BioRuralHeating

Project background

The overall objective of the BioRuralHeating Project is to promote social enterprises to uptake biomass energy activities at the community level in Armenia, and share the required knowledge to enterprises’ demand.
The specific objectives are:
  • to build strategic partnerships between farmers, R&D centers, public and private sectors, and strengthen their capacities for achieving impacts at scale;
  • to raise the level of local public awareness and acceptance, promote the benefits of use of biomass for domestic heating;
  • to develop a set of decision-making tools for social enterprises to sustainable production and utilization of biomass in heating applications in rural areas
This should lead to the creation of operational biomass processing technology markets, which would ensure sustainability to Project interventions after its completion. New jobs will be created and new income sources will be ensured as a result of setting certain biofuel distribution channels and providing with advanced technologies at the local and regional levels.
Implementation period: 2015-2017
Project Manager: Dr. Aram Vardanyan
Project Target groups and Final Beneficiaries:
Target groups Farmers, school children, rural communities, social enterprises and CSOs
Final beneficiaries - Regional administrations and local communities that manage public buildings in the villages;
- Social enterprises that need cost-efficient and sustainable community solutions and business models;
- Local agricultural entrepreneurs (in many cases farmers);
- R&D centers that are engaged in research of various aspects of biomass processing and use;
- Producers of biomass briquettes, pellets, biomass-fired boilers and gasificators.

Project components

The Project strategy is presented by a logical framework approach. The essence of this approach is that outputs are clustered by outcomes, which together will achieve the project objective. It will follow a classic model consisting of four key interrelated components: i) information, knowledge and outreach, ii) strategic partnerships and alliances, iii) support, iv) market stimulation as shown in the figure below.
The following specific results are expected to be achieved by implementing project’s activities:
  1. Established partnerships between the public and private stakeholders involved in the value chain of biomass for domestic heating production and utilization.
  2. Developed and strengthened appropriate social enterprises and networks.
  3. Developed and tested effective set of decision-making support tools of social enterprises, associated business and reinvestment strategies and technological issues.
  4. Installed and demonstrated pilot-systems.
  5. Strengthened capacity and increased awareness of local stakeholders for bioenergy development and replication in Armenia

News

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Photo-Video

Video
Participation in EU Energy Days in Armenia 03-07 June, 2017
Study visit of beneficiaries, July-August 2017
Transfer of Biomass Equipment to Beneficiaries, July 06, 2017
Participation in Europe Day Information Fair on Northern Avenue (Yerevan), 27 May, 2017
Workshop in Lori Marz, February 2017
Transfer of Biomass Equipment to Beneficiaries, December 10, 2016
Study visit of beneficiaries, 10 December 2016
Seminars in Armavir region, 05 and 08 February, 2016

Work meeting with Ludovic Ciechanowski, Representative of EU Delegation to Armenia, 16 February, 2016

Visit to Moldova, Kishinev, Moldova Energy and Biomass Project, 16-21 February, 2016

Meetings with regional administrations and local communities in Tavush region, 04 March, 2016

Workshop in National Polytechnic University of Armenia, 23 March, 2016

On-line seminar with representatives of R&D institutions from Vanadzor, Gyumri and Goris, 24 March, 2016

“Renewable Energy Technologies and Energy Efficiency" educational seminarsin the schools of various villages of Aragatsotn region, May 2016

Study visits of schoolchildren from various villages of Aragatsotn region to Barva Innovation Center, May 2016

Visit to Georgia, Tbilisi, Biomass Production and Utilization in Georgia Project, 20-22June, 2016

Participation in Europe Day Information Fair on Northern Avenue(Yerevan), 05June 2016

Steering committee meeting, April 26 2016

Participation in Expo “Made in Armenia” 19-22 September, 2016

Participation in EU Energy Days in Armenia 26-29 September, 2016

Participation in International Conference “Prospects of agricultural waste recycling” (11 October, 2016) and “Energy, Ecology, Economics and Community” 01 November, 2016

Communication materials

- Analysis of Green Houses Heating by Biomass Pellets in RA (only in Armenian) - Biomass Biological Characteristics in RA (only in Armenian)
- Biomass Solid Standards - World Production of Pellets - Syngas
Structure of biomass sown areas in various region of Armenia


Crop production volumes in Armenia
Products thousand tons
Grain and leguminous crops 5 90.2
Potatoes 7 32.7
Vegetables 9 23.2
Melon crops 2 45.7
Fruit and berries 2 91.0
Grapes 2 61.3
Forest Lands in various region of Armenia, ha

SYUNIK MARZ

No Branch Total Area Forest Cover
1 Kapan 38459 35082
2 Syunik 16530 12845,1
3 Sisian 5420 2063,4
Total 60409 49990.5

LORI MARZ

No Branch Total Area Forest Cover
1 Gugark 16146 10496.9
2 Dsegh 15330 14505.2
3 Eghegnut 14082 11826.8
4 Lalvar 26837 24339.5
5 Jiliza 15292 13851.1
6 Stepanavan 6665 5674.9
7 Tashir 6860 5105.2
Total 101205 85799.6

TAVUSH MARZ

No Branch Total Area Forest Cover
1 Artsvaberd 42837 38664.2
2 Ijev 25512 20955.8
3 Noyemberyan 29254 27001.1
4 Sevkar 20484 18236.5
Total 118087 104857.6

KOTAYK MARZ

No Branch Total Area Forest Cover
1 Hrazdan 22618.42 15068
Total 22618.42 15068

VAYOTS DZOR MARZ

No Branch Total Area Forest Cover
1 Vayots Dzor 15050.96 7656.2
Total 15050.96 7656.2

GEGHARKUNIK MARZ

No Branch Total Area Forest Cover
1 Chambarak 9022 6547.2
Total 9022 6547.2

ARAGATSOTN MARZ

No Branch Total Area Forest Cover
1 Aragatsotn 12629 5215
Total 12629 5215

SHIRAK MARZ

No Branch Total Area Forest Cover
1 Gyumri 6021 2000
Total 6021 2000

What is biomass?

Biomass generally refers to the organic matter deriving from plants and that is generated through the photosynthesis. Biomass not only provides food but also construction materials, fibers, medicines and energy. In particular, biomass can be referred to as solar energy stored in the chemical bonds of the organic material.

Where does biomass come from?

Carbon dioxide (CO2) from the atmosphere and water absorbed by the plants roots are combined in the photosynthetic process to produce carbohydrates (or sugars) that form the biomass. The solar energy that drives photosynthesis is stored in the chemical bonds of the biomass structural components. During biomass combustion, oxygen from the atmosphere combines with the carbon in biomass to produce CO2 and water. The process is therefore cyclic because the carbon dioxide is then available to produce new biomass. This is also the reason why bio-energy is potentially considered as carbon-neutral, although some CO2 emissions occur due to the use of fossil fuels during the production and transport of biofuels.The figure below shows the global carbon reservoirs in gigatonnes of carbon (1GtC = 1012 kg) and the annual fluxes and accumulation rates in GtC/year, calculated over the period 1990 to 1999. The values shown are approximate and considerable uncertainties exist as to some of the flow values.
Representation of the global carbon cycle
carbon

Biomass resources

Biomass resources can be classified according to the supply sector
Supply sector Type Example
Forestry Dedicated forestry Short rotation plantations (e.g. willow, poplar, eucalyptus)
Forestry by-products Wood blocks, wood chips from thinnings
Agriculture Dry lignocellulosic energy crops Herbaceous crops (e.g. miscanthus, reed canarygrass, giant reed)
Oil, sugar and starch energy crops Oil seeds for methylesters (e.g. rape seed, sunflower)
Sugar crops for ethanol (e.g. sugar cane, sweet sorghum)
Starch crops for ethanol (e.g. maize, wheat)
Agricultural residues Straw, prunings from vineyards and fruit trees
Livestock waste Wet and dry manure
Industry Industrial residues Industrial waste wood, sawdust from sawmills
Fibrous vegetable waste from paper industries
Waste Dry lignocellulosic Residues from parks and gardens (e.g. prunings, grass)
Contaminated waste Demolition wood
Organic fraction of municipal solid waste
Biodegradable landfilled waste, landfill gas
Sewage sludge

Plant biomass composition

The chemical composition of plant biomass varies among species. Yet, in general terms, plants are made of approximately 25% lignin and 75% carbohydrates or sugars. The carbohydrate fraction consists of many sugar molecules linked together in long chains or polymers. Two categories are distinguished: cellulose and hemi-cellulose. The lignin fraction consists of non-sugar type molecules that act as a glue holding together the cellulose fibers.
Typical values for the composition of straw, softwoods and hardwoods
Cellulose Hemi-cellulose Lignin
Softwood 45 25 30
Hardwood 42 38 20
Straw stalks 40 45 15

The energy content of biomass

The calorific value of a fuel is usually expressed as Higher Heating Value (HHV) and/or Lower Heating Value (LHV). The difference is caused by the heat of evaporation of the water formed from the hydrogen in the material and the moisture. Note that the difference between the two heating values depends on the chemical composition of the fuel. The HHV correspond to the maximum potential energy released during complete oxidation of a unit of fuel. It includes the thermal energy recaptured by condensing and cooling all products of combustion. The LHV was created in the late 1800s when it became obvious that condensation of water vapour or sulfur oxide in smoke stacks lead to corrosion and destruction of exhaust systems. As it was technically impossible to condense flue gases of sulfur-rich coal, the heat below 150°C was considered of no practical use and therefore excluded from energy considerations. The most important property of biomass feedstocks with regard to combustion – and to the other thermo-chemical processes - is the moisture content, which influences the energy content of the fuel. The figure below shows the evolution of the lower heating value (LHV, in MJ/kg) of wood as a function of the moisture content.
Possible ranges in moisture content for selected biomass resources
Biomass resource Moisture content
Industrial fresh wood chips and sawdust 40-60 wt. % (wb)
Industrial dry wood chips and sawdust 10-20 wt. % (wb)
Fresh forest wood chips 40-60 wt. % (wb)
Chips from wood stored and air-dried several months 30-40 wt. % (wb)
Waste wood 10-30 wt. % (wb)
Dry straw 15 wt. % (wb)
Biomass resources include a wide variety of materials diverse in both physical and chemical properties. Depending on the application, these variations may be critical for the final performance of the system. In particular, some advanced applications require fairly narrow specifications for moisture, ash content, ash composition. Both the physical and chemical characteristics vary significantly within and between the different biomass raw materials. However, biomass feedstocks are more uniform for some of their properties compared with competing feedstocks such as coal or petroleum. For example, coals show gross heating value ranges from 20 to 30 GJ/tonne. However, nearly all kinds of biomass feedstocks destined for combustion fall in the range 15-19 GJ/tonne for their LHV. The values for most woody materials are 18-19 GJ/tonne, while for most agricultural residues, the heating values are in the region of 15-17 GJ/tonne.
Some typical characteristics of biomass fuels compared to oil and coal
Typical characteristics GJ/t toe/t kg/m³ GJ/m³ Volume oil equivalent (m³)
Fuel
Fuel oil 41.9 1.00 950 39.8 1.0
Coal 25.0 0.60 1000 25.0 1.6
Pellets 8% moist. 17.5 0.42 650 11.4 3.5
Pile wood (stacked, 50%) 9,5 0,23 600 5,7 7,0
Industrial softwood chips 50% moist. 9,5 0,23 320 3.0 13.1
Industrial softwood chips 20% moist. 15.2 0.36 210 3.2 12.5
Forest softwood chips 30% moist. 13.3 0.32 250 3.3 12.0
Forest hardwood chips 30% moist. 13.3 0.32 320 4.3 9.3
Straw chopped 15% moist. 14.5 0.35 60 0.9 45.9
Straw big bales 15% moist. 14.5 0.35 140 2.0 19.7

Bioenergy key drivers and advantages

Some bioenergy key drivers consist in its contribution to:
  • The reduction of energy dependency on energy imports and thus, the increased security of supply
  • The climate change mitigation (bioenergy use decrease net greenhouse gas emissions and some other noxious gas emissions compared to fossil fuels, thus contributing to fulfil the Kyoto commitment) and the fight against desertification
  • Stable employment opportunities in rural areas and among small and medium sized enterprises; this in turn fosters regional development, achieving greater social and economic cohesion at community level.
Other important advantages of bioenergy are as follows:
  • Widespread resources are available
  • Biomass resources show a considerable potential in the long term, if residues are properly valorised and dedicated energy crops are grown. Bioenergy makes valuable use of some wastes, avoiding their pollution and cost of disposal
  • Biomass has the capacity to penetrate every energy sector: heating, power and transport. Bio-fuels can be stored easily and bioenergy produced when needed
  • Bioenergy creates worldwide business opportunities for EU industries
  • Biofuels are generally bio-degradable and non toxic, which is important when accident occur.

Densification-related advantages

Some practical problems are associated with the use of biomass material (sawdust, wood chips or agricultural residues) as fuel. Those problems are mainly related to the high bulk volume, which results in high transportation costs and requires large storage capacities, and to the high moisture content which can result in biological degradation as well as in freezing and blocking the in-plant transportation systems. In addition, variations in moisture content make difficult an optimal plant operation and process control. All those problems may be overcome by densification, which consists in compressing the material to give it more uniform properties.
The main advantages of densified fuels, compared to non-densified ones are the following:
  • An increased bulk density (from 80-150 kg/m3 for straw or 200 kg/m3 for sawdust to 600-700 kg/m3 after densification), resulting in lower transportation costs, reduced storage volume and easier handling.
  • A lower moisture content (humidity <10%), favouring a long conservation and minor losses of product during the storage period.
  • An increased energy density and more homogeneous composition, resulting in better combustion control possibilities and thereby higher energy efficiency during combustion.
The major disadvantage is the relatively high-energy cost for the pelleting process, increasing the price of the end product.
Densified products can be found as briquettes or as pellets. The heating value, moisture content and chemical characteristics are about the same for both but the density and strength are somewhat higher for pellets. The major difference is the size (generally Ø 6 to 12 mm, with a length 4 to 5 times the Ø for pellets), making them easy to use in fully automatic operation, from household appliances to large-scale combined heat and power (CHP) plants.
Comparison between briquettes and pellets
Pellet Briquette
Appearance
Raw Material Dry and grinded wood or agricultural residues Dry and grinded wood or agricultural residues. Raw material can be more coarse than for pelleting, due to the larger dimensions of final product
Shape Cylindrical (generally Ø 6 to 12 mm, with a length 4 to 5 times the Ø). Cylindrical (generally Ø 80 to 90 mm) or parallelepiped (150*70*60 mm)
Struture Stable, hard, without dust Relatively friable, fragile
Bulk Density Min. 650 kg/m3 600 to 700 kg/m3
Aspect "Smooth" Mostly "rough"
Forest softwood chips 30% moist. 13.3 0.32
Transport Bulk, bags, big bags Unit, palet
Straw chopped 15% moist. 14.5 0.35
Handling Manual or automatic use Manual use
Pellet characteristics for domestic use
Heating value > 4,7 kWh (>17 MJ/kg)
Moisture content Max. 10%
Ash content Max. 0.5%
Dimension Diameter: 6 mm; Length: 25 mm

Gasification process

After a long lasting development, which dates back to the 18th century, the commercial implementation of biomass gasification is still problematic. Very few processes have yet proved economically viable, although the technology has progressed steadily.
Gasification is the conversion by partial oxidation (i.e. more oxidizing agent than for pyrolysis but less than for complete combustion) at elevated temperature of a carbonaceous feedstock such as biomass or coal into a gaseous energy carrier. Gasification takes place in two main stages. First, the biomass is partially burned to form producer gas and charcoal. In the second stage, the carbon dioxide and water produced in the first stage are chemically reduced by the charcoal, forming carbon monoxide and hydrogen. Gasification requires temperatures of around 800°C or more to minimize the residues of tars and high hydrocarbons in the product gas. This gas, commonly called "producer gas", contains hydrogen (18–20%), carbon monoxide (18–20%), carbon dioxide (8-10%) , methane (2-3%), trace amounts of higher hydrocarbons such as ethane and ethene, water, nitrogen (if air is used as the oxidising agent) and various contaminants such as small char particles, ash, tars and oils.
The partial oxidation can be carried out using air, oxygen, steam or a mixture of these. Air gasification produces a low heating value gas (4-7 MJ/Nm3 higher heating value) suitable for boiler, engine and turbine operation but not for pipeline transportation due to its low energy density. Oxygen gasification produces a medium heating value gas (10-18 MJ/Nm3 higher heating value) suitable for limited pipeline distribution and as synthesis gas for conversion, for example, to methanol and gasoline. Such a medium heating value gas can also be produced by pyrolytic or steam gasification. Gasification with air is the more widely used technology since there is not the cost or hazard of oxygen production and usage, nor the complexity and cost of multiple reactors. With air gasification, the cold gas efficiency, describing the heating value of the gas stream in relation to that of the biomass stream, is in the order of 55 to 85%, typically 70%.

Technology platforms

Three categories of gasifiers can be distinguished:
  • Fixed bed gasifiers
  • Fluidised bed gasifiers.
  • Entrained flow gasifiers.
The fixed bed gasifiers are mostly small scale and come in two types, either down-draft (<2 MW) or up-draft (<10 MW). They differ in the direction of gas flow through the biomass in the reactor. In the up-draft gasifiers the raw gas contains important fractions of tar (typically 10-20 g/m3) which need to be removed before using the gas, in particular for engine operation. The down-draft reactor enables the cracking of the high hydrocarbon fraction (resulting in tar content sometimes below 0.1 g/m3) but a drawback is the high gas temperature at the outlet.
Representation of fixed bed gasifiers
The fluidised bed gasifiers, either stationary, SFB, or circulating, CFB, are in the MW-range. The circulating variety, CFB, requires a size of more than 15 MW to be commercially viable. The product gas is characterized by low tar content (typically about 0.01 g/m3) and also sulphur and chloride may be absorbed in the bed material. Thus, fluidised bed gasifiers apparently reduce significantly the problems associated with the utilization of agricultural biomas
Representation of fluidised bed gasifiers
Entrained flow gasifiers operate at very high temperatures, 1200 to 2000°C and require biomass in form of very finely ground particles. Again there are a number of different types. Because of the high temperature, gas is entirely tar free. The removal and handling of the molten ashes is proven technology, however from practice it seems only affordable in very large equipment.

Flue gas contaminants and their problems

The flue gas contaminants

Contaminant Examples Problems Clean-up method
Particulates Ash, char, fluidised bed material Erosion Filtration, scrubbing
Alkali metals Sodium, potassium compounds Hot corrosion Cooling, adsorption, condensation, filtration
Fuel-bound nitrogen Mainly ammonia and HCN NOx formation Scrubbing, SCR
Tars Refractive aromatics Clogs filters; Difficult to burn; Deposit internally Tar cracking; Tar removal
Industrial softwood chips 50% moist. 9,5 0,23 320
Sulphur, chlorine Hcl, H2S Corrosion; Emission Lime or dolomite, scrubbing, Absorption

Tar removal

The efficient removal of tar still remains the main technical barrier for the successful commercialisation of biomass gasification technologies. Different approaches can be considered for reducing the tar content of the product gas. A rough distinction can be made between physical, thermal and catalytic procedures.
The experience of one hundred and fifty years in cleaning tars from biomass gas is not convincing. Physical separation methods are unreliable and produce primarily waste streams that are environmentally unsafe. Entrained flow gasifiers produce a tar free gas. However, those systems are only feasible at very large scale. Those large scales are compatible with chemical synthesis processes, but less with biomass based power generation. Entrained flow gasifiers are proven technology for gas, oil and coal, but they are not used for direct solid biomass injection. With respect to tar free operation downdraft gasifiers are load inflexible and difficult to scale up. Catalytic systems are being demonstrated. However, demonstration systems experience technical problems and show large construction time overruns.

Firing in boilers or heat applications

Firing the raw gas in boilers or heat applications such as kilns after removal of dust and particulates is the simplest application since the gas is kept hot and the tar problem is avoided. This market is one where all types of gasifiers can compete. For these applications, a low tar content is not essential if the wall temperature of the gas pipe system can be maintained above the level where tars condense.

Devices overview

The burning of wood and other solid biomass is the oldest energy technology used by man. Combustion is a well-established commercial technology with applications in most industrialised and developing countries, and development is concentrated on resolving environmental problems, improving the overall performance with multi-fuel operation and increasing the efficiency of the power and -where ever possible- the heat cycles.
The devices used for direct combustion of solid biomass fuels range from small domestic stoves (1 to 10 kW) to the largest boilers used in power and CHP plants (>5 MW). Intermediate devices cover small boilers (10 to 50 kW) used in single family houses heating, medium-sized boilers (50 to 150 kW) used for multi-family house or building heating and large boilers (150 to over 1 MW) used for district heating. Co-firing in fossil fired power stations enables the advantages of large size plants (>100 MWe) that are not applicable for dedicated biomass combustion due to limited local biomass availability.
Most frequently used furnaces for biomass combustion
Application Type Typical size range Fuels Ash Water content
Manual Wood stoves 2 kW – 10 kW dry wood logs < 2% 5%-20%
Log wood boilers 5 kW – 50 kW log wood, sticky wood residues < 2% 5%-30%
Pellets Pellet stoves and boilers 2 kW – 25 kW wood pellets < 2% 8%-10%
Automatic Unterstoker furnaces 20 kW – 2.5 MW wood chips, wood residues < 2% 5%-50%
Pre oven with grate 20 kW – 1.5 MW dry wood (residues) < 5% 5%-35%
Understoker with rotating grate 2 MW – 5 MW wood chips, high water content < 50% 40%-65%
Cigar burner 3 MW – 5 MW straw bales < 5% 20%
Whole bale furnaces 3 MW – 5 MW whole bales < 5% 20%
Straw furnaces 100 kW – 5 MW straw bales with bale cutter < 5% 20%
Stationary fluidised bed 5 MW – 15 MW various biomass, d < 10 mm < 50% 5%-60%
Circulating fluidised bed 15 MW – 100 MW various biomass, d < 10 mm < 50% 5%-60%
Dust combustor, entrained flow 5 MW – 10 MW various biomass, d < 5 mm < 5% 20%
Co-firing Stationary fluidised bed total 50 MW – 150 MW various biomass, d < 10 mm < 50% 5%-60%
Circulating fluidised bed total 100 – 300 MW various biomass, d < 10 mm < 50% 5%-60%
Cigar burner Dust straw 5 MW – 20 MW straw bales < 5% 20%
Combustor in coal boilers total 100 MW – 1 GW various biomass, d < 2 – 5 mm < 5% 20%
To achieve complete burnout and high efficiencies in small-scale combustion, downdraft boilers with inverse flow have been introduced which apply the two-stage combustion principle. An operation at very low load should be avoided as it can lead to high emissions. Hence it is recommended to couple log wood boilers to a heat storage tank. Since wood pellets are well suited for automatic heating at small heat outputs, as needed for nowadays buildings, pellet furnaces are an interesting application with growing propagation. Thanks to the well-defined fuel at low water content, pellet furnaces can easily achieve high combustion quality. They are applied both as stoves and as boilers.
Understoker furnaces are mostly used for wood chips and similar fuel with relatively low ash content, while grate furnaces can also be applied for high ash and water content. Special types of furnaces have been developed for straw that has very low density and is usually stored in bales. Beside conventional grate furnaces operated with whole bales, cigar burners and other specific furnaces are in operation. Stationary or bubbling fluidised bed (SFB) as well as circulating fluidised bed (CFB) boilers are applied for large-scale applications and often used for waste wood or mixtures of wood and industrial wastes e.g. from the pulp and paper industry. In fluidised bed boilers, nearly homogeneous conditions of temperature and concentrations can be ascertained thus enabling high burnout quality at low excess air. The choice of different bed materials in CFB offers additional opportunities of catalytic effects. Further, the option of heat removal from the bed allows controlling the combustion temperature and hence enables an operation at low excess air without excessive ash sintering. Since similar conditions for nitrogen conversion as by air and fuel staging are attained, relatively low NOX emissions are achieved.

Pellets appliances

Small-scale pellet combustion for heating applications has made great inroads in markets in countries such as Sweden, Austria, and Germany and to a lesser extent in Finland and France. Technology development has led to the application of strongly improved heating systems, which are for example automated and have catalytic gas cleaning equipment. Such systems reduce significantly the emissions from fireplaces and older systems while at the same time they improve significantly the efficiency.

Pellet stoves

Modern pellet stoves are nowadays efficient home heating appliances. While a conventional fireplace is less than 10% efficient at delivering heat to a home, an average modern pellet stove achieves 80%-90% efficiency.
Modern pellet stoves: aesthetic design and profile

Pellet boilers

Today, many of the small boilers available on the market use two stage combustion systems equipped with ceramic refractory lining and are combined with a suitable hot water tank for heat recovery. In this configuration, the boiler can then be operated at optimal load with sufficient air and high process temperatures independently from the heating demand. This leads to relatively low emissions of all products of incomplete combustion. With this, over the last ten years, boilers thermal efficiencies have improved from an average of 60 to 80-90%, while the emissions of VOC and tars have been reduced 100 fold. For a single family house, a pellet boiler of 8 kW will burn 3,200 kg of pellets to cover the heat needs over the year. This amount requires a storage space of approximately 5 cubic meters. Bigger boilers are used in district heating systems.

Pellets burners

One common low-cost solution when changing from heating oil to pellets in small houses is retrofitting the old oil-fired furnace with a new burner designed for pellets. Several pellet burners convenient for substitution for oil burners are already available on the market. These are relatively simple but functional devices that in general give lower emissions than the best firewood boilers.

Operational problems in biomass combustion

A high combustion quality, in terms of maximal combustion of the burning gases, is very important for a low emission level. It mainly depends on the combustion chamber temperature, the turbulence of the burning gases, residence time and the oxygen excess. These parameters are governed by a series of technical details such as:
  • combustion technology (e.g. combustion chamber design, process control technology)
  • settings of the combustion (e.g. primary and secondary air ratio, distribution of the air nozzles) load condition (full- or part-load)
  • fuel characteristics (shape, size distribution, moisture content, ash content, ash melting behaviour).
Biomass has a number of characteristics that makes it more difficult to handle and combust than fossil fuels. The low energy density is the main problem in handling and transport of the biomass, while the difficulties in using biomass as fuel relates to its content of inorganic constituents. Some types of biomass used contain significant amounts of chlorine, sulfur and potassium. The salts, KCl and K2SO4, are quite volatile, and the release of these components may lead to heavy deposition on heat transfer surfaces, resulting in reduced heat transfer and enhanced corrosion rates. Severe deposits may interfere with operation and cause unscheduled shut downs. The release of alkali metals, chlorine and sulfur to the gas-phase may also lead to generation of significant amounts of aerosols (sub-micron particles) along with relatively high emissions of HCl and SO2.
The nature and severity of the operational problems related to biomass depend on the choice of combustion technique. In grate-fired units deposition and corrosion problems are the major concern. In fluidized bed combustion the alkali metals in the biomass may facilitate agglomeration of the bed material, causing serious problems for using this technology for herbaceous based biofuels. Fluidized bed combustors are frequently used for biomass (e.g. wood and waste material), circulating FBC are the preferred choice in larger units. Application of biomass in existing boilers with suspension- firing is considered an attractive alternative to burning biomass in grate-fired boilers. However, also for this technology the considerable chlorine and potassium content in some types of biomass (e.g. straw) may cause problems due to deposit formation, corrosion, and deactivation of catalysts for NO removal (SCR).
Currently wood based bio-fuels are the only biomasses that can be co-fired with natural gas; the problems of deposition and corrosion prevent the use of herbaceous biomass. However, significant efforts are aimed at co-firing of herbaceous biomass together with coal on existing pulverized coal burners. For some problematic fuels, esp. straw a separate auxiliary boiler may be required. In addition to the concerns about to deposit formation, corrosion, and SCR catalyst deactivation, the addition of biomass in these coal units may impede the utilization of fly ash for cement production. In order to minimize these problems, various fuel pretreatment processes have been considered, including washing the straw with hot water or using a combination of pyrolysis and char treatment.