Direct Application of Fast Pyrolysis Bio-oil in Combined Heat and Power
Creators: name: Wang, Yu
Publisher: name: Eindhoven University of Technology
Year: 2024
Subjects: Fast pyrolysis bio-oil
Combined heat and power
Diesel genset engine
Spray combustion
Combustion characteristics
Two-stage Lagrangian model
Constant-volume combustion chamber
Ignition delay
Emissions
Thermal efficiency

description: Small-scale combined heat and power (CHP) is an efficient technology to provide decentralized, dispatchable heat and electricity. Along with other applications that utilize large quantities of fossil energy sources, replacing these fuels with renewable alternatives in CHP plants is of great importance to contribute to a clean energy transition. In the EU funded SmartCHP project, fast pyrolysis bio-oil (FPBO) derived from agricultural and forestry residues is used to fuel modified stationary diesel engines for combined heat and power applications. The special physicochemical properties of FPBO, such as its high viscosity, strong corrosivity, and low chemical reactivity, make direct application of this energy source in engines challenging. Therefore, the present thesis aims at characterizing the ignition and combustion characteristics of FPBO and implementing dedicated modifications to achieve successful engine operation.

 

The spray combustion characteristics of FPBO and its blends with alcohols and/or ignition enhancers are first experimentally investigated in a combustion research unit (CRU). Major modifications are made to the CRU to facilitate testing FPBO, including the installation of a high-temperature chamber and a dual-circuit fuel injection system. The pressure-based heat release analysis is used to evaluate fuel ignition and combustion characteristics, while a high-speed imaging technique is adopted to visualize natural luminosity of spray flames through a borescope. In these recordings, more light is indicative for more soot. Results show that FPBO has lower sooting tendency while it significantly alleviates the fuel dribble phenomenon observed with benchmark experiments using diesel. At injection pressure of 300 bar, the poor atomization of FPBO leads to rather grainy-looking spray flame images, and nozzle orifices becoming clogged within couple of injections. The increase of FPBO injection pressure to 900 bar results in an obvious improvement of the atomization quality and nozzle durability. Compared with diesel, FPBO has a prolonged ignition delay due to its low chemical reactivity and a much lower sooting tendency due to its high oxygen content. Adding ethanol into FPBO shortens its ignition delay, improves nozzle durability, and further reduces the sooting tendency. Adding n-butanol into FPBO improves its ignitability. FPBO samples from different feedstocks show different ignition and combustion characteristics, while all samples have lower sooting tendencies than diesel. The FPBO sample made from sunflower husk has the lowest ignitability and the lowest natural luminosity during combustion, while the volumetric LHV of FPBO sample made from miscanthus is shown to be lower than others.

 

To further investigate FPBO as a future fuel alternative, a reduced-order, two-stage Lagrangian (TSL) model is developed to investigate the ignition process of FPBO sprays. The ignition delays of n-dodecane and n-heptane sprays predicted by the developed TSL model are first validated with Engine Combustion Network Spray A and Spray H data over a wide range of ambient temperatures and oxygen concentrations. The results show that the two reactor TSL model is able to capture the multiple-stage ignition process of a fuel spray caused by the complex interaction between turbulent mixing and chemistry at the spray periphery and core regions. Then, the validated TSL model is employed to study the influences of ambient temperature and ethanol addition on ignition processes of an FPBO spray using a 9-composition surrogate mechanism. Results show that FPBO requires an ambient temperature over 1000 K to initiate combustion reactions due to its low chemical reactivity. In the FPBO surrogate, methyl oleate and vanillin are two important components which play crucial roles in the FPBO ignition process. Ethanol addition into FPBO has slight influences on ignition delay values, leading to a retarded 1st-stage (low-temperature) ignition but an advanced 2nd-stage (high-temperature) ignition.

 

The obtained information regarding FPBO ignition and combustion derived from CRU experiments and TSL simulations is finally used to implement dedicated modifications on an FPBO fueled genset engine. A 500-hour engine durability test using FPBO blended with 10 wt.% ethanol has been successfully achieved for the first time after modifying the air preheating, fuel switching, and fuel injection systems. The needle lift, fuel pressure, and cylinder pressure data are collected for evaluating fuel injection and combustion characteristics at different injection timings, electric loads, and intake temperatures. After replacing diesel by FPBO, the injection timing is advanced by around 1.4 crank angle degree, the maximum injection pressure is increased by around 30 bar, and the injection duration is almost doubled. The increased intake temperature slightly reduces the ignition delay of FPBO but has negligible influence on diesel combustion. FPBO reduces the amount of NOx by nearly a factor two when compared to diesel, but increases CO emissions at the same operation conditions with a factor two as well. Intake temperature plays an important role in engine emissions using FPBO, and the increased intake temperature elevates NOx emissions but slightly reduces CO and HC emissions. The optimized net indicated efficiency for the engine running on FPBO is 53.6%, higher than that of diesel (49.1%), since FPBO has a faster burning rate. It is observed that the engine suffers from a derating problem during FPBO experiments, which requires additional modification of the fuel injection system. This problem can be solved by enlarging the nozzle orifice diameters, but it results in an increased liquid length which needs to be mitigated by elevating the intake temperature to avoid wall-wetting.

 

Through the research work presented in the thesis, the technology readiness level (TRL) for fueling an engine with FPBO has been increased from between 3 and 4 to a TRL of 5. The fundamental spray combustion research with FPBO and its application to the modified diesel engine have provided a firm foundation to develop new standards for using FPBO in these applications. The design, testing, and optimization of the holistic SmartCHP system, including engine, boiler, aftertreatment system, and heat exchangers, will be the focus of future work.