COMBUSTION OF COAL AND ALTERNATIVE FUELS IN OXY-FUEL ATMOSPHERE SPALANIE WĘGLI I PALIW ALTERNATYWNYCH ATMOSFERZE TLENOWEJ

. In this paper, results of fuel behavior during combustion of coal and alternative fuels in oxy-fuel atmosphere are presented. The objective of this work was to evaluate the dependencies between properties (ignition, explosion parameters, emissions of NO x and SO 2 ) of coal and alternative fuels. The exchange of air for recycled flue gas and oxygen mixture, leads to changes of combustion behavior i.e. lower emission of NO x , SO 2 and increased ignition delay time. Nevertheless, performed experiments do not indicate significant differences in explosion parameters among different fuels at air and 30%O 2 /70%CO 2 atmosphere.


Introduction
Oxy fuel combustion is one of the leading carbon capture and storage (CCS) technology. In this process, the fuel is combusted in a mixture of recycled flue gases and nearly pure oxygen, instead of air. The main attraction of this technology is production of a flue gas containing mainly CO 2 and water vapour. Since the H 2 O can be easily removed by condensation, the almost pure CO 2 stream becomes suitable for compression, transport and storage.
During the last decade, the oxy fuel technologies have been significantly developed to the point where several demonstration projects have begun [11]. A number of state-of-the-art reviews have been presented [1,7,10]. Nevertheless, while oxy fuel conditions differ considerably from air conditions, several issues, i.e. ignition mechanism, explosion risks, SO 2 and NO x emissions still need clarification.
The ignition of solid fuel particles is an important step of combustion mechanism due to its influence on flame stability and pollutant formation. The ignition temperature and ignition delay time are not an inherent property of the fuel, but they depends also on the operating conditions (i.e. atmosphere composition, heating rate, etc.). In oxy fuel atmosphere, poor ignition quality has been often noted during pilot scale experiments [9]. As shown in the paper [8] the ignition delay time is longer and the ignition temperature is higher in an O 2 /CO 2 environment than in an O 2 /N 2 . Authors suggested that the longer ignition delay in O 2 /CO 2 mixtures is mainly due to the higher heat capacity of CO 2 . The same effect has been attributed also to endothermic char-CO 2 gasification reaction [6].
The explosion parameters of pulverized solid fuels under oxy fuel conditions are important for burner design and development, and power plant safety considerations. The amount of experimental data in the literature is very limited and ambiguous. As presented in [4] substitution of air by oxy fuel atmosphere with higher O 2 concentration increases the rate of pressure rise. On the other hand, results presented in [3] do not indicate differences between explosion parameters in air and oxy fuels atmospheres.
The reduction of emissions is an important environmental issue due to the NO x and SO 2 contribution to acid rain formation. Many of researchers [5,7,10] indicated that oxy fuel combustion conditions may decrease the NO x emission intensity due to: (1) lower partial pressure of N 2 , (2) different radical and gas compositions within and around the char particles and (3) reburning of recycled flue gas containing NO. As shown in [2] the SO 2 emission intensity does not change significantly under oxy fuel and air conditions. However, recycling of flue gasses may increase the SO 2 concentration in the furnace and raise potential corrosion issues.
The aim of this paper is to examine the behaviour of various coals and alternative fuels during the oxy fuel combustion and to compare obtained results with data possessed in referenced air atmosphere. The principal objective of the study is to determine the impact of oxy fuel conditions on the ignition mechanism, explosion risks and SO 2 and NO x emissions.

Fuel samples
Eight fuels were selected for the research, i.e. hard coals Janina and Sobieski, lignites Turów and Bełchatów, alternative fuels: wood pellet (PelD), sunflower pellet (PelS), sewage sludge (OsadS) and solid recovery fuel (SRF). The chosen alternative fuels represent three groups, defined according to source and origins: wood/woody biomass (PelD), herbaceous/agricultural biomass (PelS) and solid waste (OsadS and SRF). Physicochemical properties of investigated fuels differ substantially and are summarized in Tab. 1. Comparing to coals, alternative fuels contain high amount of volatiles and lower fuel ratio FR. Moreover, sewage sludge and SRF contain high amount of ash, caused by the process of their production. The ratio between fractions of the proximate analysis is shown on the Fig. 1. All of the investigated alternative fuels are characterized by the fuel ratio lower than 0.5, thus the predominant form of combustion is gasphase oxidation of the volatile matter. Fuel ratio lower than unity, may increase the fire and explosion risks during combustion and co-combustion of these fuels. Particularly exposed to the hazard of fire and explosion are the feeding and the preparation systems. Nevertheless, high amount of volatiles (low fuel ratio) may promote ignition and increase flame stability.

Ignition of coal particle suspension in oxy-fuel atmosphere
Ignition measurements in oxy-fuel (O 2 /CO 2 ) atmosphere were performed using monodisperse coal dust clouds experimental technique. An electrically heated vertical furnace (Fig. 2) coupled with optical particle detection has been applied to study the ignition of clouds of pulverized coal. The experimental program involved the variation of furnace temperatures, T g (ranging from 400°C to 900°C), type of atmosphere (air, oxy 30%O 2 /70%CO 2 ) and fuel type.

. Vertical furnace diagram
A series of experiments were undertaken to determine the sensitivity of the cloud ignition delay time to adjustable experimental variablesthe furnace temperature and oxy-fuel (O 2 /CO 2 ) atmosphere. The variation of the ignition delay time with respect of the furnace temperatures for fuels is reported in Fig. 3.
As shown in the Figure 3, ignition temperature and ignition delay time vary depending on the fuel type. Regardless of the atmosphere composition, the lowest ignition parameters were measured for biomass PelS, PelD and for lignites Turów, Bełchatów. On the other hand, long ignition delay time and high ignition temperature were obtained for hard coal Janina, Sobieski and sewage sludge OsadS. air atmosphere oxy atmosphere

Fig. 3. Ignition characteristics for fuels in different atmosphere
In order to investigate the impact of atmosphere composition on ignition parameters, results obtained for air and 30%O 2 /70%CO 2 are summarized in Tab. 2. Obtained results revealed that exchanging air for 30%O 2 /70%CO 2 atmosphere, in general, leads to slight decrease of ignition temperature (MTI C ) and to increase of ignition delay time (t z650 ). The highest difference between ignition temperature in air and oxy fuel conditions, measured for hard coal Sobieski, was 30°C, the mean difference calculated for all investigated fuels was about 15°C. For 5 of 7 analysed fuels, presence of O 2 /CO 2 mixture increase ignition delay time by average about 0,06s. For lignite Turów and Bełchatów oxy fuel conditions decrease the ignition delay time by 0,04s and 0,02s respectively.

Dust explosion characteristic in oxy-fuel atmosphere
Ignition of fuels dust cloud leads to the explosion. To cause the explosion, three conditions must be fulfilled, i.e. proper dust concentration, access to oxygen and a source of explosion (embers, fire). Knowledge about characteristics of the explosion is important due to prevention (lower explosive limit, LEL) and providing information on the consequences (maximum explosion pressure p max , maximum rate of pressure rise (dp/dt) max , the K st parameter). The explosion behavior was determined using 20 litresphere presented in Fig. 4.
Results obtained for investigated fuels are presented in Fig. 5 and Tab. 3.
It may be concluded that biomass and sewage sludges are characterized by lower explosion parameters than coals and lignites but their explosion tendency is similar. This can be explained by the high content of volatiles and low onset temperature of devolatilization (from range 200-400°C).

NO x and SO 2 emission in oxy-fuel atmosphere
Predicting of the NO x and SO 2 emission during the combustion of fuels is more difficult than for coal. Some fuels such as SRF may have lower nitrogen content, what can result lower NO x emission. Sewage sludges have higher nitrogen content than typical coal, but the emission of NO x does not depend only on the nitrogen content in the fuel, but also on the path of emission and combustion temperature. If the greater part of the nitrogen is emitted with the volatile matter, it will results in lower emission of NO x , especially for combustion in Low-NO x burners. Furthermore, NO x is created by the thermal reaction between nitrogen and oxygen. Hence, higher moisture content in sewage sludges may decrease the maximum flame temperature and thereby reduce thermal NO x formation. Evaluation of the emission of sulfur and nitrogen oxides was performed in the 3 meter long Plug Flow Reactor, where fuel and blend were combusted. Fig. 6 shows the entire rig.

Fig. 6. Gas installation diagram
In the second stage of experiment, fuels were combusted in oxy fuel atmosphere containing 30%O 2 and 70%CO 2 . Emissions of NO x and SO 2 in oxy and air fuel atmosphere for Janina are shown in Fig. 7. For chosen experiment conditions it was showed that emission of NO x and SO 2 is higher in oxy fuel atmosphere than in air (in mg/m 3 ). However, the volume of flue gases from oxy fuel combustion is reduced due to higher content of O 2 and higher density of CO 2 than N 2 . Taking into account lower volume of flue gases, the overall emission of NO x during oxy fuel combustion is lower than air condition combustion (Fig. 8) (in g/GJ). The NO x reduction is thought to be the result of non-NO x formation via fixation of atmospheric nitrogen. Similar to NO x case, the SO 2 emission per energy input of coal in oxy fuel atmosphere is lower than in air. IAPGOŚ 4/2014ISSN 2083 The emission reduction is proportional to the share of alternative fuel. Figure 9 shows the degree of conversion of fuel sulfur to SO 2 in the flue gas. It presents that the lower emission was noted in the case of oxygen-enriched atmosphere and the emission of sulfur compounds is directly proportional to the amount of elemental sulfur.

Conclusion
Analysis shows that the use of coals and alternative fuels in oxy-fuel combustion technology is possible and does not pose a risk than the use of these fuels in conventional systems. The obtained results have confirmed the following facts: 1) ignition parameters depends on the fuel type, the lowest ignition parameters were measured for biomass and lignites, the highest ignition parameters were obtained for hard coals and sewage sludge; 2) exchanging air for 30%O 2 /70%CO 2 atmosphere, in general, leads to slight decrease of ignition temperature (MTI C ) and to increase of ignition delay time (t z650 ). The differences in ignition temperature and ignition delay time are a consequence of the higher density and heat capacity of CO 2 than N 2 . The impact of CO 2 presence is visible even though the reactivity of the fuel-oxidizer mixture in oxy fuel conditions is higher (due to higher concentration of oxygen); 3) biomass and sewage sludges are characterized by lower explosion parameters than coals and lignites but their explosion tendency is higher. This can be explained by the high content of volatiles and low onset temperature of devolatilization; 4) regardless of the fuel type, no significant impact of the atmosphere composition on the explosion parameters (the maximum explosion pressure, the maximum rate of pressure rise, the K st parameter) were found; 5) the overall emission of NO x related to energy input (in g/GJ), during oxy fuel combustion is lower than air condition combustion. The NO x reduction is thought to be the result of non -NO x formation via fixation of atmospheric nitrogen; 6) the emission of SO 2 related to energy input (in g/GJ), during oxy fuel combustion is lower than air condition combustion ant it is proportional to the amount of elemental sulfur. The SO 2 reduction is a result of fixation of sulfur in the ash.