Combustion of mixtures of biogases and syngases with methane in strong swirl flow

The paper presents data from experimental studies o n combustion of mixtures of methane and biogas and sy gas in the swirl burner. The swirl of the stream of mixtur e of air and gas in the burner was generated by a s wirler with helical/ blades. The study was focused on the impact of mola r fraction of carbon dioxide (for biogas) and hydro gen (for syngas) on flame stability and emissions of noxious compounds during combustion. The results, both ex perimental and numerical, show that a low content of carbon di xi e in methaneair mixture leads to better flame stability through an increase of the volume of the recirculation zone . It also leads to the lowering of emission of nitr ic oxides caused in the first place by the lowering of the bulk tempera tu e of flame. Co-burning of natural gas with synga s containing hydrogen results in a considerable increase in flam e stability due to a strong increase in velocity of flame propagation in laminar flow. However, when a limit of molar fra ction of hydrogen is passed, it leads to flashback. Introduction Energy policy of the EU [1] and Poland [2] emphasizes the use of renewable energy. Special emphasis is placed on the use of biogas and other g ases from biomass gasification. Increasing the share of biomass in overall energy consumption is especially important in Poland, where 90 % of energy generatio n is based on fossil fuels (coal and lignite). One way t o raise this share is through using biogas or syngas as fue l . Biogas has two main components CO 2 and CH4. The specific content of the fuel depends on the place a nd method of its extraction and can range from 50 – 7 5% CH4 and 2550 % CO2 [3]. Additionally, biogas contains small amounts of nitrogen and oxygen and trace amou nts of hydrogen sulphide, ammonia and hydrogen. Synthes is gas is formed in the process of gasification of coa l, biomass or waste hydrocarbon fractions. The main flammable components of syngas are hydrogen and carbon monoxide, with their molar fractions strongl y dependent on the kind of gasifier feed, time and temperature of the process. The process of biogas a nd syngas combustion for energy generation industry is a very complex one because of changes in gas composit ion during its production [3]. The gas can be used dire ctly in cogeneration installations based on gas engines or can be burnt in boiler units. The process of stabilization of the composition of biogas-based fuel can be completed b y mixing it with natural gas. This makes it possible to maintain stable operating parameters for energy generating units. Polish law [4] allows to pump bio gas obtained from processing of farm waste into the pip eline system for natural gas. Hence the need to examine t he effect of specific components of biogas and syngas on the combustion process. The authors have mostly concentrated on the process of co-combustion of biogas and syngas with natural gas in strong swirl flow. Flame stabilization by produc ing a strong swirl is the most common method of assuring flame stability in gas turbines [5]. Due to their molar fraction, fuels like biogas or syngas differ as to various physical parameters describing the combustion process, such as adiabati c combustion temperature T a , or laminar speed of combustion SL. The numerical value S L for pure methane is about 42 cm/s (p=1bar, T sub=300) [6] and Ta is about 2230K. For the same conditions of combust ion of syngas with chemical composition as given in Tab le 1 the value of SL = 52 cm/s and T a = 2360 [7] . Different values of the adiabatic temperature of combustion a nd of laminar combustion speed have some direct effect on flame stability described by such phenomena as flam eout or flasback [8] and also on the volume of emissions f toxic compounds and pollutants sent into the atmosp here. Nomenclature c – bulk velocity csyn – bulk velocity for syngas Sswirl number λ – excess of air φ – equivalence ratio Tsub – substrates temperature T – temperature r – radius uvelocity p – pressure in the combustion chamber Ta– adiabatic flame temperature SL – laminar flame speed [cm/s] LHV – Low Heating Value