The article presents an attempt by a group of researchers to contribute to a larger and more organized application of available biomass for energy purposes. The group of researchers is part of a research team from the following educational institutions and institutes: Vinca Institute of Nuclear Sciences - Laboratory for Thermotechnics and Energy, Innovation Center of the Faculty of Mechanical Engineering in Belgrade, Faculty of Mechanical Engineering in Belgrade, Faculty of Technical Sciences in Novi Sad, Faculty of Agriculture in Novi Sad and Land Institute from Belgrade. The article is also a kind of compilation of several papers in domestic and international professional journals, conferences and discussions at many meetings and public forums and as a result of this group of researchers on the implementation of two very successful projects of the Ministry of Education, Science and Technological Development (III42011 and TR33042) . Both of these projects have concrete results in the development of technologies for the application of biomass and other renewable energy sources. The presented "View" is given with the best intentions and with the desire to help in the adoption of solutions for the popularization and concrete application of biomass for energy purposes.
The view is focused on three basic issues that have been taken into consideration and which guide the overall thinking on how to best use biomass resources, how to popularize it and how to help implement it. The main issues discussed are:
1. Available resources and possible combustion technologies,
2. Multidisciplinary approach to the problem,
3. Concept of plant construction.
1. Available resources and possible combustion technologies
According to the official data of the Ministry of Mining and Energy of the Republic of Serbia, the greatest potential for renewable energy in the Republic of Serbia is biomass. Its potential is at the level of ≈60-63% of the total potential of all renewable energy sources. Translated into millions of tons of oil equivalent (MToe) that is about 3 MToe. As the total energy consumption in the Republic of Serbia is around 15 MToe, the amount of 3 MToe is by no means negligible and due attention must be paid to it. On the other hand, out of the total available biomass, ≈60% refers to the remnants of agricultural production and the remaining part to forest biomass. Most of this biomass is most suitable for use in combustion systems. Therefore, further analysis deals with only the application of biomass in combustion systems.
- Mechanical properties of these biomass,
- Chemical composition,
- Thermal powers,
- Average moisture content,
- Bulk weights,
- Quantities and composition of ash,
- Solubility of ash,
- Method of collection, transport, storage and preparation for combustion, etc.
All these differences directly affect the combustion characteristics, which in turn affects the possible combustion technologies that are to be applied. For example, it cannot be expected that forest biomass-wood chips burn as e.g. chopped straw when the sintering temperature of forest biomass ash is 1000-1100 0С and wheat straw around 800 0С. The bulk density of wood chips is ≈500 kg / m3, chopped straw 100-150 kg / m3, so that from that side it is difficult to think that such different fuels are used in the same boiler from the point of view of using the same dosing and transport systems. . If different systems of transport, storage, preparation and dosing of biomass are applied in one boiler, it greatly complicates the boiler itself and work on it. With the reduction of boiler power to a couple of megawatts, the problems grow. All other similar parallel comparisons of certain characteristics of different biomass, which are not mentioned on this occasion, indicate that more different biomass should not be used in the same combustion system. If work with several types of biomass is envisaged, then those biomass, according to their stated characteristics, should be as similar as possible. Environmental problems must be taken into account when considering combustion technologies. Forest biomass has ≈1-1.5% of ash while agricultural biomass has many times more ash. Agricultural biomass ash hatches more easily from the forest than forest biomass ash. So particle flue gas purification systems are not the same.
When ash is mentioned, it would be desirable to return the biomass ash to the country from which the biomass was collected. Ash is a mineral part of a plant that it takes from the soil during its growth. It is logical then to return that mineral part to where it came from, especially since that would also reduce the use of artificial fertilizers. The ash can also be used as a manure stabilizer. When recovering ash, attention should be paid to the content of heavy metals, which are mostly present in the smallest fractions of ash. The smallest particles are separated from the flue gas in bag or electric filters. If the content of heavy metals in that ash exceeds the permitted limits, then such ash should be treated as waste and its safe disposal must be resolved.
On the other hand, attention must be paid to gaseous combustion products. Combustion technologies must be selected so that gaseous combustion products meet all environmental standards. It must be taken into account that the combustion of agricultural biomass can lead to an increased concentration of nitrogen oxides in the flue gas, so systems must be provided for the neutralization of these gases. These problems in agricultural biomass occur due to the use of nitrogen fertilizers for their growth. When using forest biomass, there are usually no problems with excessive concentrations of nitrogen oxides in the flue gas.
All of the above also affects the choice of combustion system. A study by the EU Energy Institute based in Petten (Netherlands) provides an overview of possible combustion technologies for different types and forms of biomass. The following table provides an overview of possible technologies.
* Straw means all biomass collected in the form of bales
** The last row in the table was added by the author of this paper
Indications of acceptability of combustion techniques from the table are:
(-) extremely unfavorable, (-) unfavorable, (0) possible, (+) favorable, (++) very favorable.
The table above shows that there is no combustion technology that is suitable for all forms of biomass. Based on: the explanations presented so far, as well as the data from the table above, it is recommended to choose those technologies that have the label "++" (very favorable) for the form of biomass to be burned.
In general, frequent switching on and off is not suitable for all types of biomass power plants, but it is recommended to organize the work in such a way that it enables continuous operation with as small and slow oscillations in power as possible.
2. Multidisciplinary approach to the problem
As the application of biomass is a multidisciplinary problem in considering an organized approach to the use of biomass, as the largest renewable energy source in the Republic of Serbia, experts from several Ministries and institutions of the Republic of Serbia should be synchronized, as follows:
- Ministry of Mining and Energy,
- Ministry of Agriculture, Forestry and Water Management,
- Ministry of Economy,
- Ministry of Education, Science and Technological Development,
- Ministries of Labor, Employment, Veterans and Social Affairs,
- Ministry of Environmental Protection,
- Cabinet of the Minister without portfolio in charge of innovation and technological development,
- Chamber of Commerce of the Republic of Serbia and
- Local governments where a biomass plant would be built.
Since we are talking about the use of biomass as a renewable energy source, the role of the Ministry of Mining and Energy should be crucial in the part where we talk about combustion technologies that would be applied and energy efficiency, all in accordance with previous considerations. When it comes to energy efficiency, overall energy efficiency should always be considered. Thus, for example, it is not possible to look at the energy efficiency of plants that use pellets as fuel from the amount of energy that is used to e.g. make pellets out of raw wood. That amount of energy is at the level of 30-35% of the total energy potential of raw wood. Looking at pellets in this way, it is reasonable to use them in smaller plants up to e.g. 100 kW. The reasonable strength to which the use of pellets is recommended should be determined by this Ministry. When building biomass cogeneration plants, this Ministry must oppose the construction of biomass cogeneration plants, which would, for example. during the summer produced only el. energy and heat released into the environment using e.g. cooling tower. In such regimes, only 15-25% of the total biomass energy potential would be used. In many places of the Republic of Serbia, especially AP Vojvodina, gasification is carried out. We believe that the Ministry of Mining and Energy, in addition to gasification, should support and stimulate biomass-based heating. Biomass-based district heating can automatically lead to the reindustrialization of the Republic of Serbia, the reduction of fossil fuel imports, all with a favorable impact on environmental protection. So the Ministry of Mining and Energy should support all projects that are in line with the recommendations on the choice of combustion technology and maximum reasonable energy efficiency and prescribe the criteria for that.
Biomass, in general, whether of agricultural or forest origin as energy raw materials falls under the responsibility of the Ministry of Agriculture, Forestry and Water Management. This Ministry, by the nature of things, should take into account that everyone who sells and distributes forest biomass, in any form (firewood, wood chips or pellets) must have an obligation to afforest. Criteria for how many tons of used wood must be afforested, e.g. 1 ha of land and what type of wood must be brought by this Ministry and in accordance with that prescribe rigorous penalties for those who do not respect these measures. Only in that way can the forest fund of the Republic of Serbia be preserved. Similarly, criteria must be set as to what percentage of crop residues may be used as fuel and which must remain on the land so that the land is not devastated. In the case of agricultural biomass, re-seeding (equivalent to afforestation) does not have to be taken into account because it is implied. This Ministry must also give guidelines for the return of ashes to the country.
The construction of biomass power plants, whether they are only thermal or cogeneration, can significantly contribute to the reindustrialization of our country. This key role must be played by the Ministry of Economy, because within each energy plant on biomass, a small industrial zone for processing can be built, e.g. agricultural products, wood drying and the like. With such a concept and good design, the produced energy could be used for heating, electricity production. energy and industry. The equipment for those industrial zones could be produced to a good extent in our country. This Ministry may provide guidance in the construction of such plants in the sense that it favors those that can produce and distribute energy, in any form, throughout the year.
Within the Ministry of Education, Science and Technological Development, there was a "National Energy Efficiency Program", which was abolished. We believe that this program should be revived. In that sense, it would not take much effort to invest in its revival. It would be enough to use old experiences and continue working on that principle. Within that Program, development projects were formed that concerned all segments of energy production and consumption, as well as the use of renewable energy sources, and within that, biomass. This would help a lot for the development of the biomass application system to go in the right direction and for the development results to be used by the domestic industry
3. Concept of plant construction
The approach to the construction of biomass power plants depends on many circumstances such as: existing resources in biomass, identified need for energy and energy carrier, phased construction, return on investment, ownership relations (involvement of public - private partnership, etc.)
The concept of construction according to resources can be on:
- forest biomass,
- agricultural biomass and
- those that use several types of biomass.
In earlier considerations, it has been said to go for those biomass that are most abundant in the environment and those forms of biomass that require the least preparation for combustion. This automatically means the highest, overall, energy efficiency. In addition, it was said that work with several types and forms of biomass in one plant should be avoided as much as possible. Delivery and storage must be taken into account when considering the use of biomass as a fuel. Warehouses for forest biomass can be significantly smaller in capacity than those for agricultural. The reason for that is that agricultural biomass is collected in a much shorter period of time, ie at the time of harvest. If agricultural biomass is collected in the form of bales, the moisture and the possibility of self-ignition must be taken into account. Therefore, smaller storages of that biomass are recommended next to the plant itself and more storages near the collection point. In that way, the consequences of self-ignition can be reduced and the engagement of mechanization at the time of harvest can be relieved, when it is most needed. Wood biomass can be collected almost all year round, if there is no heavy snowfall. Storage facilities for this biomass may therefore be smaller. With that biomass, the possibilities of self-ignition are less.
The concept according to the need for energy refers to
- thermal and
- cogeneration plants.
Heat plants produce only heat that can be transferred, hot flue gases, hot air, hot or hot water, steam (co-saturated, superheated) or hot oil. Thermal plants can be used to heat some spaces or in industry. With those systems that are used only for heating, it is known that the average engaged heating power for our climate is about 45% of the project, 18 hours a day, in the heating season. Therefore, care should be taken when designing plant optimization and power control options. The use of heat accumulators greatly helps in choosing the optimal solutions. It is very advantageous, if possible, to harmonize the operation of the heating plant with the needs of the industry. Good harmonization of work can extend the operation time of the plant in one year, which certainly has a favorable effect on the payback time of the investment. Thermal plants intended for industrial needs are usually specialized and work throughout the year. In most cases, heat accumulators are not required for these plants, or are significantly smaller than in heating-only plants.
Cogeneration plants can be attractive to investors because the Republic of Serbia has introduced a preferential price for electricity produced from renewable sources, including biomass. The task of all investors is to review the profitability of such plants and based on that make decisions on their construction. What the "State" should decide on is to prevent the construction of such plants that do not have the guaranteed consumption of "waste" thermal energy for the entire period of operation of such plants. If such plants e.g. flies only produce el. energy then it means that 75-85% of the energy input by biomass is thrown away in some way. We are of the opinion that in such cases it is better to leave biomass in the fields and rot there. We think that pedologists would agree with our position. We also think that in such cases, money (eventual earnings of investors) must not be a decisive factor.
By the way, two types of cogeneration plants are mostly in use: steam with classic steam turbines and so-called ORC systems (Organic Renking Cycle) with hot oil boilers and steam turbines that work with silicone oil steam. Both have their advantages and disadvantages. We are generally of the opinion that steam turbine systems are better for larger systems (10-30 MWe and more) while ORC systems are better for smaller plants.
Conception according to the established need for energy and energy carrier
The need for energy is always combined with the energy carrier. If plants are built for the production of thermal energy, care must be taken where that energy is consumed. In heating systems, the energy carrier is usually hot (below 1100C) or hot water (above 1100C). Where energy is used for both heating and industrial needs, plants are usually adapted to industry requirements. In industry, there may be a need for hot gases (eg brick industry), hot air (eg dryers for agricultural products), hot and hot water and steam (eg slaughter industry), etc. We should always strive for the simplest solutions that meet the given energy needs. The simplest solutions are, as a rule, the cheapest and most energy efficient. If the needs for the energy carrier are very different (eg hot and hot water, steam, hot air) then boilers in which thermal oil is heated may be the most suitable because the later heat exchange between the thermal oil and the previously mentioned energy carriers takes place in very simple exchangers heat, and boilers work as hot water only with higher temperatures of the working fluid - thermal oil. A similar statement applies to cogeneration plants.
Conception according to the approach of building a biomass plant
The construction approach can be such as to plan the complete construction of plants that meet the energy requirements or phased construction. Phased construction can refer to the construction of thermal and cogeneration plants. What does she mean in that sense?
If we are talking about the phased construction of thermal plants, we are of the opinion that it is better to meet the energy requirements in several phases by building more smaller than one larger plant. There are several reasons for this. There is still no organized market for biomass in the Republic of Serbia, especially agricultural. Therefore, during the operation of one (first built) smaller plant, one can gain experience in: operation of the plant itself, supply, transport, storage and preparation of biomass for use, as well as ash handling and disposal. In the initial period of operation of the first plant, the distribution (sale) of energy can be offered and organized outside the heating season, which can have a very favorable effect on the cost-effectiveness of construction and attracting interested parties who would use that energy in industry. After the experience gained, the construction of other plants should be approached. In addition, the construction of several smaller plants affects the security of supply of the required energy because it is less likely that two or more plants will break down at the same time. Power regulation is also easier and can be done in a wider power range if we have multiple plants. The construction of several plants is certainly more expensive, but since the investment does not have to be secured at once, then it is more acceptable. These advantages of phase construction certainly outweigh the disadvantages due to increased investment.
The previously discussed discussion on the phased construction of thermal power plants can also refer to the phased construction of cogeneration plants. By applying the ORC system, the possibility of phase construction for cogeneration plants is increased, because a thermal plant (hot oil boiler) can be made first. If hot oil boilers are made first, they can be installed in heating systems, industry or a combination without any problems and with minimal energy losses. Hot oil boilers operate at low pressures as well as hot water boilers, only their outlet oil temperature is higher than the outlet water in hot water boilers. After the operation of hot oil boilers in the heat regime, these boilers could very easily be connected to parts for the production of electricity. Energy. Parts for the production of el. energy would arrive from the supplier in the form of a "black box" with connections for input / output: hot oil, hot water and electricity. Energy. As the pressures in hot oil boilers are low and as there is no danger of explosions, the necessary safety measures in the operation of these boilers are lower than those of steam boilers. The level of training of the staff operating a hot oil boiler can be significantly lower than that of operating a steam boiler, which is not negligible, especially if the plants are built in rural areas. The presence of highly professional staff during the operation of the steam boiler is mandatory. Phased construction of steam cogeneration plants is not reasonable because it is not reasonable to use a steam boiler with high steam overheating to heat water up to e.g. ≈900S.
Dr. Dragoljub Dakić B.Sc. eng.
Scientific advisor at the Innovation Center of the Faculty of Mechanical Engineering in Belgrade
Participant in projects III42011 and TR33042 MPNTR
Dr. Branislav Repić B.Sc. eng.
Scientific Adviser at the Vinča Institute of Nuclear Sciences - Laboratory for Thermotechnics and Energy
Project Manager III42011 MPNTR
Dr. Stevan Nemoda B.Sc. eng.
Scientific Adviser at the Vinča Institute of Nuclear Sciences - Laboratory for Thermotechnics and Energy
Project Manager TR33042 MPNTR
Prof. Dr. Milan Martinov B.Sc. eng.
Faculty of Technical Sciences, Novi Sad
Full professor, participant in project III42011 MPNTR
The authors hope that this paper can help popularize and organize the application of biomass for energy purposes. In addition, the authors are of the opinion that theses from the paper can help in the affirmation of public-private partnership as well as directing possible donations and incentive funds in the direction that this paper suggests. Finally, we note once again that the work was created as a result of work on the implementation of projects of the Ministry of Education, Science and Technological Development, for which we are grateful.
Note: The authors reserve the right to present the views expressed in this paper on other occasions when the opportunity arises.