PERSPECTIVE OF MSW TO POWER GENERATION THROUGH GAS ENGINE

DEZHEN. CHEN*, MIN. YANG*

*Thermal & Environmental Engineering Institute, Mechanical Engineering College, Tongji University, Shanghai, 200092, China. E-mail: [email protected]

SUMMARY: In this paper perspective of MSW to power generation through gas engine in China is evaluated. The waste to energy (WtE) plant based on thermal chemical conversion and gas engine technology include four important issues: preparation of MSW materials, reliable gasification or pyrolysis reactors, gas product processing and availability of gas engine. The state of the arts of these issues have been surveyed and the challenge for implementing WtE process based on gas engine technology has been analysed. It has been found that MSW pretreatment machinery is relatively mature; the gas engine products suitable for syngas are also available. While economic and reliable gasifiers and syngas scrubbing systems are very limited and they are the core challenge for implementing WtE process through gas engine.

1. INTRODUCTION

Most of municipal solid wastes (MSW) in big cities in China have been safely disposed through landfilling, incineration and other combined technologies. By the end of year of 2015, 60.2 wt.% of the MSW was disposed in landfills, 29.8 wt.% was incinerated and 1.8 wt.% was composted, there was still 8.2 wt.% of MSW piling on their generating sites and remaining untreated (Speciality committee of urban domestic refuse of CAEPI, 2016). Almost all of the incineration plants in China are equipped with boilers to recover heat released during incineration in form of steam for power generation. However in the small cities and countryside where the generation of MSW are less than 600 tonnes per day, setting up new waste to energy (WtE) plants based on incineration and Rankine cycle technology is difficult due to the economic constraints. And sanitary landfills are also difficult to find in the small cities, where cultivated lands are very close to the town. To safely dispose MSW and take full advantage of preferential policy for WtE in the small cities and countryside as well as in big cities where distributed MSW disposal is desired to avoided long distance transportation, WtE plants based on gasification and gas engine technologies are desired due to the flexibility, high efficiency and lower investment compared to the incineration plants with steam boilers. Although WtE process through gas engine fed with biogas is widely applied, few experiences are available by feeding syngas from MSW thermal chemical conversion processes, due to the fact that gas components of the syngas from gasification or pyrolysis processes are varying and their cleaning system are complicated. In this work the perspective of power generation with MSW derived syngas through gas engine in China will be evaluated by reviewing the maturity of four key issues involved: preparation of MSW materials, reliable gasification or pyrolysis reactors, gas product processing and availability of gas engines.

Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

2. PREPARATION OF MSW MATERIALS FOR GASIFICATION PROCESSES

Different from mass burn incineration, for which MSW pretreatment is not necessary; MSW pretreatment is required before feeding to the pyrolysis or gasification reactors to ensure the safe and steady operation. The pretreatment often includes sorting and crushing of MSW, to remove unsuitable components and reduce size. With the development and wide application of fluidized bed incinerators in China for decades, MSW pretreatment facilities including sorting and crushing machinery have been developed and becoming mature in China's market. Table 1 lists the important crushing machinery in China.

Table 1 Important crushing machinery for MSW. Equipment Role Supplier Primary shredder Used for crushing non-pretreated domestic and bulk waste, Lindner Recyclingtech for MSW commercial and industrial waste, mixed construction site waste, GmbH, Austria (JUPITER wood and straw, plastics of all kind, wastepaper, paper rolls, model) rubber and tires, textiles, electronic waste, cables etc. Jano Recycling High performance shredder with a capacity of up to 70 t/h; Technology (Suzhou) Co., the output size is from 70 to 600 mm. Ltd, including Model Typical products: double-shaft comprehensive shredder; 1PSM0816 and model double-shaft shear shredder. 1PSM0822. Secondary Used for crushing pre-shredded domestic and bulk waste, Lindner Recyclingtech Shredder commercial and industrial waste, mixed construction site GmbH, Austria (KOMET waste, wood and straw, plastics of all kind, wastepaper, paper Model or POWER rolls, rubber, tires, textiles and electronic scrap etc. Especially KOMET Model) used for producing RDF or SRF. Jano Recycling It is high performance shredder with a capacity of up to 28 Technology (Suzhou) Co., t/h. The output size is from 10 to 150 mm. Ltd. (Model 1PSX0828) Universal Shredder Used for crushing domestic and bulk wastes, commercial and Lindner Recyclingtech industrial wastes, mixed construction site wastes, wood and GmbH, Austria straw, plastics of all kind, foils, paper rolls, rubber, textiles and (UNIVERSO Model) electronic scrap etc. Harden Machinery, It is robust shredder with a capacity of up to 60 t/h. The Zhongshan, China output size is from 20 to 600 mm. Mobile universal Robust two shaft shredder for rootstocks, trunk wood, Lindner Recyclingtech Shredder commercial and bulk waste, scrap metal, electronic scrap, light GmbH, Austria scrap, aluminum scrap etc. The capacity is up to 150 t/h. (URRACO Model) Output size is from 150 to 500 mm. Hydrostatic driven With aggressive knives made of Hardox steel, the PreShred M & J Industries Horsens PreShred shredder shredder is capable of effectively and reliably shredding Denamrk A/S virtually any type of material. This waste shredder is designed (Model 4000 Serials) to meet the special needs of plants in which the incoming material varies greatly in terms of size and composition and can contain unknown quantities of steel, stones and concrete, can also easily shred bulky wastes.

For the sorting machinery the products mainly include bag opener, drum screen, ballistic separator, disc screen, air separator, permanent magnetic iron separator and eddy current separator for non-ferrous metals, etc, the typical suppliers include BRT-Hartner GmbH, Germany, Jano Recycling Technology (Suzhou) Co. Ltd, Shanghai Jiming Enviromental Protection Equipment Co. Ltd, Sichuan Leiming Biological Environmental Protection Engineering Co. Ltd.

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Presently there is a trend to use gasification or pyrolysis technology to replace incineration to dispose MSW in small cities in China, for example in Penglai, Shangdong province a MSW treatment plant was formerly planned to install two incinerators with capacity of 300 t/d, but now is changed to adopt pyrolysis and gasification technology with total investment of 157,330,000 RMB, including 12 gas engines of 1WM. Very recently in Anhui province, in a MSW treatment plant with capacity of 500 t/d, gasification technology has been employed after careful environmental assessment. With reference to MSW pretreatment for fluidized bed incineration, the typical pretreatment diagram for preparing MSW for gasification or pyrolysis reactors is shown in Figure 1. Generally the investment of the pretreatment facility is around 70 to 100 thousand yuan RMB for per tonne of MSW, almost accounting one third of the total facility investment for a thermal chemical conversion plant. Imported facility is more reliable but more expensive.

Figure 1 Pretreatment diagram for preparing MSW for gasification or pyrolysis reactors

3. MSW GASIFIERS AND PYROLYSIS SYSTEM OPERATING IN CHINA

The MSW gasifiers operating in China include fixed bed reactors with rotary grate and rotary kiln reactors. Most of them only produce syngas of poor quality with very small lower heat value (LHV) that cannot meet up with the requirements for feeding gas engine. But pyrolysis- gasification combined process can generate syngas with LHV higher than 15 MJ/Nm3 (Wang et al., 2017); however this gas product from the combined process is characterized with high H2 content at the same time, which arouses special requirements for the gas engine. Table 2 lists the typical gasification and pyrolysis plants operating in China. Although fluidized bed reactors have the merits of large heat capacity, uniform temperature field and intensive mixing of the air and the MSW, making the fast reaction and high quality syngas product possible, they are not available in China for MSW gasification or pyrolysis. From Table 2 it can be also seen that gas engine is seldom equipped as a final syngas consumer for the available gasification or pyrolysis processes, the only case is still under testing. The reason behind this situation lies in the three facts: 1) the syngas quality is poor from gasification

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017 process;

Table 2 Typical gasification and pyrolysis plants operating in China Reactor Characteristics Applications Fixed bed reactor Simple and reliable. The updraft and Xianju County, Taizhou, Zhejiang downdraft gasification reactors are commonly province, with capacity of 2×150 t/d, the used. The downdraft reactor corresponds to a syngas is burnt in the boiler to produce relatively low tar content in the syngas (Wu et steam and steam is used for power al., 2003); but updraft reactor can treat MSW generation. with higher moisture content. However, for the heterogeneous MSW, the fix Xiangyuan County of Shanxi province, bed reactor with fixed grate is not efficient to with capacity of 100 t/d, producing steam. supply air to the MSW to finish gasification. Therefore fixed bed reactor with rotary grate is adopted in practice. Rotary kiln Rotary kiln is simple in its structure, can A rotary kiln in size of ø3.32 m×15 m, accept MSW components with different size followed by a moving grate furnace is and no rigid requirements for pretreatment. running in Dongguan, Guangdong Province, with capacity of 150 t/d. The Its rotary speed can be easily adjustable to MSW in the kiln is firstly dried and control the duration time of MSW inside the evaporated, gasified and then burnt. reactor. Rotary kilns in size of ø1.6 m×12m are It can be used for multi- purposes including running in two stages in Qingdao, incineration, gasification and pyrolysis. Shangdong province, with capacity of 50 t/d. The MSW is pyrolyzed and syngas, oil and char are collected separately. Annular tunnel The MSW is put on a moving plate, the plate is In Bazhou, Hebei Province, China, an pyrolysis reactor moving forward following annular tunnel with annular tunnel furnace with diameter of 65 heat radiation from above m is operating to deal with MSW with capacity of 100 t/d combustibles. Gas, oil and char are collected separately. Moving bed The prepared MSW is put on an inclined grate In Anxin County, Hebei Province, China, gasifier which is unmovable but MSW is driven an moving bed gasifier provided by forward from the entrance. The syngas is Bellwether Gasification Technologies, reformed with a plasma torch and then Germany is installed to deal with MSW cleaned. combustibles with capacity of 200 t/d. Syngas is cleaned for power generation in gas engine. The furnace is still under testing.

2) its cleaning technology is expensive and 3) its components are not stable enough to ensure the steady operation of gas engine. Therefore to produce high quality syngas with LHV high enough and components being stable is very important to set up WtE plants with gas engine as the power generation unit. However pyrolysis combined gasification system, in which the MSW is firstly undergone pyrolysis and then volatile is reformed over catalyst with steam or gasified with pure O2 can provide better quality syngas, as proven by Ebara, JFE and Thermoselect technologies.

4. SYNGAS PURIFICATION TECHNOLOGIES

The syngas is usually contaminated with tar, particulates and gaseous pollutants such as H2S,

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

HCl and NH3 therefore should be cleaned before feeding to the gas engine. Based on the experiences of using syngas from biomass gasification for power generation, numerous gas cleaning technologies have been developed, as shown in Table 3.

Table 3 Syngas cleaning technologies Syngas type Technology Results or effect Purpose of Ref. syngas utilization Syngas from Filtration, washing, Sand bed filter, Venturi scrubber, IC engine Hasler et al., MSW gasifier particle separation, rotational particulate separator and applications 1999. adsorption catalytic tar cracker working at 900 oC are effective for removing tar.

Volatile from Cyclone and filters For dust removal the combination of To purify the Li et al., 2012 biomass including bag filter, cyclone and filters can achieve the bio-oil quality pyrolysis ceramic filter, porous efficiency of 99 %. metal filter and particulate filter

Syngas from To remove H2S from The steam activated carbons For feeding Sisani, et al., biomass syngas with five impregnated with Cu/Cr salts or high 2014. gasification absorbents KOH showed the highest H2S temperature removal activity. fuel cell Syngas from Particulates: cyclone Ceramic filter at 1123 K to remove To fuel solid Aravind et al., biomass and ceramic filter; particulates oxide fuel 2012 gasification Tar: catalytic Tar cracking with dolomite at 1173 cells reforming; K to several tens ppmv. H2S: metal oxides HCl cleaning with Na2CO3 at 873 HCl: spray alkali K to 1 to few ppmv. absorbents H2S cleaning with zinc titanate at Alkali metals: 873 K to 1 to few ppmv. condensation and Alkali cleaning with activated absorbents alumina at 1123 K to 1 ppmv. Ceramic filter at 1123 K to remove particulates to 1 ppmw. Volatile from Thermal cracking at Tar concentration is about 6.8 g/m3 in Syngas is the Chen,et al., sewage sludge 800oC + wet the volatile; plant oil is effective for target product 2011. pyrolysis at scrubbing removing heavy oil in the tar; and for later use. 600oC diesel is effective for removing N-containing tar, NH3 and HCN. Volatile from Thermal cracking at Thermal cracking can remove 78% of Syngas is the Phuphuakrat, biomass pyrolysis 800 oC or reforming the tar; and steam reforming can target product et al., 2010. at 600oC at 800 oC + remove 77-92 % of the tar. Sawdust for later use. adsorption is a good material for tar adsorption. Syngas from Plant oil scrubbing + 98 % of tar can be removed. Syngas withou Nakamura,et biomass char adsorption t tar is the al., 2016. gasification target product for later use. Syngas from Use biochar to Biochar is a good material for tar Syngas is the Shen,2015. biomass adsorb/catalytically adsorption,it is also a catalytic or target product gasification crack tar catalytic support for tar cracking. for later use. Volatile from coal Steam reforming Steam reforming with char as catalyst Syngas is the Małek, 2007 pyrolysis can reduce the formation of the target product carbon black and PAH. for later use. Volatile from Reforming and Through hot char reforming and For gas Chen, et al., MSW pyrolysis filtration with hot filtration, condensation, diesel and engine applica 2016 char layer; Na2CO3 solution washing, syngas tions condenser; produced from pyrolysis of MSW is Na2CO3 scrubber cleaned satisfactorily.

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Syngas from Effective The fabric filters can effectively To produce Prabhansu et gasification technologies of hot remove particulates and final syngas for gas al., 2015 gas cleanup (HGC) concentrations less than 1 mg/m3 at engine and temperatures below 250 oC. other purposes Olivine can serve as in-situ catalyst in fluidized bed gasifier for tarremoval. Nickel catalysts is 10 times more efficient than that of dolomite in decomposing tar at temperature 400–550 oC. Zinc and copper oxides can remove 99% of sulphur compounds at above 300 oC. Nickel based catalyst, combined with

MnO3 and Al2O3,can remove nitrogen compounds like ammonia and tar simultaneously in hot gas condition. HCl is removed by solid slaked lime and limestone at500-600 oC. Getter materials can be used as sorbents for the removal of alkali metals when the gas temperature is around 600 oC.

From Table 3 it can be seen that generally the particulate matter in the syngas can be removed with cyclone and filters together; tar is usually cracked over catalyst with or without steam supply; sulfur compounds and HCl can be effective removed with Na2CO3 and metal oxides; and alkali metal species can be absorbed with porous materials. Ammonia can be effectively absorbed by warm Na2CO3 solution (Chen et al., 2016) or by diesel (Chen et al., 2011). Hot gas cleanup (HGC) technology is widely adopted for cleaning gas from gasification process, however HGC is expensive and only available for coal gasification in practice. For syngas produced from MSW pyrolysis, Chen et al. (2016) introduced a sequential cleaning technology including filtration with hot char layer, washing with diesel and warm Na2CO3 solution and a final activated carbon layer absorption, which can provide clean gas for gas engine. However most of the technologies in Table 3 are of lab scale and have none experiences in practice. To make those technologies available in the market is urgent.

5. GAS ENGINE FOR SYNGAS WITH LOW LHV AND HIGH H2 CONTENT

The available gas engines are mainly based on pre-mixture of gas and fuel, which are suitable for biogas from anaerobic digestion and landfill gas that has high CH4 content and low H2 content with LHV above 20 MJ/Nm3. While syngas from MSW gasification or pyrolysis combined gasification may contain large fraction of H2, or it may be characterized with very low LHV.

When feeding the syngas with high H2 content to the conventional pre-mixing gas engines, back firing in intake pipes or explosion in exhaust pipes may happen. On the other hand when fed with syngas of very low LHV, the fuel/air ratio will be deviated from stoichiometric-burn in conventional gas engines. Many gas engine suppliers revised their products to adopt to syngas from gasification and pyrolysis processes. Table 4 lists some of those gas engine products in China market.

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017

Table 4 Typical gas engine products available in China for MSW syngas Supplier Model LHV of gas Typical requirements to the syngas Waukesha VSG11GSI/GSID 1500rpm, 16~36 MJ/m3 Not clear Capacity from 140kWe 3 Guascor Model SFGLD, from 300 kW 14-17 MJ/m O2 ≤ 2%; H2 ≤12 % Halide ≤ 60 mg/Nm3 (Cl identical) Si ≤ 4 mg/Nm3; 3 NH3 ≤ 33 mg/Nm Tar ≤ 5 mg/Nm3 Dust ≤ 5 mg/Nm3, Diameter of dust ≤ 5 µm Shengli Petroleum 12V190, 1000r/min > 3.7656MJ/m3 Tar ≤ 30 mg/Nm3

Power machinery plant 400 -1000 kW H2 ≤ 60 %; Diameter of particulates ≤ 5 µm Particulates ≤ 50 mg/Nm3 3 NH3 ≤ 20 mg/Nm 3 H2S ≤ 50 mg/Nm Zibo Zichai New X300D/ML (X=6, 8, 9) ≧ 4 MJ/m3 Tar ≤ 20 mg/Nm3

Energy Co.Ltd. ～300-1100 kW H2 ≤ 60 %; Diameter of dust ≤ 5 µm Dust ≤ 50 mg/Nm3 Moisture ≤ 40mg/Nm3 3 NH3 ≤ 20 mg/Nm 3 H2S ≤ 50 mg/Nm Pressure change ≤ 1kPa/min

In order to adopt to syngas with high H2 contents, the blending system of syngas and air is innovatively designed, where syngas and air are independently compressed and sent to the combustion cylinder. The electronic fuel injection valve is automatically controlled to avoid back firing in intake pipes caused by fast combustion of H2. Also the discharge temperature is reduced to avoid explosion in exhaust pipes. For the syngas of very low LHV, the intake pipes are expanded to intake more syngas to satisfy stoichiometric combustion. With all of these efforts the gas engine products can support the WtE plants based on gasification or pyrolysis technologies if the cleaning of syngas is good enough to meet with their feeding requirements. Table 4 also lists the limitations for the pollutants in the syngas, which provide guideline for syngas cleaning system.

6. CONCLUSIONS

The disposal of MSW in small cities and distributed MSW treatment in large cities address the imperative requirement on WtE through gas engine in China. In this paper perspective of WtE plants based on thermal chemical conversion and gas engine technologies is evaluated by surveying the state of the arts of individual steps in the system, which include preparation of MSW materials, provision of the reliable MSW gasification or pyrolysis reactors, processing the syngas product and the choice of suitable gas engine product. It can be concluded that MSW pretreatment facility is available in China’s market and with reference to preparation of MSW for fluidized bed incineration, pretreatment of MSW to fit for pyrolysis and gasification reactors is becoming mature. The gasifiers developed and operating

Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017 in China are very limited and become the core challenge of this technology. Cheap syngas scrubbing system consists of sequential steps to effectively remove tar, particulates, H2S, HCl, NH3 and alkali metals is the second crux of implementation WtE plants through gas engine.

AKNOWLEDGEMENTS

The research was supported by the China state-owned assets supervision and administration commission project (Grant No. 2013019) and Shanghai Municipal Science & Technology Commission Fund for Improving Economy in the Yangtze River Delta Region (Grant No. 12195811100).

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