.7% of all ecological effects are caused by the energy crop mixture used as input to the biogas plant. From this the importance of this module on the overall effects can be observed. The normalised and the weighted results in the comparison of energy crops show that most ecological effects per functional unit are caused by the production of grass and rye silage. Fewer ecological effects are caused by the production of silage from forage beets, maize silage and the assumed production of maize silage with a yield of 30.0 Mg dry matter per ha. 
A conclusion can be drawn in which production systems with a higher yield per area level cause less ecological effects compared to energy crops yielding lower dry matter contents per unit area. The smallest number of weighted ecological effects per functional unit were  caused by forage beets (3,120 EI '99H points), most were caused by rye silage  (27,500 EI '99H).  
Taken from the weighted results, it is evident that most effects per energy crop are caused  by the use of land, which is in this case land occupation for the production of crops. The influence of the impact category land use is discussed above in detail. The weighted results of  this impact category have a strong influence on the overall results. That is why crop production systems with high yield per area levels cause less ecological effects per functional unit  than low yield production systems produce. This same reason is the cause for the higher  consumption per unit area of cultivated soil is set at the same scale for all production  systems. Therefore, the yield per unit area creates the percentage of fossil resources consumed for the production of one functional unit. Comparable results, stressing the ecological  advantages of maize compared to other energy crops, support the findings of this assessment [TENTSCHER2004]. The production of grass silage produces the majority of effects in the impact categories respiratory inorganics and acidification/eutrophication. This is caused by higher emission of nitrogen (NH3, N2O, NOx) to the air compared to the other systems. These higher emissions are  based on the fact that fertilisers, which are applied to grassland, cannot be incorporated to  the soil; so mainly NH3 emissions are produced. 
It should be highlighted that these results are based on a functional unit; in this case 1.0 TJ  electricity produced from these energy crops. If energy crops are used as input to the biogas  process very few ecological effects are created, compared to a certain amount of produced. biogas or electricity. In this instance, the output of the biogas process is constant. If the area  under cultivation is stable the results can change. If the production of energy crops in an invariable number of unit areas is used as input to the biogas process, fewer ecological effects  are caused by extensive production systems [GAILLARD&NEMECEK2002, HAAS ET AL.2000]. 
The goal of the production system must be discussed in the assessment of ecological effects  from the production. Very few overall ecological effects are produced by low input systems.  The smallest amount of ecological effects per unit of energy output are caused by production systems with a maximum number of dry matter yields per unit area. Due to the complex assessment and debate concerning the land use impact category and its  weighting, it was decided to carry out a sensitivity analysis on this impact category. It takes  into account the standard scenario based on the Eco indicator '99H method; a modification of this method using an extensive production system as a reference system, as a substitute for  the potential natural vegetation originally used, in addition to a system that does not calculate  effects from the occupation of the land but from indirect effects.  The standard scenario of maize silage production causes 6,080 EI '99H points, the extensive  reference system scenario 2,180 EI '99H points, and the scenario without occupation of land 
1,400 EI '99H points. Thus, the consideration of effects from direct land use in agricultural production systems causes 5,630 EI '99H points for the production of maize silage. These are 77.0% of the total effects of the standard scenario. Regarding the overall ecological effects from the standard scenario of biogas production, 57.1% of all weighted ecological effects are associated with the direct effects of land use. In the scenario, which does not take into account the effects from direct use of land, only 24.4% of all ecological effects are related to this impact category. From a methodological point of view, none of the three alternatives can be considered as right or wrong. It alwways depends on the reader’s opinion on the results. If nature conservation is an important safeguard subject for the reader, he will tend to take into account the ecological effects from the direct use of land. From a pragmatic point of view, no ecological effects from the occupation of fertile land should be taken into consideration. This pragmatic point of view can be justified by the fact that the energy crop producing areas under cultivation, considered in this study, would be used to produce crops, even if no energy crops would be produced. The overall effects on the environment would not change, if no energy crops 
were produced. The ecological effects would only be related to another agricultural production system. If there were a contention between the production of energy crops and another arable production system, it would be compulsory to consider the ecological effects from the direct use of land. The transport of energy crops and biogas slurry causes weighted effects of 242 EI '99H points and 222 EI '99H points respectively. Compared to the overall effects of the system under analysis this module causes a share of 4.7% (8.3% in the system without effects from the direct use of land). Most effects in this module are caused by the consumption of fossil fuels (34%) and the emission of respiratory inorganics (25.9%). This result reflects the data 
results found in literature and are therefore regarded as valid [BORKEN ET AL.1999]. The waste input scenario causes slightly more ecological effects compared to the standard scenario; the 2.0 MW biogas plant causes further ecological effects. Only the biogas slurry treatment scenario leads to noticeable transport effort savings on the output side of the biogas plant. In this assessment, the assumption is made that all transports are done with a dead head. In existing biogas production systems, it can be assumed that some of these transports will be done carry cargo to and from the plant. This leads to ecological savings within this module. In conclusion, it can be said that the transport module has a minor input on the overall ecological effects of the system under analysis. 
The results from the module biogas plant are divided into three sub modules: installations, 
electricity consumption, and CHP emissions. The installations cause 138 EI '99H points per 
functional unit; this is a 1.0% share of the overall effects. The influence of this sub module is 
therefore seen as negligible compared to the overall results. The electricity consumption of the biogas plant causes a share of 4.4% (436 EI '99H points) in the standard system under analysis. In the system, which considers no direct land use impacts, the share of this module is 7.8% of the overall ecological effects. This is a comparable amount to the effects from the transport system. The effects from this category depend 
on the used data set for the assessment of consumed electricity. In this study, medium voltage electricity data from the grid in Germany were taken into account. If the same amount of electricity with the same voltage produced in France were to be taken into account then only 50%  of these overall ecological effects would be caused. The electricity from the European mix would cause ecological effects of the same magnitude as the German electricity mix, but it would be made up from a higher consumption of fossil fuels, but would have less radiation and land use [ECOINVENT2004]. Therefore, in further assessments, this influence should be considered when selecting the goal and scope of the object under analysis. 
The CHP plant emissions are the third sub module within the biogas plant module. In this sub module, two different technologies are calculated. The standard technology is classed as a gas engine, which causes emissions of 1,090 EI '99H points. These are 11.0% of the overall effects of the biogas standard scenario and 19.4% of the scenario, which does not calculate direct land use effects. Thus, this module has a noticeable effect on the overall results of the biogas production. The used data are taken from measurements from three CHP plants, which had been operating for two years at the time of investigation. In the case of further 
assessments, data from CHP plants within the scope definition of the object under analysis should be used, as the emissions vary depending on the kind and size of the biogas plant.  It should also be taken into account that catalytic converters can help to save emissions from biogas fed CHP plants. These converters are sensitive to sulphur contents from the exhaust and could be destroyed by them [SKLORZ ET AL.2003]. The assumption is made that most converters at CHP plants are inoperable due to this influence. A better gas purification upstream in the CHP plant can reduce the deterioration of the converter and therefore reduce 
the emissions from this module. 
The ecological effects from a fuel cell are checked within the sub module CHP emissions. The considered MCFC would cause ecological effects of around 204 EI '99 points. These are 80% less ecological effects as caused in the standard scenario. The data used are taken from literature. The first MCFC used for the conversion of biogas will be put into operation in summer 2006 in Böblingen, Germany. Data from this plant could help to validate data in literature. If these were to be validated, remarkable amounts of ecological conservation would be gained. 
The application of biogas slurry causes ecological effects of -275 EI '99H points (-2.8%) in the standard scenario and -4.9% in the scenario that does not consider land use. Herein ammonia emissions cause 7.2%, and 12.7% of the total ecological effects respectively. The positive ecological effects in this module are derived from the nutrient recycling. From literature, a share of 40.0% on the overall effects was assumed for this module [DOLMAN ET AL.2001]. This difference is caused by the storage of the biogas slurry and the application technology used. In literature, an open vessel is assumed for storage. This causes a high 
level of CH4 and NH3 emissions. In addition, the application with a broadcaster and instead of 
an additional incorporation method is assumed. This leads to high NH3 emissions, which cause most of the overall ecological effects in this study. In the here done study, the biogas slurry is stored in a gas-tight vessel.  A trail-hose is used to apply it and the slurry is incorporated directly after application. The emissions created by this method were tested in a field trial, from which the data for this LCA study come from. The NH3 emissions of this trial are 95% below the data from literature. This is possible due to the type of application used and the incorporation method.  Furthermore, the emissions from biogas slurry, which was applied with a broadcaster and not incorporated, were calculated. The results of this module, related to the overall effects of the scenario not considering direct land use effects, are of a comparable scale (29.0% of the 
overall effects) as the results from literature [EDELMANN ET AL.2001]. At this point, it must be emphasised that this scenario considers indirect effects from land use with an amount of 14.8% of the overall effect. By subtracting this influence, which is not even considered in literature, the same importance on the overall ecological effect is given. Relating these NH3emissions to the scenario, which does not calculate direct land use impacts but instead good agricultural practice for slurry application, shows that ammonia emissions have an influence of 18.7% on the overall effect. Therefore the ammonia emissions still prove to be a serious 
Finally, an evaluation was made on treated biogas slurry emissions. The effect of this scenario compared to the standard scenario leads to minor ecological savings; the share of the ammonia emissions on the overall result is 0.3% less in the treated biogas scenario than in the standard scenario. This influence is insignificant when compared to the influence of the application using an incorporation method. 
The emissions from applied biogas slurry are dependant on a wide variety of factors. These emissions cause direct damage to the environment and also nutrient losses. Therefore exact data of the emission rates are demanded.This trial reflects data from just one situation. For further assessments the data from this trial 
should be repeated with different soils, weather conditions, and biogas slurries to obtain 
more information on biogas slurry emissions.The comparison of all the scenarios shows the influence of the single impact categories per scenario, in addition to the influence of the assumptions in each scenario in comparison to  other scenarios. Taking into account the different energy crop production systems and the 
different weighting methods of the impact category land use, it can be concluded that most 
ecological effects of the biogas production are caused by the agricultural production of energy crops. 
The results of the scenario considering waste as an input to the biogas process are conspicuous. Once having considered most impact categories as well as the overall ecological effects, it is obvious that this scenario causes the least ecological effects. This scenario compared to the standard scenario causes only 12.2% of the overall ecological effects. Even if the content of heavy metals in the biogas slurry resulting from these inputs and no substitution effects such as animal foodstuff are considered in this scenario, the utilisation of biodegradable wastes is more environmentally sound than the utilisation of energy crops. The breeding efforts, resulting in a 30.0 Mg dry matter yield per ha, would cause ecological savings of 53.3% compared to the standard scenario. Therefore the breeding of high yield energy crops could lead to dramatic ecological savings in biogas production systems. 
The utilisation of a MCFC instead of a conventional gas engine reduces the ecological effects of all impact categories. The higher conversion efficiency of the fuel cell causes fewer inputs to the biogas process and therefore related fewer transport efforts. Altogether, 29.0% of all ecological effects are saved by the utilisation of a fuel cell compared to the standard scenario.  
If the waste heat of the biogas process were to be used to substitute fossil fuels, then considerable savings of ecological effects would be achieved. If only 25% of the available waste 
heat of a biogas plant were to be used to replace natural gas for heating purposes, 15.1% of the overall ecological effects of the standard scenario would be saved. That is why, the utilisation of waste heat from biogas CHP plants, as well as any other CHP process, should be promoted. It is an easy, sometimes expensive way, of saving ecological effects by state of the art technology. 
When comparing the weighted overall results of all systems under analysis, the most environmental friendly biogas production system that stands out is the scenario using waste as an input (1,830 EI '99H points). Moreover, the scenario considering the utilisation of high yield maize as an input causes few ecological effects (4,600 EI '99H points). The utilisation of a fuel cell would cause 6,990 EI '99H points; and the waste heat utilisation 8,360 EI '99H points. The standard scenario causes 9,850 EI '99H points.  
Material impact(s): 
Ozone layer depletion
Waste generation
Abiotic ressources depletion
Toxicity / Eco-toxicity
Raw material impact level: 
Manufacturing impact(s): 
Ozone layer depletion
Manufacturing impact level: 
Shipping impact(s): 
Abiotic ressouces depletion
Shipping impact level: 
Usage impact(s): 
Global warming
Abiotic ressouces depletion
Usage impact level: 

Intissar LAZHAR

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