About emissions from solid fuel boilers

It is known that any solid fuel boiler provides heat to a room by burning fuel, which produces carbon monoxide and other harmful substances.

Most modern boilers are equipped with an automatic fuel supply (including household appliances). In addition, they usually have a better control system for the combustion process compared to boilers with manual fuel supply. They usually require fuel of standard and consistent quality.

In Europe, solid biofuels based on plant biomass (mainly wood) are increasingly replacing fossil fuels (oil, coal, and gas) from the heat market every year. This is how measures to protect the environment by reducing greenhouse gas and dust emissions are being implemented in practice.

Emissions caused by incomplete combustion are mainly the result of insufficient mixing of combustion air and fuel in the combustion chamber, a general lack of available oxygen, too low a temperature, short residence time and too high a concentration.

As a result of incomplete combustion in boilers, the following components can be released into the atmosphere: carbon monoxide (CO), particulate matter (PM) and non-methane volatile organic compounds (NMVOCs), ammonia (), polycyclic aromatic hydrocarbons (PAHs), and polychlorinated dibenzoparadioxins and furans (PCBs/F).

Small amounts of ammonia can be released as a result of incomplete combustion of all nitrogen-containing solid fuels. This occurs in cases where the combustion temperature is very low (fireplaces, stoves, boilers of old design).

In most cases, emissions can be reduced through basic measures aimed at reducing incomplete combustion products and improving efficiency.

Total Suspended Particulate Matter - particulate matter in the flue gases resulting from the combustion of fuels (including solid mineral fuels and biomass) can be defined as carbon, smoke, soot, and chimney particulates.

They can be divided into three groups of fuel combustion products:

The first group is formed by the gaseous phase of combustion or pyrolysis due to incomplete combustion of fuel (products of incomplete combustion (PCC)):

• soot and organic carbon particles are formed during combustion and from gaseous feedstocks;
• CO and some mineral compounds in the form of catalytic compounds;
• resins/particles of heavy aromatic compounds due to incomplete combustion of coal/biomass, volatile matter removal/pyrolysis products (from the first stage of combustion) and secondary sulfur and nitrogen compounds.

Condensed heavy hydrocarbons (tar) are an important, and in some cases, the main source of total particulate emissions from small-scale solid fuel combustion appliances such as fireplaces, stoves, and older boilers.

The following groups (second and third) may contain ash particles or cenospheres, which are mainly formed from minerals in the fuel. They contain oxides and salts (S, Cl) of the metals Ca, Mg, Si, Fe, K, Na, P, heavy metals, and unburned carbon resulting from incomplete combustion of carbonaceous materials, black carbon or elemental carbon.

Particulate matter emissions from solid fuel boilers largely depend on the combustion conditions. Optimization of the solid fuel combustion process through the establishment of continuously adjustable conditions (automatic fuel supply, combustion air distribution) leads to a reduction in total suspended particulate matter emissions and a change in the distribution of particulate matter.

As can be seen, boilers can have a wide range of particulate matter emissions, and these emissions can be separated into filtered and condensed fractions. The proportions vary and the determination of particulate matter emissions is highly dependent on the measurement method.

Reducing emissions from the combustion process can be achieved by preventing the formation of such substances (primary measures) or by removing pollutants from the exhaust gases (secondary measures).

Initial measures have several common features:

• change of fuel composition and improvement of its quality; preparation and improvement of solid fuel quality, in particular, coal (in relation to S, Cl, ash content and fuel fractional composition); change of fuel granulometry by pressing - briquetting, pelletizing; preliminary cleaning - cleaning by washing; selection of particle size according to the needs of heating devices (furnaces, boilers) and control of its granulometry; partial replacement of coal with biomass (implementation of co-combustion technology, which reduces the amount of SO2, NOx), use of combustion modifier; catalytic and S-sorbent additives (limestone, dolomite), reduction and change of moisture content in the fuel, especially in the case of solid biomass fuel;
• replacement of coal with modern secondary solid fuels and biomass;
• optimization of combustion process control;
• managing the number of furnaces: replacing low-efficiency heaters with more modern ones and supervising their distribution through a mandatory certification system; supervising household and municipal heating systems;
• improving the design of furnaces; introducing advanced technologies in boiler design.

Additional measures to reduce emissions: for small combustion plants, secondary emission removal measures can be applied, especially for PM. Thus, emissions of PM-related pollutants such as heavy metals, PAHs and polychlorinated dibenzoparadioxins and furans (PCBs/F) can be significantly reduced by removing them together with particulate matter.

For particulate matter, the following options can be considered:

• Dust settling chambers: self-distribution is characterized by low collection efficiency and is ineffective for small particle fractions;
• cyclone separators - widely used, but have a relatively low collection efficiency for fine particles (<85%);
• for higher efficiency (94-99%), units with several cyclones (cyclone units) are used;
• electrostatic precipitators (with efficiencies of 99.5% - 99.9%) or fabric filters (with efficiencies of about 99.9%) can be used for larger facilities in the ≤50MW range, but are likely to be overkill for smaller facilities.

Emissions from burning different types of fuel

The table shows that wood fuel (primarily pellets and briquettes) has more advantages in terms of air pollution compared to fuel oil (especially coal), as it has almost minimal effect on greenhouse gas emissions, primarily CO₂.

Unfortunately, it is impossible to completely eliminate the formation of suspended particulate matter during the combustion of solid fuels. If the concentration of particulate matter can be significantly reduced at large boiler plants by improving dust collection units, cyclones, construction of high pipes, etc.for small boilers there are only two solutions :

Use of high-quality low-ash dry fuel

As for the more environmentally friendly type of fuel - pellets - the main quality criterion is the ENplus certificate. At the same time, ENplus is not just a standard that regulates the physical and chemical properties of pellets, but a whole system that controls quality along the entire chain from raw materials to the end user, which sets the most stringent requirements for the quality of wood pellets and divides them into 3 classes.

EN plus -A1 - This standard of pellets has the most stringent requirements. They can only be used for domestic purposes. The requirements for ash content are very strict - 0.5% for pellets from coniferous trees and 0.7% for pellets from deciduous trees.

EN plus -A2 - Grade plus - A2 can already be used not only in the private sector, but also in boiler houses. The ash content of these pellets can reach 1%, and the pellets can be obtained from mixed tree species, while the first grade pellets are obtained only from a certain species.

EN-B - Grade EN-B is a type of industrial or industrial pellets that can be used in thermal power plants. The requirements for them are less strict. But absolutely all pellets are subject to the rule that they should not be made from chemically treated wood.

For each grade, there are parameters such as bulk density, diameter and length, and calorific value.

High-quality pellets have a shiny surface without cracks, which indicates compliance with production technology. The color of wood pellets alone cannot serve as a quality criterion and can only be used to determine the raw materials used by the manufacturer. The color can also tell about storage and transportation conditions.

If you pour water over the pellets, after a while they should turn into a mushy mass. If this does not happen, it indicates the low quality of the raw materials and the use of external chemicals by the manufacturer. This method also makes it possible to determine the volume of heavy particles, which indicates the presence of dust and sand in the pellets.

To test the pellets for strength, a lignotester is used, in which the pellets are loaded to 70 mAtm and placed in a fast air stream. After that, the weight of the pellets is compared before and after the test. High-quality pellets lose no more than 2.3% in weight. In general, the harder the pellets, the better, as soft pellets crumble when overloaded into the hopper and fed into the burner area.

As a rule, pellets smell like the wood from which they were made or do not smell at all. On the other hand, pellets, when stored for a long time, absorb moisture from the surrounding air and, accordingly, odors.

When choosing firewood as a fuel, you need to remember that for optimal combustion and efficiency of your boiler, you need to use high-quality, well-dried firewood for at least six months in a well-ventilated room or under a canopy naturally. The moisture content of firewood should be more than 30%.

Ash content is one of the main indicators that determine the quality of coal. The less mineral impurities coal has, the less harmful substances are released into the atmosphere. For high-grade coal, the percentage of ash content is approximately 25%.

For the most efficient combustion of biofuels such as wood chips, it is advisable to use boilers with a chamotte furnace, which provide high-temperature combustion of fuel. Such are the ALTEP BIO biofuel boilers.

Introduction of new combustion technologies

The operation of any solid fuel boiler involves such processes as ignition, combustion and smoldering. A buffer tank - the ALTEP heat accumulator - will help minimize the smoldering process (and significantly reduce the amount of harmful emissions). Its use will provide:

• optimal combustion of solid fuels;
• increase the system's operating time on a single boot;
• increase the efficiency of the boiler.

The use of a torch burner will also help to ensure long-term fuel combustion. A dosed supply of high-quality fuel to the combustion zone will make the entire process of operating heating equipment more automated and preserve the ecological balance of the environment.

Boilers with automatic fuel supply have a wide range of capacities (both for domestic needs and industrial scale). According to the international standard GOST 33016-2014 (EN 303-5:2012), when ALTEP boilers operate in compliance with all the manufacturer's requirements for connection and operation, using a high-quality fuel (in particular, pellets), the value of permissible emissions does not exceed the norm.