The reduction of emission production of small heat-sources for solid-based fuels by applying control mechanisms

INTRODUCTION: Production of emissions from small heat sources burning solid fuels in households in Slovakia is relatively high. At the present time, the operation of such devices are neither controlled nor regulated properly and e ﬀ ectively. OBJECTIVES: This work presents a research analysis of current monitoring and maintenance procedures and certiﬁed norms for all solid-fuel small-scale heating systems utilized in households. METHODS: The research compared data from over 13 countries in the European Union, evaluating information regarding certiﬁed emission limits, e ﬃ ciency levels, normalized monitoring procedures for both fuel standards and heating system performance. A proposition is therefore presented for a new monitoring and inspection guide for solid-fuel heating systems in Slovak Republic households, where regulations, such as Ecodesign or the EN 305-5, were integrated. RESULTS: Using a mathematical-statistical model, the implementation of the proposed regulations brings an emission saving of almost 140 tons per 10,000 inhabitants for both central and local heating systems. CONCLUSION: The study demonstrated substantial opportunity in decreasing operation cost, prolongation of product lifetime, and last but not least, the reduction of health risks of local citizens


Introduction
To attain thermal comfort in households, it is necessary to ensure a sufficient amount of thermal energy, that is commonly created by burning fuel: Solid (wood, coke, coal), liquid (oil) or gaseous (natural gas) fuel. During the chemical reaction, emissions are formed and released into the air. Based on the knowledge of the greenhouse effect, adverse effects are present affecting public health and brings forth operational risks. The European energy organizations have introduced standards and standardized procedures for the control of household heating appliances in aims of reducing emissions while extending operation lifetime.
Currently, there is a high number of outdated and unmaintained equipment in operation. This creates a high concentration of harmful substances in the environment and directly within a household as well.
According to a study carried out on a sample of Slovak households in 2018, the average age of solid fuel boilers in Slovakia is 12 years, and for fireplace stoves up to about 21 years old [17].

Emissions
All chemical reactions in which energy conversion takes place produces pollutants or emission substances that are harmful to health and air purity. The main pollutants are considered to be particulate matter (PM), sulfur dioxide (SO 2 ), nitrogen oxides (NO), carbon monoxide (CO), carbon dioxide (CO 2 ) and heavy metals [15]. The types of emissions that are released into the air are divided according to the Slovak Air Act no. 137/2010 para. 11 § 20 : 9 groups in total based on the chemical origin of the given substance. Particulate matter (PM) is the sum of all particulate matter and liquid particles in the air, namely dust, pollen, soot, smoke and liquid droplets. This is further divided into particle sizes up to 10 µ m (designated as "PM10") and up to 2.5 µm (designated as "PM2.5") [13] [8]. Heavy metals are defined as metals with a density greater than 4.5 g / cm 3 (Pb, As, Cr, Cu, Hg, Cd, Ni, Se, Zn, Sn, Mn) [10].
The harmfulness of individual substances for global warming can be described by the GWP -"Global Warning Potential" coefficient [8]. The GWP value expresses the equivalent amount of CO 2 released needed to achieve the same warming effect as when burning one kilogram of a given substance [1]. Emissions are measured with respect to a certain concentration of oxygen in the air, which allows the correct comparison of the values of emission substances under different conditions (most often to 12% O2).

Emission standards
The European standard STN EN 305-5 is focused on solid fuel boilers with a rated power output of up to 500 kW. This standard is in use in the Slovak republic. The standard defines five emission groups, according to which the efficiency and emission purity of any heating device is determined. "P n " denotes the normalized output in units of MBtu / h. Emission classes 1 to 2 will be banned from operation, effective from 1.1.2022 [12].

Emission control methods
Production of gaseous emissions (CO, NOx and OGC) is measured by a flue gas analyzer which is able to record the concentration of specific substances in units ppm (parts per million). These values have to be converted to mg/m 3 according to the equation 1: where Y represents a calculated emission production in mg/m 3 , X is the measured emission in ppm units, M represents the molecular weight of the components in g/mol, the value 22.41 represents the standard molar volume in dm 3 /mol and p represents the pressure in Pa. The normalized concentration of oxygen in the flue gas from a boiler O 2n is considered to be 10 %. Therefore, the measured concentration values are recalculated according to the formula 2 to meet homogeneity requirements.
where Y(10%O2) is the normalized emission concentration in mg/m 3 , Y represents the emission concentration, which is calculated by a previous equation (Eq. 1) in mg/m 3 , O 2n represents the normalized concentration of oxygen in the flue gas in % and O 2avg represents the mean value of the oxygen concentration in the flue gas in %. These measurement devices have to be regularly calibrated by authorized personnel.
The production of particulate matter may be measured via the optic method or gravimetric method. The latter consists of multiple sampling by the use of a complex sampling apparatus under isokinetic 2 EAI Endorsed Transactions on Energy Web 11 2021 -11 2021 | Volume 9 | Issue 37 | e12 conditions, following the gravimetric analysis itself. Isokinetic sampling is a sampling technique in which the velocity of gas in the nozzle of the sampling probe is the same as the velocity of gas in the flue-gas ducting [11]. The samples are collected on so-called flat filters.
For the measurement of particulate matter production, the analyzer Tecora ( Fig. 1) can be used alongside the three-stage separation impactor ISOSTACK. Such devices allow for automatic isokinetic measurements of low, medium and high concentrations of particulate matters complying to norms EN 13284-1 and STN ISO 9096. The use of the aforementioned impactor enables to determine the particle size distribution: Concentration above 10 µm, concentration between 2.5 µm to 10 µm, and below 2.5 µm (PM2.5). The regulation of isokinetic condition for flue gas sampling is done via modifying the flue gas velocity. The flue gas velocity is measured by the Pitot tube. These measurement devices have to be regularly calibrated by authorized personnel.

Procedures in Slovak republic
In Slovakia, Act no. 17/2007 Coll. presents procedures and intervals for regular inspection of boilers, heating systems and air conditioning systems (Table 4) [6]. These inspections must be carried out before the boiler room is put into operation, after each overhaul and reconstruction of the boiler. It must be also inspected when the type of fuel is changed, after the first year in operation, and a seasonal inspection is conducted at the beginning of each season.

Control procedures in Czech republic -"Semaphore/Stoplight" Methodology
In the Czech Republic, according to Act no. 172/2018 on air protection § 17 paragraph 1 entitled "Obligations of the stationary source operator", states that the boiler operator is obliged by law to burn solely fuels that meet the requirements for fuel quality set by the manufacturer and corresponding legislation party. The method called "Semaphore", in translation "Stoplight", is intended to indicate the combustion of waste or contaminated fuel (CF) in stationary boilers with a rated output of up to 300 kW via ash analysis. The method measures 7 standard indicators: Pb, Cu, Zn, Cl, Sb, Sn, Ti [5]. The result of the analysis of each indicator is evaluated with three possible scenarios (depicted using a stoplight): • Significantly exceeded value (A) -RED: The exceedance index of the given indicator is greater than 200% • Suspicious value (B) -ORANGE: The exceedance index of a given indicator is greater than 100% and less than or equal to 200%.
• Permissible value (C) -GREEN: The exceedance index of the given indicator must be less than or equal to 100%

Control procedures in Nordic countries (Finland, Sweden and Denmark)
In Denmark, they regulate emissions utilizing the standard STN EN 305-5 [12].

Fuel control
Decree no. 228/2014 Coll., introduced by the Slovak Ministry of Environment, sets requirements for the quality of fuels and their operational records. The decree sets limits on the content of sulfur, water and ash. For each type of fuel, it defines the procedures for the formation of bulk samples (STN SIO 5069-1 for brown coal), and the creation of samples for general analysis (STN 44 1314). Additionally, it drives the official form which records the amount of solid fuel sold [4] [14].

Exceptional awards from Germany
German energy agencies award prizes to boiler suppliers once in a while after meeting strict emission limits. For example, the "The Blue Angle" award is an exceptional award from the German Federal Environmental Agency intended for pellet stoves and boilers. Specific limits are shown in table 5 [12].

Proposal of a new methodology for the control of small heating devices in Slovakia
The integration of new Ecodesign requirements over the coming years will eliminate the operation of Class 1 and 2 boilers in Slovakia in early 2022. Basic operational control will consist of efficiency, particulate matter concentration, OGC, CO, CO2 and NOx measurements. The ash is sampled and a chemical analysis is made to determine combustion of unsuitable fuel. Additionally, it recommends measurements of smoke particles and its opacity [18].
The aim of this work is to design a theoretical model of heating in the future. This aim will be achieved by using previous knowledge about various methodologies of managing small heat devices abroad, in the Table 6. Distribution of fuel types between inhabitants of the mathematical model introduction of new equipment and decommissioning of older, already incompatible, equipment. It is also visioned to introduce stricter measures to control flue gases and emissions, which are a by-product of heating. The control of airborne substances and the reduction of the use of incorrect / unsuitable fuel will be the focus conditions for improving the ecological, environmental and technical aspects of heating in the near future.

Mathematical model of heating in Slovakia
In order to anticipate and evaluate the positive effects of the implementation of the proposed boiler control methodology in Slovakia, a mathematical heating model was created for a "town" with 10,000 inhabitants, which evaluates the savings in emissions and energy per year. It is assumed that 50% of the population lives in family houses (self-heating) and 50% in apartment buildings (central heating). Based on the professional experience of the authors, the distribution of fuels is assumed as presented in table 6.
The values of emissions per kWh of burnt wood were evaluated by a Slovak study [9]. The specific energies of fuel were chosen at 15 MJ/kg for wood and coal and 22.7 MJ/kg for gas [3]. According to a Slovak project carried out in 2018, a sample of 1549 households assessed the average wood consumption per year (8.7 tonnes/household), where energy consumption is based on 366 MJ/day/household.The apartment buildings in the model have 64 apartments, where the energy consumption is 309MWh per year, specifically 72.42 MJ / day / household [7].
It has been estimated that up to 56% of households use obsolete and unmaintained boilers, stoves and fireplaces, which produce a high percentage of emissions [17]. The mathematical model also assumes a new division of boilers in operation -class 3 would be used by 20% of the population, class 4 by 40% and class 5 by 40% of the population.

The results of analysis
Self heating in family houses recorded energy savings of 60.4% and 62.5% less emissions per year. Apartment 4 EAI Endorsed Transactions on Energy Web 11 2021 -11 2021 | Volume 9 | Issue 37 | e12 The reduction of emission production of small heat-sources for solid-based fuels by applying control mechanisms

Conclusion
At present, the situation in Slovakia is insufficient in terms of citizens' knowledge of the correct way of heating, cleaning boilers / other heating equipment and their correct maintenance. There are cases when residents do not have the necessary knowledge on how to properly burn a particular fuel, and this often results in a state where a very unsuitable fuel is burned, for which a particular boiler is not designed and damages the particular boiler and the air. Mandatory inspections of heating systems are ignored and, as a result, old boilers and stoves, which should have been taken out of service a long time ago, are often used for many years there on. The results, which result from the mathematical model, suggest an average improvement of 30% in air quality, which has a positive effect on both the exterior and interior of households, as the excretion of harmful substances during heating is significantly reduced. An important conclusion is more than 30% energy savings, which represents significant fuel savings. It can be assumed that the implementation of these incentives and the reduction in unsuitable fuels would result in a significant improvement in air quality and thus in the overall well-being of humans.