How to write data to excel file and download it in asp.net

I use the EPPlus package, which you can install via Nuget. It allows you to load data onto an Excel worksheet directly from your datatable, and it includes support for things like formatting on the worksheet (fonts, column widths etc). See their documentation page here on using it inside a web application.

For your case, I would suggest something like:

DataTable ProductDetails = sql.ExecuteSelectCommand("SELECT *  FROM Products_Details_View WHERE Supp_Id = " + Session["SuppID"].ToString() + " and Is_Available = 1"); 

using (ExcelPackage pck = new ExcelPackage())
{
    //Create the worksheet
    ExcelWorksheet ws = pck.Workbook.Worksheets.Add("Demo");

    //Load the datatable into the sheet, starting from cell A1. 
    //Print the column names on row 1
    ws.Cells["A1"].LoadFromDataTable(ProductDetails, true);

    //Write it back to the client
    Response.ContentType = "application/vnd.openxmlformats-officedocument.spreadsheetml.sheet";
    Response.AddHeader("content-disposition", "attachment;  filename=ProductDetails.xlsx");
    Response.BinaryWrite(pck.GetAsByteArray());
}

Try this

string attachment = "attachment; filename=xxxx" + DateTime.Now + ".xls";
Response.ClearContent();
Response.AddHeader("content-disposition", attachment);
Response.ContentType = "application/ms-excel";
StringWriter stw = new StringWriter();
HtmlTextWriter htextw = new HtmlTextWriter(stw);
ProductDetails.RenderControl(htextw);

GridView dg = new GridView(); //Create an empty Gridview to bind to datatable.
dg.AutoGenerateColumns = true;
dg.DataSource = ProductDetails;
dg.DataBind();

dg.RenderControl(htw);
Response.Write(stw.ToString());
stw.Close();
Response.End();