Posted by: SATYASRINIVAS | January 11, 2014

250 kW Biomass Gasification based Power Plant

What is Biomass and Biomass fuel?

Biomass fuels are organic materials from living plants and animal waste. They

Include:

·   Wood and wood residues such as sawdust, wood pieces etc.

·   Crop residues such as straw from grains, rice husks, sugarcane etc.

·   Residues from livestock production such as excreta, slaughterhouse residues, 

   garbage.

In this 250 kW power plant project, rice husk is used as biomass fuel.

Biomass gasification technology

Biomass gasification is viewed today an alternative to conventional fuel. Biomass gasification is the process of converting solid fuels (rice husk, wood/ wood-waste, agricultural residues etc.) into a combustible gas mixture usually called the “Producer Gas” i.e. biomass materials are gasified to produce “Producer Gas”. The technology can be used for both thermal applications and power generation.

The advantages of gasification are:

·         It ensures efficient usage of traditional biomass products by converting it into a high quality, combustible gaseous fuel.

·         Biomass gasification technology is environment friendly which significantly reduces environmental pollution caused by traditional usage of biomass products.

·         Reasonable cost of energy/ power production to operate as a commercially viable entity.

Biomass is a renewable source and its application as a fuel does not result in net CO2 emission because the CO2 released during the conversion of biomass is equivalent to that which has been absorbed by the biomass from the atmosphere. Thus, unlike fossil based fuels, it is a CO2 neutral energy source.

Biomass gasification technology: Power Generation

For Power generation, biomass gasification process consists of 3 major components:

  1. Gasifier unit

  2. Gas purification unit

  3. Internal Combustion (IC) Engine

At first, rice husk is fed into the gasifier and gas is produced inside the gasifier. Produced gas is then cooled and cleaned in the purification unit and finally clean gas is fed into the engine to generate power.

1.   Gasifier unit

The gasifier is essentially a chemical reactor where various complex physical and chemical processes take place i.e. drying of fuel, pyrolysis, combustion and reduction. There are mainly two types of gasifier: 1. Updraft 2. Downdraft. In updraft technology, the produced gas is drawn from the top of the gasifier while in down draft technology; the gas is drawn from the bottom of the gasifier. In this project downdraft technology is used. Biomass is fed into gasifier at specified intervals. Biomass is burnt inside of gasifier and gas is produced which is known as “Producer gas.” The equipment is designed in such a manner that it takes in air in controlled quantities, resulting in partial oxidation of biomass. One kg of biomass can produce 2.5 to 3.0 Nm3 of producer gas with a calorific value of 1000-1300 Kcal per Nm3. Specifications of the gasifier used in this project given below:

Parameter

Description

Gasifier Type

Downdraft

Capacity

Total 250 kW

Rated Gas Flow

625 Nm3 /hr (up to total 250 kW capacity)

Average Gas calorific value

> 1,050 (Kcal/Nm3)

Rated Biomass consumption

Up to 300 kg/hr (for total 250 kW capacity)

Gasification Temperature

1050oC-1100oC

Gasification Efficiency

Up to 75%

Temperature of Gas at Gasifier Outlet

250 to 400oC

 Biomass Feeding

Manual

Desired Operation

Continuous (minimum 300 days/yr)

Typical Auxiliary Power Consumption

Up to 11 kW

Typical Gas composition

CO-20.62%, H2-10.62%, CO2-13.61%, CH4– Up to 4%, N2-52.62%

2. Gas purification unit

The gas coming out of the gasifier has a temperature of 450oC or higher. The hot gas generally contains tar and fine particles of ash, which need to be cleaned before feeding to the engine. Since this project uses downdraft technology, therefore gas cleaning and cooling system is less complicated and easy for maintenance. Following gas purification stages and filter element have been used in each stage of this rice husk based power plant:

Stage 1: Coarse Filter: uses rice husk char as filter element to partly clean the gas

Stage 2: Fine Filters: sawdust is used as filter element to trap all the particulate and ash particles

Stage 3: Safety Filter (‘SF’): a special kind of fabric (rated for 5 micron particulate size) is used as filter element

3. Internal Combustion (IC) Engine: Duel-fuel Engine

In this project, a 300 kW capacity duel-fuel generator is used to generate electricity. Mainly the gas that has been produced by the gasification of biomass, generates electricity. However, in this rice husk based power plant, to run the generator certain amount of diesel is required. Because, the producer gas has relatively lower heating value and needs to be supplemented by diesel to get the necessary power output. That’s why the IC engine has been converted into duel fuel mode, i.e. it can run both on producer gas and diesel. Using this technology Producer gas to diesel ratio is. 70:30. During start up of the plant, main generator is started first on diesel and then changed over to duel fuel mode when the producer gas is available for charging to the engine.

Power distribution Network

A mini grid has been constructed to sell the power to the adjacent area. The plant is able to deliver power at least 200 households and over 100 commercial entities of that area.

Environmental impact

Generally 4 types of effluent are generated from the gasification process like ash, char, tar, and waste water. Ash is collected in wet condition. Around 20% of rice husk is made up of ash and the ash coming from the gasifier contains 10 to 15% carbon by weight. The ash-laden water can be used as organic fertilizer or land filling purpose. The plant has on site storage facility for ash. Char can be transformed into charcoal which is used as a domestic fuel for cooking and heating. Tar can be either recycled or burnt in the gasifier or used as black paint for the wooden materials like boat, wooden structures and construction of roads. In case of a downdraft gasifier, there will be little liquid emission from the plant. The plant has onsite storage facility to deposit waste water which needs to be changed in every three month.

 

Source : Renewable Energy Information Network, Bangladesh

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