Considering the advantages of indirect co-combustion technology, many researchers focused on the reduction of NOx emissions with co-combustion coal and biomass gasification gas.

Fan et al. (2006) performed experiments with simulated biomass gasification gas (CO/H2/CH4/C3H6) and found that the reduction effect on NO emissions was decreased with the concentration of H2, CO in biomass gas increased; the proportion of CH4, C2H6, C2H4 increased, the NO reduction efficiency increased, where the reaction CH; + NO ^ N2 + M played a leading role.

Duan et al. (2006), performed experiments with simulated biomass gasification gas (CO/CO2/CH4/H2/N2) co-combustion for reduction of NO in an electrical heating corundum tube flow reactor. It is confirmed that the mixing of biomass gasification gas can improve NO reduction rate when the oxygen content changed in a range of 0-5% in reactor entrance, temperature changed from 1000 to 1400°C

Dong and Hu et al. (2009) performed some experiments to study the influence biomass simu­lation gas played on emissions of N2O in a small fluidized bed reactor. Also, N2O reduction was effected by the co-combustion ratio of biomass gas (0-1.4%), co-combustion temperature (800- 1000°C), residence time in co-combustion zone (0.16-0.32 s), O2 initial concentration of gas (4-8%), bed material height (0-50 mm), and other conditions changed. The results showed that the higher the co-combustion temperature was, the higher the thermal decomposition rate of N2O was. With the biomass gas content of 1.0%, when the co-combustion temperature was 850°C, N2O decomposed absolutely; the oxygen concentration of flue gas plays an inhibitory action on N2O decomposition, but the injection of biomass gas can effectively avoid this problem. Based on

this, the further research showed that in the process of biomass gas reducing N2O, the following reactions took the more important influence on N2O decomposition:

N2O + H< = >N2 + OH (4.1)

N2O(+M)< = >N2 + O(+M) (4.2)

N2O + CH3< = >CH3O + N2 (4.3)

A integrated system, including a circulating fluidized bed subsystem and a fixed bed biomass gasifier subsystem, was built by Zhang (2011). The circulating fluidized bed subsystem mainly includes a circulating fluidized bed reactor, a hot air ceramic electric heater, a fluidized bed start heating furnace, a spiral feeder, a spray desuperheating tower, a tubular heat exchanger and a mechanical vibration type bag dust extraction, which is shown in Figure 4.8. In order to describe different nozzles, Rh is defined as the ratio of its height away from air distributor to the furnace diameter. Corresponding to nozzle A, B, C, D, E and F, the value of Rh was 4.3, 6.3, 8.3, 10.3,

12.3 and 14.3 respectively. Six temperature probes were distributed at the nozzles.

Figure 4.9 is the flow diagram and photograph of the fixed bed biomass gasification subsystem, which mainly includes fixed bed gasifier, catalytic tower, spraying tower, purification tower, water ring type vacuum pump, drying tower and connection pipes. Rice husk, the raw material for gasification, was fed into the gasifier from the hopper. The produced gas flowed into the catalyzing tower from the bottom of the gasifier. Some activated carbon particles were added to the catalyzing tower for adsorbing and catalyzing decomposition of the tar produced. Then the gases were purified further through the water tower for protecting other equipments and pipes, and were dried by the drying tower with a water-ring vacuum pump. Finally, the dried gas was used for co-firing with coal in the circulating fluidized bed, which helps to reduce N2O and NO emissions. The air used in the gasifier was supplied from the medium of the gasifier at the top of the hopper. After gasification, the ash fell into the ash hopper at the bottom of the gasifier. At the end of experiments the ash was cleaned. During operating the pilot plant system, firstly the water ring vacuum pump is run before starting the gasifier system to ensure the gasifier system operated with a slightly negative pressure, and then starting the circulating water pump in the water tower. After checking all the relevant components, the gasifier was ignited with a electric igniting torch.

From an analysis of the results, it is concluded that:

1. With the increase of the proportion of the reburning, the theoretical air requirement was decreased, and in contrast the theoretical flue gas was increased accordingly, as was the furnace temperature and exhaust temperature. However, the boiler efficiency was decreased with the increase of exhaust volume and exhaust temperature.

2. By injecting gasified biomass from the nozzle with a length to diameter ratio of 8.3, the highest N2O removal rate of 99% was achieved, while its NO removal rate was 44%.