In order to reduce the environmental pollution caused by municipal sludge and realize the resource utilization of sludge, this paper expounds the current situation of sludge disposal at home and abroad, and introduces the research status of sludge mixing with coal to prepare sewage sludge coal water slurry, mixing with coal (semi coke) to prepare briquette fuel, and mixing with plant straw to prepare fuel to prepare biomass fuel.

The results show that the preparation of biomass fuel from sludge can make full use of the effective calorific value of sludge, not only replace a small amount of coal, but also provide an effective technical way for the rational utilization of sludge. It is analyzed that there are still some problems in the preparation of biomass fuel from sludge, such as small scale, high energy consumption, complex process and low calorific value of fuel. At the same time, in view of these problems, it is proposed to strengthen the deep modification of sludge to prepare high concentration fuel coal water slurry, the development of high-efficiency binder for sludge and fuel straw granulation and sludge and coal (semi coke) to prepare briquette fuel, the dehydration and drying of sludge before briquetting The waterproof treatment of briquette fuel and the industrialization of sludge to biomass fuel are studied to accelerate the industrial application of sludge to biomass fuel.

Municipal sludge is a by-product of sewage treatment with a large number of pollutants produced by sewage treatment plants in the process of treating urban domestic sewage and industrial wastewater. With the rapid growth of sewage treatment rate and the resulting rapid increase of sludge production in China, the annual municipal sludge has reached more than 30 million tons (water content 80%). From the appearance, urban sludge is a floc in black or dark brown fluid, semi fluid or mud cake shape.

It is characterized by high moisture content, up to 25% ~ 98%, huge volume and complex composition. Sludge contains solid particles such as mud sand, fiber, animal and plant residues and their condensed flocs, and contains a large amount of organic substances (mainly benzene, chlorophenol, etc.), toxic and harmful heavy metals, pathogenic microorganisms, parasitic eggs, salts, radionuclides and other refractory substances, causing serious pollution to the environment. At present, landfill is the main treatment method of sludge at home and abroad, but the increasing sludge production not only occupies land, but also pollutes groundwater quality; The obvious reduction of incineration is one of the international directions of sludge treatment, but the initial investment in the construction of incinerator is large and the operation cost is high. Therefore, how to effectively treat and dispose of urban sludge is an urgent problem to be solved.

Because a large amount of organic matter contained in sludge has a certain calorific value (the high calorific value on dry basis is 8 ~ 10mJ / kg) and rich nutrients such as nitrogen, phosphorus and potassium, the resource utilization of sludge is of great significance to realize sludge reduction, stabilization and harmlessness. At present, the developed ways of sludge resource utilization mainly include land utilization, building material utilization and preparation of biomass fuel. The preparation of biomass fuel is to mix sludge with industrial and agricultural waste or clean fuel to make fuel with high calorific value for boiler combustion, which is the main way of sludge resource utilization. The preparation of biomass fuel from sludge mainly uses the characteristics that sludge has low calorific value and can not be burned directly. Mixing it with combustibles to prepare biomass fuel as energy, which not only makes full use of the calorific value of sludge, but also effectively reduces the cost of sludge treatment. It is an effective way to make sludge harmless and resource.

1、 Current situation of sludge disposal at home and abroad 1.1 current situation of sludge disposal abroad. At present, the methods of sludge treatment abroad mainly include agricultural utilization (fertilizer and feed), landfill, dry incineration and landfill into the sea. Western developed countries have strong economy, advanced technology and high degree of sludge treatment. For example, indirect thermal drying is mainly used in Western Europe, landfill and agriculture are mainly used in the United States and Britain, while incineration is mainly used in Japan. European countries such as Germany and the Netherlands have built large-scale sludge pre drying plants. After the water content of pre dried sludge reaches 60%, it enters the power plant for incineration or composting for agriculture to realize energy reuse. According to statistics, in the sludge disposal of seven developed countries in the world, land use accounts for 9% ~ 54%, landfill accounts for 16% ~ 55%, incineration accounts for 15% ~ 55%, and others account for 0 ~ 8%. The changes of sewage sludge output and the proportion of corresponding treatment and disposal methods in Japan from 1999 to 2008 are shown in Figure 1.

The treatment and disposal methods of sludge in Japan are divided into two categories: Disposal (landfill, open stacking and other disposal methods) and resource utilization (building materials, farmland, green space and fuel). Due to the lack of resources and energy, Japan attaches great importance to the recycling and energy utilization of sludge. Therefore, the proportion of resource utilization has increased rapidly in recent years, reaching 77.9% by 2008. The organic components in the sludge are mainly used for agricultural green space (fertilizer and soil conditioner) and energy utilization (biomass gas, solid fuel, etc.), while the inorganic components are used for building materials.

A 5t / D low-cost sludge power generation technology was built in 2010 [1]. 1.2 current situation of domestic sludge disposal in China, due to financial and technical reasons, there is no reasonable way out for sludge at present, which is mainly landfill and stacking. According to statistics, the current situation of sludge disposal in China is: agriculture accounts for 44.8%, landfill accounts for 31%, others account for 10.5%, and untreated accounts for 13.7%. Data show that more than 90% of the sewage treatment plants built have no supporting facilities for sludge treatment. In some places, due to the abuse of sludge, heavy metals, organic matter, diseases and pests directly endanger human health and cause secondary pollution to the environment.

As landfill not only causes land shortage, but also pollutes land and marine resources, the proportion of sludge landfill is decreasing year by year. The obvious reduction of incineration is one of the directions of sludge treatment in the world, but the heat value of sludge itself is low and usually can not be burned directly. At present, there are problems such as investment, high operating cost and environmental pollution [2], which is still in its infancy. With the increasingly stringent environmental protection policies, China is also experimenting with the resource utilization of sludge, including the proper treatment of sludge as land use such as soil conditioner and compost, as building materials such as brick making, cement and new materials, and as energy utilization such as fuel, power generation and gas generation. Using the ultra-fine particles of sludge and the characteristics of certain calorific value, the sludge is mixed with agricultural wastes such as straw and coal to prepare bio briquette fuel or fuel coal water slurry, which is an effective way of sludge fuelling.

2、 Status of domestic sludge fuel technology with the rapid increase of sludge production and its adverse impact on the environment, the resource utilization of sludge has attracted more and more attention. In recent years, domestic scholars have studied the preparation of biomass fuel from sludge, mainly including the preparation of briquette fuel by mixing sludge with coal and agricultural waste straw, and the preparation of fuel coal water slurry by mixing sludge with coal. Because the research in this field has just started, most of them stay in the laboratory research stage.

2.1 technology of preparing briquette fuel by mixing sludge with coal and agricultural straw sludge has high moisture, low calorific value and difficult direct combustion. Mixing sludge with coal and agricultural waste for direct combustion or briquette 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 thermobalance reaction device to study the combustion characteristics such as ignition temperature and burnout temperature of coal, sawdust and sludge mixture. The results show that the combustion performance of sludge alone is poor. When coal or sawdust is added for common combustion, its comprehensive combustion characteristic index increases, the ignition point increases, the burnout temperature decreases, and the combustion performance is improved, It is proved that the mixed combustion of sludge and biomass waste is a good way of sludge resource utilization.

Zhang Changfei et al. [4] mixed two kinds of sludge with moisture content of about 80% with coal and sawdust, prepared sludge briquette fuel with certain strength and calorific value with filter press under certain pressure, and conducted thermogravimetric test on the briquette fuel. The results show that the pyrolysis processes of the two sludge briquette fuels are drying stage, volatile analysis stage, combustion stage and burnout stage. The maximum combustion rate and temperature of briquette fuel prepared by sawdust and sludge are about 100k lower than that of briquette fuel prepared by sludge and pulverized coal. The purpose of preparing briquette fuel after mixing sludge with straw and an appropriate amount of additives is to improve the combustion calorific value and heat energy. A variety of additives need to be added during molding, mainly including the igniting agent that makes the molding fuel easy to ignite, the loosening agent that improves the porosity of the molding fuel, the catalyst that promotes the complete combustion of carbon particles, the curing agent that ensures the curing effect of the molding fuel, and the deodorant that reduces the odor pollution in the combustion of the molding fuel [5].