Nowadays energy generation is the heart of most critical, economic, environmental, and development issues of the world. Challenges are faced by the global community and national governments due to energy security, climate change, health impacts, and poverty. Shifting toward green energy production is supposed to play a critical role in solving some of the most prominent contemporary challenges the world is facing at present [1]. As power demand increases, power generation increases, which results in increased emission of NO_x, SO_x and CO₂. The largest emission source of these pollutants comes from coal-fired power plants [2]. These pollutants cause various environmentally harmful effects such as acid rain, ozone depletion and urban smog [3] [4]. Biomass gasification coupled with a coal-fired power plant is one of the solutions to the problems. In Pakistan, there produce 82 million tons of biomass. It is best to use this biomass in power generation as it reduces both fossil fuel consumption and mitigates climate change [5]. Biomass gasification is a process for converting carbonaceous material to a combustible or synthetic gas [6]. The solid biomass is converted into a gaseous product that can be handled with maximum convenience, and low cost and readily be purified to a clean fuel [7]. It is a complex process and the operational parameters in the gasification process play an important role in improving the quality and quantity of syngas [8]. As compared to the pure coal case, co-firing of syngas results in decrease of furnace combustion temperature and the increase of flue gas temperature and volume flow rate. Thus, boiler efficiency slightly decreases; however, emissions of NO_x and SO_x decline also.

Aspen Plus is a widely used process simulation software nowadays and can simulate the gasification process of biomass fluidized bed gasifier and the co-firing process of bio-gas with the coal-fired boiler. Meanwhile, the simulation results were compared with test data and the rationality of the model was verified successfully [9]. Based on chemical balance and phase equilibrium, a fixed bed gasifier model was developed using Aspen Plus. Study shows that the model can be used to predict the syngas component at optimized working conditions [10].

Biomass gasification coupled with a coal-fired power plant is more useful than directly co-firing. The co-firing of most kinds of biomass may reduce the boiler efficiency. Wet wood chip for biomass gasification was studied and gasification efficiency increased for biomass with 20% [11]. It solves boiler fouling and corrosion problems up to some extent. As biomass ash is removed before syngas is brought in a furnace, ash characteristics of the boiler remain the same [12].

In this work, the operation processes of the biomass gasification coupled with a coal-fired boiler system were simulated. Straw, Food Rubbish and Wood Pellet are employed as feedstock at gasifier. They were gasified at the air to biomass ratio varying from 0.6 to 2.4. The comparative study between three cases is done to find the best of them. The effects of the air to biomass ratio on bio-gas compositions, bio-gas low calorific value, biomass gasification efficiency, and the influences of excess air ratio on the furnace combustion temperature, flue gas compositions, boiler efficiency and the system efficiency of biomass utilization have been studied. The findings will give a reference to apply the co-firing technology in coal-fired power plants.

The operational performance of biomass gasification coupled with coal-fired boiler directly depends upon the biomass gasification and co-firing of biomass and coal. The system includes one coal-fired boiler and one biomass gasifier. The block diagram of biomass gasification coupled with the coal-fired boiler is shown as Figure 1.

Three kinds of biomass are studied, which are Food Rubbish, Straw and Wood Pellets. They were gasified in a gasifier, whose capacity is constant and its mass flow rate is 10 t/h. The air is used as a gasifying agent. The cooling temperature of syngas after it leaves cooler is 450˚C. Then obtained syngas from each case was entered the coal-fired boiler for co-firing with coal. Then flue gases were passed through the heating surface of the boiler. The air leakage is considered at the tail heating surface of the boiler. The boiler is a subcritical pressure and natural circulation boiler with a maximum capacity of 330 MW production. Three different co-firing cases have been studied at that of 100% Boiler Rated Load (BRL).

The proximate analysis, Ultimate analysis and Lower heating value of the sub-bituminous coal, Food Rubbish, Straw, and Wood Pellets are shown in Table 1.

A 330 MWe boiler and a 10 t/h gasifier are employed to simulate the co-firing system performance and pollution emissions. It found the syngas from straw had maximum boiler efficiency as compared to that of Food Rubbish and Wood Pellets in co-firing cases, and the co-firing of straw syngas had a maximum system efficiency of 83.88% while Food Rubbish and Wood Pellets were 83.25% and 82.56% respectively. As compared to pure coal case it is observed that in co-firing cases the furnace combustion temperature decreased and the flue gases temperature increased, but in case of Wood Pellets the flue gases temperature decreased. The volume flow rate also increased in the case of Food Rubbish and Straw but decreased in the case of Wood Pellets. Boiler efficiency and emission of NO and SO₂ reduced in the case of co-firing.

The authors declare no conflicts of interest regarding the publication of this paper.

Abid, S., Zhang, X.T., Zhang, W.D., Mu, H.L., Zhang, C.Y. and Wang, A.J. (2020) System Performance and Pollution Emission of Biomass Gas Co-Firing in a Coal-Fired Boiler. Journal of Power and Energy Engineering, 8, 18-25. https://doi.org/10.4236/jpee.2020.810002