Biomass (Solid Waste/Sludge) Gasification
This study is supported by the Economic Development Board (EDB) of Singapore under grant number: R-261-501-003-414 through the Minerals, Metals and Materials Technology Center (M3TC) of the National University of Singapore and the National Research Foundation under the SJTU CREATE program grant number: R-706-001-101-281 through the National University of Singapore.
The fossil fuels, such as coal, petroleum and natural gases, are being consumed rapidly, and will be depleted in the near future, the biomass is gaining more attention because it is renewable, CO2 neutral and environmentally friendly. Through gasification biomass can be converted into gaseous, liquid and solid fuels, convenient for storage and transportation. Gasification of solid waste can be considered as an effective and economical clean technology for the production of bio-syngas (CO+H2) as a source of clean energy.
According to the statistics of National Environment Agency (NEA), in 2012 Singapore has generated 7.3 million tons of Municipal Solid Waste (MSW), and average 60% of them is recycled, but recycle rates of some types of MSW are quite low, for example, paper/cardboard is 56%, horticultural waste is 44%, food waste is 12%, plastics is only 10%, and there is no recycling for sludge. For the non-recycled MSW, incineration is the main method, but there is less and less spaces for land fill of the ashes in Singapore, furthermore, the incineration may produce a large amount of pollutant gases, such as CO2, NOx and SOx etc, the heavy metals in the ashes also cannot be removed, which will pollute the water resources. Actually in some MSW, such as wood, paper, and plastics, some energy can be recycled as the clean power supply, through the gasification this aim can be reached; furthermore, the pollutant emission can also be minimized greatly.
We have made some achievements in these studies. The 10 kW gasifier has been set up (Fig.1), and its performance has been tested with wood chips as feedstock. According to the experiment, 1kg wood chips can produce about 2 m3 of synthetic gases (15% CO and 15% H2), or 0.75 kWh electricity, the cost of electricity generated from this gasifier with biomass is in the range of SGD 0.024 to 0.06 per kWh, much lower than the cost from coal, natural gas or other fossil fuels. The consumption rate of wood chips for this gasifier is about 216 kg/day with 75% load. This gasifier can be used to gasify the horticultural waste, paper/cardboard even plastics and sludge, the relevant experiments are being carried out. The existing gasifier is based on the fixed bed technology, but it is only suitable for the small-scale gasification, now we are planning to develop the fluidized bed gasification technology, and it can be used in the medium or large scale gasification, suitable for future commercialization, furthermore it has high gasification efficiency and great adaptability to the feedstock.
Demonstration of our biomass gasification system
Figure 1 shows the diagram of our 10 kW biomass gasifier which is used to convert biomass waste (sludge/wood) into electricity
1. Biomass Gasification Process for Waste Conversion into Energy-Clean, Efficient, and Low Emission
As mentioned earlier, gasification of biomass (solid waste/sludge) materials may produce a large amount of pollutant gases, such as CO2, NOx and SOx etc. Therefore, biomass gasification process which can minimize the emission of those pollutant gases has been developed in our group (as shown in Fig.2).
At the end of process, hydrogen (H2) is used as a fuel source to generate electricity rather than syngas (a mixture of CO and H2). Water is the only emission product from its combustion. Therefore, the overall process can be considered as the low emission process for biomass (waste) conversion into clean energy.
Figure 2 shows the schematic diagram of the developed biomass gasifier system
2. Dual Fluidized-Bed Reactor for Biomass Gasification
As compared to downdraft (or updraft) gasifier (Fig.3), fluidized-bed gasifiers represent a balance for potential large-scale applications, ability to handle wet biomass (e.g. sludge), low impurity formation, ability to minimize tar formation, and high thermal efficiencies.
Figure 3 shows the downdraft fix-bed gasifier and the formation of tar (high molecular weight hydrocarbons such as toluene, naphthalene, etc.) and char during the biomass gasification.
Recently, we have been developing dual (two-stage) fluidized-bed gasifier (Fig.4) for biomass gasification. Addition of a second zone is believed to further increases efficiency and fuel quality. In the first zone, biomass is gasified with air (and steam) into gaseous products (CO, CO2, H2, etc.), volatile compounds (Tar), and residual solid char. At the same time, tar is reformed in the second zone. As a result, dual (two-stage) fluidized-bed gasifier is believed to be able to minimize the formation of tar and improve producer gas.
Figure 4 shows the schematic diagram of our 10 kW biomass gasifier which has been used to produce electricity from biomass.
3. Numerical Simulation of CO2 Gasification of Biomass for Clean Energy
Biomass gasification is an effective way to alleviate the shortage of fossil fuels and reduce the pollutions. In order to reduce CO2 emissions during the utilization of biomass, we carry out pioneer work on CO2 gasification of biomass. CO2 gasification has several advantages, for example, no energy is required for vaporization unlike steam, different H2/CO ratios in syngas can be achieved easily, use of CO2 can results in a reactive char, helping improving the gasification efficiency, etc. In group we conduct numerical simulation for CO2 gasification of biomass in the bubbling fluidized bed. The effects of CO2-to-biomass ratio, the percentage of CO2 in gasifying agent, biomass size, and moisture contents in the biomass on the biomass gasification are investigated systematically. It is found that when the CO2 mass percentage is 60% in the gasifying agent of mixture of CO2 and air, the produced gases have the highest heating values, and the cold gas efficiency is also the highest. The moisture content and biomass size have negative effect on the biomass gasification.
Figure 5 Mole fractions of gas species during CO2 gasification of biomass [Cited from: Y. P. Cheng, C.H. Wang, Numerical simulation of coal/biomass gasification for clean energy applications, 9th European Congress of Chemical Engineering, World Forum, The Hagen, Netherlands, April 21-25, 2013.]