The status, development and prospect of whole wheat straw alkali recovery technology

I. Introduction:
Since 1996, under the leadership and support of the State Planning Commission, the National Environmental Protection Agency and the former China Light Industry Association. In Shandong, Henan, and Anhui provinces, a batch of 75-ton and 100-ton straw pulp alkalis were processed to recover the energy-saving and pollution-control project. The pulping and papermaking black liquor was used to control river, lake and sea pollution. Most of these projects were successfully completed and put into operation after 1997. Shandong has formed Shouguang Chenming, Dongying Huatai, Linqing Yinhe and Zhangzhou Sun as typical successful experiences; Henan has formed Luohe Silver Dove as a typical successful production plant. Their experience is worth learning and learning from. They better handled the pollution load of pulp and papermaking wastewater in wheat straw pulp mills and recycled chemical products, promoted the further improvement and development of wheat straw black liquor alkali recovery technology, and made a bright future for wastewater treatment of many wheat straw pulping plants. .
As we all know, the wheat straw alkali recovery is the most difficult one for alkali recovery in various kinds of slurry at home and abroad. In the past six years, after many production experience summaries and technical equipments have been continuously reconstructed and improved, the wheat straw alkali recovery technology in China should reach a more desirable level: the removal rate of CODcr and BOD5 from the cooking black liquor pollution load is over 80%. Alkali recovery rate of 70-74%, fuel consumption down to 30kg / t below the alkali, with the alkaline slurry recovery consumption indicators comparable, the cost of recovery of alkali in the 1000 yuan / t alkali. The success of wheat straw pulp alkali recovery lays a solid foundation for the whole plant waste water treatment of wheat straw pulping and paper mill. After the black liquor from wheat straw pulp mill is recovered by alkali recovery and extraction, the mid-stage wastewater can be processed after primary and secondary biochemical treatment. Discharge.
Second, wheat straw black alkali recovery technology needs to be optimized issues Process, equipment selection and combination of optimization 1 black liquor extraction is the key process of wheat straw black liquor governance, but also the current restrictions on the removal of black liquor pollution load and improve the black liquor extraction rate The bottleneck project. Black liquor extraction equipment model selection, from the mature, reliable and advanced and equipment capacity indicators, drum vacuum pulp washer is still widely used at home and abroad, washing slurry and black liquor extraction equipment. "95th Five-Year Plan" Scientific and Technological Research (Project) "Research and Development of Wheat Straw Soda Alkali Recovery Technology" Project, Huateng Group's 75t/d wheat straw pulp alkali recovery demonstration project and Huajin Group's new (cone valve) drum vacuum washing The unit test results showed that the black liquor solids equipment extraction rate reached 83.5%, 6.7Be/20C, solids concentration of 9.68%. Only the minimum black liquor quality standard for the evaporation section is met. However, higher black liquor extraction rate and concentration are required. Whether it is from production practice or mechanism, the drum vacuum pulper can hardly achieve the black liquor extraction rate of 85%. This is due to the physicochemical properties of wheat straw and its black liquor. We believe that it is difficult to solve the black liquor extraction rate alone with certain extraction equipment.
The alkali recovery rate is restricted, and it is difficult to reduce the pollution load value into the middle-stage wastewater. If some manufacturers increase the amount of washing water to achieve the purpose of multiple extraction of black liquor. Although the black liquor extraction rate increased a little, but a large amount of dilution water into the black liquor, increase evaporation station evaporation load, the need to consume more steam, but also need to increase the evaporation area, from the economic point of view, the flower is not come, the annual operating costs are quite high . Therefore, our institute, Beijing Pulp and Paper Industry Research Institute and Shandong Sun Paper Industry and other units have begun to use the combined process of the two extraction equipments, adopting squeeze-diffusion displacement washing or adopting diffusion displacement-extrusion washing mechanism, achieving The purpose of improving the extraction rate of wheat straw black liquor. Shandong Sun Paper Group 100t/d wheat straw pulp alkali recovery project, in order to increase the extraction rate, the use of drum-type vacuum washing machine (three sets in series) and a Swedish Shunzhi DWA-type dual-roller pulper combination process. The system has been put into operation in June 2000 and has achieved good results. The extraction rate of black liquor has reached more than 85%. Shandong Anqiu Lantian Paper Industry Group Co. Stainless steel spiral mesh belt machine combination process, the black liquor extraction rate reached more than 85%, Henan Huaxian County Huanhua Paper Group transformation of old black liquor extraction, the use of 40m2 drum vacuum pumping unit before adding 4 pulper combination process The black liquor extraction rate also reached more than 85%. Reflect our new ideas in engineering design and achieve good results. This also brings good news for wheat straw alkali recovery evaporation process and mid-stage wastewater treatment.
2 Evaporation station adopts full-plate evaporator group Currently, domestic wheat straw and straw alkali recovery and evaporation stations mostly use tube-plate combined “two-plate three-tube” and “three-plate three-tube” evaporator group concentrated black liquor, all achieving satisfactory results. This evaporation station has an average evaporation intensity of about 9.5 kg water/m 2 ·h. The evaporative heat efficiency is 3.3-3,skg water/kg steam, and the concentration of outbound black liquor is between 42-44%. Before entering the furnace, it needs to be evaporated directly to waste in the disc evaporator using waste heat to 46- About 48% can be burned. The production operation adopts the secondary circulation method to break out the concentrated black liquor, but the process is complicated and the operation level is high. In the past two years, due to the reduced manufacturing costs of plate evaporators, the prices of tube evaporators are close to those of tube evaporators. Abundant funds require companies to use the "five-body-five-effect" and "seven-five-effect" full plate evaporator group concentrate black liquor, process production operations still use the "two-step steam concentration" method, concentrated black liquor concentration can reach 45-46 % Because of the good heat transfer effect and the saving of steam, it is currently the preferred process and equipment for the alkali recovery and evaporation station of wheat straw pulp.
3 Burning section Alkali furnace is one of the core equipment for alkali recovery project. In recent years, wholegrain grasses developed for black liquor caustic recovery furnaces have been operated through production practices. Basically it can meet the requirements of wheat straw black liquor incineration, so that the normal production throw away the oil gun. This is mainly due to the fact that a plate-type air preheater is installed under the furnace tail economizer to increase the primary and secondary air temperature of the furnace from 150C at the outlet of the steam air preheater to more than 250C, thereby improving the quality of wheat straw. Pulp black liquor drying and ignition conditions increase the temperature of the furnace and remove the waste heat from the flue gas back to the alkali furnace. However, this also brings some negative effects to the operating conditions of the disc evaporator and the electrostatic precipitator. The temperature of the electrostatic precipitator smoke is too low, which affects the normal operation of the electrostatic precipitator and becomes the cause of the low operation rate of the electrostatic precipitator for wheat straw slurry recovery. Alkali-fired furnaces are only enough to meet the purpose of being able to burn and open. It is necessary to use black liquor to generate more by-product steam with a calorific value. Large-scale alkali furnaces (above 150t/d slurry) should produce medium-pressure steam and use the principle of generating electricity first. The amount of steam recovered by the alkali is related to the direct cost of recovering the alkali. If the alkali furnace produces more steam, less steam is added from the external network (the steam consumption accounts for 1/3 of the recovered caustic soda). Therefore, I think that evaluation of black liquor jet kiln (alkaline kiln) mainly uses two indicators to measure: First, the amount of alkali consumption per ton; First, the amount of by-product steam after burning per ton of solids (t steam solids): According to The wheat straw pulp was calculated to be 2.7t steam solids (1.27MPa saturated steam) and the mash was 2. Gt Steam/t solids, wood pulp is 3.3 t steam/t solids. When the concentration of wheat straw black liquor is 47-48%, 75t/d wheat straw alkali furnace should produce 10t/h steam, 100t/d wheat straw pulp alkali furnace should produce 15t/h steam, 150t/d wheat straw pulp alkali furnace Should produce 22t/h steam. Only in this way can we really evaluate whether the alkali furnace is energy efficient. It is hoped that the design and manufacturing unit of the alkali furnace will summarize its successful experience, carry out scientific and technological innovations, develop a highly efficient and energy-saving alkali furnace suitable for wheat straw pulp, reduce the cost of alkali recovery, improve the economic benefits of recycling, and create a brilliant future.
4 Causticization is the key part of alkali recovery production. Its product, white liquor, is the pulping cooking liquid. The amount and concentration of recycled liquor directly affect the pulping cost. Causticization is also the last process of alkali recovery. If the process flow, equipment selection is improper, and production management is not good, there will be a bumper harvest. When the equipment is determined, the selection of the process flow is the key to optimizing the process design. At present, the continuous causticization process recommended by our institute is the use of three clarifiers (green liquor, white liquor, dilute white liquor), a vacuum slag scrubber, and a pre-mounted mud mud filter to wash the white mud in tandem. . In the process, the white mud from the concentrated white liquor clarifier is put into the agitation tank (diluted with the second filtered filtrate), washed by a vacuum slag scrubber, and the filtrate (concentrated white liquor) is returned to the white liquor clarifier inlet. The filter cake is used Pre-loading white mud filter Filtration Dispersion Mixing The white liquor clarifier (white mud scrubber) is used to perform the first white mud washing, and the mud is extracted and then washed by the pre-mounted white mud filter, which greatly reduces the access The amount of dilute white alkali (about 8g/l), at the same time, the final white mud dryness reached more than 55%, easy to transport. White mud residual alkali <1 other from the green liquid clarifier to extract green mud into the mixing tank and send the first vacuum scrubber filter, the filtrate into the green storage tank and then into the digestion and causticization system. The green mud was diluted into a white mud scrubber and the green mud residue was rinsed with the filtrate into a white mud scrubber. Green mud intermittent filter. In this way, the conversion rate of green liquor is increased, more caustic soda is produced, and the amount of thin white liquor is also reduced.
III. Prospects of wheat straw alkali recovery technology during the “10th Five-year Plan” 1. Breakthroughs in wheat straw alkali recovery technology 1 Process of combining two or more types of black liquor extraction processes and equipment, ie, extrusion— - Diffusion displacement washing mechanism handles wheat straw slurry washing and black liquor extraction; and is equipped with a closed screening process. Save water and achieve clean production. The black liquor extraction rate is increased to about 90%. So that cook black liquor pollution load COD and BOD; removal rate of more than 88%. Relieve the middle-stage wastewater pollution load and meet the requirements of high-quality black liquor in the evaporation section. (3) Breakthroughs in black liquor evaporation technology - At present, the “two-plate three-tube” and full-plate multi-body five-effect evaporators used in domestic evaporation stations are affected by the physicochemical characteristics of viscosity and other properties of black liquor. In the range of 42-46%, the black liquor has a large amount of moisture, and it needs to be directly concentrated in the disk evaporator to around 48%, affecting the heat load of the alkali furnace. In order to overcome the difficulties caused by the viscosity of black liquor, Oslo Finland developed and studied the “black liquor heat treatment” technology in 1985. The first black liquor heat treatment production plant was put into operation in a Finnish pulp mill in December 1989. There are currently five cases. In 1997, he applied for a patent in China. Because this system is expensive, it is not transferred separately. In recent years, domestic colleges and universities and scientific research institutes have also conducted experiments on the black liquor thermal viscosity reduction of wheat straw pulp and obtained some valuable scientific research data. This applies to the plate evaporation system and the black liquor heat treatment (ie, thermal viscosity reduction) for evaporation stations in China. The development and application of processes and equipment play a catalytic role. It is expected to be practically practical in the project during the “Tenth Five-Year Plan” period, so that the concentration of wheat straw black liquor after evaporation will increase to more than 55%, and a breakthrough in evaporation technology will be completed. 2, wheat straw pulp alkali recovery technical and economic indicators of the standard 1 black liquor extraction section black liquor extraction rate> 90% black concentration: 9.25Be/15 (11.5%) black liquor temperature ≥75C 2 evaporation section outbound thick black Liquid concentration 33Be/110°C (55%) Evaporation intensity 11 kg water/m·h Evaporation heat efficiency ≥ 4.5 kg water/kg Steam 3 Combustion furnace black concentration 33Be/110°C (55%) Steam production 3t/ t Solids among them: 150t/d alkali furnace steam production 25t/h (3.82MPa) 200t/d alkali boiler steam production 32t/h (3.82MPa) Electrostatic precipitator rate >98% 4 Caustic section causticity 88% white mud residual alkali <0.5% 5 Main raw material consumption index (150t/d slurry size as an example and recovery of one ton of caustic soda) Lime 1.03t (Cao> 75%) Heavy oil <20kg Clear water <303 Steam supplement 2t soft water 5t electricity <400kwh 5 alkali recovery rate> 85% 1 alkali recovery cost <800 yuan 3 total project investment 65 million yuan