Performance analysis of power generation by wood and woody biomass gasification in a downdraft gasifier

Received Jun 24, 2020 Revised Oct 14, 2020 Accepted Oct 19, 2020 An equilibrium simulation model was developed by applying Aspen Plus to evaluate the performance of 28 wood and woody biomass (W&WB) gasification in a downdraft gasifier integrated with power production unit. The developed simulation model does not focus the gasification process as a closed box, it considers important processes in gasification like drying, pyrolysis, combustion, gasification and integrated with power production plant (combustion chamber plus gas turbine). The results for the 28 W&WB alternatives show that the net power produced from 1-ton feedstock entering to the gasification system is between the interval [0-400 kW/ton] and among them, gasification system derived from Tamarack bark biomass significantly outranks all other systems by producing 363 kW/ton, owing to the favorable results obtained in the performance analysis. Moreover, effect of various operating parameters such as gasification temperature and air to fuel ratio (AFR) on the system performance was carried out. Finally, the developed model is applied as an effective tool to assess the impact of so many biomasses and operating parameters on output power.


INTRODUCTION
The finite nature of fossil fuels, high prices and their negative effects on environment and public health, are encouraging to find renewable energy sources and alternative technologies to produce power [1][2][3]. Biomass is a renewable energy option that is abundant and can create low net CO2 emission and it has also the greatest potential to substitute for transportation fuels [4][5][6][7]. Woody biomass is also one of the important energy sources and it is currently the most important source of renewable energy, globally. In 2010 worldwide use of woody biomass as energy resource was about 3.8 Gm 3 (30 EJ/year), which consisted of 1.9 Gm3 (16 EJ/year) for household fuel wood and 1.9 Gm 3 (14 EJ/year) for large-scale industrial sector. During this period, global primary energy consumption and global renewable energy consumption were 541 EJ and 71 EJ, yearly. Hence, in 2010 woody biomass formed roughly 9% of world primary energy consumption and 65% of world renewable primary energy consumption [8,9].
Gasification has attracted attention as one of the most efficient methods for utilizing woody biomass, as CO2 emission has become an important global issue [10][11][12][13][14]. Biomass gasification means incomplete combustion of biomass resulting in production of combustible gases consisting of carbon monoxide (CO), hydrogen (H2) and traces of methane (CH4) [15][16][17][18][19][20]. This mixture is called syngas. Syngas can be used directly to run internal combustion engines, it can be consumed as substitute for furnace oil for heat production and can be used as chemical feedstock to produce methanol [21][22][23][24][25]. Therefore, the development of small-scale, woody biomass gasification is a great social need in a resource-poor country or for regions which are far from the central energy networks and require district heat and power systems [11,26,27]. The primary aim of this work is to develop a steady state computer model by using Aspen Plus for performance analysis of 28 wood and woody biomass (W&WB) gasification in a downdraft gasifier integrated with power production. The objective is to find the most efficient W&WB for power production.
Then as a sensitivity analysis we examine the effect of operating parameters of temperature and air to fuel ratio (ARF) to know where the overall system is optimal to reach the maximum net output power.

MATERIAL AND METHODS
A simulation model relied on equilibrium approach is developed for wood and woody biomasses (W&WB) gasification linked with power generation plant by employing Aspen Plus. In order to compute physical properties of the components in the gasification, equation of state of PR-BM, Penge Robinson-Boston-Mathias alpha is applied. In addition, for modeling of enthalpy and density of biomass and ash as non-conventional materials, HCOALGEN and DCOALIGT models are used. It is worth to say that MCINCPSD stream includes 3 streams of MIXED, NCPSD & CIPSD classes, to define the biomass structure and ash streams that are not in Aspen Plus database [12,[28][29][30][31][32][33]. Figure 1 shows the flow chart of the system simulated in Aspen Plus. The feedstock stream has been described as a nonconventional material and it was defined by determination of the elemental and proximate analysis of feedstock. In order to have a detailed study, 28 W&WB were considered as feedstock for gasifier. Table 1 shows the proximate and elemental analyses of all these feedstocks [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Temperature through the drying is around 150 °C to reduce moisture of the original biomass to less than 5 wt%. This step is done by utilizing a RSTOIC, stoichiometric reactor in Aspen Plus. RSTOIC module is practical for chemical reactions with known stoichiometry [50][51].

Figure 1. Modules and streams of gasification simulated in Aspen Plus
At the next stage, RYIELD, the yield reactor is simulated for feed pyrolysis. In this part, the studied biomass is transformed to volatile materials (VM) and char. VM includes mainly carbon, hydrogen, oxygen and nitrogen. Char also includes ash and carbon [12,[52][53][54][55][56]. After pyrolysis, RGibbs is applied for simulation of the biomass gasification. This reactor computes the syngas composition by minimizing the Gibbs free energy based on complete chemical equilibrium assumption [57][58][59][60][61]. Input streams to the RGibbs are decomposed biomass and air then combustion and reduction reactions will be occurred inside of the reactor. For combustion level, another RGibbs reactor needs to be simulated with minimum air mixing. This process will be also based on minimization of Gibbs free energy. To generate power, the combustion chamber has to be connected with a gas turbine [62][63][64][65][66]. The thermal content of the flue gas, achived as the combustion heat is recovered to preheat the input air to the combustion chamber as well as to supply the heat required in dryer. The recovered heat can be also utilized for convertion of water to high pressure steam to be  [40,41] (this part was not considered in this study). The solid lines and the dashed lines in the Figure 1 present the mass streams and heat streams, respectively. The system is also assumed to be auto-thermal that means a part of the feedstocks is combusted inside the gasifier to provide the heat required in situ. Heat is also obtained by the hot syngas and the combustion chamber then it is consumed wherever required.

RESULTS AND DISCUSSION
The simulation model results for the 28 W&WB alternatives, ranked regarding their contribution to output net power ( net gas turbine compressor W W W =−) for 1 ton feedstock are shown in Figure 2. This ordering is based on the net power that it is between the interval [0-400 kW/ton], values highlighting the lowest and the highest efficient options, respectively. Class 1 includes 5 wood and woody biomass gasification systems based on Land clearing wood, Fir mill residue, Forest residue, Eucalyptus bark and Alder-fir sawdust that produce the lowest amounts of output power (it is in the range of 0-100 kW/ton). Many of the studied W&WB gasification systems are located in class 2 which their output power is in span of 100-200 kW per one tone of feedstock. Class 3 contains 7 wood and woody biomass gasification systems relied on Spruce wood, Pine bark, Spruce bark, Balsam bark, Hemlock bark, Poplar bark and Birch bark which generate relatively higher net power.
Obviously, the gasification system derived from Tamarack bark biomass significantly outranks all other systems from the viewpoint of power production (363 kW/ton), owing to the favorable results obtained in the performance analysis. This is mainly due to Tamarack bark has the highest percentage of carbon and hydrogen, see Figure 3. Percentage shares depicted in Figure 3 are contributions of carbon and hydrogen, oxygen, ash and nitrogen and sulphur in elemental analysis of each feedstock. Carbon and hydrogen are key elements in each biomass. So that the higher C and H2 content, the more carbon monoxide and hydrogen will be in the syngas and also leads to the improvement of heating value (LHV) of syngas. CO and H2 are combustible substances which are converted to flue gas (mainly CO2 and H2O) through the combustion chamber. Therefore, improving LHV of syngas leads to enter gases at high temperature to the gas turbine. Raising the turbine inlet temperature ameliorates output power from that as well as more net power will be resulted. The impact of temperature of gasifier on power generated from the 28 wood and woody biomass gasification systems, is presented in Figure 4. The sensitivity results in Figure 4 are all at fixed conditions of 1-ton biomass input, air to fuel ratio (AFR) of 2 and temperature of gasifier in the range of 600-1500 ˚C. Obviously for all studied W&WBs, output power from each system is increased by growing temperature. At low temperature (600 ˚C), the present carbon in the feedstock is not completely oxidized, so the syngas product can not be in an acceptable quantity. In fact, at low temperature, CH4 and unburned carbon will stay in syngas then by growing temperature much more carbon is combusted and transformed to CO relied on partial combustion reaction. CH4 is also converted into H2 by reverse methanation reaction. Moreover, water gas reaction (WG) moves toward CO and H2 production at high temperature. So, increasing the temperature of gasifier is so proper for H2 and CO production which causes to the improvement of lower heating value (LHV) of product gas. Then, modified LHV of syngas makes higher quality gases entering to the combustion chamber as well as high temperature gasses entering to the gas turbine. Consequently, growing TIT, turbine inlet temperature increases power output from the system. However, at a specific temperature, yield of H2 and CO are satisfied, both reach to an approximately fixed rates that is called the optimum temperature for the gasifier. Output power also increases in a gradual way near the optimum temperature. The optimum temperature of the down draft gasifier for W&WB is in the range of 900-1000 ˚C. Changes in the air values used in the system have a critical impact on the quality and composition of the product gas and following the generated power. The amont of air arriving the gasifier can be represented in the form of air to fuel ratio (AFR), which is the amount of air needed to burn a unit of dry biomass. The effect of AFR on power produced for the gasification systems derived from 28 wood and woody biomasses, is shown in Figure 5. In this assessment, the operating conditions are fixed on gasifier tempretare of 900 ˚C and 1 ton from each feedstock. The optimum AFR for W&WB gasification is between 1,8-2. At low AFR, biomass reactions will approach to the pyrolysis, charcoal remains with corresponding energy losses. However, at much amount of AFR the additional oxygen is reacted that caused the reduce in syngas production. So, it is critical to find the proper range of AFR for W&WB gasification.

CONCLUSION
An integrated gasification simulation model is developed containing a series of modules that models processes individually through the gasification of biomass. These processes are drying, pyrolysis, combustion and gasification linked with power generation plant (combustion chamber plus gas turbine). The established model is relied on thermodynamic equilibrium approaches and it is employed for 28 wood and woody biomasses. The studied model is practical for prediction of some outputs such as net power in a variety operating conditions like air flow rate and temperature and for different kinds of biomass materials with a specified ultimate and proximate analysis. The extablished simulation model can be a helpful tool for principally evaluation, assessment, and operation of down draft biomass gasifiers. Furthermore, the model can be employed as an assessment of various alternatives at an early stage to allow the decision-makers to carry out efficient oriented decisions.
The simulation model results for the 28 W&WB alternatives show that the net power produced from 1-ton feedstock entering to the gasification system is between the interval [0-400 kW/ton] and among them, gasification system derived from Tamarack bark biomass significantly outranks all other systems by producing 363 kW/ton, owing to the favorable results obtained in the performance analysis. At the end, a sensitivity analysis was directed to evaluate the influence of temperature of gasifier, and AFR on output power production from each system. Extention of temperature modifies the gasifier performance, it grows the production of both CO and H2 that causes to higher power on output. While, growing AFR reduces CO and H2 production which makes in degrading of system performance.