Thermochemical gasification of local lignocellulosic biomass via fixed-bed and fluidized-bed reactors

Abstract

The government in Norway aims to double the bioenergy use from 14 TWh to 28 TWh between the year 2008 and 2020. This calls for major changes in the current energy practice and an increase in the biomass based energy applications in the total energy mix. One effort to this end is the replacement of existing space heating oil burners with heat from biomass combustion. However, this strategy alone could not be able to adequately meet the target of further bioenergy expansion. Neither will it help in achieving the Kyoto aim of reducing CO2 emission. So an alternative such as gasification could be a part of the solution. Gasification offers many advantages, but a major concern towards this technology is the logistics. A highly efficient gasification process would mean very little to a technical viability if efficient biomass supply is not guaranteed. Thus maintaining a smooth biomass supply to the conversion plant is important in perspective of which local lignocellulosic biomass could be an interesting option. In terms of the availability of lignocellulosic biomass, Norway is strategically well placed due thanks to its abundant forest reserves. Norway and forest are the two sides of the same coin which makes this country ever attractive in developing and practicing local bioenergy research, which is partly why this study is conducted. The main objective of the present project was to utilize local forest crops birch, oak and spruce and energy crops like poplar and willow as feedstocks for labscale fixed bed downdraft gasification. Further to evaluate the gasification performance interaction with biomass characteristics and process parameters, experimental work consisted of series of gasification tests was optimized over a range of air equivalence ratio (ER) between 0.19 and 0.45. The key results in terms of gas lower heating value (LHV), gas yield, cold gas efficiency (CGE) and carbon conversion efficiency (CCE) indicated that the producer gas obtained from all types of woodchips species gasification exhibited a great potential to be utilized for further downstream applications. Besides wood, in many places of the world the lion’s share of the biomass is contributed from herbaceous biomass. Hence this research also focused on two local herbaceous biomasses, alfalfa and wheat straw (in pellets) from Spain and one local herbaceous biomass common reed (in briquettes) from Norway for investigating gasification via two fluidized bed configurations being pilot-scale and lab-scale, and one fixed-bed downdraft gasifier respectively. Pilot-scale 4.7 kg/h air-blown gasification of sole alfalfa pellets was carried out at a variable ER between 0.25 and 0.30. The resulting bed temperature was ca 780 C and the maximum producer gas lower heating value (LHV) was ca 4.2 MJ/Nm3 which suits for combined heat and power application. Pilot-scale test also yielded producer gas composition rich in H2 composition (around 13 % dry basis vol.), indicating the potential of this gas to be utilized for hydrogen production. In addition to the pilot-scale tests, a series of lab-scale fluidized bed gasification on alfalfa and wheat straw pellets were conducted for a wide range of ER (0.20- 0.35) achieved by varying both fuel and air input. The optimized gasification performance attained for the two feedstocks differed in respect to the operational ER with a higher ER (0.35) for alfalfa and a lower ER (0.30) for wheat straw respectively. At the optimum condition, the gas produced from both the feedstocks exhibited a good LHV value above 4.1 MJ/Nm3 which can be recommended for engine or, turbine application. However, owing to the poor H2 composition of 4 %, this technology may not suit downstream H2 production. The fixed bed gasification on common reed briquettes on the other hand is a very recent addition to the current project which is not fully developed yet. Therefore the preliminary results obtained from a solitary test run were not representative. The measured parameters so far indicated that the gas LHV, bed temperature and ER range of 2.9 MJ/Nm3, 498 C and 0.35-0.86 respectively can be achieved. Last but not the least, a techno-economic modeling considering a producer gas generator in a 100 % renewable energy hybrid plant was performed for a standalone household in Grimstad by using HOMER (a commercial hybrid energy optimization tool). The results showed that producer gas generator together with the other renewable energy options is capable of fulfilling the annual electricity demand of a remote household with a constraint of a producer gas price less than 0.1$/Nm3. This system also provided environmental gain worth ~22,000 kg/year of CO2 equivalent savings. All in all, the influence of diverse local biomass feedstock on diverse gasification techniques is proved to be feasible in aspects of technology, economy and sustainability.