Resultado final
The establishment, cultivation and harvesting of all four species were performed successfully using conventional agricultural methods.
Yields of each crop varied considerably according to site and climate. High ash contents for all crops were measured compared to woody feedstocks and also significant variability in both ash content and ash composition. Great variability exists in biomass production costs, mainly caused by water and soil quality. Multi cropping scenarios may be more costly than single cropping systems, minimize storage and secure operation of the conversion plants. Cost of biomass production, harvesting and transportation is -100 euro per d.t, with cardoon and switchgrass being the cheapest forms.
To obtain a high environmental benefit, high, ecologically- sound crop yields should be aimed from multi cropping systems that enhance biodiversity, higher conversion efficiencies and co-products use.
Energy crops should be studied at all appropriate scales and on a long-term basis to identify appropriate habitats for desired species, both for crops and lands (agricultural, managed forest and natural).
An LCA study was performed to compare the environmental impacts of the different bioenergy chains of perennial crops with respect to their fossil alternatives and among each other, considering their life cycles from the growing region and agriculture to conversion and usage.
Biofuels can help saving energy and greenhouse gases. To some extent they are, however, associated to environmental disadvantages. An objective decision for or against a particular fuel cannot be made. However, based on a subjective value system a decision is possible.
Depending on the crop, the conversion technology, and usage, the environmental effects show at times large or even very large differences. The largest environmental advantages with the smallest environmental disadvantages at the same time can be achieved as follows:
- For obtaining a high environmental benefit, one should grow crops permitting high yields while being ecologically sound.
- In order to produce heat, one should combust biomass directly instead of using the products of gasification or pyrolysis. Only for very advanced technology conversion efficiency can get significantly higher and there is still waste heat left over for heating uses. If one desires to produce electric power or combined heat/power from biomass, the decision whether direct combustion, gasification, or pyrolysis is the best option depends on the individual case.
- For utilising the bioenergy crop the optimal way, every effort should be taken to optimise the efficiency of gasification and pyrolysis and to utilise the waste heat of the conversion processes and pyrolysis residues.
- Given a certain conversion technology, the optimal usage (heat, power, or combined heat/power) depends to a large extent on the individual case.
- If biomass is to be gasified it should be used for heat production at the site of the plant; if it is pyrolysed it should be used rather for power production. If the biomass is to be combusted directly it depends on the single case if heat production or combined heat/power production are preferable.
- For the sake of the greenhouse effect especially light oil should be replaced, only in the second run natural gas.
- If one attaches highest environmental value to the saving of depletable energy resources one should, in the best case, produce electric power from biomass in France in order to replace the French power mix. If, instead, one deems the greenhouse gases most important one should replace the Greek power mix.
The general outcome, e.g. the advantages for biofuels regarding energy savings and greenhouse gases, can generally speaking also be transferred to other bioenergy chains. An exact quantification of the respective life-cycle assessment and
a site-specific environmental impact assessment, however, have to be undertaken for each individual case.
Environmental impact assessment is a procedure for examining proposed activities early in planning for their potential to impact the environment. The aim of environmental impact assessment is to identify, describe and assess the effects of a project on environmental issues. In general, EIA is working on a site specific level, i.e. environmental effects of projects are investigated on the basis of detailed analysis of different environmental factors, e.g. soil or vegetation at a certain site.
Energy crops can be used to manage or direct regional landscape ecology if the system is properly designed. Potential services include buffers around similarly structured natural habitats, linkages between fragments of natural habitat, or the creation of new habitats.
How effectively the energy crop serves such roles does not only depend on the particular crop, but also on how it is managed (including use of chemicals, equipment, and harvesting cycle).
Some general recommendations include (Wolfe 1993):
- Energy crops should be concentrated on current, idled, or former agricultural, pasture, or other simplified or marginal lands. Energy crops should not be grown on naturally structured primary-growth forest land, wetlands, or other natural lands.
- Energy crops should combine multiple vegetative structures to enhance landscape diversity as needed by particular species. This could include various combinations of short-rotation woody crops, perennial grasses, and other dedicated energy crops, as well as inclusions of natural habitat.
- If possible, energy crop fields should be arranged as buffers around similarly structured natural habitats and linkages between fragments of natural habitat.
- Ideally, energy crops should provide artificial environmental functions important for a given location (erosion control, enhanced or reduced transpiration, wastewater recycling systems).
- Energy crops could also be used to provide structure to conventional agricultural monoculture through the addition of shelterbelts and fencerow plantings.
- Similarly, monoculture of energy crops should have shelterbelts or fencerows of other types of vegetation.
- Landscape structure can also be made more diverse by harvesting adjacent stands on different rotation cycles, including leaving some stands for longer periods if possible.
- Energy crops should be studied carefully at all appropriate scales and on a long-term basis to better understand the best means of improving appropriate habitats for desired species, both for the energy crop itself and for related agricultural, managed forest, and natural lands. This should also be done on a regional basis, as appropriate.
Within the project BTG evaluated the suitability of different crops in a standard fluidized bed gasifier. Based on the properties of the energy crops, the results gained in this project and experience from other projects, it is believed that this approach (energy crop standard fluidized bed) will not lead to commercial feasible operation.
The high minerals content (and in particular Cl) may cause bed agglomeration on the short or longer term and relative low temperatures must be maintained. Low gasification temperatures will result in a dirty, tar-rich gas, which needs severe cleaning.
BTG expects that for a commercial system minimum requirements are:
- Minimum capacity of 1-2 t of dry biomass per hour to be economic feasible
- Robust (at least 6,000 hrs of operation must be feasible)
- Multi fuel system (reduced dependency on one specific feedstock, and most likely even not limited to energy crops yields may vary substantially per year)
To fulfil these requirements BTG has started the development of a multi-step gasification process. In this process biomass is vaporized at low temperature, and subsequently the vapors are converted into a clean gas. Initial development work has started in May 2005. Experimental work has started recently on a scale of - 20kWth.
The low temperature part is based on BTGs pyrolysis process; the high temperature part strongly relates to BTGs earlier work on catalytic tar reforming. Therefore, the time-to-market is expected to be relatively short.
The information collected so far from RTD programmes regarding the energy crops feasibility for electricity and/or heat generation presents great variations with respect to biomass yields, feedstock physico-chemical properties and financial aspects. These variations are mainly due to crop species, the crop/site-specific cultivation techniques and the soil-climatic conditions.
Additionally, harvesting of the crops is signalled by the senescence of the crops on the field and thus climatic conditions prevailing at that time are very critical factors influencing the feasibility to successively harvest a selected number of crops and ensure a raw material throughout the year. To eliminate the site effect, which is a major source of variability, this project is focused on South European areas with Mediterranean climate, namely Greece, Italy, France and Spain. Under these conditions, according to previous trials, the selected perennial herbaceous crops (cardoon, switchgrass, miscanthus, giant reed) complete their growing cycle and reach physiological maturity in successive periods of time.
While most information collected from previous trials referred to experimental and consequently small scale - plots, where all works were carried out manually, the information generated in this project covers the establishment, cultivation and harvesting of all four perennial crops in large scale fields of 10 h approximately, using conventional equipment for planting and harvesting each crop.
It is worthwhile to mention here that this successive harvest is not effective in North European countries. In North European countries switchgrass, miscanthus and giant reed do not complete their growing cycle in the year and their senescence is not caused by their natural maturity, but by the autumn or winter killing frost. Therefore, their harvesting is done simultaneously for all the selected crops. In this case cardoon is not accounted, because it is a perennial herbaceous crop especially adapted to the Mediterranean climate.
The four selected crops were established in large-scale fields in Central Greece (by CRES), Italy (by UNIBO) and Spain (by UPM). The establishment, cultivation and harvesting of all four species were performed successfully using conventional agricultural methods. Yields of each crop varied considerably according to site and climate.
The scope of this task was to gain detailed information about the characteristics of the energy crops switchgrass, giant reed, cardoon and miscanthus by means of chemical analysis of fuel samples provided by the biomass growing partners.
The characteristics investigated are heating value, ash content, chemical composition, concentrations of main elements (C, H, N, S, Cl, Si, Ca, Mg, K, Na, P, Al), concentrations of heavy metals (Fe, Mn, Cu, Zn, Ni, Cr, Pb, Cd, Hg) and moisture content.
The main quality aspects of biomass fuels are the heating value and the ash content.
On an ash free and dry basis the heating values of the crops investigated are similar to wheat straw but lower than wood. The ash content of perennial crops is comparable with the ash content of wheat straw and significantly higher than the one of wood chips. However, there is a very strong influence of the harvesting method used and the conditions during the harvest.
Based on the analyses performed, the concentration levels of relevant elements (especially K, Na and Cl) in the perennial crops imply that generally high particulate emission levels, corrosion and deposit formation rates are to be expected during combustion of these fuels.
The chemical composition and also the morphology of the perennial crops investigated are comparable to the commonly used biomass fuel wheat straw and significantly different to wood. According to this, combustion technologies developed for straw combustion should be applicable for the combustion of perennial crops as well.
First of all, the harvest methods for the perennial crops have to be further developed and tested in order to avoid contamination of the fuel. Secondly, for large-scale applications the fuel should be stored and transported as bales and presumably also combusted as bales. For small-scale applications pelletising of the fuel prior to transportation is the best solution as the chopped material has much too low mass and energy densities for efficient transport, storage and fuel feeding.
When combusting these fuels, care has to be taken concerning emissions (especially NOx and particulates). Also operational problems such as slagging and fouling are to be expected. Especially in small-scale applications these problems are difficult to solve and thereby techniques for improving the quality of the fuel such as leaching, usage of additives or blending with other fuels should be investigated in the future work.
Cost analysis of agricultural production.
The economic evaluation of agricultural production is tracing all costs incurred in the field, by agricultural activity of energy crops. The economic analysis of biomass production has been greatly assisted and standardised by the Bioenergy Economic Evaluation (BEE) computerised model, which was developed within the project for this purpose. The economic analysis of agricultural production covers all cultural practises before biomass harvesting.
For this analysis two basic scenarios were made. The first scenario covers the production of every crop as a monoculture.
The result of this analysis is the cost of production for every crop in each country
(i) Per cultivated hectare and per tonne,
(ii) Per agricultural operation and per production factor/input.
This part also covers the economic comparison between energy and selected conventional crops that estimates for each case the break even price of energy crops that makes their production economically viable for the farmer.
The second scenario covers the joint production of (i) all four crops and (ii) a mixture of Arundo, Miscanthus and Switchgrass. The cultivation of more than one crop is in general more costly.
However, due to reduction of storage needs, possible increase in the use of agricultural equipment and more efficient operation of the transformation plant during the year, it may be preferable in some cases.
Harvesting, Storage and Transport cost analysis Energy crops can be harvested and transported in different ways and forms.
For this study five forms were considered:
- Pellets,
- Bundles,
- Bales,
- Billets and
- Chips.
Economic analysis of harvesting storage and transport was performed for every crop and every form in each country. The most cost effective form was selected for more detailed analysis.
Due to economies of scale, three basic scales were analysed:
- Small scale: 4,000 tonnes of dry biomass for 3 km average distance
- Medium scale: 15,000 tonnes of dry biomass for 6 km average distance
- Large scale: 50,000 tonnes of dry biomass for 10 km average distance
Equal quantities of each crop in each case were assumed.
This analysis was also performed for (i) single crop and (ii) combination of crops, as mentioned before.
Combustion test runs with the four perennial crops were performed at a laboratory scale fixed-bed reactor as well as in a pilot-scale fixed-bed combustion plant.
The investigations at the pilot-scale plant focused on the identification of possible operational problems such as fuel feeding, ash melting on the grate and in the furnace, ash transport systems, emissions (NOx, SOx, HCl and particulates), formation and growth of deposits (fouling tendencies), fractionation of elements in the different ash fractions.
The results from the evaluation of the test runs performed indicates, that a temperature below about 1,100°C is necessary in order to prevent ash melting on the grate. It is therefore recommended that technologies used for wheat straw combustion such as water cooled furnace walls and a water cooled grate should be used for the combustion of these crops.
Another possibility to avoid slagging problems would be to blend these fuels with a less problematic fuel such as wood pellets.
However, the test runs at the pilot-scale combustion unit have shown, that Switchgrass, Miscanthus and Giant Reed could be utilised in such a combustion unit but not Cardoon samples, which was characterised by extremely high K, Si and Clcontents.
From the results of the test runs it can be concluded, that the combustion characteristics as well as the problems occurring during combustion are similar for all four perennial crops investigated. The design of a combustion plant using all these crops would be similar as well except for the Cardoon tested, which is not suitable for combustion.
The main problem would be to design a suitable fuel feeding system. Bales are probably the easiest and most efficient way for transporting for large-scale combustion plants. In the case of small-scale plants pelletised fuels are the best suitable solution in order to ensure high enough bulk and energy densities of the fuels, which makes them suitable for the fuel feeding systems usually applied. Although pelletised perennial crops have a higher energy density than chopped fuels, still problems with reaching the full load of the plants designed for wood pellets are observed.
Consequently, new designs with a larger combustion chamber in relation to the boiler capacity would be necessary for an optimised combustion of these crops.
Moreover, also the increased amount of ash produced from these crops has to be considered in the design of the ash removal systems of the plant.
According to the test run at the pilot-scale combustion plant about 87 to 95% of the ashes produced remained as bottom ash. Due to the generally low concentrations of environmentally relevant heavy metals in these ashes, the ashes could be recycled and used as fertilisers on the soils where the crops were grown.
Pyrolysis tests were performed with all four perennial grasses at a lab-scale pyrolyser such as those studied in this project have the potential to be converted into a useful bio-oil energy product.
The result from the test show that these crops have alkali metals contents sufficiently high that pyrolysis yields would be much lower than for most low ash wood feedstocks.
Crops with low alkali metal content (perhaps less than about 0.3wt% d.b.) will provide pyrolysis yields similar to those obtained from clean wood feedstock i.e. an organic liquid yield of over 60% and a total liquid yield of over 70% of the dry ash fee mass of the feedstock. The oils produced from such feedstocks appear very similar
to oils produced from woods such as beech and poplar, but further work needs to be done to understand their physical and chemical properties in more detail. In particular, their ability to dissolve small amounts of pure hydrocarbons may enable quality improvements to be made through the use of additives.
To obtain low alkali metal content with reliability would probably require a pre-treatment such as cold-water washing since natural rain leaching (as experienced by some of the project crops) cannot be relied upon.
The work has shown that the washing process could be fairly simple given the availability of sufficient clean water. However there is much work to be done to determine the technical feasibility and costs of such a system.
It will be important to establish the relationships between increase in oil yield and improvement in quality and the incremental cost of washing.
Even without washing, and with low pyrolysis liquid yields of 40 to 50% (d.b.) there may be some applications for which poorer quality bio oil could be suitable, for instance it may have value as an industrial fuel oil for process or space heating applications provided that its storage stability could be managed, for instance by blending with other biofuels such as biodiesel and /or bioethanol.
At the moment the cost of fast pyrolysis for producing high-grade energy such as electricity is very high (even assuming high pyrolysis efficiency and without the cost of pre-treatment of the feedstock) and would require substantial subsidy to be economic.
Buscando datos de OpenAIRE...
Se ha producido un error en la búsqueda de datos de OpenAIRE
No hay resultados disponibles