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 . As power demand increases, power generation increases, which results in increased emission of NOx, SOx and CO2. The largest emission source of these pollutants comes from coal-fired power plants . These pollutants cause various environmentally harmful effects such as acid rain, ozone depletion and urban smog  . 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 . Biomass gasification is a process for converting carbonaceous material to a combustible or synthetic gas . 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 . 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 . 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 NOx and SOx 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 . 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 .
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% . 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 .
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.
2. The Basic Theory of Coupled System
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.
3. System Modeling
3.1. Biomass Gasification and Syngas Cooling Process
The biomass gasification is composed of four sequential step processes which include biomass pyrolysis, biomass gasification, separation of syngas and biomass ash and syngas cooling, as shown in Figure 2.
The feed biomass is gasified as a non-conventional stream. The biomass stream is passed to the DECOMP block where biomass pyrolysis is done to get conventional components such as carbon (C), sulfur (S), oxygen (O2), hydrogen (H2), water (H2O), nitrogen (N2) and ASH. Then block GASIFY is used for biomass gasification. It is used to calculate the chemical and phase equilibrium of
Figure 1. The block diagram of biomass gasification coupled with the coal-fired boiler system.
Figure 2. The simulation flow chart of biomass gasification and syngas cooling process.
Table 1. Proximate, ultimate analysis of fuel (Received basis)  .
the system by minimizing the Gibbs free energy. Its main purpose is to gasify solid biomass into a gas-solid stream PRODUCT. The block SEPARATE is used to separate biomass ash from syngas to get gaseous stream GASES. Then this gaseous stream enters the heat exchanger EX block where syngas is cooled to 450˚C. Then this cooled stream SYNGAS, is the syngas entered into the boiler.
3.2. Co-Firing Process and Heat Exchange Process in the Tail Flue
In a co-firing process coal pyrolysis, co-firing of syngas with coal and heat exchange process in the tail are carried out. Latter this process is split into three further steps: separation of coal ash and flue gases, air leakage and heat exchange in tail flue gases channel, as shown in Figure 3.
Coal is pyrolyzed in the block DECOMP 1 into the products such as C, S, O2, H2, H2O, N2 and ASH. Then preheated air, syngas and coal products are co-fired in block BURN. Coal ash is separated from flue gases by the block SEP to obtain gaseous stream GAS. Then the stream GAS is introduced in the tail flue. Meanwhile, the model of the heat exchange process of tail heating surfaces of the boiler is established. It has the heat exchanger block (HEATER type) and two mixer block (MIXER type). The heat exchanger blocks HEATER is used to simulate the process in super-heater, re-heater, and economizer. The heat exchanger block PRE-FLUE and PRE-AIR are used to simulate the cooling process of the flue gases side and heating process of the air side respectively in air pre-heater.
Figure 3. The simulation flow chart of co-firing and heat exchange in tail fuel.
4. Results and Discussions
4.1. Biomass Gasification Process Performance Index
The maximum heat efficiency can only be obtained at optimum air to biomass ratio. The gasification temperature at optimal air to biomass ratio of Food Rubbish, Straw and Wood Pellets is 650˚C, 703˚C and 652˚C the syngas characteristics parameters obtained at optimum air to biomass ratio is shown in Table 2.
4.2. The Basic Parameters and Boiler Efficiency of the Co-Firing Process
Syngas co-firing ratio, coal consumption mass flow rate, and the theoretical air mass flow rate were obtained, as shown in Table 3.
As boiler load is constant, the coal consumption mass flow rate and theoretical air mass flow rate decrease in co-firing cases as compared to the pure coal case. In Food Rubbish there is a 5.06% decrease in coal consumption while 4.76% and 4.15% decrease in the case of Straw and Wood Pellets. The theoretical air mass flow rate in Food Rubbish case decreased by 1.60% while in the case of Straw and Wood Pellets it decreased 1.53% and 1.49% respectively.
4.3. Performance of the Coupled System
The highest operating efficiency of the boiler, boiler operation parameters, flue gas components and characteristics at air pre-heater outlet and system efficiency are shown in Table 4.
Furnace combustion temperature in pure coal case is at 1345.81˚C while it decreases slightly in co-firing cases, Food Rubbish gas at 1340.87˚C, Straw gas at 1342.83˚C, and Wood Pellets gas at 1340.23˚C. Flue gas temperature increases in the case of Food Rubbish gas and Straw gas co-firing but decreases in the case of Wood Pellets gas co-firing, as compared to pure coal case. There are slight oscillations in the flue gases flow rate. Boiler efficiency slightly decreases in co-firing cases. The volume fraction of the main components of flue gases H2O and CO2 increases while N2 and O2 decreases in co-firing cases. The volume fraction of pollutant components nitrogen oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), sulfur dioxide (SO2), and sulfur trioxide (SO3) decreases.
Table 2. Optimal syngas components and gasification characteristic parameters.
Table 3. Mass flow rate of coal consumption and theoretical air mass.
Table 4. Flue gas components, flue gases characteristic and coupled system performance.
The reason behind the reduction of NOx and SOx in flue gas is that the presence of reductive gases carbon monoxide (CO), H2, and methane (CH4) in syngas. It reduces NOx to N2 along with the generation of H2O and CO2 under reducing temperature. That is why the H2O and CO2 content increase in flue gases after co-firing. The decrease in combustion temperature and the presence of N and S in biomass fuel is the most important factor in the reduction of volume fraction of NOx and SOx    .
System efficiency is 83.25% in the case of Food Rubbish gas co-firing, 83.88% in the case of Straw gas and 82.56% in Wood Pellets gas case.
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.
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