NOx reduction by co-combustion and reburning of biomass in coal- and gas-fired furnaces

다국어 초록 (Multilingual Abstract)

The use of biomass as carbon-neutral fuel for power generation has been increasing rapidly to displace the fossil fuels. It also has advantages of diversifying the fuel portfolio in a power plant and of reducing SO2 and NOx emissions. One common method for using biomass is to cofire in existing boilers. It can be achieve by direct co-firing the biomass and coal blend or by indirect co-firing after conversion of biomass into synthetic gas (syngas) using a separate gasifier.
NOx is one of the main gas-phase pollutants released by oxidation of fuel-bound N and N2. For solid fuels such as coal and biomass, the fuel NOx formation is dominant over that for the thermal NOx. Although NOx released from the furnace can be removed by flue gas cleaning, it is required to minimize the NOx formation during combustion for lower operational costs. Reburning is one of technologies for in-furnace de-NOx, which can be applied to various systems with low capital/operational costs. The reburn fuel supplied immediately downstream of the combustion zone establishes a fuel-rich region for reduction of NOx to N2.
The present study aims to investigate the contribution of biomass to reducing NOx emission by direct and indirect co-firing with fossil fuels. For this purpose, various biomass types and simulated syngas were tested in a bench- and pilot-scale furnace using natural gas and coal as the main fuel. The effects of key combustible species in the syngas (CO, H2, CH4, etc) and various biomass on NOx reductions were evaluated quantitatively based on detailed measurement within the furnaces.
When gas components simulating syngas from biomass were co-fired by reburning, NOx emission decreased as the co-firing ratio increased for both natural gas and coal combustion. CH4 was the most effective component in syngas for NOx reduction, while the contribution of CO and H2 were also significant. Syngas reburning can be more effective when combined with fuel- and air- staging, even with CO- and H2-rich syngas. The actual reburn reactions through the conversion of NO into HCN would take place mainly by in the syngas and partially by CO at high reburn ratios. In reburning cases, CO and NOx measured within the furnace were inversely proportional with each other, indicating the NOx reduction reactions.
When biomass was directed co-fired with the main fuel or injected as reburn fuel, NOx emission decreased as co-firing and reburning ratios increased mainly due to the low fuel N contents of various biomass types. Compared to direct co-firing, NOx reduction was more effective in reburning. Among tested biomass types, wood pellets and empty fruit bunch were found suitable for co-firing fuel due to good grindability and low fuel N content. However, torrefied biomass was ideal for co-firing with high grindability, high heating value and low moisture content. The effects of air staging was still dominant in NOx formation, regardless of biomass co-firing ratios.
Further investigations are required to develop numerical models such as computational fluid dynamics (CFD) for application to commercial-scale boilers such as power plants. This requires more fundamental study to characterize the pyrolysis and char conversion properties of various biomass for derivation of rate constants. The experimental data accumulated in this study can be used for validation of CFD modeling.

목차 (Table of Contents)

• Abstract
• Nomenclature
• 1. Introduction
• 1.1 Background
• 1.2 Objectives
• 1.3 Content of research
• 2. Technical review
• 3. Experiment methods
• 3.1 Analysis of fuel properties
• 3.2 Analytical instrument
• 3.2.1 Gas analyzers
• 3.2.2 Thermocouple (Temperature analyzers)
• 3.2.3 Heat flux measurement
• 3.3 Experiment setup–Bench-scale combustion system
• 3.3.1 Gas combustion system
• 3.3.2 Coal combustion system
• 3.3.2.1 Solid fuel pulverizer
• 3.3.2.2 Burner
• 3.3.2.3 Pulverized solid fuel feeder
• 3.3.2.4 Air preheater
• 3.3.2.5 Bag filter and line cooler
• 3.3.2.6 Ring blower and ID fan
• 3.4 Experiment setup–Pilot-scale combustion system
• 3.4.1 Furnace
• 3.4.2 Pulverized solid fuel feeder
• 3.4.3 Burner
• 3.4.4 Primary and secondary air preheater
• 3.4.5 fans
• 3.4.6 Bag filter
• 3.5 Uncertainty analysis
• 4. Natural gas/syngas reburning in bench-scale furnace
• 4.1 Test condition for reburning
• 4.2 Results and discussion
• 5. Coal/syngas reburning in bench-scale furnace
• 5.1 Test condition for reburning
• 5.2 Results and discussion
• 6. Coal/biomass co-combustion in bench-scale furnace
• 6.1 Test condition for co-combustion
• 6.2 Results and discussion
• 7. Long term (72hr) Coal/biomass co-combustion test in bench-scale furnace
• 7.1 Test condition for co-firing
• 7.2 Results and discussion
• 8. Coal/biomass co-combustion and reburning in pilot scale furnace
• 8.1 Test condition for bituminous coal
• 8.2 Results and discussion for bituminous coal
• 8.3 Test condition for subbituminous coal
• 8.4 Results and discussion for sub-bituminous coal
• 9. Conclusions
• Reference
• Abstracts in Korea