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Energy from biomass and waste
authors Faaij, A.P.C.
source Scheikunde Proefschriften (2001)
full text [Full text]
document type Dissertation
disciplines Scheikunde
abstract Biomass, a broad term for all organic matter of plants, trees and crops, is currently regarded as a renewable energy source which can contribute substantially to the world's energy supply in the future. Various scenarios for the development of energy supply and demand, such as compiled by the World Energy Council (WEC), the Intergovernmental Panel on Climate Change (IPCC), Shell and the Stockholm Environmental Institute (SEI), indicate that biomass has the potential to make a large contribution to the world's energy supply. Estimates of this potential in the year 2050 vary from 14% to 50% of the total supply, or from 100 to about 300 EJ/yr. It is estimated that currently biomass contributes 10-14% of the energy supply, which is equivalent to about 40-55 EJ/yr. The use of firewood in developing countries makes up a large part of this 40-55 EJ, but there it is for a large part non-commercial and non-sustainable use of biomass. In recent years there has been renewed interest in biomass as a commercial and sustainable source of energy. There are three main reasons for this: 1. Technological developments, in the field of crop production and conversion technology permit the more efficient and cleaner utilisation of biomass at lower costs. These developments make bioenergy more competitive with energy produced from fossil fuels. 2. The agricultural systems of especially the European Union and the United States are producing food surpluses. This situation has led to policies whereby agricultural land is 'set-aside', resulting in depopulation of rural areas. The continuously increasing productivity in agriculture might strengthen these trends. There is therefore a desire to develop alternative crops. Energy crops could be a suitable alternative since there is virtually an infinite market for this, provided the costs are competitive with those of fossil fuels. 3. There is a threat of global climate change due to the rapid increase in the concentration of greenhouse gases, especially CO2 in the atmosphere, resulting mainly from the large scale use of fossil fuels. If produced sustainably, biomass can be a carbon neutral alternative for fossil energy carriers. If biomass is to make a substantial contribution to the world's energy supply it will have to include not only biomass residues - such as from commercial forestry (e.g. thinnings) and agriculture (e.g. straw) - and organic wastes, but also energy crops. Perennial crops seem to be a particularly promising energy source. Crops like Short Rotation Coppice (e.g. Willow and Eucalyptus) and grasses (e.g. Miscanthus) give a relatively high net energy yield per hectare, have a low environmental impact and produce relatively cheap energy. The use of such crops in a Biomass Integrated Gasifier/Combined Cycle (BIG/CC) plant to produce electricity or combined heat and power, and the gasification of these crops to produce fuels like methanol and hydrogen appear to be promising routes for achieving high energy conversion efficiency at relatively low cost. However, despite the promising outlook, various barriers are hampering the large scale development and implementation of commercial biomass energy systems. Currently, the commercial use of biomass to generate electricity is limited mainly to the utilization of zero- or low-cost biomass waste or residues. At the moment specially cultivated biomass is too expensive an option. However, biomass is able to compete on a significant scale in countries, like Sweden, Denmark and Brazil, where government policies support its use financially or have actively discouraged the use of fossil fuels (such as by the introduction of a carbon tax). The complexity of large scale bioenergy systems is also a barrier. Furthermore, biomass has a relatively low energy density. The production of biomass is bound up with seasons and makes high demands on organization and logistics. Furthermore, it involves many different actors involved in the production and utilisation of energy crops: farmers, utilities, industries, governments, etc. Difficulties concerning public acceptability and uncertainties concerning the ecological effects of the large scale production of use of biomass be form another problem. Last but not least, the availability of land may be a major problem if the large scale production of energy crops is being considered. If agriculture is not modernizing, especially in developing countries, there might be very little room left for alternative crops. Energy farming may then conflict with food production, a situation which is highly undesirable. This thesis focuses on a number of aspects relating to the utilization of biomass and waste for energy purposes. The general objective of this work is as follows: "To analyse the possibilities for biomass (both crops and wastes) as a modern energy carrier in the Dutch energy system." Therefore this thesis has the following specific objectives: 1. To analyze of the technical, economic and environmental characteristics of Biomass Integrated Gasifier/Combined Cycle technology for the conversion of biomass and waste streams. 2. To examine the potential energy supply of energy farming, biomass residues, organic waste streams and waste in the Netherlands. 3. To analyze of the potential costs and benefits of different biomass energy systems (including waste treatment) in the Netherlands. In chapter II, the characteristics and availability of biomass waste streams and residues for power production by means of integrated gasification/combined cycle technology (BIG/CC), are evaluated with respect to the situation in the Netherlands. Four main categories are investigated: streams from agriculture, organic waste, wood and sludges. Altogether 18 different streams are distinguished. An inventory is made of gross availability and net availability. Various properties (composition, heating value, supply patterns) are analysed and the suitability of these streams for conversion in a BIG/CC unit is studied. The costs at which various streams are likely to be available are assessed. The gross energetic value of the biomass wastes and residues amounts annually to approximately 190 PJ (HHV) primary energy. Part of this gross potential is used for applications such as fodder and fertilizer, which have a higher value than if the biomass was used as a fuel. The resulting net availability for energy purposes is slightly less than 90 PJ (HHV). For a number of biomass waste streams the energy costs are negative due to the current costs of waste treatment. The energy costs of waste streams vary from -10 to 5 ECU/GJ. The costs of a small fraction of those streams is expected to be higher than the costs of energy crops (estimated to be approximately 4.5 ECU/GJ). Because there are large variations in the properties and contaminants of the various streams, the conversion system needs to be flexible enough to handle a diversity of streams. Some streams require to be mixed with cleaner fuels to make them suitable for use in a direct atmospheric Biomass Integrated Gasifier/Combined Cycle system. Important technical limits on the use of biomass fuels in the system studied are the moisture content (maximum 70% of wet fuel) and ash content (maximum 20% dry matter content) of the fuel. Chapter III investigates in more detail concerning the performance of BIG/CC technology for different biomass fuels. The technical feasibility and the economic and environmental performance of atmospheric gasification integrated with a combined cycle to convert the energy from biomass wastes and residues to electricity are investigated for Dutch conditions. The system selected for study is an Atmospheric Circulation Fluidized Bed Gasifier/Combined Cycle (ACFB/CC) plant based on the General Electric LM 2500 gas turbine and atmospheric gasification technology, including flue gas drying and low temperature gas cleaning (similar to the Termiska Processer AB process). The performance of the system is assessed for clean wood, verge grass, organic domestic waste, demolition wood and a wood/sludge mixture as fuel input. System calculations are performed with an ASPENplus model. The composition of the fuel gas was derived by lab scale fuel reactivity tests and subsequent model calculations. The net calculated efficiencies for electricity production are between 35.4 - 40.3% (LHV) for the fuels studied, with a potential for further improvement. Estimated investment costs, based on vendor quotes, for a fully commercial plant are 1500 - 2300 ECU per kWe-installed. Electricity production costs, including the costs of logistics and, in some cases involving a negative fuel price, vary between minus 6.7 and plus 8.5 ECUct/kWh. Fuel costs are negative if the current costs for waste treatment can serve as income to the facility. Environmental performance is expected to meet the strict emission standards for waste incineration in the Netherlands. The system seems flexible enough to process a wide variety of fuels. The kWh costs are very sensitive to the system efficiency but only slightly sensitive to transport distance; this is an argument in favour of large scale power plants. As a waste treatment option the concept seems very promising. There seem to be no fundamental technical and economic barriers that can hamper the application of this technology. In chapter IV we studied the optimization potential in both economic and energetic terms of the final waste treatment system in the Netherlands for the year 2010. In this evaluation the performance of new technologies that may be available within a time-frame of about 15 years is taken into account. Projections of the final waste supply and waste treatment technologies are combined to construct several alternative waste treatment systems for the year 2010. Technologies used in these systems include processes currently in the demonstration or pilot phase. It is concluded that it should be possible to perform final waste treatment both at lower costs and with substantially increased energy recovery, than in the present situation. Low treatment costs do not prevent increased energy recovery. In a minimum cost scenario, the final waste treatment might cost 300-600 MECU per year, compared to 1,000-1,600 MECU/year in a reference scenario. The reference system is based on the same waste treatment technologies as are currently used, possible improvements being taken into account. A maximum energy recovery scenario might save 80-90 PJ primary energy per year compared to 39-47 PJ/yr for the reference case. Compared to the current energy savings of 17 PJ/yr through energy recovery from waste, the results indicate that substantial improvements can be made. Furthermore it is concluded that the separation of integral waste may lead to further energy credits. However, composting, such is currently applied on a large scale in the Netherlands for organic waste treatment, does not have the energy or cost benefits likely to accrue from potential future alternatives like gasification. The separate collection of organic domestic waste is therefore of doubtful value. According to our study, two major competing technologies are gasification of biomass waste and integral waste, and fluidized bed incineration of integral waste. Further development of these technologies integrated with electricity production is recommended. In chapter V we assessed the spatial and energy potential in the Netherlands for energy farming as well as for a number of biomass residues. Various government memorandums and analyses of the expected future land use in various sectors have served as the basis for the assessment of the supply of and the demand for land in the future. In this study the potential supply of agricultural land is based on expected productivity increments in agriculture and assumptions with respect to the future demand for agricultural products. Various future claims for infrastructure, forestry, urban areas and nature are substracted from the expected supply. The net projected supply of land ranges from zero to 52,000 ha in 2000 to 110,000-250,000 ha in 2015. The supply of agricultural land depends however on a number of supra-national factors such as the European agricultural policy, world market developments and the agricultural production in the countries in Eastern Europe. Uncertainties remain therefore and the projected supply of agricultural land should be considered as a possible scenario based on current trends. If the calculated land potential is used for energy crops like Miscanthus and Short Rotation Coppice, this land could contribute 0-10 PJ in 2000 and 27-59 PJ in 2015. Secondary biomass yields such as those from forestry, agricultural residues, wood from prunings, etc., could contribute a further 34 PJ in 2000, decreasing to approximately 28 PJ in 2015. Taken together these potentials could satisfy 1-1.5% of the energy requirements of the Netherlands in 2000 and 1.5-2.5% in 2015, provided that energy farming is an economically feasible activity for farmers. In the Dutch context biomass production by means of energy farming is at present clearly an expensive option compared to the use of fossil fuels. However, according to a number of studies the use of fossil fuels leads to social and environmental costs (e.g. emissions cause damage to crops and human health) which are generally not reflected in the cost of these energy carriers. Furthermore, renewable energy carriers are often said to involve far lower social and environmental costs than fossil fuels. In this context, in chapter VI the external effects of bioenergy are evaluated for the Dutch context. In particular the costs of electricity production from biomass are compared with the costs of coal based power production in order to obtain an idea of the influence that the inclusion of such external costs (or benefits) may have on the cost of electricity produced by each option. In chapter VI the effects of the two fuel cycles on the Dutch economy and employment are evaluated with help of Input/Output and multiplier tables. The valuation of damages from emissions to air is performed using generic data from other studies. Furthermore, estimates are made of the external costs connected with nitrogen leaching and the use of agrochemicals for energy crop production. The average private costs of biomass and coal based power generation are projected to be 68 and 38 mECU/kWh respectively in the year 2005. It is assumed that biomass production will take place on fallow land. Because coal is imported its mining is excluded from the analysis. When the quantified external damages and benefits are included, the calculated cost range for bio-electricity amounts to 53-70 mECU/kWh and for coal 45-72 mECU/kWh. Indirect economic effects (increment of Gross Domestic Product) and the difference in CO2 emissions are the most important factors that differentiate between coal and biomass in economic terms. Damage costs of other emissions to air (NOx, SO2, dust and CO) are of the same order of magnitude for coal and for biomass (coal mining excluded). Environmental impacts from energy farming depend highly on the reference system and can both result in economic benefits or damage depending on the land-use that is replaced. The quantitative outcomes should not be considered as the external costs of the two fuel cycles studied. Many impacts have not been valued and large uncertainties persist e.g. with respect to the costs of climate change and numerous dose response relations. More detailed analysis is desired with respect to macro-economic impacts. The results serve as a first indication, but the outcomes plead for support of bio-electricity production and/or taxation of coal based power generation. From this thesis it can be concluded that the total energy potential of net available biomass wastes and residues in the Netherlands amounts to about 70 PJ (LHV). Waste streams like integral waste add about a further 90 PJ. Around 2015, energy crops could supply 30-60 PJ, depending on developments in agriculture and possibilities of combining biomass production with nature development. A total contribution of 190 - 220 PJ (in 2010-2015) to the Dutch energy supply seems therefore in principle possible. This is equivalent to about 6 - 8% of the current primary energy demand of the Netherlands. The government objective for the contribution of biomass and waste to the Dutch energy supply in 2020 is currently set at 120 PJ. It is expected that a substantial part of this biomass will be imported from other countries. This thesis shows that the objectives are modest compared to the technical potential. Furthermore, the projected contribution can be realized without import of biomass. Final waste treatment can be performed at considerable lower cost and with substantially increased energy production than in the current final waste treatment system. Integrated gasification/combined cycle technology can play a key role in obtaining such a situation. The energy potential from energy crops is directly linked to the potentially available land. This thesis indicates that in the Netherlands a certain surplus of agricultural land may become available for energy farming. To enable this potential to be realized, however, some preconditions must be fulfilled: one condition is that energy farming should be an economically attractive option for farmers. Furthermore, the assumed productivity increments (ranging from 15 to 30% in the time-frame considered) in the Dutch agricultural system must be realized. It should be noted that the actual demand for agricultural products (including for export) depends on various, mainly supranational, factors. These factors may lead to a larger and a smaller land potential for energy farming than what is projected here. In the Dutch context cultivated biomass is currently more expensive than fossil fuels. However, when the social and environmental costs and benefits are included in the costs of electricity from (cultivated) biomass and the figure is compared to the costs of coal based power generation we find that the cost difference between the two becomes far smaller. Despite the uncertainties, this outcome could lead to increased support for electricity production from biomass. Further research into the possibilities of fitting energy crops into the current agricultural system and landscape is recommended. Furthermore, larger scale demonstration projects are needed so that experience can be gained with advanced dedicated biomass fuel supply systems. Such research and demonstration activities are essential to lower the costs of such options and increase their efficiency. Directly related to the desire for the demonstration of BIG/CC technology on a substantial scale, is the so called Noord-Holland gasification project, which aims to realize a 30 MWe BIG/CC unit. Such a project is of crucial importance to develop this technology further. The research reported in this thesis indicates that Atmospheric Circulating Fluidized Bed BIG/CC technology is a promising conversion technology from the point of view of energy efficiency and environmental and economic performance. These benefits can already be achieved on a modest scale. The technology seems flexible enough to treat a diversity of potential wastes and biomass fuels with varying characteristics. No attempt has been made in this thesis to evaluate the import of energy from biomass from countries with a potential biomass surplus. Since this option is being seriously considered in the Netherlands, attention should be paid to the environmental and economic consequences for both the importing and the exporting country. The production of fuels like methanol and hydrogen, which may form a promising conversion route to supply energy production from on longer term, has hardly been investigated in this thesis either. More attention needs to be given to this option in future work.
keywords bio-energy, energy from biomass, waste treatment, renewable energy, gasification technology, biomass wastes and residues, energy crops, external costs of bio-energy