In order to reduce the pollution of municipal sludge on the environment and achieve the resource utilization of sludge, the current situation of sludge disposal at home and abroad is elaborated. The research status of sludge mixing with coal to prepare sewage slurry, mixing with coal (semi coke) to prepare formed fuel, and mixing with plant straw to granulation to prepare fuel to produce biomass fuel is introduced.
The results indicate that the preparation of biomass fuel from sludge can fully utilize the effective calorific value of sludge, replace a small amount of coal, and provide an effective technical approach for the rational utilization of sludge. Analyzed the problems of small scale, high energy consumption, complex process, and low fuel calorific value in the preparation of biomass fuel from sludge, and proposed strengthening the deep modification of sludge to prepare high concentration fuel coal water slurry, the granulation of sludge and fuel straw, and the development of efficient binders for the preparation of formed fuel from sludge and coal (semi coke), as well as the dehydration and drying of sludge before forming Research on the waterproofing treatment of formed fuels and the industrialization of sludge to biomass fuel, in order to accelerate the industrial application of sludge to biomass fuel.
Introduction
Urban sludge is a byproduct of sewage treatment with a large amount of pollutants generated by sewage plants in the process of treating urban domestic sewage and industrial wastewater. With the rapid growth of China's sewage treatment rate and the resulting rapid increase in sludge production, municipal sludge has reached over 30 million tons per year (with a moisture content of 80%). From the appearance, urban sludge is a black or black brown fluid like, semi fluid like, or cake like flocculent.
The characteristic is high moisture content, up to 25%~98%, large volume, and complex composition. Sludge contains solid particles such as mud and sand, fibers, animal and plant residues, as well as coagulated flocs. It contains a large amount of organic substances (mainly benzene, chlorophenols, etc.), toxic and harmful heavy metals, pathogenic microorganisms, parasitic eggs, salts, and radioactive nuclides, which are difficult to degrade and cause serious environmental pollution. At present, the main treatment method for sludge at home and abroad is landfill, but the increasing production of sludge not only occupies land, but also pollutes groundwater quality; The significant reduction in incineration is one of the international directions for sludge treatment, but the initial investment in the construction of incinerators is high and the operating cost is high. Therefore, how to effectively treat and dispose of urban sludge is an urgent problem to be solved.
Due to the large amount of organic matter contained in sludge having a certain calorific value (dry based high calorific value of 8-10MJ/kg) and abundant nutrients such as nitrogen, phosphorus, and potassium, the resource utilization of sludge is of great significance for achieving sludge reduction, stabilization, and harmless treatment. At present, there are three main ways to develop sludge resource utilization: land use, building material utilization, and preparation of biomass fuel. Among them, preparation of biomass fuel is to mix sludge with industrial, agricultural waste, or clean fuel to produce high calorific value fuel for boiler combustion, which is the main way of sludge resource utilization. The preparation of biomass fuel from sludge mainly utilizes the low calorific value of sludge, which cannot be directly burned. Mixing it with combustible materials to prepare biomass fuel as energy use not only fully utilizes the calorific value of sludge, but also effectively reduces the cost of sludge treatment. It is an effective way for sludge to be harmless and resourceful.
Therefore, domestic and foreign scholars have conducted extensive research on the preparation of biomass fuel from sludge. Foreign sludge is mainly used for land use, landfill, and incineration, with limited resource utilization. Research on the resource utilization of sludge in China has just begun. Therefore, this article reviews the current status of sludge disposal at home and abroad, introduces the research status and existing problems of sludge to biomass fuel, and proposes the development direction of sludge to biomass fuel.
1、 Current situation of sludge disposal at home and abroad
1.1 Current Situation of Sludge Disposal Abroad Currently, the main methods for sludge treatment abroad include agricultural utilization (fertilizer, feed), landfill, drying and incineration, and landfill disposal. Developed Western countries have strong economies, advanced technologies, and high levels of sludge treatment. For example, in Western Europe, indirect thermal drying is mainly used, while in the United States and Britain, landfill and agriculture are mainly used, while in Japan, incineration is mainly used. Large sludge pre drying plants have been established in countries such as Germany and the Netherlands in Europe. After the moisture content of pre dried sludge reaches 60%, it enters power plants for incineration or composting for agriculture, achieving energy reuse. According to statistics, land use accounts for 9% to 54% of sludge disposal in seven developed countries around the world, landfill accounts for 16% to 55%, incineration accounts for 15% to 55%, and others account for 0 to 8%.
The treatment and disposal methods of sludge in Japan are divided into two categories: disposal (landfill, open-air storage, other disposal methods) and resource utilization (building materials, farmland, green space, fuel). Japan lacks resources and energy, and attaches great importance to the resource and energy utilization of sludge. Therefore, in recent years, the proportion of resource utilization has increased rapidly, reaching 77.9% by 2008. Mainly, the organic components in sludge are used for agricultural green spaces (fertilizers and soil amendments) and energy utilization (biomass gas, solid fuel conversion, etc.), while the inorganic components are used for building materials utilization. A low-cost sludge power generation technology of 5t/d was built in 2010.
1.2 The current situation of domestic sludge disposal in China. Due to financial and technological reasons, there is currently no reasonable way out for sludge, and landfill and stacking are still the main methods. According to statistics, the current situation of sludge disposal in China is as follows: 44.8% for agriculture, 31% for landfill, 10.5% for others, and 13.7% for untreated sludge. There is data indicating that over 90% of the sewage treatment plants built do not have supporting facilities for sludge treatment. In some places, due to the misuse of sludge, heavy metals, organic compounds, and pests and diseases directly endanger human health, causing secondary pollution to the environment.
Due to landfill not only causing land tension, but also polluting land and marine resources, the proportion of sludge landfill is decreasing year by year. Incineration has significantly reduced the amount of sludge and is one of the international directions for sludge treatment. However, sludge itself has a low calorific value and usually cannot be directly burned. Currently, there are problems such as high investment, operating costs, and environmental pollution, which are still in the initial stage. With the increasingly strict environmental protection policies, China is also trying to utilize sludge as a resource, including treating it appropriately as a soil amendment and compost for land use, using it as building materials such as bricks, cement, and new materials, and using it as energy for fuel, power generation, and gas production. Utilizing the ultrafine particles and certain calorific value of sludge to mix with agricultural waste such as straw and coal to prepare bio formed fuel or fuel coal water slurry is an effective utilization method for sludge fuel conversion.
2、 The current status of domestic sludge fuel technology has attracted increasing attention due to the rapid increase in sludge production and the adverse impact on the environment. In recent years, domestic scholars have conducted research on the preparation of biomass fuel from sludge, mainly including the preparation of formed fuel by mixing sludge with coal, agricultural waste straw, and the preparation of fuel water coal slurry by mixing sludge with coal. As research in this area has just started, most of it remains in the laboratory research stage.
2.1 Technology of preparing shaped fuel by mixing sludge with coal and agricultural straw. Sludge has high moisture content and low calorific value, making direct combustion difficult. Mixing sludge with coal and agricultural waste for direct combustion or shaped combustion is an effective method to improve the combustion characteristics of sludge. In order to understand the combustion characteristics of sludge and agricultural waste, Wu Hongxiang et al. [3] used a thermal balance reaction device to study the combustion characteristics of coal, wood chips, and sludge mixtures, such as ignition temperature and burnout temperature. The results showed that the combustion performance of sludge alone was poor. When coal or wood chips were added for co combustion, the comprehensive combustion characteristic index increased, the ignition point increased, the burnout temperature decreased, and the combustion performance improved, Prove that mixed combustion of sludge and biomass waste is a good way to utilize sludge as a resource.
Two types of sludge with a moisture content of about 80% were mixed with coal and sawdust using a filter press to prepare sludge briquettes with a certain strength and calorific value under certain pressure, and thermogravimetric tests were conducted on the briquettes. The results indicate that the pyrolysis process of the two types of sludge briquettes involves the drying stage, volatilization analysis stage, combustion stage, and burnout stage. The maximum combustion rate of formed fuel prepared from wood chips and sludge is about 100K lower than that of formed fuel prepared from sludge and coal powder. The purpose of preparing briquette fuel by mixing sludge with straw and appropriate additives is to increase the combustion heat value and heat energy. Various additives need to be added during the molding process, mainly including igniting agents that make the fuel easy to ignite, loosening agents that improve the porosity of the fuel, catalysts that promote the complete combustion of carbon particles, curing agents that ensure the solidification effect of the fuel, and deodorants that reduce odor pollution during the combustion of the fuel.
