Biomass power generation technologies

Steam boiler vs Gasifier vs Digester

1. Biomass power technologies overview

Biomass is a kind of solar energy through photosynthesis , converting solar energy into chemical energy stored in plants, the common biomass is wood chips, crop stalks, rice husk, bagasse , husk and so on, and the usual methods for power generation are biomass combustion, gasification or fermentation, etc., directly or indirectly to convert the biomass energy stored in plants to power energy. Compared with wind power, solar power and others,biomass can be efficiently stored in a tangible way, biomass power is good quality without volatility and intermittent. Power generated by these abandoned agroforestry crop byproducts, not only can solve the energy shortage to some extent , but also to solve a certain degree of environmental pollution, turning waste into treasure.

Biomass power generation, turns original agricultural production " open-loop chain " into "closed loop chain" , forming a self- loop benign closed-loop with virtually no waste efflux, can handle all wastes of agricultural production ; and the left ash after burning recycled in the form of fertilizer to field; and the major expenditures for straw as raw fuel biomass on power generation returned to farmers directly at last.

Using the biomass gasification gas for power generation, to obtain the purposes of achieving high-grade energy from low-grade energy without producing pollution, is a comprehensive utilization and energy saving projects, reduce environmental pollution, reduce atmospheric greenhouse effect, moreover additional income available via selling CERS indicators on biomass utilization. 

Biomass power generation , there are three commonly used forms , the first is the direct combustion of biomass in boilers to produce steam to drive steam turbine driven generators. The second is themolysis of biomass in gasifier to produced biomass combustible gas and drive internal combustion engine driven generators. The third way is the fermentation of biomass to produce biogas , biogas-driven internal combustion engine driven generators.

2 . Steam boiler vs Gasifier
Biomass burning steam boiler power generation with biomass gasification power generation are the two main ways the current use of biomass sources , each with its advantages and disadvantages and applicability. Here is the source material rice husk power generation as an example of these two techniques were compared :

2.1 pollution problems caused in different technologies and the resulting
Steam boiler power generation technology is the use of biomass direct combustion, in the way of generating steam, then the steam turbine to generate electricity and then drive to produce electricity ; biomass gasification technology is a confined pyrolysis of biomass at high temperature conditions to produce combustible biomass gas, then the gas to  engine to drive generator for power generation .

Direct combustion of biomass will inevitably produce a lot of smoke and toxic substances " dioxins " , its toxicity is very large as 130 times of cyanide, and 900 times of arsenic,which is said the " poison of the century " . Due to the highly toxic dioxin , national environmental protection departments have set strict emission standards.

As for biomass gasification power generation, there’re two inevitably environmental issues, firstly namely tar produced in the gasification process can not be completely removed, resulting generators can not run stably for long-term ; Second, gas cooling and cleaning process will made sewage generated, causing secondary pollution to the environment . These two issues became a bottleneck constraint to biomass gasification power generation project development .

But with the gasification technology of continuous improvement and perfection , these two issues cab be properly solved. Several major manufacturers of domestic gasifier , have all a set of relatively mature technology.
Problems for the removal of tar , conventional physical methods by water washing can only remove about 80% of the tar, the tar fraction left will still hinder long-term stable operation of the gas engine . Besides the existing technology in the use of physical methods, high-pressure electrostatic tar precipitation techniques is developed at the same time, that can remove more than 98% of the tar , so that the tar content of the gas in the gas engine to be controlled in the allowable range .

Emissions in the waste water , during the biomass power generation processes , sewage water resulting from the removal of solid particles and tar generated is the major source of pollution. Conventional approach is to discharge sewage into the settler , grease before discharge , this will cause more serious secondary pollution on environment. The new technology uses a water recycling processing technology, the treated effluent sewage water is recycled, re- used for gas cleaning and cooling , in order to achieve no effluent discharged outside . Combustion engine exhaust gas compositions are mostly nitrogen, carbon dioxide and a small amount of very small amounts of carbon monoxide and other gases,which are very small environmental pollution.

2.2 Application and energy efficiency
Steam boiler power generation technology, its energy efficiency is relatively low compared with the biomass gasification technology. For example rice husk power generation, 3 to 4 kg of rice husk could generate one kWh with steam boiler combustion technology, but also extra large amount of rice husk combustion boilers to raise steam temperature in the turbine start-up phase. Therefore using this technology, the project must be large-scale, generally above 15MW or more to produce economic benefits.
Gasification technology using 1.5 to 1.8 kg of rice husk can generate one kWh of electricity, energy efficiency is relatively high, and the equipment is relatively simple. Rice production line at 100 tons processing each day, using a 400kW gasification generators can meet their basic demand for electricity, while solving the rice husk processing problems. 

2.3  Investment scale and cost
Steam boiler power generation technologies , because of economies of scale reasons, the general scale of not less than 10000kW, requires a lot of upfront investment, the investment of unit of power generation is relatively high.
Biomass gas power generation technology, the scale of biomass energy projects can range from 200kW-20MW, does not require much investment in addition to equipment purchases , smaller investment of unit of power generation .For example,to invest in a 3000kW of rice husk power generation projects, if adopting steam boiler power generation technology, only investment in equipment about not less than 30 million RMB, while the use of biomass gas power generation technology is not more than 15 million RMB.

2.4 applicability and investment risk
Steam boilers biomass power generation technology projects requires large-scale, needs higher one-time investment and higher requirements on the amount of biomass feedstocks and supply stability , raw material collection radius is relatively large, so the risk is higher. The use of such technology must work closely with the local government , especially in the biomass feedstock to ensure long-term stable supply , as well as competitive electricity purchase price for the projects. Due to the low efficiency of energy use relatively and large investment costs, investment recovery period is quite long.
Scale of projects using biomass gasification technology can range from 200kW to 20MW, lower investment, less restrictions on sources of biomass feedstock, which is generally built in underdeveloped industrial agroforestry areas, near food or timber processing enterprises , to facilitate the collection of raw materials . It can be big or small scale with step by step investment according to the actual demands , the power generated can be for private use , you can also deliver to power grid, very flexible. Therefore, the use of biomass gasification power generation technology projects are more suitable for private investment , like agricultural and forestry products processing and manufacturing enterprises and the ones need more electricity .

2.5  Biomass gasifier gas power generation system

System Concept
The biomass power generation system is a biomass energy plant used to convert solid biomass with low heating value into gas by gasification, and then uses the gases for power generation after purification processes such as dust removal and decoking. The compatible biomass materials include wastes of agriculture and forestry industries, organic wastes of daily life, and more.

Energy Efficiency 
The production of 1 KW of power requires only 1.2~1.85kg of rice husks, wood dust or crop stalks as fuel. The water used in the cooling system of this process can be recycled after undergoing a water treatment and thereby having no harmful effects to the ecological system and the environment. Self power consumption of this systems is low as percentages ranging between 5~8% of the generated total power. 
This project is an ideal solution for regions where there are large quantities of biomass waste resources, farms, nearby forests etc. 
Typical energy converting efficiency:
Raw material               Feedstock input (kg/kWh)
Saw dust                      1.3~1.5 kg/kW h 
woodchip                      1.2~1.5  kg/kW h 
Coconut fiber dust          1.5~2.0 kg/kW h 
straw                           1.6~1.8 kg/kW h 
Palm bunch                   1.6~1.9 kg/kW h 
Rice husk                      1.6~1.8 kg/kW h 
Crop stalk                     1.6~1.8 kg/kW h 

In consideration of the above mentioned particulars, the feasibility of such power station will give a high ROI (Return of Investments) in a short period of time. The reasoning to this high ROI is due to the cost of the fuel being negligible – near ZERO(0), only labor, drying and grinding has to be considered. In most power stations with other kinds of fuels, fuel cost corresponds to approximately 60-75% of power selling price, in our case, there is a saving of approximately 70% compared with other forms of power stations. 
The technology of producing power by gasification of biomass is easy to operate, simple and economical in maintenance and service, has no high pressure steam, no turbines and is easy to start up and run the standard internal-combustion generator set. 

Construction of the Biomass Power Generation System
This system is mainly made up of the following three parts.
Biomass gasifier 
The biomass gasifier serves to convert solid biomass into gases. Through control the air supply into the gasification plant, the biomass undergoes incomplete combustion to achieve solid to gas conversion. Finally, combustible gases such as H2, CO, CH4, and CnHm are generated. 

Gas purification system 
The gases generated by the biomass power generation system have high temperature and contains major impurities such as the ash and the tar. So, they need cooling and purification before they are filled into the international combustion generating set. Generally speaking, they should be cooled down to 40℃ or below, and the impurity content should be controlled within 50mg/Nm3. 

Biomass electric generator (internal combustion engine generator
Inside the internal combustion engine, the fuel gas mixes with the air to generate energy which acts to drive the spindle rotate at a high speed. Then, the spindle drives the electric generator for power generation.
For improved performance, our biomass power generation systems never use turbine engines in the electric generator, and the installed engine typically runs at low speed to ensure long term stable operation. 

3. Digester for biogas power generation 
Biogas, produced through organic anaerobic digestion, which are mainly farm animal manure, garbages, sewage water or other organic wastes, the main purpose is environment protection, to reduce emissions of methane and other harmful gases, as the pollution of CH4 in the atmosphere is 25 times that of CO2.
Disadvantages of biogas power generation:
1. Users need to dig digesters, lay piping, etc., also need to regularly clean up with professional engineers.
2 large investment on biogas generation, enough raw materials needed, even if there are a lot of biomass, there must be a sufficient proportion of organic waste in order to achieve the desired fermentation.
3. Trouble and professional to manage, less gas production at low temperature season, gas production is subject to seasonal ambient temperature.
4. Biogas contains sulfur, if desulfurization not enough, engine block and other metal parts will be corroded, serious damage to the life of engine block. 
Biogas contains silicone material, this material is very hard, if not removed before enter into engine, cylinders will be worn up severely.

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