5Scheme of Biomass Gasifier Outlet Secondary Tar Removal
and Gas Generator Exhaust Purification
Because of enormous quantity all over the world, the usable solid biomass is mainly based on agriculture wastes and wood wastes. The biomass is hard to be collected and transported due to its scattered distribution. In the current conditions, the large-scale combustion technology is difficult to be adopted, so the small and medium-scale biomass gasification power generation technology (200-50,000kW) has its unique advantages. At present, due to serious waste and low price of biomass wastes, the costs for biomass gasification power generation are about 0.2-0.3 yuan/ KWH, being equal to or superior to conventional power generation. Its specific investment is only about 3,500 to 40,000 yuan /kW, occupying 60% to 70% of coal-based power generation, so it has the conditions for entry into market competition. Currently, the capacity to build MW-level biomass gasification power generation projects has been available.
The biomass gasification power generation technology cannot solve the secondary environmental pollution problem better (including the second pollution of tar to water and exhaust purification of generating set), so it greatly slows down the pace of development. Seen from the current technical condition, the reduction of tar content in the fuel gas and the purification of generator exhaust as well as avoidance of secondary environmental pollution is a key issue for the development of biomass gasification technology for power generation.
2. Analysis on the main pollutants arising from biomass gasification power generation technology
In view of current technology, the main pollutants arising from the process of biomass gasification power generation technology include as follows:
I. Tar, also called coal tar, is a kind of black or black brown sticky liquid obtained from the dry distillation process of coal, which is combustible and corrosive with some kind of special odor. It is a kind of mixed compound of hydrocarbon (voc) with high aromaticity.
II. Sulfur dioxide (SO2), colorless, is a poisonous gas with penetrating odor and is easily absorbed by the wet mucosal surface to generate sulfurous acid and sulphuric acid. It has strong stimulating effects on eyes and mucous membrane of respiratory tract. It can cause pulmonary edema, throat swelling and vocal cord cramping so as to result in suffocation if it is breathed in a lot. In case of mild intoxication, such symptoms as lacrimation, photophobia, begma and throat thermalgia, etc.; in case of severe intoxication, pulmonary edema can take place in several hours; it can cause reflex glottis cramping so as to result in suffocation if it is breathed in at an extremely high concentration. It also can result in inflammation or ambustion if contacting with skin or eyes. Chronic effects: In case of long-term contact at a low concentration, it causes constitutional symptoms like headache, dizzy and malaise as well as chronic rhinitis, faucitis, bronchitis, hyposmia and hypogeusia, etc. “London smog” is mainly caused by sulfur dioxide, and it causes serious pollution to atmosphere.
III. Nitrogen oxide (NOX). The anthropogenic emission of a large number of volatile organic matters and NOx and so on will disturb equilibrium and result in the generation of O3 and any other oxidative substances like PAN, namely photochemical pollution. This is a main pollution mechanism of nitrogen oxide. Nitrogen oxide is also an important index of atmospheric pollution. “Los Angeles type smog” is a kind of simulating light blue smog that is caused from the nitrogen oxide and hydrocarbon in the atmosphere under the effect of solar ultraviolet.
IV. Hydrocarbon (CH), mostly the VOC gas, is also commonly called “fragrant killer” by people. In a normal temperature, it exists in the air in a form of vaporization. Its toxicity, stimulation, carcinogenicity and special odor will impact the skin and mucous membrane and cause acute impairment to the human body. In a short time, it can make people feel headache, nauseous and vomitory. If serious, it may cause convulsion, coma and memory failing, and impair the people’s liver, kidney, brain and neural system.
V. Carbon Monoxide (CO). In the event of carbon monoxide poisoning, the carbon monoxide will combine with hemoglobin in the blood after entering into the human body to make hemoglobin unable to combine with oxygen, so as to cause oxygen deficit of organism tissues and then cause people dying from suffocation. Therefore, carbon monoxide is toxic.
VI. Carbon dioxide (CO2). Currently, the reason why the air temperature in the world is becoming higher and higher is the increase of carbon dioxide. Because carbon dioxide has heat preservation effect, the temperature rises up. In almost 100 years, the air temperature is increased by 0.6℃ all over the world. At this rate, it is predicted that the global air temperature will rise up by 1.5-4.5℃ in the middle of the 21st century. The sea-level rise is also caused for the increase of carbon dioxide. In almost 100 years, the sea level rises up by 14cm. In the middle of the 21st century, the sea level will rise up by 25-140cm. If the sea level rises up, Amazon Rain Forest will disappear and the ice blocks of the oceans in the Antarctic and the Arctic will also melt. All of those changes are a catastrophe for the human beings and wild animals.
VII. Dust. Dust pollution is absolutely not a little thing. It directly threatens the people’s lives. Especially in an environment with dust pollution, it will cause various kinds of cardiovascular diseases and respiratory tract diseases, etc. The incidence of asthma and bronchitis is rapidly rising steadily in the recent years, which is a typical embodiment of dust pollution aggravation. Dust pollution has a larger effect on old people and children. Moreover, excessive dust will easily produce bacteria and virus, which is extremely adverse to people’s health and especially a serious hazard to the skin.
VIII. Kakosmia. The influence on human environment by obnoxious gas is a very important issue in the modern society and it has aroused more and more attention by people. In some developed countries, people lodge a higher and higher complaint for kakosmia. In Australia, the complaint for kakosmia occupies 91.3% in total complaint for environmental pollution; and the complaint for kakosmia occupies over 50% in total complaint for air pollution in USA, and is second only to noise in Japan. In major cities of China, the complaint for kakosmia lodged by the residents is showing a sharp rising tendency, which has aroused the high attention of relevant departments.
The influence on people’s health by obnoxious gas arising from the production process is manifested in many aspects: It brings obnoxious feelings to people. If things go like this, it will cause various mental or physical symptoms, like emotional instability, dysphoria, inappetence, olfactory imbalance, insomnia, and even nausea, headache and induced asthma, etc. The high concentration kakosmia can cause important physiological barrier and mechanism barrier and pathological changes to human body as well as chronic diseases, shorten the life span and even cause acute diseases to the contaminated people and then cause death.
3. Status of existing technology
The traditional biomass gasification power generation system is mainly composed of purifying apparatuses such as screw conveyor, down-draft gasifier, cyclone dust collector and spray eliminator as well as roots blower, secure water seal, internal combustion generator, wet gas storage cabinet and pipe network, etc. Shown as Figure 1, the produced gas mainly includes H2, CH4 and CO, which is used for power generation so as to realize co-production of gas and electricity. As the gas stays a short time in the biomass gasifier, the too low temperature cannot satisfy the conditions for complete reaction of reduction zone and a large amount of tars (100～150mg/m3) is produced; through dry and wet purification and filtration, the tar content is controlled at about 50mg/m3. The excessive tar and ash content in the biomass gasification gas will pile up the charcoal in the sparks, failing to strike fire. The experimental data shows that the produced gas with tar and ash content of less than 15mg/Nm3 will less likely to pile up the charcoal, thereby meeting the reliable operation of generating set.
If it is operating according to the existing technology, there are two most urgent problems: the first one is that the tar content (50mg/m3) cannot satisfy the operation of internal combustion generating set; the second one is that the secondary pollution caused by tar and the environmental pollution caused by emission of internal combustion generating set is even more serious.
(Figure 1) Technology Diagram for Biomass Gasification Power Generation System
II. Tar removal purification technique
From the source, tar purification radically solves the structural and technological problems of biomass gasifier, which makes the tar meet the optimum conditions of cracking reaction at the cracking zone to generate CH4 and H2 through thorough reaction, thus not only solving the problems of environmental pollution, but also improving the thermal value of gas and the contents of combustible constituent in gas. However, it is not possible that the fuel gas at the outlet of biomass gasifier is clean without any tar due to the limitation of technology, and secondary tar removing problem exists technically. The common-used tar removing technique includes static tar removal technology and filtering tar removal technology which are briefly introduced in the following.
① Static tar removal technology
Working principle: when the coal gas with tar (dust) flows through the interpolar space under the condition of gas ionization, the dust and tar particles in the gas are charged forcibly during collision and friction with electron. The free electron which is accelerated not only is in large quantities, but also flows to the anode (receiving electrode) through the overall interpolar space, while the flow area will account for 99% area of interpolar space, thus most of the gas dust and tar particles have a negative charge and flow to the anode (receiving electrode). The charged dust and tar particles at the receiving electrode are collected after neutralization, and few dust and tar particles with a positive charge flow to the cathode (corona electrode) , and then are collected after neutralization. Ionization only occurs at a highfield not far away from the cathode, thus the iron with a positive charge is easily attracted by the cathode so as to flow to the cathode. The flow area of positive iron only accounts for a small area of overall interpolar space.
Advantages: it has large treated wind volume and only consumes electrical energy.
Disadvantages: ① The negative plate is easily absorbed with oil and scale due to the adsorption of static electricity, thus resulting in such problems of hard cleaning and reduction of tar removal capability, and the effect is not ideal even a steam cleaning unit is provided; ② it has high equipment design and quality requirements, difficulty in stable operation of equipment, large investment, greatly difficult maintenance, and secondary pollution problem; ③ it has no guarantee of security. The requirement of static tar removal for oxygen content is equal to or less than 1, but in actual operation, the oxygen content in fuel gas is often more than 1 at some moment. At the same time, the high-voltage discharge of discharge filament brings potential safety hazards due to the gas in the biomass gasifier being flammable and explosive gas; ④ the efficiency of tar treatment is equal to or less than 20mg/m3, which can’t meet the requirements of generator set. Please see Figure 2 for main structural principle of static tar remover.
② Shengda high-foam multi-aperture tar removal filtration technology
Shengda gas & tar filter is exclusively used for biomass gas-making device to remove such impurity as dust and tar etc. Our company adopts the high-foam multi-aperture materials with proprietary intellectual property rights as the main filter material of tar filter. It has the advantages of excellent mechanical strength, uniform structure, high porosity factor, high pass rate, large specific area, high chemical and electrochemical activities, large dust containing capacity, small differential pressure, easy cleaning, long service life, temperature resistance and corrosion resistance etc., thus may operate normally under difficult working condition. Moreover, it can effectively remove the water, dust, tar and other solid impurity in the fuel gas with high filtering precision, excellently improve the combustion efficiency (thermal value) of gas, and is environmental friendly and energy saving. Please see Figure 3 for the structural principle of Shengda gas & tar filter.
III. Purification technology for exhaust of internal combustion generating set
1. Main pollutants
The components of the exhaust of internal combustion generating set mainly include: sulfur dioxide （SO2）, nitrogen oxides （NOX）, hydrocarbon compounds（CH）, carbon monoxide（CO）, carbon dioxide（CO2）, dust and obnoxious gas.
2. Shengda SD-SG exhaust purification series
Such equipment can conduct effective disposal such as collection, capture and degradation for the harmful ingredients such as SO2, NOX and CH, dusts and obnoxious materials in the exhaust of internal combustion generating set, the dust removal efficiency reach more than 95%, and the concentration of obnoxious gases is no more than 1,000 dimensionless, which conforms to GB14554-93 emission standards of the People’s Republic of China.
1) Working principle
See (figure IV) for the equipment principle, the exhaust of internal combustion generating set is transmitted to this device through air duct, and most dusts and partial water soluble molecules are involved in water through the impact and wash of numerous form when passing through the duster unit, and most of the remaining CH and obnoxious materials transfer to odorless and harmless materials through surface action of special packing suction and the synergic oxidation action generated by the photolysis pipe. The gases after disposed by the aforesaid unit carry lots of water fog which after condensed and dripped by the defogger become clean, nontoxic and odorless gas and discharged from the air outlet pipe through fan installed integrally.
2) Appearance and performance of the equipment
See (figure V) for the equipment appearance
(Figure IV) Figure of Equipment Principle
(Figure V) Figure of Equipment Appearance
High performance of the equipment
1. The prominent characteristic of this equipment is combining the VOC degradation, dust removal and peculiar smell removal, and with compacter and more reasonable structure.
2. Overhead-type air inducing structure minimizes the land occupancy of the generating set and greatly decreases structural wind resistance.
3. It removes peculiar smell with spray, foam and photolysis while removing dusts, thus realizing thorough and effective gas purification.
4. The materials of the equipment are of stainless steel, which ensures high strength and long service life.
5. Its whole operating process is controlled by PLC, which can be operated by
pressing the touch screen for prompts, and it is simple and convenient.
IV. Process flow for secondary tar-eliminating of fuel gas of biomass gasification furnace and for tail gas purification of internal combustion engine set
In sum, a complete biomass generating system shall include the secondary tar-eliminating device for fuel gas of biomass gasification furnace and the purification device for tail gas of internal combustion engine set, and the detailed process flow is shown in Figure 6. Straw and other biomass materials are delivered to the down-draft gasification furnace through screw conveyer, and the produced fuel gas will be firstly subject to the dust removing treatment through the dust collector cyclone, spray eliminator and other purification devices provided by the gasification furnace manufacturer and the primary tar-eliminating treatment, and then enter the secondary tar-eliminating device of the biomass gasification furnace (fuel gas)（the red device in the Figure）to make the tar consistency reduced to be smaller or equal to 10mg/Nm3. Finally the processed fuel gas will enter the generation system of internal combustion engine to be burned for generating power. The tail gas produced by the generation system of internal combustion engine will be discharged into the air through the chimney after being treated by the purification device for tail gas of internal combustion engine set (the red device in the Figure). The entire system can realize the win-win of economic benefit and environmental-protection benefit.
(Figure 6) Process Flow for Secondary Tar-eliminating of Fuel Gas of Biomass Gasification Furnace and for Tail Gas Purification of Internal Combustion Engine Set