Heat Production in Considering Boilers and their Influence on CO and NOx Emission Values

At present, there is a tendency to make greater use of hot water heat sources for the combustion of solid fuels, such as lump wood, coal, coke, instead of heat sources for the combustion of natural gas. This tendency is due to the high price of natural gas as well as the availability of cheaper solid fuel. In many cases, as part of saving on heating costs, respectively. as part of waste disposal, municipal waste of various compositions is also co-incinerated with solid fuel. This co-incineration entails increased emissions, such as CO (carbon monoxide), NOx (nitrogen oxides), TZL (particulate matter), PM10, HCl (hydrogen chloride), PCDD/Fs (polychlorinated dibenzodioxins and dibenzofurans), PCBs (polychlorinated biphenyls) and others which, with a relatively large number of heat sources for the combustion of solid fuel in municipalities, have a significant impact on the quality of the environment in the locality. Between the population and representatives of the state administration as in Žilina, Thus, the Moravian-Silesian Region also lacks awareness of how the environment is degraded due to the co-incineration of solid fuel and municipal waste. At present, there is no qualitative comparison of the effect of combustion of solid fuel and municipal fuel waste to the emission burden on the environment. There are no real measured emission factors for determining the production of the above emissions from the coincineration of solid fuels and a certain amount of municipal waste. The combustion of fossil fuels and the consequent occurrence of harmful emissions on the environment are therefore becoming a central theme of current heating technology.


Introduction
The cycle of each atmospheric component consists of emissions to air, dispersion, physical and chemical transformation in the atmosphere, and deposition on the surface. The balance of the basic components of the atmosphere has stabilized over tens of millions of years. As a result of anthropogenic activity, completely new substances have entered the atmosphere, which fundamentally affect it on a global as well as a regional scale.The increase in the , share of gasified municipalities and the gasification of heat production sources and their innovation also have a positive effect on improving the state of the air, especially in terms of a significant reduction in emissions of sulfur oxides, carbon and solids. In order to further reduce mainly gaseous emissions of heat sources burning natural gas, it is necessary to approach new combustion systems, new design principles of boiler units.In terms of construction, gas boilers are divided into the following: -classic (standard) boiler, -low temperature boiler, -closed gas boiler, -condensing boiler (new condensing boilers meet the strict economic and ecological criteria required within the EU for heat sources and are thus promising in the near future, when the installation of conventional heat sources will be banned in Slovakia. They are more economical in consumption compared to conventional modern boilers natural gas (up to 15% difference), which means significant savings in heating and hot water costs in the long run, and from an environmental point of view they burden the environment with significantly lower COx and NOx emissions.

Combustion of fuels
Combustion of fuel involves a number of physical and chemical processes in which the chemical reactions of the individual combustible components of the fuel with oxygen take place simultaneously at high temperature, the energy chemically bound in the fuel being converted into heat and combustion by-products (flue gases and ash). Oxidation of flammable components of fuel is most often carried out by oxygen from atmospheric air. The process of oxidizing flammable substances with oxygen, in which heat and light are released in exothermic reactions, is called combustion. Fossil fuels are and, as experience to date has shown, will remain a major source of energy for a long time to come. When burning fossil fuels, the latent, chemically bound energy in the fuel is converted into thermal energy, which usually has a high temperature potential. The working substances of the combustion process are: fuel (mostly of organic origin), oxidizer (mostly atmospheric air and combustion process products (gaseous and solids) [1]. Combustion is a combination of often very complicated exothermic reactions of fuel and oxidant components, taking place with intense heat release and a sharp rise in the temperature of the reacting mixture. With perfect combustion, there are no flammable gaseous and solid components of the fuel in the flue gas. The burden on the environment by consuming fresh air and generating the amount of flue gas in the production of heat by burning fossil flue gases is considerable. E.g. to burn 1 liter of light heating oil resp. 1 Nm 3 of natural gas requires 11 m 3 of fresh air, while at a flue gas temperature of around 200 °C about 20 m 3 of flue gas is produced. An essential component of flue gases is carbon dioxide (CO2), a gas that is currently receiving increased attention due to its contribution to the "greenhouse effect" while constantly increasing its concentration in the Earth's atmosphere and its high absorption capacity for infrared radiation. The amount of CO2 emissions in units of measure, e.g. (kg / kWh) or (kg / GJ) depends on the hydrogen to carbon ratio in the fuel. The higher the proportion of carbon in the energy carrier, the higher the proportion of CO2 emissions in the flue gas. Based on the composition of natural gas, an essential component of which is methane, is the combustion of natural gas with the lowest carbon dioxide emissions compared to other fossil fuels.

Composition of combustion products
Due to the fact that CO2 emissions are specific for a given fuel, their reduction is possible only by such physical methods that allow the reduction of primary thermal energy per unit of the required form of the most common thermal energy [3]. It is therefore necessary to use such boiler units in which, on the basis of their very working principle, there is a coherent reduction in specific fuel consumption (e.g. by modernizing heating systems by introducing condensing technology). Combustion of sulfur-containing fuels (coal, heating oils, some gases) also produces oxidation of sulfur dioxide (SO2), which is a major cause of 'acid rain' due to its combination with steam and the formation of aggressive acid. Reducing sulfur dioxide emissions is possible by reducing it in the flue gas and also by reducing the specific fuel consumption.
Due to the insufficient amount of combustion air, the non-formation of a homogeneous stoichiometric mixture or the insufficiently dimensioned combustion space, highly toxic carbon monoxide (CO) is also formed during combustion. Its reduction in flue gases is achieved especially in gas boilers relatively easily by increasing the excess combustion air (Fig. 3), increasing the combustion temperature or by increasing the geometric dimensions of the combustion chamber [4]. However, the reduction of CO concentrations is closely related to the reduction of another group of pollutants -nitrogen oxides. Nitrogen oxides (NO x) usually refer to the chemical bonds of nitrogen (N) and oxygen (O), with a mixture of nitric oxide (NO) and nitrogen dioxide (NO2) occurring in the flue gas.
In the combustion process, NO is formed almost exclusively, less than 10% by volume is still oxidized to NO2 in the boiler space [5]. The rest of the NO is oxidized to NO2 only in the atmosphere and here, with atmospheric humidity, it causes the formation of nitric acid. The mechanism of nitrogen oxide formation is strongly dependent on combustion conditions [2], e.i. temperature, concentration, reaction time and fuel quality. Nitric oxide is formed in three ways ( Fig. 1):  thermal method of NO formation from atmospheric nitrogen at high reaction zone temperature,  prompt method of NO formation from atmospheric nitrogen by reaction with hydrocarbons in a flame,  by reacting the nitrogen contained in the fuel with hydrocarbons in the fuel zone. Reduction of NOx concentration in natural gas combustion can be achieved by several measures [2], [4]:  by decreasing the combustion temperature by external cooling of the flame (using mainly the radiant component of heat transfer from the flame zone),  by reducing the temperature and partial pressure of oxygen by internal cooling of the flame (return of cooled flue gases),  shortening the residence time of the reactants in high temperature areas (shortening the flame front, adjusting the geometric parameters of the combustion chamber),  using catalysts.

Hot water considering boiler
A very effective way of reducing the ecological burden on the environment by the abovementioned emissions from hot water boilers is also the use of condensing technology in conjunction with suitable types of burners for natural gas combustion [6][7]. The physical principle of applying condensing technology to more efficient use of primary heat lies in the use of the energy content of natural gas in the form of combustion heat. Combustion heat (H0) expresses the total amount of heat released during perfect combustion, including the latent heat contained in water vapour and which is released during its condensation. Calorific value (HU) refers to the amount of heat that is released during perfect combustion when the resulting water remains in the form of steam [8]. The energy of water vapour is lost in conventional boilers because it escapes into the air through the chimney in the flue gas [9]. Combustion of one cubic meter of gas produces about 1.63 kg of water vapour.By condensing this amount of water vapour, approximately 4.1 MJ of heat can be obtained, which is sufficient to heat 10 litters of water from 5 °C to the boiling point. However, this energy is left unused in boilers of a classic design. It represents about 9.1% of the value of the heat of combustion of natural gas, which is the maximum amount of heat that can be obtained by its perfect combustion. 1 kg for solid and liquid fuels and 1 Nm -3 are chosen for the agreed unit amount for determining the calorific value and combustion heat. In principle, the higher the ratio between the heat of combustion (H0) and the calorific value (HU), the higher the utilization of condensing heat. This proportion is 6% for heating oil and 11% for natural gas, as shown in Tab. 1.
The advantage of natural gas over heating oil is the higher hydrogen content and thus the water vapour in the flue gas. Natural gas does not release any sulphur oxides (SO2, SO3) during combustion, which form aggressive acids with condensed water and is therefore a more environmentally friendly fuel, unlike liquid fuels [10]. At the same time, the dew point temperature is 10 K higher than the heating oil, which affects the use of energy released by condensation. Maximum use of sensible and latent heat released during the combustion of noble fuels is possible by changing the design of the combustion chamber. The innovation of the geometry of the combustion chamber shape and the change of the material of the heating surfaces of the boilers, where the flue gas paths are extended by creating return drafts in the additional heating surfaces, are also characterized by the character of modern conventional boilers [11]. The graded heat transfers in additional convection surfaces, which are multilayered, thermally well conductive, ensures that the surface temperature always remains above the dew point of the flue gas. By optimizing the multi-layer heating surfaces, the heat loss through the flue gases is reduced below 6% and the standard degree of utilization reaches 94%. The efficient use of the energy value of fuel depends mainly on the construction of the combustion chamber of the boiler, the shape and materials of the heating surfaces, the method of flue gas flow, as well as the temperature of the heat transfer medium in the heating system. By continuously lowering the heating water temperature in new boiler constructions, the value of the flue gas temperature will be affected, which reduces the chimney heat loss and increases the degree of utilization of the boiler.
In the combustion chamber of modern boilers, return chambers of various constructions are applied, the purpose of which is graded heat transfer and flue gas cooling. The flue gas temperature, which is measured at the end of the boiler extension or behind the last heating surface, is a decisive criterion for the categorization of new boilers and their use in heating systems.
Depending on the flue gas temperature and the use of their energy, the distribution of hot water boilers can then be extended as follows: -classic, heat losses by sensible heat of exhaust gases are over 10%. The latent heat of water vapour is unused and is dissipated into the air by the flue gases. The boilers operate with a constant return heating water temperature, which is maintained at a temperature above 60 °C. The high content of solid and gaseous harmful substances in the flue gas significantly burdens the environment. -low temperature, form an intermediate stage between conventional and condensing boiler. The combustion space of the boiler is divided into several chambers for a directed flow of flue gases [12]. The flue gases in the additional chambers remain for a longer time, which reduces their temperature below 160 °C. Unlike conventional boilers, low-temperature boilers are operated with a constantly decreasing return heating water temperature, which is maintained above the dew point temperature of the fuel. A reduction in the flue gas temperature below 160 °C has the effect of reducing the chimney heat loss below 6%, which results in an increase in the degree of utilization to 94%. The latent heat losses remain unchanged, which means that water vapour condensation does not occur. -condensing, eliminate latent heat loss. The total heat recovery of the condensing boiler at a return water temperature below 40 °C reaches up to 109%. The boilers are energy efficient and very environmentally friendly. The flue gas temperature is below the dew point temperature of the fuel, i.e. below 60 °C. Condensing heating respectively the heat exchanger surface is made of stainless steel and is usually placed vertically, so that the condensed water can flow freely in the same direction as the flue gas (downwards). This prevents the concentration of acids contained in the condensate from increasing. The resulting condensate washes the heating surfaces and keeps them clean. The vertically arranged stainless steel heating surfaces with a network of oppositely crossing overhangs have a high heat exchange efficiency for the condensation of water vapour [13]. Large heat exchange areas enable intensive heat transfer to the boiler water, and thus very high energy utilization.

Conclusion
Condensing technology uses a significant part of the heat that escapes in conventional heating systems in the form of sensible and latent heat in the flue gas through the chimney. Latent heat is transferred to the boiler water by suitable arrangement of the heat exchange surfaces and return water flow in the hot water boiler, which results in significantly lower consumption of primary chemically bound energy in the fuel and also lower environmental load by carbon dioxide emissions. Condensing boilers are designed so that they almost completely use the heat of combustion and thus the energy contained in the fuel. Combustion technology with consistent use of the heat content of noble fuels in modern boilers makes it possible to produce heat energy significantly more efficiently in the whole spectrum of operating conditions occurring during the heating season, as well as to reduce fuel consumption and emissions [14].In the average household, heating and heat supply account for 60 to 80% of energy consumption, hot water preparation or supply for about 30% and electrical appliances with gas appliances for only 10%. Of course, these values may vary from household to household, but the order is the same [15]. The constant rise in energy prices, whether fossil fuels or electricity, brings with it the effort of people to reduce energy consumption to a minimum in various ways, especially for heating. Expanded gasification in Slovakia has brought new possibilities for heating with natural gas, but currently interest in gasification is declining. Today, there are about 500 smaller municipalities without gas in Slovakia, mostly in mountainous areas, and due to current natural gas prices, the interest of local governments in expanding gas supply has also declined rapidly [16]. In most family houses, obsolete solid fuel boilers have been retained as an alternative to additional heating, resp. compensation for natural gas. Heating with solid fuels is currently on the rise again and these boilers are being used again in most households. Likewise, the current increasing production of municipal waste, and especially its flammable components, causes people to try to make large-scale energy recovery of municipal waste. In particular, when burning in such small solid fuel heat sources, in order to save on heating costs. In places where heat supply is solved by decentralized local heat sources, especially in municipalities, this method of waste management is very widespread. The incineration of waste generates a number of undesirable substances that adversely affect not only the environment, but especially human health. In waste incinerators, emission minimization facilities are included in the process due to the formation of these pollutants. The flue gas passes to a mechanical flue gas cleaning unit, which consists of, for example, an electrostatic precipitator and a fabric filter. The incineration of municipal waste also produces persistent substances, which are removed, for example, by adsorption, catalytic filtration. These technologies are classified as mechanical or chemical flue gas cleaning. The problem therefore arises when municipal and garden waste, including plastics, are mixed with solid fuels in a solid fuel boiler, where no method of filtration and elimination of the leakage of undesirable substances into the air is implemented. An ordinary family house in the village thus becomes an illegal waste incinerator. As a result, the environment is exposed to odors and harmful emissions from combustion. In general, it is only a small source of pollution. However, if it is taken into account that in each municipality, respectively. There are several dozen, respectively, inhabited localities. Hundreds of households burning solid fuel together with municipal waste, we receive a significant source of air pollution with a significant impact on the environment and human health. In the Žilina self-governing region, this fact represents a significant share in the quality of the environment. The aim of the study is to point out the given issue and to clarify the impact on the environment through experimental measurements in the co-incineration of solid fuel and components of municipal waste with significant energy potential. The experiment was focused on emission factors in the combustion of fossil fuels in combination with municipal waste in conventional boilers designed for the combustion of solid fuel, which are commonly used for heating in municipal houses. It was based on real conditions and methods of fuel combustion in simple boilers, which are the most widespread and most often used due to their price and combustion possibilities.