Biomass: Combustion and Gasification for Substitution of Natural Gas
A considerable need for natural gas is one of the key problems of Ukraine’s energy balance. Considering high volume and high cost of import, it is clear that the efforts of scientists, engineers and entrepreneurs are focused on solving the issue of energy saving and natural gas substitution with alternative energy sources.
Biomass is considered to be one of the most significant alternative energy sources. The evaluations of Ukraine’s bioresource potential vary widely. Based on an optimistic scenario, the potential comprises not less than 4.16M metric tons of oil equivalent, including peat at 0.52M; wood waste at 1.1M; and straw at 2.54M metric tons. An opportunity to grow the biomass on empty land is taken into account, and its yield is expected to be over 21M metric tons of oil equivalent.So, Ukraine’s biopotential could enable the substitution of the natural gas volume ranging from 4.3 up to 25.6 billion m3.
The Problems and Prospects of Biofuel Use
Cost efficiency of the project is the key criterion for the decision on biofuel use. In the Ukraine, the cost of natural gas is two or three times higher than the cost of biofuel in a volume equivalent to the natural gas in terms of its energy potential. Availability of proven technologies and equipment is another important driver, which determines the use of biofuel in technological processes. Therefore, the task of creating reliable and cost-efficient industrial objects (with the minimum payoff periods) that use biomass for the substitution of natural gas was instrumental in defining the biofuel “research” work program.
Prospective investors are primarily interested in a reference facility where the proposed solution is already used so that they can find out about it on-site. It was, therefore, decided to initiate the development of demo projects.
Two types of technologies of biofuel use in thermal processes exist. They are based on two different principles –
direct combustion and gasification. The highest effect is reached for direct combustion of pre-treated biomass in boiler furnaces or kilns. Biomass pre-gasification technology competes with direct combustion in a number of applications. Any energy transformation causes irreversible thermodynamic losses, which makes the fuel’s primary energy utilization somewhat less efficient than direct combustion.
However, gasification has an indisputable advantage over direct combustion when the project is specifically intended to generate electric energy, process steam, in cogeneration technologies and combined processes of producer gas combustion simultaneously with natural gas and in the same burner. The study of technological chains using natural gas as a fuel enabled the development of the criteria to assess applicability of the specific natural gas-substitution technology.
Direct Biomass Combustion
The solution implemented on the rotary kiln of refractory clay firing at Vatutinsky Refractories in the Cherkassy region of the Ukraine can serve as an example of using biofuel direct-combustion technology. The factory’s major shareholder is A.G.S. Corporation (France). Technical re-equipment of the system was targeted at the maximum reduction of natural gas usage through its substitution with pre-treated biofuel. The furnace specifications were 75 meters in length, 3.5 meters in diameter and 15 ton/hour output of final product. Average gas consumption before the project was 2,200 m3/hour. Based on the evaluation of the local biomass resources by the factory’s experts, sunflower husk was selected as the principal biofuel with the possibility of using waste wood (sawdust) as well. The task of shifting to the use of biomass presupposed the development of technical requirements of the fuel and the technology of its co-combustion with natural gas.
As the first step in the project implementation, technical requirements that the fuel had to meet were defined to ensure complete combustion of particles in the combustion space. At the same time, the type of fuel was to be taken into account. The parameters that determine the combustion speed are fraction composition and moisture of solid particles. The time of the particles’ combustion was experimentally determined on the fluidized-bed installation with inert heat carrier during the combustion in the air at 900˚CС(1652˚F).
It follows from this experiment that wood waste burns out about two times quicker than the particles of sunflower husk. The deviation in the moisture level from the natural moisture (10-12%) results in the increase of their burnout time.
The results of the research are shown in Fig. 1. The experimental data regarding the wood sawdust have completely agreed with the results of mathematical process modeling. The sunflower-husk burnout process was not modeled because an insufficient source data was available on this fuel’s physical properties.
Another difference between the processes of biomass and gaseous fuel combustion is the varying theoretical combustion temperatures and the amount of air needed for complete burnout of the fuel. In the process of shifting to biomass use, the specific features of its burning process are crucial. These were experimentally determined through combustion of fuel particles in the derivatograph. A 162.6-mg sunflower husk was burned in the open platinum cup. Separate intervals with alternating-sign heat flux can be singled out on the thermogram (Fig. 2).
The heating of the biofuel mass and moisture evaporation initially occur. This process is accompanied by thermal-energy consumption (the peaks 1 on DTA and DTG curves). After heating the fuel’s particle to over 200°C (392˚F), the process of its destruction starts (peak 2 on the DTG curve). With the extraction of volatile matters, the combustion ensures intense heat radiation (peak 2 on the DTA curve). The combustion of biomass particles is completed with coke residue burnout, and the heat-emission peak of the latter in the process of burning is indicated with point 3 on the DTA curve.
The practical conclusion drawn on the basis of the thermogram analysis was as follows: The combustion of solid particles should have been organized in such a way as to reduce the time of moisture evaporation and distillation as much as possible. This was accomplished with the help of a special burner design, which ensured intense recirculation of fuel in the root zone of the flame.
The research has enabled us to formulate the requirements for efficient burning of biofuel in the rotary kilns (Table 1).
The results of the biofuel investigation and the combustion specifics were used as fundamentals for creating the automated complex of biomass use as fuel – natural gas substitute – in the rotary kiln. The complex includes a fuel depot, biomass pneumatic conveying line, a burner device, and a control and automation system (Fig. 3). The burner device for solid fuel supply is installed in the kiln head above the gas burner (Fig. 4).
The results of partial natural gas substitution with biofuel are shown in Table 2. This work to optimize the kiln operating modes for joint combustion of natural gas and biofuel has demonstrated little dependence of kiln temperature on the ratio of gas and biofuel.
During the burning of mid-temperature fireproof compounds, the degree of natural gas substitution reached 70%, while in the process of burning high-temperature fireproof compounds it was up to 50%.
The complex has been successfully operated since 2010. Annual volume of natural gas substitution with biofuel is over 10 million m3. Any funds invested in the project were repaid (ROI) in less than one year. As an added benefit of the project, new jobs have been created to provide the factory with biofuel.
The technical solution set out above can be successfully used in a number of companies in the metallurgy and construction-materials sectors.
Biomass Gasification: Substitution with Producer Gas
Setting up a complex with 1.8 MW of installed capacity for wood-chip gasification is an example of the development and implementation of biomass gasification technology and equipment. The complex is designed for partial substitution of natural gas in the steam boiler heating system. It includes a gas generator (Fig. 5a); a gas purification and transportation system; a dual-fuel burner for simultaneous combustion of natural and producer gas (Fig. 5b); and a system of boiler operation automatic support and the steam boiler.
The complex was developed by the Institute of Gas of the National Academy of Science of Ukraine and has been operated at JSC “Malyn Paper Mill – Weidmann” (Malyn, Zhytomir region, Ukraine) since January 2011. Significant thermal-load fluctuations are specific to the company’s technological process.
Average producer gas output, taking into account the power change, was 120 m3/hour in natural gas equivalent. On average, 4.2 kg of woodchip with natural moisture content was used for the substitution of 1 m3 of natural gas. The substitution of natural gas with producer gas did not result in boiler productivity reduction (Fig. 6). The development of an effective system for producer gas purification from resins and resin-containing items was an important result of this effort. The use of woodchip for natural gas substitution enabled natural gas cost reductions of over 30%.
The gas generator described above is a continuously operating facility. At the same time, a pilot batch-oriented gas generator for gasification of different biomass types was developed and tested.The fundamentals used for the generator development were based on technology by Sibtermo Company (Krasnoyarsk) applied for Kansk-Achinsk brown-coal gasification for semi-coke production purposes. The testing results are presented in Table 3.
About 20% of energy is spent on the cooling of the gas generator body. This energy can be used for heating the boiler feed water or for technical needs.
The substitution of natural gas with biofuel is commercially viable and attractive for many countries. Two technologies of biomass use are considered in this article: direct combustion and gasification. Each of them has its advantages and areas of application. Technical solutions piloted in industrial scale could be efficiently applied in manufacturing and energy sectors. IH
For more information: Contact Prof. Karp Igor, The Gas Institute of Nat. Ac. Sc. of Ukraine, 39 Degtyarivska St., 03113 Kiev, Ukraine; tel: (380 44) 456 02 83; e-mail: firstname.lastname@example.org
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