Disposal of Wastewater from Mazout-Fired Boiler Plants by Burning Water–Mazout Emulsions

Abstract

Liquid fuel can be an alternative to solid fuel in non-gasified locations. The mazout for municipal-energy boiler plants is much cheaper than fuel oil. However, this relates to the problem of wastewater disposal from such a boiler plant. A proprietary system for the disposal of bottom water and other sewage-containing mazout has been developed. The solution depends on the proper preparation and burning of a water–mazout emulsion (WME). The emulsion is prepared in a specially designed and developed dosing ejector. By burning the emulsion using the developed system, an efficiency increase of 2–3% was achieved for boilers operating on the mazout fuel. Such a result was achieved due to a reduction in the excess air factor. Moreover, emissions of nitrogen oxides and carbon monoxide were reduced by 30–35% and 70–80%, respectively. In addition due to cleaning the heat-exchange surfaces obtained when working on the WME, the amount of time for productive and more efficient use of the equipment has increased.

1. Introduction

In many European countries, the use of solid fuel within cities and towns is already prohibited. In countries where the current energy economy is based on stone coal, it is only a matter of time. For example, Poland, where stone and lignite coal in municipal energy constituted 71% of fuels used in heat sources in 2019 [1], has announced that it would follow the European Green Deal, which aims at achieving climate neutrality by 2050 [2]. Nuclear energy can be an effective solution for decarbonization and increasing energy security [3]. However, the transition from solid fuel to nuclear energy is not a quick process as it requires large investments and public consultation.

Due to the situation in Poland, the most natural method for a fuel change is to switch to natural gas. A similar trend is characteristic of most European countries. According to the forecast, the consumption of natural gas in the Union will grow [4]. Therefore, methods of its effective and ecological use are under current investigation. In the literature [5,6,7], the authors presented the results of their investigations on the reduction in nitrogen oxide emissions during the combustion of natural gas in industrial and heating boilers. In [8], a method of fuel combustion with a high excess of air was proposed as an effective method for increasing the efficiency and environmental-friendliness of fuel combustion. This method also allows one to reduce the size of pre-furnaces used in industrial boilers. Articles [9,10] present issues related to the improvement of environmental-performance and combustion-efficiency indicators of gaseous fuel. The EU intends to use gas as a transition fuel in the switch from solid fuels [11,12,13]. An additional advantage of such a solution is the continuous diversification of the sources and directions of natural gas supplies. These radical changes in the structure of the fuel balance in Poland [4] have been presented in Figure 1.

In the locations that use the supplied gas, the problem may be the irregularity of supplies. In such cases, it is possible to burn gas and mazout together. In studies [14,15,16,17,18], the authors discuss in detail the technology and advantages of joint combustion of gaseous fuel and mazout. The problem is that not all countries and not all of their localities are supplied with gas. Therefore, in many cases, the only technically and economically viable alternative to solid fuel is to use liquid fuel. In this case, the most economically attractive fuel is mazout (compared to fuel oil) [19,20,21]. For many European countries, this fuel and methods of its efficient and ecological combustion are not known on a large scale.

In this situation, it is worth taking advantage of the experience of countries with many years of tradition in the use of mazout [22,23]. Thus, the issue of optimizing the combustion of mazout fuels is still within the scope of intense investigations [19,24,25,26,27]. Achievements of countries with great experience in the use of mazout show that, even with the boiler room equipment in good technical conditions, a significant problem is the generated water sewage. This form of sewage contains mazout and, by law, cannot be discharged into the sewage system. Water accompanies crude oil from the beginning of its extraction, during its processing and transport to its use as fuel. In larger municipal boiler plants, the basic fuel is fuel mazout obtained during the primary processing of crude oil (once-run mazout) or during deep cracking. Deeper processing of the raw material changes a number of physicochemical properties of the mazout, which is the final product. As a result of these changes, as well as the processes taking place during the storage and transport of mazout, the user receives fuel with increased moisture content.

Mazout consists of high-molecular-weight hydrocarbons and has a calorific value of about 41.5–45.0 MJ/kg [28]. The refinery usually produces mazout with a moisture content of up to 1.5% (the content of up to 5% of water in the mazout does not cause negative effects during combustion) [29]. When transported from a refinery, this humidity can increase up to 3–5%. The critical moment is the stage of heating the mazout with “hot” steam in order to allow it to be poured from transport tanks to road tankers. At this stage, the water content may be up to 10% or more. In this state, the fuel travels to the mazout tanks in the boiler plant. In reserve tanks in which mazout is stored for a long time, the so-called bottom waters are created (in the form of layers or lenses). The amount of this water increases during operation, sometimes reaching even 30%, reducing the usable capacity of the reservoir. From these tanks, the mazout is next taken to the working tanks. The jumping of the water lenses into the boiler room’s fuel track causes flame pulsation and incomplete combustion, which may cause the flame to break and, consequently, even stop the boiler’s operation [30]. However, this results in the production of a large amount of sewage that contains mazout in its composition. Additionally, the amount of such sewage increases during the periodic cleaning of equipment (tanks, filters, and heaters).

The second important source of the formation of sewage containing mazout is the mazout plant. These are sets of devices for heating and pressing the prepared mazout into the boilers. Mazout heaters work by saturated steam coming from the boiler room supplied with mazout. According to current regulations, the condensate leaving the heaters may not be returned to the boiler water and steam line. Thus, together with the bottom waters, such condensate is collected in special “wells”. Due to the fact that such sewage cannot be discharged to the municipal sewage system or to natural water reservoirs, each mazout boiler room is equipped with its own treatment plants.

Paradoxically, technological sewage produced in this way may contain at least 10–15% organic substances and during routine cleaning of equipment, up to 60–70%.

The authors believe that the comprehensive solution of the above problems is the disposal of sewage containing mazout by specially preparing and then burning it in boilers operating on a water–mazout emulsion (WME). This would not only reduce the amount of discharged technological sewage to zero but also allow the full use of the heat of combusting organic substances contained in the sewage.

It should be stated that the combustion of water–mazout emulsions is currently an implemented (used) technical solution [19,25,31]. It allows a significant improvement in the efficiency and ecological use of mazout, which is generally considered to be an environmentally aggressive fuel. The results confirming the appropriateness of this solution were described in detail in [32,33,34,35,36]. The core of this new unique solution is the use of the WME-combustion method for the disposal of very environmentally troublesome wastewater from mazout boiler plants.

2. Materials and Methods

2.1. Experimental Objects

The investigations were performed in a “Szuwałkowska” boiler plant belonging to GUPTEK Saint-Petersburg (State Unitary Oil and Energy Complex). The location of the boiler plant did not allow it to be connected to the gas network. In the original version, the boilers in the boiler plant were designed to run on solid fuel and then were modified to run on mazout fuel. The boiler plant is intended for heating and occupies a detached building. There are two DKVR 2.5-14 steam boilers in the boiler plant, equipped with two GMG-1.5 gas and mazout burners located symmetrically at the front of the boiler. The nominal steam capacity is 2.5 t/h. The permissible steam overpressure due to the technical condition of the boilers is 8 bar. The theoretical gross boiler efficiency is 90.4%.

Each boiler has an individual D-8 exhaust fan with a capacity of 8000 m³/h and a pressure of 1000 Pa with a 4 kW motor. The blower system is common for both boilers and consists of two parallel WD-8 fans, with a capacity of 10,000 m³/h each, a pressure of 900 Pa, and an engine power of 4 kW.

The boilers produce saturated steam, which is then used in the exchangers. The exchangers prepare the network water for the heat network supplying nearby housing estates and utility buildings. The mazout economy consists of four reserve tanks and two working tanks. The mazout boiler room is equipped with appropriate equipment for heating, filtering, and pumping mazout to the boilers.

2.2. Technical and Technological Solution

The authors have experience in the preparation and combustion of water–fuel emulsions [30,35,36,37]. As a rule, an emulsion with a water-dispersed phase size of 1 to 70 μm is prepared for fuel purposes. At an optimal water content of 8 to 12% [38], the first signs of emulsion delamination were observed after 25–30 h. During this time, the entire fuel content of the reserve tanks was replaced. In addition to the disposal of water from mazout management, EWM combustion has a positive effect on the operation of boilers, increasing their efficiency.

This process occurs due to the phenomenon of micro-explosions of water droplets contained in a drop of fuel emulsion. There is an additional mixing of the fuel with the air. The dissociation of water vapor liberated as a result of micro-explosions results in an increase in the content of active radicals in the combustion zone (O⁻, H⁺, and OH⁻). As a result of the above phenomena, the coefficient of excess air in the EWM combustion can be reduced, even to values characteristic for gaseous fuel, and amount to 1.05 ÷ 1.07 [30]. Thus, the ecological indicators of the operation of mazout boilers were also significantly improved. Based on the presented assumptions, a proprietary system for the disposal of bottom water and other sewage containing mazout was developed. The system prepares and then burns EWM [30,35,36,37]. The technological scheme of the developed and implemented sewage-treatment system is shown in Figure 2.

Sewage-containing mazout is collected in a level-regulated tank (1). This reservoir can be omitted when required. The emulsion is prepared in a specially prepared and designed dosing ejector (2). The principle of operation of this device, allowing for an adjustable level of sewage, ensures constant water content at any rate of flow of mazout. The mazout is supplied to the system from working tanks through filters (3) after being heated in mazout heaters (4). The ejector performs the first stage of coarse-water dispersion. The further grinding of water droplets takes place in a gear mazout pump (5). Final dispersion is achieved with the use of a self-constructed emulsifier (6), which is shown in Figure 3.

The operation of the emulsifier is based on the effect of a water hammer during multiple openings and closings of opposite holes (1a) of the emulsifier in a thin gap between the rotor (2a) and the stator (3a). The rotation of the rotor is ensured by a turbine (4a) driven by a stream of pressed mazout. The degree of water dispersion obtained in this manner is 8–20 μm and the emulsion’s stability is not less than 40 h. The emulsion prepared in this manner is gathered in the tank (7). Maintaining the required temperature of the emulsion is ensured by the emulsion heater (8). The emulsion from the tank is pumped to the main boiler house by a gear pump (9). Finally, the emulsion from the boiler-plant circulation line returns to the tank (7).

All measurements and chemical analyses were carried out in accordance with the methodology of the regime and adjustment tests of the State Energy Company GUPTEK in Saint-Petersburg [39]. The composition and characteristics of mazout fuel and water–mazout emulsion, as well as the water content in the emulsion, were determined on the basis of samples taken in a certified laboratory [28,38].

The chemical composition of combustion products (oxygen, CO₂, CO, SO₂, and nitrogen oxides), their temperature along the boiler gas line, and the excess air coefficient were determined with the use of an Optima 7 MRU flue gas analyzer. The concentration of carcinogenic substances in the exhaust gas was determined in a certified laboratory. The results of the measurements obtained during the tests were statistically processed in order to eliminate the erroneous results. Assuming the normal distribution and confidence interval (p = 0.95), the measurement results obtained were the arithmetic mean with the uncertainty interval maintained.

First, extensive investigations were performed to determine the optimal water content in the emulsion. The research confirmed the previously obtained results [37], which indicated that the optimal emulsion humidity is 10–15%. A further clarification of the recommended water content was obtained in the optimization process by maximizing the boiler’s efficiency and minimizing the emission of nitrogen oxides and products of chemical unburning [39,40,41,42,43]. In this way, an optimal WME moisture content of 11–13% was achieved. The dimensions and characteristics of the dosing ejector (see Figure 4) were calculated based on the appropriate WME composition.

Due to natural reasons (fluctuations in the moisture content of the mazout itself), the actual WME moisture content fluctuates. Moreover, the WME humidity was influenced by the unpredictable content of organic substances in the sewage. As a result, during the investigation, the moisture content in WME for boiler No. 1 was 11–13%, and the calorific value was 34.75 MJ/kg. In turn, for boiler No. 2, the moisture content in the WME was 11–12% and the calorific value was 34.96 MJ/kg.

3. Results and Discussion

As part of the long-term research and tests of boilers, a large amount of experimental data has been obtained. The most characteristic results (indicators, parameters, and values) were chosen. The results obtained prove the effectiveness and efficiency of the proposed and developed method of sewage disposal. The results of the selected series of tests for boiler No. 1 for the combustion of the water–mazout emulsion utilizing the boiler-plant wastewater according to the proposed method are presented in

Table 1. Fuel parameters.

No	Parameter	Unit	Combustion of Mazout	Combustion of Water–Mazout Emulsion (WME)
				1	2	3	4
1.	The number of working burners	pcs.	2	2
2.	Fuel characteristics	-	low-sulfur mazout Wp = 5%	Water–mazout emulsion Wp = 11–13%
3.	Fuel calorific value	MJ/kg	38.10	34.75
4.	Density	kg/m3	0.95	0.955
5.	Temperature	°C	100–110	100–110
6.	Overpressure before injectors	bar	2.0	1.0	2.0	3.0	3.5
7.	Actual fuel consumption	kg/h	152	109	156	191	206

,

Table 2. Steam and water parameters.

No	Parameter	Unit	Combustion of Mazout	Combustion of Water–Mazout Emulsion (WME)
				1	2	3	4
1.	Overpressure in the upper-boiler drum	bar	7.0	7.0
2.	Saturated-steam temperature	°C	169.6	169.6
3.	Enthalpy of saturated steam	MJ/kg	2.76	2.76
4.	Steam capacity	t/h	1.98	1.29	1.94	2.34	2.53
5.	Feed-water temperature	°C	102	102
6.	Thermal capacity of the boiler	MW	1.21	0.79	1.19	1.38	1.46
7.	Thermal capacity of the boiler unit	MW	1.29	0.84	1.27	1.52	1.63

,

Table 3. Pressure parameters of the air and flue gas duct.

No	Parameter	Unit	Combustion of Mazout	Combustion of Water–Mazout Emulsion (WME)
				1	2	3	4
1.	Air pressure before the burners:
	- primary	Pa	400	150	280	460	300
	- secondary	Pa	600	400	500	600	650
2.	Air temperature	°C		20
3.	Negative pressure in the boiler furnace	Pa		15–20
4.	Negative pressure after the boiler	Pa	90	70	100	140	180
5.	Negative pressure after the economizer	Pa	550	400	500	650	730
6.	Boiler aerodynamic drag	Pa	70	50	80	120	160
7.	Economizer aerodynamic drag	Pa	460	330	400	510	550
8.	The aerodynamic drag of the boiler unit	Pa	530	380	480	630	710

,

Table 4. Flue gas parameters.

No	Parameter	Unit	Combustion of Mazout	Combustion of Water–Mazout Emulsion (WME)
				1	2	3	4
1.	Maximum content of triatomic gases	% vol.	16.5	15.5
2.	The composition of exhaust gases after the boiler:
	-RO2 (sum of CO2 and SO2)	% vol.	9.4	10.6	11.5	12.3	12.6
	- oxygen	% vol.	9.0	6.6	5.4	4.4	3.9
	- nitrogen	% vol.	81.6	82.8	83.1	83.3	83.5
	- carbon monoxide	% vol.	0.016	on average (below 0.01% vol.)
3.	The composition of exhaust gases after the boiler unit:
	-RO2 (sum of CO2 and SO2)	% vol.	7.7	7.1	7.9	8.6	8.9
	- oxygen	% vol.	11.2	11.4	10.3	9.4	9.0
	- nitrogen	% vol.	81.1	81.5	81.8	82.0	82.1
4.	Concentration of nitrogen oxides (at α = 1)	mg/m3	310	195	220	240	260
5.	Air-excess coefficient:
	- after the boiler	-	1.71	1.43	1.32	1.25	1.21
	- after the boiler unit	-	2.08	2.11	1.90	1.76	1.70
6.	Suction air in the economizer	-	0.37	0.68	0.58	0.51	0.49
7.	Exhaust temperature after the boiler	°C	615	590	610	630	640
8.	Exhaust temperature after the boiler unit	°C	210	185	200	220	230
9.	Cooling exhaust in the economizer	°C	405	405	410	410	410

and

Table 5. Heat balance.

No	Parameter	Unit	Combustion of Mazout	Combustion of Water–Mazout Emulsion (WME)
				1	2	3	4
1.	Heat loss after the boiler:
	- with the exhausts	%	40.24	32.70	31.64	31.08	30.83
	- to environment	%	2.45	3.67	2.45	1.98	1.85
2.	Gross efficiency of boiler	%	57.31	63.63	65.91	66.94	67.32
3.	Heat loss of the boiler unit:
	- with the exhausts	%	14.69	12.96	12.83	13.29	13.56
	- to environment	%	4.96	7.34	4.91	3.95	3.69
4.	Gross efficiency of the boiler unit	%	80.36	79.70	82.26	82.76	82.75
5.	Increasing the efficiency with the use of an economizer	%	23.05	16.07	16.35	15.82	15.43

. The results obtained were compared with the results of burning pure mazout. For this purpose, setting No. 2 for WME combustion was compared with the combustion of pure mazout with a moisture content of 5% at the same pressure level in front of the injectors, which was 2 bar.

It should be emphasized that for the emulsion work mode, the humidity was 6–8% higher than in the case of pure mazout. Achieving almost the same efficiency, it is possible to save 6 to 8% of mazout consumption. Compared to the operation of the boiler on pure mazout, the efficiency of which was 80.36%, the efficiency of the boiler was 82.26% (Table 5). The emission of nitrogen-oxides concentration during EWM combustion decreases by almost 30% (Table 4).

The comparison of the results obtained for the mazout-combustion process and WME proves that finely dispersed water exerts both physical and chemical influence on the combustion processes. The physical effect is based on the aforementioned phenomenon of micro-explosions of water droplets contained in a droplet of a fuel emulsion. During this phenomenon, the additional mixing of fuel and air takes place. The dissociation of water vapor released as a result of micro-explosions results in an increase in the content of active radicals in the combustion zone. First of all, the increasing concentration of the hydroxide radical significantly accelerates the combustion of carbon monoxide [30,37]. All this allows the excess-air coefficient to be reduced to practically critical values of 1.21 after the boiler and 1.70 after the boiler unit, thus increasing the efficiency of the boiler (see Table 4).

The results obtained for boiler No. 1 are shown in Figure 5, Figure 6, Figure 7 and Figure 8. The results in Figure 6, Figure 7 and Figure 8 are presented for the boiler itself and for the boiler unit consisting of a boiler and an economiser. Figure 5 presents the dependence of the concentration of nitrogen oxides as a function of exhaust gas on mazout pressure.

It was observed that with the decrease in mazout pressure, the nitrogen content in the exhaust gas decreased. The maximum content of nitrogen oxides for boiler No. 1 was 260 mg/m³, which corresponded to a mazout pressure of 3.5 bar.

Figure 6 presents the dependence of the total RO₂ concentration and the excess-air coefficient, after the boiler and the boiler unit, as a function of the mazout’s pressure.

The dissociation of water vapor released as a result of micro-explosions results in an increase in the content of active radicals in the combustion zone. First of all, the increasing concentration of hydroxide radicals significantly accelerates the combustion of carbon monoxide.

All this allows the excess air to be reduced practically to the critical values of 1.21 after the boiler and 1.70 after the boiler unit.

Figure 7 presents the dependence of the exhaust gas’s temperature after the boiler and after the boiler unit as a function of mazout pressure.

The exhaust gas temperature after the boiler ranged from 590 to 640 °C after the boiler and from 185 to 230 °C after the boiler unit.

Figure 8 presents the dependence of thermal efficiency as well as boiler efficiency as a function of mazout pressure.

Figure 9 presents the dependence of steam and thermal efficiency as well as boiler efficiency as a function of mazout pressure. As a result of reducing the value of the excess-air coefficient, the boiler’s efficiency increased to 82.76%, which corresponded to a mazout pressure of 3 bar.

4. Conclusions

A system for the disposal of sewage containing mazout was developed and implemented. This system is based on the method of burning a water–mazout emulsion. On the basis of the experimental results obtained, the optimal water content in the emulsion was established in the range of 10 to 13%. Economic and ecological indicators of emulsion combustion have been determined. The results of pure M100 mazout combustion, obtained experimentally for the selected work regime of boilers, have been taken as the reference level. When operating on a water–mazout emulsion, the efficiency of the boiler increases by 2–3% compared to operations on pure mazout. This effect was achieved due to the reduction in the excess-air coefficient, reduction in losses from chemical unburning, and reduction in the exhaust gas’s temperature (due to the reduction in contamination of the screen surfaces and convection bundles). It was found that the emission of nitrogen oxides decreased by 30–35% and the emission of carbon monoxide decreased by 35–38%. The above-mentioned ecological effects accompany the overall improvement of the boiler operation and the increase in fuel efficiency. Increased time for a more efficient operation of the equipment has been observed. The problem of water content in mazout has been resolved. Due to the proposed solution, it is easy to conceptualize the use of water generated during the consumption of mazout, which cannot be discharged into the sewage system.