Bioenergy in wine/beverage production

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Among renewable energy sources, biomass plays the most important role and creates with about 80% by far the biggest share of the energy generated by renewable sources. The main difference between biomass and other renewable is the possibility of its utilization as a fuel. Biomass is the only carbon based renewable energy source which can directly substitute fossil fuels. Biomass is also the only one renewable energy source that can be stored and applied to produce heating, electricity and fuels when they are needed. The environmental benefit of using biomass as energy source is the ability to reduce CO2 emissions. The liquid or solid biomass feedstock can be directly used to provide energy or converted to more convenient energy carriers. Basically the processes utilized for energy production from biomass can be divided into three main categories: thermochemical, physical-chemical and biological conversion routes. The thermochemical conversion processes of biomass include: combustion, pyrolysis and gasification. The physical-chemical conversion processes can be applied for the production of biodiesel. The biological conversion routes of biomass are used to convert biomass into biogas or bioethanol [1].

Biomass combustion

Among the technologies for energy production from solid biomass, combustion is the most advanced and market-proven application. Consequently combustion of biomass has been found to be the most promising method for biomass utilization. Modern biomass combustion plants and biomass boilers achieve an efficiency of over 90% and utilize sophisticated technology to control the process in order to minimize its environmental effects and promote efficiency [2]. In the recent years utilization of biomass as a fuel in combustion appliances has been of great interest as an alternative to burning fossil fuels [3]. However, there are still unsolved problems related to the complexity of the biomass burning process, which include among others relatively low ash melting temperature or, variation in the fuel properties. Furthermore, efficiency enhancement and economic feasibility of bioenergy projects are often crucial issues [4]. One of the problems with using solid biomass as a fuel is the varying fuel parameters like moisture content, calorific value and geometric shape and size [5].

Fixed bed combustion

Fixed bed wood combustion boilers have been developed to produce heat with the aim of optimised energy generation. Modern biomass combustion systems ensure complete combustion with small amounts of pollutant emissions. The biomass combustion appliances are producing small quantities of ash, which can be disposed of with relative ease for example for use as a fertiliser.

Wood pellet appliances and burners

Pellets are small cylindrical pieces made by compressing sawdust and wood shavings. Pellets can be used for automatically charged stoves because they are flowable and well suited to automated feed systems. Another advantages of pellets are their high energy content, low moisture and high density which allow a significant reduce of the transport and storage costs. Due to their advantages pellet burning appliances offer an ease of operation which is similar to fossil-fuel-fired boilers.


Figure 1: Wood pellet combustion system [6]

Pellet boilers are automatic combustion systems, which range in size from a few kilowatts up to large megawatt size units. They allow a continuous combustion appliance operation with high quality fuels and a system output performance controlled by the rate of the fuel supply. The operation of a pellet boiler can be controlled relatively easily and the system maintenance is similar to fossil-fuelled boilers. Pellet-fired systems offer a high fuel conversion efficiency, which can achieve values of over 80 per cent. Even at part-load operation the combustion efficiency remains considerably high.

Woodchip appliances

Wood chips are mainly produced from round wood or by chipping wood residues containing bark by using specialised wood chippers. The chippers are designed to produce wood pieces with a uniform size, with a longest dimension of 20-50 mm, that work well in automated fuel feeding systems. The production of wood chips requires less effort than pellets and the handling of wood chips requires less effort than wood logs. Since the ease of maintenance and operation of wood chip furnaces is similar to fossil fuel boilers and therefore they can be the most attractive and energy efficient use of biomass in combustion appliances. Other advantages of wood chip combustion systems are the relatively high combustion efficiency, modern appliances achieve efficiencies of over 80% and a relatively low pollutant emissions.


Figure 2: Woodchip combustion appliance [7]

Wood chip boilers are fed by an automatic feed of chipped wood and their heat release rate is controlled by the use of the fuel feed rate. The size of wood chip boilers ranges from small domestic systems of few kilowatt up to power station boiler of 100 MW and more. Wood chip combustion appliances are most appropriate for medium scale and large scale bioenergy projects. Good quality wood chips with relatively low moisture contents (between 15 and 30%) are burned usually in smaller appliances with underfeed stokers. Large scale appliances, which can deal with varying fuel properties, use moved or stepped grate systems.

Combustion technologies for industrial systems

Combustion technologies for industrial systems are using fixed bed combustion (underfeed stoker and grate furnaces) as well as fluidised bed combustion (BFB and CFB furnaces).

Underfeed stoker

Underfeed stokers are an economical and technically mature biomass combustion technology which can be applied up to a power capacity of 6 MW. In this type of furnace, a screw conveyor feeds the fuel from below. Primary air is supplied through the grate, where the fuel drying, gasification and charcoal oxidation occur. The secondary air is introduced to the combustion gas mixture at the entrance of the secondary combustion chamber in order to ensure complete fuel oxidation [8].


Figure 3: Underfeed stoker for biomass combustion [9]

Underfeed stokers are suitable for combustion of woody biomass with a water content between 5 and 50%, however, the design of the combustion chamber has to be adapted to a certain fuel water content. The underfeed stoker burning appliances offer the advantage of efficient part load operation and relatively simple load control. The output performance of the combustion system can be controlled relatively easily, because the amount of fuel in the combustion chamber is relatively low [10].

Table 1: Underfeed stoker characteristics [11]

Advantages Disadvantages
low investment costs for plants < 6MWth low flexibility in regard to particle size
simple and good load control due to continuous fuel feeding and low fuel mass in the furnace sustainable only for biomass fuels with low ash content and high ash-melting point (wood fuels) (<50mm)
-low emissions at partial load operation due to good fuel dosing

Grate furnaces

Grate firing is one of the mainly used technologies for biomass combustion as it combines relatively high efficiency with reasonable investment and operating costs. Biomass grate furnaces can deal with varying fuel properties and thus represents a promising alternative for energy generation from biomass. In large-scale biomass combustion appliances separation of different stages of thermal decomposition of the fuel on the grate as well as staged air combustion is applied, where the combustion air supply in the grate zone is divided into sections according to the requirements of the individual steps of thermal decomposition of the fuel on the grate. This complex combustion air management allows smooth operation of grate furnaces at partial load down to the 25% of the nominal loadFehler! Verweisquelle konnte nicht gefunden werden.


Figure 4: Grate furnace for biomass combustion [12]

In modern biomass furnaces staged air combustion is also applied in order to simultaneously reduce both the emissions from incomplete combustion and NOX emissions. The combustion chamber is divided into primary and secondary zone. In the primary zone thermal decomposition of fuel on the grate occurs in fuel-rich conditions (λ<1), where the heat release from the fuel bed is determined by the amount of air fed to the grate zone. Due to the sub-stoichiometric conditions unburned fuel components leave the bed and are transported with the gas flow to the secondary combustion zone. The hot gases from the primary zone are oxidised in the secondary zone where the mixing of the combustible gases and secondary air should be as complete as possible. If good mixing is ascertained the emissions from incomplete combustion can be close to zero.

  Table 2: Grate furnace characteristics [11]

Advantages Disadvantages
low investment costs for plants < 20 MWth usually no mixing of wood fuels and herbaceous fuels possible (only special constructions can cope with such fuel mixtures)
low operating costs efficient NOX reduction requires special technologies (combination of primary and secondary measures)
low dust load in the flue gases high excess oxygen (5-8vol%) decreases efficiency
less sensitive to slagging than fluidised bed furnaces combustion conditions not as homogenous as in fluidised bed furnaces
low emission levels at partial load operation require a sophisticated process control

Bubbling fluidized bed (BFB) combustion

Bubbling fluidised bed (BFB) combustors are interesting for larger boiler capacities of over 20 MW. BFB furnaces are suitable for combustion of fuels with particle size of up to 100 mm and this furnace type allows achieving a complete fuel oxidation. The bed material consists of particles with the size of about 0.5-1 mm and is located at the bottom of the combustion chamber. The primary air is supplied from below over a distribution plate and fluidises the bed. The temperature of the fuel bed can be controlled by heat exchangers that are integrated in the fluidised bed. Nozzles located in the upper part of the combustion chamber introduce the secondary air in order to achieve complete combustion [8].


Figure 5: BFB furnace [13]

The advantage of BFB furnaces is the possibility to combust fuel with different particle sizes and moisture. Mixtures of different kinds of biomass fuels can be burned in BFB furnaces. Another advantage of BFB combustors is the possibility to operate with a relatively low fuel bed temperature (650-850°C) which allows to burn fuels with low ash melting temperature [10].

Table 3: BFB furnace characteristics [11]

Advantages Disadvantages
no moving parts in the hot combustion chamber high investment costs, interesting only for plants > 20MWth
NOX reduction by air staging works well high operating costs
high flexibility concerning moisture content and kind of biomass fuel used reduced flexibility with regard to particle size (< 80mm)
low excess oxygen (3-4 Vol%) raises efficiency and decreases flue gas flow utilisation of high alkali biomass fuels (e.g. straw) is critical due to possible bed agglomeration without special measures
high dust load in the flue gas
loss of bed material with the ash without special measures

Circulating fluidised bed combustion

In circulating fluidised bed (CFB) furnaces the bed particles are carried with the flue gases, separated and fed back into fluidised bed. The temperature of the fluidised bed is controlled by heat exchanger which are cooling the bed particles. Due to the better heat and mass transfer conditions in the fuel bed, optimal combustion reaction conditions can be achieved. CFB furnaces are applied to combust a wide variety of biomass fuels and this type of combustion appliances is frequently used to burn residues [8].


Figure 6: CFB furnace [10]

The CFB furnaces have, however, the disadvantage of larger size and higher investment cost. The fuel size has to be small (0.1-40 mm), which often causes higher investments in fuel pretreatment. Due to their high combustion efficiency and lower gas flow produced, CFB furnaces can be an interesting option for plants of more than 30 MW thermal output [11].

Table 4: CFB furnace characteristics [11]

Advantages Disadvantages
no moving parts in the combustion chamber high investment costs, interesting only for plants > 30MWth
NOX reduction by air staging works well high operating costs
high flexibility concerning moisture content and kinds of biomass fuels used low flexibility with regard to particle size (< 40mm)
homogenous combustion conditions in the furnace if several fuel injectors are used utilisation of high alkali biomass fuels (e.g. straw) is critical due to possible bed agglomeration
high specific heat transfer capacity due to high turbulence high dust load in the flue gas
use of additives easy loss of bed material with the ash without special measures
very low excess oxygen (1-2 vol%) raises efficiency and decreases flue gas flow high sensitivity concerning ash slagging

Direct combustion CHP systems

In the recent years rapid technological progress has been made in the research and development of biomass combustion equipment with respect to the combustion efficiency and reduction of the environmental impact of biomass burning appliances. If operated in a sustainable manner decentralised energy generation systems based on biomass combustion can contribute to an improvement of the quality of environment in the respective regions. The sustainability of energy generation from biomass can in particular be achieved by using natural biomass fuel sources in combination with newly developed control systems of modern biomass boilers [14]. The relatively low cost of fossil fuels and the relatively high investment cost of biomass combustion based plants are the main reasons why bioenergy does not contribute a higher proportion of the energy generated in the EU.

Among market available technologies the ORC-modules (Organic Rankine Cycle) and Stirling engines are the only commercialized technologies for heat and power production based on biomass combustion in decentralized cogeneration plants.

ORC plants

The most well-proven and commercial available technology for decentralized cogeneration based on biomass combustion is the Organic Rankine Cycle with its main advantages, which are excellent part-load operation and reduced operating costs. The principle of energy generation with the ORC technology is similar to the classical water-steam process, with the main difference that instead of water an organic fluid (silicone-oil) is used as the working medium. The utilization of silicone oil as the working fluid has the advantage that electricity can be generated at much lower temperature and pressure in comparison to the water-steam process.


Figure 7: ORC cogeneration plants based on biomass combustion.

The ORC plant is a closed system which is connected to a heat source. Thermal-oil is mainly used as a heat transport medium between the heat source and the evaporator of the cogeneration module. Thermal energy produced in a biomass combustion appliance is used to generate superheated organic steam in the evaporator. The steam flows to the turbine which is connected to a generator. The condensation of steam takes place in the condenser and the waste heat from the electricity production process is utilized as district heat. The electrical efficiency of ORC systems ranges between 6 and 20% and is linked with the maximum heat recovery and the thermal efficiency of the boiler [15].

Table 5: Technical specifications of ORC plants

ORC plant parameters
Typical performance range 300 kW to 10 MW
Energy output Hot water 90°C/60°C

Electricity (η el ca. 15%)

Fuel Wood-chips, pellets, sawdust

Stirling engines

Stirling engines are indirectly heated gas engines operating by cyclic gas compression and expansion, with air, helium or hydrogen as the working medium. Stirling engines enable to generate energy in a closed cycle with the advantage that various heat sources can be used to power the system. The engine can be driven by almost any kind of heat source with a flue gas temperature of approximately 1000 °C with low levels of particles and ashes. The typical electrical capacities of Stirling plants range between 1 kW and 100 kW and the efficiency of power production is in the range of 12-30 per cent [16]. In comparison to the internal combustion engine Stirling engine can be characterized by relatively low maintenance requirements, high electrical efficiency, good part load performance, low emission level, low vibration and low noise level [17].


Figure 8: Biomass fuelled Stirling CHP plant

The cost of Stirling plants is relatively high; equating to approx. 3300 – 3900€ per kW electric output which is higher than comparable ORC systems. Another drawback of Stirling engines is the relatively low electrical efficiency; when solid fuels are used as a fuel the power generation efficiency can be as low as 15% [18].

Table 6: Technical specifications of Stirling plants

Stirling CHP plant parameters
Typical performance range 1 kW to 100 kW
Energy output Hot water 90°C/60°C

Electricity (η el ca. 12-30%)

Fuel Wood-chips, pellets, solid biomass

Anaerobic digestion

Additional renewable technologies in the food sector

Biomass gasification
Biomass pyrolysis
Biofuel production

Case studies


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  2. M. Leskens, L.B.M. Van Kessel, O.H. Bosgra, Model predictive control as a tool for improving the process operation of MSW combustion plants, Waste Manage. 25 (2005) 788-798.
  3. T. Madhiyannon, P. Sathitruangsak, S. Soponronnarit, Combustion characteristics of rice-husk in a short-combustion-chamber fluidized-bed combustion (SFBC), Appl. Therm. Eng. 30 (2010) 347e353.
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  18. Alanne, K., & Saari, A. (2004). Sustainable small-scale CHP technologies for buildings: the basis for multi-perspective decision-making. Renewable and Sustainable Energy Reviews (8). , pp. 401-431.
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