Optimization of gasification process and waste heat recovery for power and distilled water production

Abstract

Two major issues concerning the world presently are the depletion of fossil fuels due to an increase in energy requirements and global warming mostly caused by the rejection of flue gases (waste heat) to the atmosphere. In many developing countries, people are still using biomass inefficiently for heating and cooking purposes which causes an indoor air pollution and diseases to people. Now, one of the major arising issues for the environment is stubble burning (combustion process) in the field and many other places. Many industries are still using biomass for power generation through combustion process which causes environment pollution as well as global warming. Additionally, most of the industries, automobiles, power plants and combustion engines produce waste heat (WH) caused by inefficient use of fossil fuels and dump into the atmosphere. This WH represents about 20-50% of the fuel energy consumed by a conversion system and is capable for sustainable energy use. For intense, an amount of 440 TWh/year is released by the industrial sector of the United States (US) as reported by the Department of Energy, the US while India releases about 160 TWh/year from cement, iron and steel industries only. Further, the availability of fresh drinking water is also a universal problem because the quantity of freshwater available is only about 2.5% of the total quantity and the remaining is saline water. The increase in population and subsequently the decrease in freshwater arise the major attention of researchers towards it. Reverse Osmosis/RO (membrane-based) to produce fresh drinking water has limited output and also it is driven by the power consuming pump. It is such an extensive energy process that it requires about 10,000 tons of fossil fuel every year to produce 1000 m3 of fresh water per day. These concerns motivate the researchers to develop more efficient and clean energy technologies. This work is aimed at the assessment of effective energy (heat and power) and fresh drinking water production from the renewable energy as well as WH sources through the development of efficient systems. This study is carried out fully experimentally on the various developed systems and performance parameters have been analyzed. Based on the research gaps identified in the literature survey, the present work has been accomplished in the following steps. The first study (chapter-3) reveals the energy cogeneration study of locally available biomass red mulberry (Morus Rubra) and other biomasses (dried grass, leaves and dead branches) in a 10 kW downdraft biomass gasifier (power plant). The optimal operating condition of a gasifier is obtained by performing experiments at variable equivalence ratios (ER), and by analyzing the characteristics of syngas (produced) along with gas composition, calorific value (CV) and cold gas efficiency (ηcg). Before processing various biomasses for gasification, the potential capability of these biomasses was analyzed by determining higher heating value (HHV), ultimate (C-Carbon, H-Hydrogen, N-Nitrogen, Su-Sulphur and O-Oxygen) and proximate (moisture content/MC, ash content/AC, volatile matters/VM and fixed carbon/FC) analyses. The characterization of gasification end products including bio-oil and bio-char have been done by gas chromatography and Fourier-transform infrared techniques. Furthermore, the economic analysis (electricity generation cost and payback period) of the biomass gasification-based power plant of various capacities (10 kW, 500 kW and 1000 kW) are assessed. The outcomes of this study specified that the various biomasses mentioned above are capable in producing thermal energy and power within the substantial range (15.58-18.36 MJ/kg) of HHV. The ultimate and proximate analyses show that red mulberry biomass is relatively superior to other biomass reported in the published literature, while dried grass, leaves and dead branches are also comparable with others. The maximum values of CV and ηcg are obtained as 5.846 MJ/m³ and 68.45%, respectively for red mulberry biomass at the optimum ER of 0.296. The economic analysis indicated that the electricity production cost (Rs.4.34/kWh or 0.055 USD/kWh) and payback period (3.12 years) are minimum for the largest capacity plant i.e., 1000 kW. In the second study (chapter-4), use of biomass energy as an external heat source for the heat recovery system (HRS) to produce continuous and long-term electric power generation aimed at end-use applications. This study is carried out in three stages to accomplish the objective. In the first stage, the electrical power is generated from the WH of biomass energy driven engine-generator using two thermoelectrical generators (TEGs)-thermosyphon-based HRSs. The proficiency of the TEGs-thermosyphon-based HRS is recognized for power generation by performing the experiments at variable conditions of heat source water temperature (TS) and thermosyphon filling ratio (TFR). The performance is analyzed by measuring the different electrical parameters such as open circuit voltage (V), short circuit current (IS) output power (Pₒ) and conversion efficiency of TEG (ηTEG). The effort is made to charge a 12 V, 7 Ah uninterruptible power source (UPS) battery for end-use applications. The results showed that the maximum values of V, IS, Pₒ and ηTEG are found as 17.12 V, 0.152 A, 0.615 W and 2.218% respectively at the maximum TS of 87 ºC and an optimum TFR of 0.496. It has been realized that this HRS is able to charge a 12 V, 7 Ah UPS battery and found a minimum value of IS as 0.118 A to charge it. In the second stage, these two TEGs-thermosyphon-based HRSs are installed in a salt gradient solar pond (SGSP) for power generation since the literature study found on this system are performed either theoretically or electrically at the simulated conditions. The performance of HRSs has been investigated by carrying out the experiments for 40 days under the actual weather conditions. Before installing them, the capability of a fabricated SGSP is examined for storing the solar energy through measuring the profiles of key thermal and electrical parameters such as temperature, specific heat (cSW), thermal (kSW) and electrical conductivity (EC) of salt water along with the thermal efficiency of SGSP (ηSGSP). The results indicated that the SGSP is able to store a high amount of solar energy in its lower convective zone (LCZ) with 6.06% of ηSGSP. It has been found that this thermosyphon-based HRS is not able to generate the minimum required output to charge a 12 V, 7 Ah UPS battery at the maximum achievable temperature gradient obtained in SGSP (∆TLU) due to the involvement of various thermal resistances (a total of nine) in the heat flow. The maximum V of 1.44 is obtained corresponding to a heat source (here LCZ i.e. TLCZ) temperature of 49.99 °C. Further, it is also suggested that an external heat source is mandatory to meet the same that can be fulfilled by the WH of biomass based engine-generator of a gasifier. Subsequently, the biomass energy required to produce the enough output is computed theoretically under certain conditions and found that 15.60 kg of biomass is needed corresponding to a minimum obtained average (over a day) ∆TLU of 7.0 °C for charging a UPS battery. Moreover, the economic analysis is also performed in order to viability of the proposed system (a SGSP combined with a 10 kW gasifier) and found that this system can replace the conventional power generation from diesel with a significant amount of profit. In the third stage, a new TEGs-array-based HRS is developed for the effective power generation from SGSP through minimizing the thermal resistances. This new HRS is integrated with a small capacity SGSP (proposed system) which is firstly operated by the solar energy under actual weather conditions and then externally heated by the WH of biomass based engine-generator of a 10 kW gasifier at variable conditions of an engine load (WL) and frequency (f) levels. Before integration of a new HRS with the SGSP, a study is carried out to analyze the capability of fabricated small capacity SGSP for recovering and storing the WH of an engine-generator. The results revealed that this SGSP has efficiently recovered the WH with a significant range (0.75 0.77) of effectiveness and a high ηSGSP of 47.73%. The outcomes of the proposed system when operated by the solar energy explored that this new HRS has generated much higher (9.19 times) output compared to the TEGs-thermosyphon-based (1.44 V at 49.99 °C) and found a maximum V of 7.97 V corresponding to 43.36 °C of TLCZ. But this output is still not enough and also it doesn’t remain constant due to the intermittency of solar energy over the day and unavailability during the night. Therefore, the proposed system is further operated by the WH of biomass engine-generator for constant power generation and obtained the maximum V, IS, Pₒ and ηTEG as 81.62 V, 0.272 A, 7.483 W and 4.63% respectively at 68.04 ºC (TLCZ), 3 kW (WL) and 51 Hz (f ). This high performance is obtained due to upgrading in HRS along with the cooling system where seven thermal resistances (out of total nine) have been totally eliminated with the aid of an array-based system and the remaining two are minimized by using a high thermal conductive material (copper). A 12 V, 80 Ah heavy-duty battery is successfully charged from the Pₒ of TEGs-array-based HRS and the minimum IS required to charge this battery is recognized as 0.155 A under the given conditions. Therefore, the TEGs-array-based HRS has effectively generated the power from heat sources, however it has been realized that pumps are required when it was connected with the SGTSD. Thus, to make it more effective for power generation without using pumps, a proposal of arrangement has been suggested in which the TEGs-array-based HRS can be applied directly to WH available at high temperature via common rail waste heat pipe and then the leftout low temperature WH can be stored in SGSP. A demonstration study has been carried out to validate the concept of direct connection of TEGs-array-based HRS with the WH of biomass engine generator. The results showed that this proposed HRS has produced 12-14 times more performance than the TEGs-thermosyphon-based HRS when operated under similar temperature limit and obtained V, IS and Po of 74.78 V, 0.261 A and 4.93 W respectively. In the third study (chapter-5), the use of biomass energy as an external heat source (WH) for water desalination using a developed distillation system. Therefore, a new wick and copper-finned distillation system (CFDS) is fabricated which is driven by the WH of biomass based engine generator. The performance of proposed CFDS is analyzed in the form of an amount of distilled water (md) produced at variable operating conditions of waste heat inlet temperature (TWH, inl.), glass inclination angle (ϴg) and height of basin water (Hbw). Furthermore, the response surface based correlations (linear and quadratic) are also developed using BBD, ANOVA as well as regression methods and the accuracy or goodness of generated correlations is checked by the coefficient of determination (R2). The deviation in the response parameter (md) is analyzed in terms of input factors (TWH, inl., ϴg and Hbw) through main effect, interaction, surface and contour plots along with the percentage contribution of each input factor (linear, square and 2-way interaction). The outcomes of this study indicated that the maximum value of md is obtained as 2.407 kg at 321.53 ºC, 45º and 0.08 m of TWH, in, ϴg and Hbw respectively. The employment of wick and fins in the distillation system has a positive effect on the yield of md and increased the productivity from 2.054 kg to 2.407 kg with 17.18% improvement at the optimal conditions of input parameters. It can be suggested that the highest level of TWH, in is always preferred to obtain a large value of md, but the moderate levels of ϴg and Hbw provide maximum performance for the same. It has been realized that each factor has its certain influence on md, but Hbw and TWH, in are found to be the most dominant factors over ϴg. The quadratic correlation has been found to be in good agreement with the experimental values with a maximum error of 12.03%.