Spray Combustion Characteristics and Gaseous Emissions of a Wood Derived Fast Pyrolysis Liquid-Ethanol Blend in a Pilot Stabilized Swirl Burner

Abstract

Biomass fast pyrolysis liquid (or bio-oil) is a cellulose based alternative fuel with the potential to displace fossil fuels in stationary heat and power applications. To better understand the combustion behavior and emissions of bio-oil, a 10 kW spray burner was designed and constructed. The effect of swirl, atomization quality, ignition source energy, air/fuel preheat, and equivalence ratio on the stability and emissions of bio-oil spray flames was investigated. A blend of 80% pyrolysis liquid and 20% ethanol by volume was used during the tests. Since the fuel is not fully distillable, it is important to have good atomization, thorough mixing, and increased recirculation to promote the burnout of nonvolatile material and decrease CO and hydrocarbon emissions. Air and fuel preheat are also important for reducing these emissions, although subsequent fuel boiling within the nozzle should be avoided in order to maintain flame stability. The amount of total primary air and atomizing air that can be used to improve turbulence, mixing, droplet burnout, and overall combustion quality is limited by the low volatility and tighter lean blow-out limit associated with bio-oil. The NO x produced in these flames is dominated by the conversion of fuel bound nitrogen. In order to reduce the NO x emissions without refining the fuel, the use of staged combustion is recommended.

On the atomization and combustion of liquid biofuels in gas turbines : towards the application of biomass-derived pyrolysis oil

“…At elevated temperatures, the oil becomes increasingly reactive due to its high oxygen content. Secondly, the presence of water and components with a low boiling point can result in vapor formation in the fuel line at high oil temperatures [90,143]. Since these phenomena may lead to problems with the fuel supply, pyrolysis oil is generally preheated in the fuel line to a maximum temperature of 80-90 • C in order to avoid any undesired changes in physical and chemical state [32,67].…”

“…Solid matter can be formed inside the droplets during the evaporation process [31,126,143], which results in char particles that require long burnout times and potentially cause damage to certain turbine components. However, previous studies [31,126] showed that the charforming tendency of the droplets reduces with higher evaporation rates.…”

“…The heating rates in these experiments were estimated to be 10 6 and above 10 5 K/min, respectively. In spray combustion experiments only 0.1% of the original mass was obtained as char [143], although it must be noticed that the hot product gases were not quenched so that part of the char could have been consumed via surface reactions. Nevertheless, these studies clearly show that TGA underpredicts the mass release in the upper temperature regions.…”

“…Gas turbines are relatively fuel-flexible and are capable of generating power on both large and small scales, suiting the demands of the future energy system. Pyrolysis oil combustion in gas turbines has therefore been subject of a few experimental studies in the past years [10,95,97,122,138,143]. These studies, conducted in both scientific and industrial test rigs, have shown that the use of pure pyrolysis oil often leads to major problems.…”

“…For this reason, pyrolysis oil is not directly suitable for use in combustion devices, including gas turbines. The few attempts that have been made revealed major problems as incomplete combustion, flame instability and coke formation on combustor walls [10,95,97,122,138,143]. Therefore, it is generally concluded that fuel blending or thorough modification of the gas turbine is required for successful application.…”