Biomass gasification gas is used as a way of co-combustion so that biomass is converted into combustible gas, and then sent into the boiler. Biomass gasification gas is rich in H2, CH4 and CO etc. with low ash content, very low sulfur content. It is an ideal co-combustion fuel and effective to decrease the emissions of nitrogen oxides.

The co-combustion based biomass gasification can avoid most of the problems associated with direct co-combustion, such as boiler fouling, corrosion, and ash characteristics altering. As shown in Figure 4.3, biomass was gasified in a gasifier and the product gas was fed into a coal fired boiler for co-combustion. The technical economical feasibility of co-combustion with biomass gasification has been verified. The most important thing was to make clear the possible effect of co-combustion on burnout, emissions and what retrofit work should be done. Therefore, a CFD modeling study of coal and product gas (from biomass gasification) co-combustion was carried out Dong et al. (2010).

In the study, 14% by heat basis of product gas from biomass gasification was injected from the lowest layer burner and co-fired with coal in a 600 MW tangential PC boiler in Yuan Baoshan power plant (China). Figure 4.4 shows a sketch of the Yuan Baoshan Boiler. The size of the boiler is 20.1 m (deep) x 20 m (wide) x 73.9m (high). The burner is a tangential swinging burner with size of 0.747m (wide) x0.838m (high). There were eight layers of burners and six layers of secondary air inlets. The designed coal was Yuanbaoshan lignite.

The simulation results showed that: (1) The combustion temperature in the furnace was lower and the flue gas volume was higher for co-combustion cases. The convection heat transfer area should be increased or the co-combustion ratio of product gas to coal should be limited to keep the rated capacity. (2) NO emission was reduced about 50-70% when the product gas was injected

7400mm

Figure 4.4. A sketch of the Yuan Baoshan Boiler.

through the lowest layer burner. The NO emission also depended on the burner design and oper­ation level. (3) The fouling problem caused by high temperatures can be reduced for the lower co-combustion temperature.

The biomass gasification and co-combustion process has been studied by Huang (2011). A model of co-combustion of coal and biomass-gas was established focusing on the research of co-combustion power generation of corn stalk gasification gas and coal. Based on the first and the second law of thermodynamics, co-combustion with corn stalk gasification gas 5% was analyzed. In the research process, an exergy flow graph of the boiler in co-combustion is shown in Figure 4.5. Comparing with pure coal burning, the research of co-combustion (5-30%

Figure 4.5. Exergy flow graph of boiler in co-combustion.

biomass-gas) shows that the theoretical burned gas mass flow will reduce and the theoretical air mass flow will increase and both boiler heat efficiency and exergy efficiency will decrease when there is more biomass gas co-combustion, which was shown at Table 4.5.

Dong (2011) modeled the co-combustion integrated system, which combined a biomass gasi­fication system and a 300 MW circulating fluidized bed boiler system, as shown in Figure 4.6. The optimized gasification gas is sent into the boiler with the temperature 598°C heating value 5401 kJ/Nm3 and gasification efficiency 72.25%. The results reveal that with increase of the co-combustion rate, theoretical air volume decreases, fuel gas volume increases, combustion temperature and exhaust gas temperature increase, boiler efficiency decreases. Adding some heating surfaces at backpass should be used to improve boiler efficiency. The simulation results are shown in Figure 4.7.

Figure 4.7 shows the influence of reburning ratio on furnace temperature, flue gas temperature and boiler efficiency. The average temperature of the furnace was proportional to the biomass gasification gas. When the reburning ratio was increased from 0 to 20%, the furnace temper­ature went up from 840 to 872°C. Given that the total heat value of biomass gas and coal is invariable, the average furnace temperature was rising because the temperature of biomass gas injected was 598°C. When the reburning ratio was increased from 0 to 20%, exhaust temperature also grew from 137.2 to 176.2°C, which agreed with the change of furnace temperature.

When the reburning ratio was increased from 0 to 20%, boiler efficiency reduced from 93.8 to 91.3%. It can be inferred from Figure 4.7 that the flue gas temperature and heat losses of exhaust were proportional to the biomass reburning ratio. The boiler efficiency was inversely proportional to the biomass reburning ratio.

The technology of large coal-fired units is mature and it has a high power generation efficiency, so the integration of straw gasification and coal-fired power generation can take the advantage of increasing the efficiency and reducing cost of electricity. In Europe and United States, the technology has some commercial applications, and has become a new efficient way of reducing greenhouse gas emissions. The gasifier is the key equipment for straw gasification and coal — fired power generation, which at the operating level has a large impact on the effect of straw utilization.